journal of geophysical research, vol. 92, no. b10,...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. B10, PAGES 10,237-10,266, SEPTEMBER 10, 1987

CHRONOLOGY AND PYROCLASTIC STRATIGRAPHY OF THE MAY 18, 1980, ERUPTION OF MOUNT ST. HELENS, WASHINGTON

C. William Criswell

Department of Geology, University of New Mexico, Albuquerque

Abstract. Many timed observations make it possible to subdivide the 9-hour Plinian eruption of Mount St. Helens on May 18, 1980, into six phases, defined by eruption style. The phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits. The suite of pyroclastic deposits indicates that the eruption became more pumice-rich and compositionally diverse with time, perhaps owing to concurrent eruption of less evolved, gas-poor parts of the magma body with the more evolved, gas-rich parts. The paroxysmal phase I (0832- 0900) consisted of landslides, lithic pyroclastic flows of a lateral blast and other explosions, and a weak pre-Plinian column.' Phase I pyroclastic deposits include lithic ash flow deposits intercalated with and overlying •the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer. The early Plinian phase II (0900-1215) consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks (1010-1035), a brief period of lithic ash ejection (1035-1100), and a pumice-rich pulse that accompanied growth in height of the eruption column (1100-1215). Deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that

(1715-1815) eruption intensity waned but included a brief episode of small pyroclastic flows (1745- 1815). Phase V deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice, and minor fine-ash deposits. Phase VI activity (1815 to May 19, 1980) consisted of a low-energy ash plume, with transient increases in intensity, while seismicity continued at depth. Sustained vertical discharge of phase II produced evolved dacite with high S/C1 ratios. Ash flow activity of phase III is attributed to decreases in gas content, indicated by reduced S/C1 ratios and increased clast density of the less evolved, gray pumice. Climactic events are attributed to vent clearing and exhaustion of the evolved dacite.

Introduct ion

On May 18, 1980, Mount St. Helens (MSH) erupted catastrophically with massive landslides and devastating pyroclastic surges and flows, followed by more localized pyroclastic flows and widespread tephra falls during a 9-hour Plinian eruption. Although the initial sequence of events was resolved to a few seconds [Voight, 1981], descriptions of the 9-hour period are vague. Harris et al. [1981] reported hourly

include the lower pumice lapilli layer, the lower measurements of eruption column height but did lithic ash layer, and the middle pumice lapilli layer, all of which consist of evolved white dacitic pumice (63-64% Si02). During the early ash flow phase III (1215-1500) the height of the eruption column decreased, vertical ejection of tephra ceased, and pyroclastic flows were fed from intermittent fountains. Phase III deposits consist of a poorly exposed sequence (_<12 m) of ash flow tuff that consist of many thin flow units (_<2 m each) containing pumiceous white dacite (63-64% SiO 2) and denser, gray silicic andesite (61-62% Si02), and fine-grained ash cloud deposits interbedded with a nongraded middle pumice ash layer. The climactic phase IV (1500-1715) developed in two stages: fountain-fed pyroclastic flows, followed by a short pulse (1625-1715) of vigorous vertical ejection of tephra. These stages were accompanied by the peak seismic energy release and peak eruption column height, respectively. Climactic deposits consist of a thick (_<35 m) sequence of thick, lapilli-rich ash flow sheets (4-12 m each) with white and gray pumice, and streaky scoria bands (60% SiO 2) in pumice breccia clasts, and the reversely-graded, upper pumice lapilli layer that is interbedded with fine-grained ash cloud deposits. During the late ash flow phase V

Copyright 1987 by the American Geophysical Union.

Paper number 6B5942. 0148-0227//87/006B-59425 05.00

not interpret processes of column formation. Rowley et al. [1981] reported observations that a change in eruption style from tephra ejection to ash flows occurred at about 1217 and that variable rates of magma discharge and seismic tremors [Malone et al., 1981] continued until 1730 but stated that heavy cloud cover prevented detailed observations. I take the position that compilation of visual sightings from many observers resolves critical changes in eruption behavior and that the changes are manifest in specific chemical and physical characteristics of the pyroclastic deposits.

This paper presents a chronostratigraphic account of the Plinian eruption in which the geologic record is keyed to observed eruption phenomena. I subdivide the 9-hour eruption into six phases (Table 1), based on observed eruption behavior, correlate each phase with stratigraphic characteristics of the pyroclastic flow and tephra deposits, and interpret eruption mechanisms from empirical correlations of eruption behavior, compositional variations (this paper), seismicity [Shemeta and Weaver, 1986] and soluble volatile data [Stoiber et al., 1981].

The character and c•hronology of the eruption was compiled from eyewitness interviews, analysis of timed photographs (Figure !) and excerpts from the U.S. Forest Service (USFS) radio log (Table 2). The log records timed, radio transmissions from USFS and U.S. Geological Survey (USGS) personnel aboard light aircraft in the vicinity of MSH and the devastated area; only information

10,237

10,238 Criswell: Chronology and Pyroclastic Stratigraphy of Mount St. Helens

TABLE 1. Summary of Phases of the May 18, 1980, Eruption of Mount St. Helens and Correlations of Pyroclastic Deposits

Time hours

Eruption Phase/Style a D•Dosits

Pyroclastic Flow a,b,c - Tephraa, c

I 0832-0900 Paroxsymal: Massive landslides; lateral blast and 6ther explos$ons; slow growth of pre-Plinian column.

Basal Sequence (bpf): blast in widespread devastated area; !ithic rich ash flows in NFTV.

Morning Sequence: basal Pumice lapilli (tl); pisolitic and silty lithic ash (t2);

II 0900-1215 Early Plinian: nonvigorous minor vertical ejection of lithic and pumice tephra; small py•o- clastic flows on upper flanks of cone.

lower pumice lapilli (t3); lower lithic ash (t 4); middle pumice lapilli (t5); lower distal layer.

III 1215-1500 Early Ash Flow: intermittent ash fountains, ash cloud plume from pyroclastic flows in NFTV.

Lower Sequence (lpf): thick? sequence of thin pumice flows in NFTV

Afternoon Sequence: lower fine ash (t 6); middle pumice ash (t 7); upper distal layer;

IV 1500-1715 Climactic: continuous? ash fountains, pyroclastic flows on all flanks, then strong vertical ejection of tephra (late Plinian).

Middle Sequence (mpf): upper pumice lapilli (ts); thick sequence of upper •ine ash (t 9); thick lapilli and ash upper distal layer; flows in NFTV, Plains of Abraham, and upper Muddy River.

V 1715-1815 Late Ash Flow: waning intensity; small pyroclastic flows in NFTV.

Upper Sequence (upf): thin sequence of lapilli and ash flow tongues in NFTV.

upper fine ash (t9); distal deposits doubtful;

VI 1815-5/19 Posteruption: weak tephra emission with transient intensity fluctuations.

None capping fine ash (t!0); no distal deposits.

Timing and relative intensity are shown schematically in Figure 25; stratigraphic correlations are d.epicted in Figure 2. PDT, Pacific Daylight Time; NFTV, North Fork Toutle Valley,

aThis paper. bHoblitt eta!, [1981], Waitt [198!], and Moore and Sisson [1981]. CRowley et al. [1981]. d•r•a-Wojcicki et al. [1981a, b], and Waitt and Dzurisin [1981].

televent to the present discussions are reported. The published characteristics of the proximal

tephra deposits are augmented in this paper with new data and interpretations concerning the proximal deposits. The composite stratigraphic column of Waitt and Dzurisin [1981] is refined on the basis of new field evidence and correlations with the eruption chronology. I group the deposits of the multilayered tephra lobe into morning and afternoon tephra sequences and correlate these with the lower and upper distal tephra layers, respectively. I recognize four major sources of tephra: (1) fine ash from the initial lithic pyroc!astic surges and flows• not completely addressed in this paper, (2) Plinian

lapilli and ash lofted by a vertical eruption column over the central vent that was be•t developed in the midmorning and a brief, vigorous interval in the late afternoon, (3) fine ash removed from the pyroclastic flows of the afternoon deposited in areas near source, as well as transported hundreds of kilometers by high- altitude winds, and (4) ash and rare lapilli convectively removed from the ash fountains that supplied the pyroclastic flows. Ash clouds from the lateral blast achieved the greatest altitude (>25 km) as a mushroom cloud over the devastated area [Hoblitt et al., 1981; Waitt, 1981; Moore and Sisson, 1981] that was mistaken for the Plinian•column by Harris et al. [1981] and Moore

Criswell: Chronology and Pyroclastic Stratigraphy of Mount St. Helens 10,239

Satellite note 9

i

__ SLAR

note 8

.... D.A. Swanson notes

-- 2, 3, 4 - --

i

i . Seibert note 1

Notes:

1. 35 mm color ground oblique (position 20 km S of MSH); photo times estimated during interview; photos privately owned, copies on file USGS Photo Library, Cascades Vol- cano Observatory, Vancouver, Washington.

2. 35 mm, color aerial oblique photographs, selected frames available USGS Photo Library, Denver, Colorado.

3. Time blocks depict aircraft departure and arrival times at Pearson Air Park, Van- couver, Washington (from USFS radio log).

4. Photo times from field notes. 5. Clock registered with frame.

2100 Sunset i R.P. Hoblitt notes

2, 3, 5 R.M. 2000 - Krimmel

s__--- 1900

-_

R.L. Christiansen _ notes 2, 3, 4 $•

$• s•

s---- •= s•

• ! J'G' Rosenbaum $• notes 2 3, 5

• A. Post

i notes 6, 7 •t R'M' Krimme 1

1800

1700

1600

1500

1400

1300

1200 1100

1000

0900

Eruption 0800

6. 240 mm b/w aerial oblique photographs; photo times estimated by frame position on roll; copies available EROS Data Center, Sioux Falls, South Dakota.

7. Time blocks from Krimmel and Post [1981]. 8. X-band, Side-Looking Airborne Radar images by

Oregon Army National Guard [Rosenfeld, 1980; Criswell and Elston, 1984].

9. GOES-West satellite images (scale 1:13,000,000); after 1700 only full-disc IR images available (scale 1:50,000,000); National Oceanic and Atmospheric Administration, Washington, D.C.

Fig.1. Summary of photographic record of the May 18, 1980, eruption of Mount St. Helens used in this study. Time blocks indicate observers-photographers presence in eruption area, dashed where approximate; s, stereo photo pairs; m, 16-mm movie film.

and Albee [1981]. Ash clouds from the pyroclastic flows formed an elongate column that extended 4-8 km north of the vent and supplied the high-altitude ash plume during much of the afternoon when no vertical (?linian) flux occurred from the central vent.

Most pyroclastic flows ponded in the upper North Fork Toutle Valley, hereinafter referred to as the Toutle Valley. Deposits are informally subdivided in this paper into four sequences, each consisting of several flow units. The basal pyroclastic flow sequence includes lithic-rich deposits of the lateral blast and other previously unrecognized explosions. The pumiceous pyroclastic flows of the Pumice Plain [Rowley et al., 1981; Kuntz et al., 1981] emplaced during the afternoon eruptions, are subdivided into lower, middle and upper pyroclastic flow sequences, defined by distribution, surface morphology, textural and compositional features. Observations of flow emplacement were limited to positions of flow margins, but I believe that these allow correlations of flow unit contacts to observed flows. At this date,

the lower sequence remains poorly exposed beneath the thick, middle sequence associated with the climactic eruptions.

In this paper, the term Plinian describes a vertically directed jet of gas and particulate materials, in the sense of Wilson et al. [1978, 1980], and does not imply eruption intensity or tephra dispersion. Lithic describes relatively dense, slightly vesiculated, juvenile clasts, as well as rock fragments accidentally incorporated during eruption. Pumice describes vesicular juvenile magma and does not imply grain size. Pyroclastic terms block (>64 mm), lapilli (<64 to >2 mm), ash (<2 mm), and fine-ash (<1/16 mm) are used for all clasts, whether lithic or pumiceous [Schmid, 1981]. Distances are measured from the preeruption summit of MSH; times are hours, Pacific Daylight Time (?DT).

Phases of the May 18, 1980, Eruption

In this section I describe the chronology and character of eruptive events, summarize the stratigraphy (Figure 2) and interpret the events


TABLE 2. Excerpts from the USFS Radio Log and Other 0bsmrved Events of May 18. 1980

Time hours PDT Ob s ervat ions Refer ence s a 0832 massive landslides; lateral blast 1, 5 0842 2 ash columns; ash shroud over vent 2 0852 dark-toned, convective clouds in NFTV 2, 3 0920 ash flow in NFTV stopped 3 0925 vertical eruption column, not too vigorous 4 1015 pfs upper S flanks; some lightning 4, 6 1033 slight intensity increase, column black 6 1036 intensity decrease, tone lightened 6 1115 eruption intensity decrease, not too violent;

darker tone 6 1200 lahars forming in NFTV 12 1217 intensity increase, column tone lightened 4 1218 pf SE flank 6 1220 pf on lqW flank to NFTV, not moving rapidly;

column colored dirty white 4, 6 1222 pf to South Fork Toutle; not to timberline 6 1225 pf volume to NFTV increasing; pf to SE 6

dissipated; some mudflow 1228 pf to NFTV, very intense and vigorous 4, 6 1236 wall of water going down NFTV 6 1248 pf increase in volume continuously 6 1253 pf SW side, not to timberline; some mudflow 6 1320 pf on SW flank, not to timberline 6 1358 pf on NW flank to South Fork Toutle 6, 8 1407 ash column extends 7 km N of mountain 6 1420 pf down E flank; some mudflow 6 1453 pf continue intermittent 6 1501 pf to E side Spirit Lake 6 1511 column tone change, intensity increase 5, 6 1521 slight intensity increase 6 1527 pf on NW flank to South Fork Toutle 6 1528 pf to NFTV and South Fork Toutle 6 1544 pf W side; pf E side to Plains of Abraham 6 1546 pf SE flank and Shoestring Glacier 6, 11 1548 increase in lightning activity 6 1551 large pf on W side to South Fork Toutle 6 1554 pf on SW flank toward Kalama River 6 1556 pf S rim 6 1557 pfs on W, SW, and SE flanks 6 1605 slight intensity decrease

pf on E and SE flanks and down Shoestring Glacier 6

1611 pf W flank to South Fork Toutle Valley 6 1622 intensity increase 6 1628 tremendous vertical acceleration; intense

lightning 6, 7 1630 vertical event continuing 6 1632 very vigorous eruption column sucking in

cumulus clouds 6, 7 1635 pf flux abated in NFTV 6, 7 1644 column still vigorous 6, 9 1654 eruption continuing 6, 9 1726 intensity decreased; no discrete bursts

as before 6 1745 pf in the upper NFTV 6, 9 1747 pf to Spirit Lake 6 1752 force of eruption diminished 6 1801 slight intensity increase 6 1808 intensity increase, lots of volume 6 1810 pf to NFTV 6 1830 intensity decreased considerably 9 1905 secondary explosions in NFTV 9 1958 eruption diminished considerably 6, 10 2015 continuous lahar in NFTV, water gushing out

isolated holes 2054 many secondary explosions in NFTV

10 6, 10

Criswell.' Chronology and Pyroclastic Stratigraphy of Mount St. Helens 10,241

TABLE 2. (cent inue4 ) Time hours PDT Observat ion• References

2114 secondary explosions continuing 6 2209 slight intensity increase in very low

eruption column 6 2301 slight intensity increase then return;

lightning increased 6 2323 slight intensity increase, then return;

some lightning 6 2343 slight intensity increase 6 2349 ash plume died down, •ot much left

The log does not identify individuals, although most observations were made from light, fixed-wing aircraft containing USGS personnel of Figure 1. pf, pyroclastic flow; NFTV, North Fork Toutle Valley.

aReferences: 1, Voight [1981]; 2, eyewitness account by K. Seibert, Vancouver, Washington; 3, eyewitness account by M. Huntting as cited by Foxworthy and Hill [1981]; 4, eyewitness account by D.A. Swanson, USGS; 5, Rosenbaum and Waitt [1981]; 6, USFS radio log, May 18, 1980; 7, eyewitness account by J.G. Rosenbaum, USGS; 8, Criswell and Elston [1984]; 9, eyewitness account by R.L. Christiansen, USGS; 10, eyewitness account by R.P. Hoblitt, USGS; 11, Rooth [1980]; 12, eyewitness account by H. Glicken, USGS.

Proximal Deposits Flows

Phase Time

VI '

V 1745_ -- 1715-

1625-

IV rnpf

-- 1500-

-- 1215-

Tephra Eruption Phenomena

Climactic events

1035- I010- • minor pyroclastic flows

-- 0900- t2-' ' Pre-Plinian lother explosions I .

I

-- 0852-

tl% • Weak ash emission t9

Late Plinian

pyroclastic flows ash cloud plume

t5

t4 Early Plinian t$

I debris avalanche I lateral blast hours P DT

Eruption Phases Pyroclastic Flow Deposits Proximal Tephra Depos{ts VI Posteruption t10 capping fine ash V Late Ash Flow upf Upper Sequence t 9 upper fine ash IV Climactic mpf Middle Sequence t 8 upper pumice lapilli III Early Ash Flow lpf Lower Sequence t 7 middle pumice ash

t 6 lower fine ash II Early Plinian pf undivided t 5 middle pumice lapilli

t 4 lower lithic ash t 3 lower pumice lapilli

I Paroxysmal bpf Basal Sequence t 2 pisolitic ash t 1 basal pumice lapilli

Distal Tephra

Upper

Lower

Afternoon Sequence

Morning Sequence

Fig. 2. General stratigraphic and chronologic correlations of eruption phases and pyroclastic flow (Figure 6) and tephra (Figure 8) deposits of the May 18, 1980, eruption of Mount St. Helens.


lower : i i , , . , i & . i ,. , .

•ass flux i fine-ash tephra la...p.i•i tephra : ......._ •;e nt -'•\: : ! lapilli tephra •.•'•-' erosion -v , _.J• :ooo •"

I"blast" •- '" • • \ I f i ash flows iapilli flow ./. :'. : , ,! , ,: , : ß

Eruption column height (over sea level)(2) / Imushroom cloud(3) i ! __,,.--•..• t20 /• z • ß Late'X. : I... krn Early Plinian :ash-cloud plume Plinian " ' ' I I I I ! I I I .•1: :

I. Relative intensity of : ß ß . - . ._

,i •' !

•.5ICumuletive •.eismic :' 3-6 km ß . IO25• ß '

12+• d9oo' 5oo

Time (hours-Pacific Daylight) Fig. 3. Empirical correlations of eruption phases (Table 1 and Figure 2) and relative mass flux (Figure 25) with other data sets. The dashed line connecting the sulfur/chlorine (S/C1) points represents time series samples, discussed in the text; other points represent values from lower (morning) and upper (afternoon) tephra at other localities. Processes of eruption col•n fo•ation are superimposed on the modified column height curve. References are (1) Stoiber et al., 1981 (2) Harris et al. [1981], (3) Moore and Rice [19843, (4) Scandone and Malone [1985], (5) Shemeta and Weaver [1986].

in summary discussions. I emphasizes new information but draw on all relevant observations. The assigned times of phase boundaries represent given events, although changes occurred over periods of 15-20 min.

Paroxsvmal Phase I (0832-0900 _

Observed events. Descriptions and discussions of the events, effects and deposits of the momentous landslide and lateral blast can be found in the works by Hoblitt et al. [1981], Waitt [1981], Moore and Sisson [1981], Rosenbaum and Waitt [1981], Voight [1981], Voight et al. [1981], Hickson and Barnes [1982] and Moore and Rice [1984]. These studies indicate that the eruption began at 0832 with sector collapse;

III I• V' 'Vl' explosion cloud formed rapidly and expanded into a fast moving pyroclastic surge that devastated

I.•6•t nearly 600 km 2 of conifer forest in rugged S/CI ratios of I

upper ; leached tephra(•) '2• terrain. The blast reached its maximum lateral ß extent in 5-7 min. Ash clouds from the flow formed an annular, mushroom cloud that lifted to 30 km height by 0900 (Figure 3).

My investigation of the beginning of the ¾1inian eruption indicates that the lateral blast was followed by a relatively light-toned ash fountain that required about 30 min to form a vertical (Plinian) eruption column. The Seibert photos (Figure 1) clearly record events that were not apparent or were overlooked in other photo sequences. Figure 4a, taken about' 0845 shows a relatively weak column of ash that deposited tephra in areas immediately east of the volcano (in midground); this subvertical eruption column is termed the pre-¾1inian event in this paper. The dark curtain of ash in the background is due to the mushroom cloud. About 0845, the pre- Plinian column was accompanied by eruption clouds that collapsed into the Toutle Valley to produce pyroclastic flows; ash clouds from the flow unit are clearly recorded in Figure 4b. These ash clouds were witnessed from the NW by M. Huntting [Foxworthy and Hill, 1982], who described a sluggish flow of ash in the Toutle Valley from 0852 to 0920, but he could not determine its origin. The Seibert photo sequence confirms the timing of this event and establishes an origin from the MSH crater and not the mushroom cloud. I consider this flow the end of phase I.

Stratigraphy of phase I deposits. Deposits of phase I include (1) the debris avalanche and associated lahars [Voight et al., 1981; Janda et al., 1981; Cummans, 1981], (2) the basal pyroclastic flow sequence, which groups the lateral blast [Hoblitt et al., 1981; Moore and Sisson, 1981; Waitt, 1981] and lithic pyroclastic flow deposits in the Toutle Valley [Criswell and Elston, 1984; Glicken, 1986], (3) tephra deposits that include (1) pisolitic ash [Waitt and Dzurisin, 1981; Hoblitt et al., 1981; Moore and Sisson, 1981; Waitt; 1981] and (2) basal lapilli layer from the pre-¾1inian column. Complete descriptions of phase I deposits are beyond the scope of this paper.

Basal py•oclastic flow sequence: The basal pyroclastic flow sequence is a composite map unit of the Toutle Valley (Figure 5) that groups pyroclastic flow deposits of the lateral blast and later explosion events. Probable deposits of the lateral blast include local deposits on the flanks of MSH and friable, fines-poor lenses, typically _<0.5 m thick, interbedded with the debris avalanche deposits near Johnston Ridge. In the NW parts of the map area (Figure 5), deposits of the lateral blast underlie the debris avalanche. South of Spirit Lake, pyroclastic deposits _<3 m thick possess characteristics and stratifications typical of deposits of the

progressive block failure of the deformed north widespread devastated area [Hoblitt et al., 1981; flank and summit regions of the volcano continued Waitt, 1981; Moore and Sisson, 1981]. These for a few minutes. The massive landslides farmed a large debris avalanche in the Toutle Valley that required about 10 min for deposition, although settling and dewatering continued for hours, producing local a•nd far-reaching lahars [Janda et al., 1981; Cuemaas, 1981]. Seconds after the first slide block, a dark-toned

deposits consist of a fines-poor ground layer (_<1/2 m), underlain by preeruption rocks, overlain by a massive, ash-rich and poorly sorted massive layer (1-2 m) that is overlain by a thin (_<4 cm) layer of bedded ash and fine ash. These are overlain by the basal lapilli and pisolitic tephra layers, described below.


Fig. 4. Photographs by K. Seibert of phase I volcanic events that followed the initial lateral blast. Views to north from 30 km south. (a) Weak pre-Plinian column about 0845; (b) ash clouds and pyroclastic flow in the Toutle Valley, photo taken about 0900.

Probable deposits emplaced after the lateral blast, <1 to _>10 m thick, appear to be largely confined to the central and western map areas (Figure 5), where they widely overlie the debris avalanche (figure 6) and possess a smooth surface morphology that locally subdued the irregular topography (Figure 7). Deposits are texturally similar to those of the lateral blast, being locally stratified with massive, ash-rich deposits (_<12 m thick) locally underlain by fines-poor, clast-supported lithic ground layers

and widely overlain by the pisolitic ash layer. Juvenile clasts comprise 25-50% by volu•e locally .

and consist of lithic, gray dacite similar in texture and composition to clasts in the lateral blast deposits, as well as local accumulations of white pumice (see section on chemical varia.- tions). Relatively low eraplacement temperatures are suggested by the abundance of incipiently charred timber fragments. The stratification characteristics and locally abundant degassing pipes, rooted in the ground layers suggest


122ø14 ' I

Ridge nested explosion crater

1984 level

May 18, 1980 0 .5 I km 2 i i i i

::'•n• S F -

'•""•• + s•'- /.•. fault

scarp 1400 K

Time (hrs, PDT) of observed flows

I

Fig. 5. Generalized geologic map of upper Toutle Valley; deposits same as Figure 2; localities: SFT, South Fork Toutle Valley; SC, Sheep Canyon, K, Kalama river; MR, upper Muddy River; and PA, Plains of Abraham. Margins and times of observed pyroclastic flows shown on flanks of MSH. Topo- graphy and vent from USGS maps, July 1980.

similarities to "typical' pyroclastic flows [Sparks et al., 1973]. However, deposits differ from a pumiceous pyroclastic flow in that the dominant component is comminuted older rocks and crystals and the juvenile components are only locally pumiceous.

The pyroclastic units described above are rarely observed in contact with each other, and other deposits in the area serve to complicate the field relationships. Ash-rich deposits, up to 6 m thick, of comminuted older rocks and crystals, with •5% by volume juvenile materials, intercalate with the pyroclastic flow and debris avalanche deposits. Whether these ash-rich, juvenile-poor deposits are facies of the lithic pyroclastic flows, or facies of the debris avalanche, or combinations of both or are the products of phreatomagmatic explosions during the pre-Plinian event is not clear.

Basal oumice laoilli laver: The basal lapilli _ . -

layer (Figure 8) consists of a typically clast- supported, well-sorted layer of pumice and lithic lapilli that ranges in thickness from 3 to 10 cm on the NE flank of MSH and thins to a few millimeters of pumice and crystalline ash at distances •10 km eastward. Clasts range in size up to 10 cm locally (Figure 9) and consist of white pumice and denser, gray dacite similar in texture to those in the basal pyroclastic flow sequence.

Deposits rest on massive deposits of the basal pyroclastic flow sequence, on debris avalanche deposits, or on older bedrock. This layer may correlate with the pumice ash observed by Waitt [1981] in the pisolitic ash layer.

Pisolitic ash layer: The pisolitic ash layer forms a relatively thin (1-5 cm) but widespread marker bed that consists of finely comminuted old crystals that locally accreted to form ash- to lapilli-sized pellets. Throughout most of the devastated area, deposits of fine ash and accretionary lapilli form the capping layer of the lateral blast deposits [Waitt and Dzurisin, 1981; Moore and Sisson, 1981; Hoblitt et al., 1981; Waitt, 1981], and were shown to be thickest in areas just north of the Toutle Valley [Moore and Sisson, 1981]. In the central and western parts of the map area (Figure 5), deposits (!2 cm) of accretionary lapilli rest on the basal pyroclastic flow sequence; in the eastern parts of the map area, deposits rest on the basal lapilli layer (Figure 9).

Interpretations and discussions of the latter oarts of phase I. My investigation indicates that the timing of events after the lateral blast have been poorly described and are locally inaccurate. The combined eyewitness accounts and stratigraphic relationships argue against a simple, short-lived event, but suggest a complex sequence of explosions that produced surges and flows that continued for several tens of minutes. At least two similar flow units were emplaced, but flows may have been emplaced that were not observed. No observational or photographic evidence supports the interpretation that the Plinian column rose to a height >25 km immediately after the blast [Harris et al., 1981]. Many witnesses mistook the mushroom cloud (Figure 3) that rose from the devastated area [Sparks et al., 1986] for the eruption column, although the partial account of P. and C. Hickson [Rosenbaum and Waitt, 1981] indicated that several minutes elapsed before the vertical column developed. The Seibert photo sequence definitively depicts events during the latter part of phase I and indicates that the pre- Plinian column that followed the lateral blast grew in a sluggish manner and was accompanied by one or more pyroclastic flows that probably served to clear the vent.

Deposits of the pre-Plinian column are represented by the basal pumice lapilli below the pisolitic ash layer. As the column slowly grew vertically, the prevailing westerly winds winnowed the tephra eastward, where it was deposited prior to the pisolitic marker bed. The deposition of the pisolitic layer formed an interbedded time marker, which may also young slightly downwind. The pisalites are probably the result of moist accretion of the fine dust in the air after the phase I events and probably correlate with wet "mud balls" observed to fall in the areas east of MSH about 0900 [Rosenbaum and Waitt, 1981]. I believe, however, that the layer is probably related more to the lithic pyroclastic flows in the Toutle Valley than to the lateral blast. Evidence to support this is largely stratigraphic: (1) the pisolitic layer rests directly on deposits of the second flow unit (those on top of the debris avalanche deposits), but has intervening layers of coarse


locations .310 ;56

ph

ph unconformity • I• C

e•.e pe

•540 ac •o• - ph -•-•_.= • SI, SI7, . .., -. 18,19 .•••' '•- ""' .)) ..o' base

I J •)') locally • base I exposed • not i J•) ',exposed

I I j I I

ps - 15

0900 •isoliti• Qsh

pumice banded

S5 sample location

115/82 134/135 59

S20

SI3

SIO S7

S9

ac

0o breccia

IiO 142

Sequence

ß •--?J base base not locally exposed exposed

Middle Sequence

Lower Sequence

unconformity

ß '"( Basal Sequence

.•, lithics debris avalanche •)•, 1500(time)hours PDT '-?.•!"::.' ash

ac-ash cloud ps-pyroclastic surge ph-phreatic crater rim

Fig. 6. Representative stratigraphic sections of pyroclastic flow deposits in upper Toutle Valley. Lower unconformity is locally represented by fluvial and reworked deposits _<2 m thick; upper unconformity is manifested by channels incised into the middle sequence and partially eroded deposits of a phreatic pit crater. Chronologic and stratigraphic correlations with tephra deposits are shown in Figure 2.

to fine ash on deposits of the lateral blast [Waitt, 1981; Moore and Sisson, 1981; Hoblitt et al. 1981]; (2) the pisolitic ash layer is thickest in areas directly downwind of the Toutle Valley, as shown by isopachs of Moore and Sisson [1981], suggesting a source of ash from the Toutle Valley.

Stratigraphic relationships in the Toutle Valley suggest emplacement of at least two pyroclastic flow units during phase I. The first and most energetic event (the lateral blast) produced deposits that occur below and within the debris avalanche deposits, and extend into the widespread devastated area. The second and less energetic event was confined to the Toutle Valley, where deposits ponded in the lows of the irregular topography. I believe that the eruption of the second flow unit accounts for the fresh pulse of hot material from the vent [Moore

and Rice, 1984] and cloud IV in the report of Sparks et al. [1986] that extended the height of the mushroom cloud. My field investigations indicate that deposits of the second flow unit differ from deposits of the lateral blast by the presence of juvenile clasts that approach the texture of pumice; both deposits contain a large amount of older lithic debris.

I support the hypothesis that the phase I eruptions were probably phreatomagmatic and involved the depressurizing of the volcano's hydrothermal system, as well as magmatic gas release. This is indicated by (1) the presence of slightly vesiculated dacite, suggesting that gas exsolution may not have been the principal source of energy; (2) the presence of abundant comminuted older rocks as fine ash; and (3) the presence of pisolitic ash, suggesting the accretion of dust in a moist environment. These


Fig. 7. Photograph of phase I deposits in the upper Toutle Valley. Basal pyroclastic flow sequence mantles the hummocky debris avalanche deposits near location 310 (Figure 5); view to west.

tlo

t;; :-.F••t s ' ..'•s•,4 Q / t•

p u m iceous . •':•'• M I• .'•' '- '•1S2 • ;.•-, 625 '• • •.- •.,•_.•.'• • I•J ," ::• J" •;'•'- "•-,=,, .... •,m t• // _- ' ,•,.•• •:•,; ,. '". '• Morning t9 '-:'--• t6 - :- - - ;;-•,• •- •"•-o9oo t '"'•t2 'i'•bpf- • •'t2 Li'i t• t8 ••• 12 4 123 127 131 133 199 ....... locations

•54o •pf (I-2m) Scale 2 cm ] sts .. ß •= ß ß 1215 time hours PDT

, . b •?•.¾:.xxx (rio) • S2 sample location t5 :, ?]-•'-• •t7 .... •:,.,, ...... " - '½-. •,,•.•;• Afternoon Sequence

t2-•o• - - .• •-Z __ Morning Sequence t•- __•bpf / 137 107 lOG 104

locations

•g. 8. Eep•eseata•ve stratigraphic sec•oas, (a) ao•al •o aad (b) across •he d•spe•s•oa ax•s o• p•ox•mal teph•a deposits. C•a•a s•zes a•e d•ag•a•a•c. Heasu•ed sec•oa at locat•oa 133 •s sho•a •a bo•h •ave•ses •o• •efe•eace. S•a•g•aph•c ch•oaolog•c co•ela•oas •h p7•oclast•c •1o• deposits a•e sho•a •a •[gu•e 2.

Criswell.' Chronology and Pyroclastic Stratigraphy of Mount St. Helens 10,247

Fig. 9. Basal pumice lapilli (t 1) layer from the pre-Plinian column lies below the pisolitic ash (t 2) layer (location 345, Figure 11).

criteria have generally been accepted to indicate water-magma interactions.

•,:- -; - -, . The eruption column from the central vet required more than half an hour to achieve its vertical configuration and was fully vertical by 0925 when first observed by D.A. Swanson (personal communication, 1983). The vertical eruption column of the morning eruptions, referred to as the early Plinian column in this paper, was composed of numerous colloform cells that tended to rise in a clock- wise spiral motion that expanded and broadened upward (Figure 10); a mushroom-shaped cap formed near the tropopause [Rosenfeld, 1980]. The tephra was transported eastward as a relatively dark-toned plume by prevailing westerly winds [Sarna-Wojcicki et al., 1981a; Quinn, 1982].

For the most part, Swanson described the early Plinian activity as nonvigorous, with short-lived changes in the tone and texture of the column, apparem. tly related to variations in pumice and lithic contents. From about 1010 to 1035, Swanson observed small pyroclastic flows on the upper flanks of MSH produced by small, bulbous masses upwelling at the base of the column and spilling over the crater rim. These small flows descended only a few hundred meters. Small pyroclastic flows may have been emplaced in the crater breach area and possibly the upper Toutle Valley but were not observed in detail. The Swanson photo sequence indicates that during the period from 1035 to about 1100 the column assumed a darker tone, with rather small, poorly developed convection cells. The cells had distinctive "feathered" lower edges produced by particles

falling earthward as the cells lifted. From 1100 to between 1200 and 1215, the column exhibited a pumice-rich nature indicated by relatively large, light-toned, billowing cell structures.

• "• .;, , ,,..- ;-,,•'. Deposits attributed t phase II inclu e ( ) proximal tephra deposits of the morning tephra sequence that groups the lower and middle pumice lapilli layers, and the lower lithic ash layer, (2) lower distal tephra deposits [Sarna-Wojcicki et al., 1981a, b], and (3)minor pyroclastic flow deposits on the upper flanks of MSH. Small pyroclastic flows may have been eraplaced in the upper Toutle Valley, but no deposits have been recognized; deposits on the upper flanks of MSH have not been examined. The morning tephra sequence forms a relatively simple, graded fan that thins and fines with increasing distance from MSH. The generalized isopach map (Figure 11) shows the distribution in the proximal areas where layer distintion is prominent. My measurements indicate that (1) thicknesses are slightly greater than reported by Waitt and Dzurisin [1981], which I attribute to more detailed sampling, and (2) thicknesses increase toward MSH. At distances >--40 km, the sequence thins to 4-5 cm of coarse pumice ash (--<4 ram) that rest on -<1 cm of fine to pisolitic ash. The lower pumice and lithic ash layers appear to merge to form a lithic-rich layer, -<1 cm thick, overlain by -<3 cm of the middle pumice lapilli layer.

''' '•'"' i ';' ' - .' The lower pumice lapilli layer Fig re 8) consists of clast- supported, angular white pumice lapilli that is typically normally graded but is locally reversely graded. Many of the pumice clasts have a ragged, quenched surface texture with only a few broken sides, suggesting that the lapilli

10,248 Criswell.' Chronology and Pyroclastic Stratigraphy of Mount St. Helens

..::•.:•:::•::<::<.,..,:•.:: :.... ..<•::•.:;.•.-

ß :•...s•. .........

.... ...:?? :,:.:::*':"*;, ,, ......::..,:?, .***.......:

.............

Fig. 10. Photograph by D.A. Swanson of Plinian column during early Plinian phase II about 1100; view to east with west crater rim in foreground. Note vertically directed eruption style.

represent small, whole clumps of quenched magma. In most areas along the dispersion axis, the lapilli rest on a thin (_<1 cm) layer of reversely graded lithic crystal ash but, in southern areas of the tephra fan, rest on pisolitic ash.

' i ' ,' , . - : The lower lithic ash layer Figure 8) forms a distinct dark horizon through the morning tephra sequence. It consists of _>60% ash- to lapilli-sized andesite and minor basalt fragments (_<5 mm) and scattered

I I I I

- ---- :5 Afternoon • 104-22'3ø"- -- -:5 Morning • __•..6__ •-••_ isop(•ch contours in cm. •• .•6; -:sx ' ) '

Spirit .Lake / 12

_ ' ß . • ; ß ß w• ; ,

MSH 122 ø 45' ß I I I I

Fig. 11. Distribution and generalized thickness map of proximal tephra deposits sundivided into the morning and afternoon tephra sequences, discussed in the text. Large dots and numbers denote locations of measured sections (Figure 8); small dots are other data points.


Fig. 12. Photograph of proximal tephra deposits near location 352 (Figure 11). Arrow depicts contact of morning and afternoon tephra sequences; layer symbols same as Figure 2. Total thickness of section is 35 cm.

pumice lapilli (--<2 cm). The contact is locally gradational to the underlying lower pumice lapilli layer, suggesting that some of the ash filtered down through the coatset pumiceß

•',, - . .,' - o,' ' o - .' The middle pumice lapilli layer consists of moderately well sorted, reversely graded to nongraded, angular, clast-supported white pumice lapilli and a small proportion (20-30%) of accidental lithic lapilli and ash; the higher proportions of lithics are typical of the upper part of the layer. Pumice clasts are almost entirely bounded by broken surfaces, probably cooling cracks, suggesting that the clasts were once much larger. A few small (_<2 cm) fragments of gray pumice occur locally and appear similar to blocks in the lower pyroclastic flow sequence (see section on chemical variations). The contact with the lower lithic ash layer is sharp and marked by an increase in pumice size and a decrease in lithic size and abundance (Figure 12).

ß o

The morning tephra sequence exhibits multiple layers that correlate with observed variations in the tone and texture of the early Plinian column (Figure 2). The lower pumice lapilli layer was probably deposited from the Plinian column from about 0900 to about 1100. Waitt and Dzurisin [1981] attributed this pumice-rich layer to a strong initial pulse, which I believe occurred not directly after the lateral blast but prior to, or contemporaneous with, the local mass, flux maxima from 1010 to 1035, when small pyroclastic flows were observed on the upper flanks of MSH. The lower lithic ash layer that forms a dark horizon through the tephra sequence probably represents the darker and more pumice-poor

eruption column between 1035 and 1100. These changes in the eruption column may represent slight vent widening that subsequently allowed a greater mass flux or possibly ,increased lithic loading by rockfalls from the crater wall. The latter interpretation is supported by similarities between andesite and basalt in the modern crater wall and the lower lithic ash layer. I concur with Waitt and Dzurisin [1981] that the middle pumice lapilli layer was then deposited by the late morning Plinian column (1100-1215) and reflects the light-toned, billowy convection cells. The light tone of the late morning column is, in turn, interpreted to have resulted from an increase in pumice abundance and sizes as well as possible reduced rockfall activity and, hence, reduced lithic loading of the column.

The overall reverse grading of the morning tephra sequence appears to reflect the increasing height of the eruption column, which may have been caused by increasing magmatic gas content (Figure 3). The height of eruption columns has been associated with the degree of eruption intensity [Wilson et al., 1978]. Stoiber and Rose [1970] and Rose [1977] showed a positive correlation between eruption intensity and calculated S/C1 ratios. At MSH, Stoiber et al. [1981] reported that calculated S/C1 ratios from ash leachates increased from 0.80 to 1.22 in time series samples of the morning tephra sequence, indicating increased magmatic gas contributions during phase II. I correlate the high S/C1 ratios with increasing column height (Figure 3) and attribute the vertically sustained eruption column of phase II to high magmatic gas release from the upper part of a zoned magma column. The (volatile-poor?) top of the magma column was


Fig. 13. Photograph by J.G. Rosenbaum of elongated ash cloud plume from pyroclastic flows during phase III at 1433. View to east. Note morphologic differences between ash cloud plume and Plinian columns (Figures 10 and 15b).

involved in the paroxysmal activity of phase I; [1980], shows the ash cloud plume to be displaced during phase II, the volatile-rich portions of northward from the vent. The spiral effects of the chamber were partly evacuated. the early Plinian column were not observed; the

Scandone and Malone [1985] defined the possible colloform cells were vigorously convective, with shape of the magma chamber on the basis of little or no loss of material or "feathering ' hypocenter locations. They hypothesize a chamber observed earlier. The light-toned tephra of 10-20 km 3, with a top at 7-9 km depth continued to be transported eastward by the connected to the surface by a narrow conduit of 50 prevailing winds (see section on distal tephra). m radius. Shemeta and Weaver [1986] reported that During the early part of the phase, Swanson seismicity during phase II consisted of numerous observed that the mass eruption rate, indicated by low-magnitude, high-frequency earthquakes that the vigor, seemed to increase continuously. Later were mostly from shallow depths (-<3 km) but however, J.G. Rosenbaum (personal communication, included events with deeper hypocenters (-<6 km), 1983) observed that from 1330 to 1500, pyroclastic possibly representing the initial disruption of flows erupted more episodically; the flows the rocks adjacent to the top of the chamber. The originated as buoyant masses that rose within the gray pumice clasts in the middle pumice lapilli summit crater and moved northward through the layer may have come from the initial tapping of deeper, less evolved parts of the chamber.

Early Ash .. ',.-- - •! -

e, - -; - -, . After 1200, a gradual change in the color and vigor of the eruption column occurred. The photographs by Krimmel and Post [1981] indicate that very light toned cells began to erupt discontinuously and eventually formed a

breach, much as a "pot boiling over" [Rowley et al., 1981]. Most of the pyroclastic flows were emplaced in the upper Toutle Valley, although a few small flows spilled over the west crater rim at the onset of the phase. A small flow on the west flank recorded by a side-looking airborne radar (SLAR) image at 1350 [Criswell and Elston, 1984; Rosenfeld, 1980] correlates with a pumiceous lahar observed in the South Fork Toutle Valley at 1400 [Janda et al., 1981]. The Rosenbaum photo

much lighter toned column; the tonal lightening is sequence (Figure 1) suggests that between 1430 and apparent in the 1215 NOAA satellite image of 1215 (Figure 1 [Foxworthy and Hill, 1982]). By 1217, D.A. Swanson (personal communication, 1983) observed that the tonal lightening was accompanied by pyroclastic flows that were poured through the crater breach. The configuration of the eruption column was significantly changed from that of phase II. A pall of ash rising from the pyro-

1500, the times between flows seem to decrease and the sizes of flows increased, suggesting the transition to the climactic phase IV eruptions.

'' ,, . .,..- ,-..-' . Deposits of phase II• consist of (1) several pumiceous ash flow tuffs ponded in the upper Toutle Valley, informally termed the lower pyroclastic flow sequence in this paper, (2) proximal tephra

clastic flows created an elongated ash cloud plume deposits of the afternoon tephra sequence that that extended 4-7 km north of the crater; no include the lower fine ash and middle pumice ash vertical flux was observed from the central vent. layers, thin deposits of interbedded fine and Figure 13, typical of numerous photographs taken coarse pumice ash in the proximal areas (_<40 kin), by Swanson and Rosenbaum (Figure 1) and Rosenreid and (3) upper distal tephra deposits [Sarna-


In the western area of the Pumice Plain (Figure 6), the lowest flow unit (_<3 m thick) is exposed in a tributary south of the former North Fork Toutle River and contains banded white and gray pumice in lobate concentration zones. This unit rests on undisturbed pisolitic ash of phase I and is overlain by its ash cloud layer (_<2 cm) and a thick sequence of middle pyroclastic flow sequence.

On the west side of the nested explosion crater (Figure 5), lower sequence deposits are exposed

-J below the lithic breccia zones of the middle • sequence. These deposits consist of at least 10 .

:;• graded flow units, typically 20-160 cm thick ., each, that accumulated to a thickness of 10-12 m •' (Figure 14). Each flow unit is vertically graded

-•-•?•._•!. from a massive, poorly sorted, ash-rich zone '"•'• containing a high proportion of glass shards to a • pumice-lapilli zone at the top. Lithic fragments

comprise _<2% of clasts in the massive zone and --. rarely exceed 1 cm; pumice clasts at the flat

flow tops rarely exceed 15 cm. Pumice clasts consist of both the expanded, white pumice and the denser, gray pumice. These deposits locally

• rest on reworked, fluvial deposits (_<1.5 m thick) • or on a thin veneer of bedded ash cloud, or piso- •' •' ' ' % litic ash. Slightly west, within the deep .:'•'•'•. • erosional exposures along the North Fork Toutle . -•,..,;j•' ........ .... ; - -;?..'••'• .......... •:•:. •'•:•"- . River, deposits terminate abruptly against the ':• ........ '::?•%;%"•" •"•"-'•'•:'• rim of a collapse pit that is filled with

*':•'•';• ......... •':•:•;• .... -•'• deposits of the middle pyroclastic flow sequenceß ---:• ..... ,,- ,--.:, - : The lower fine-ash

:.•. ':'•'• .... layer consists of bedded, light gray, fine ash of •//•- ..... e-', •' . . : mostly glass shards, broken crystals, and rare ...... •%.•:.]-... •z...:•:•[• j• •:•::-'[: ........ - •.•••.•••••••w• ....... ..• • L..• ........... ' -•.s white pumice lapilli (•5 •). In the deep ......... . .......

" .'•:"%:•4•<:":•:•.•.;.:•.•..•:..•½•'•?': ........ . ............. •.•q•;i':4• ................................................................ .....•.•...•.t•:..• exposures of the Pumice Plain, deposits underlie Fig. 14. Photograph of pyroclastic flow deposits exposed in deep gully on the western side of the nested explosion crater (Figure 5). Large arrow denotes contact of thick deposits of the middle sequence (mpf) overlying thin flow units of the lower sequence (lpf). Lithic breccia zone of mpf lies about 1 m above the contact; the full thickness of the mpf deposits is not shown but is truncated by bedded deposits of a phreatic crater rim (ph) at top of frameß Note meter stick at small arrow for scaleß

Wojcicki et al., 1981a, b]. The afternoon tephra Sequence forms a relatively broad fan of interbedded fine and coarse pumice ash attributed to

the lower pyroclastic flow sequenceß In the areas east of Spirit Lake (Figures 5 and 11), deposits _>3 cm thick widely overlie lapilli and lithic fragments of the morning tephra sequence and locally rest on the pisolitic ash layer. Across the axis of the tephra fan (Figure 11), deposits thin rapidly to a few millimeters (Figure 8)where they coat the topmost clasts in the middle pumice lapilli layer and form a meager matrix in the middle pumice ash layer. They are absent in the southern parts of the tephra fan.

•',, - , ,,' - .., . - : The middle pumice ash layer consists of nongraded, coarse pumice (40%) and lithic (30%) ash with rare scattered pumice lapilli (_<1 cm). Throughout most of the proximal tephra area (Figure 11), deposits are <2-3 cm thick; rarely, the ash layer filled gaps

removal of material from the pyroclastic flows and between the coarse underlying lapilli and did not feeder ash fountains. The layers are distinct to form a unique layer. Near Spirit Lake deposits, distances of _<25 km but appear to merge into a few rest on the lower fine-ash layer and have a millimeters of fine ash at distances of 40 km.

.,' , . - ' ., ', -, -: The lower pyroclastic flow sequence consists of a poorly exposed sequence (_<12 m) of relatively thin ash flows (<1-2 m each) that unconformably overlie deposits of the basal pyroclastic flow sequence (Figure 6). Present exposures are limited to the western and central parts of the Pumice Plain along the North Fork Toutle River and its tribu- taries near the nested explosion crater (Figure 5). Based on preliminary topographic and stratigraphic extrapolations, thicknesses may be as great as 20 m in the southern parts of the Pumice Plain where no exposures exist at the current erosion level.

matrix of fine ash that coats the fragments. Eastward, the fine-ash matrix becomes less abundant, and the middle ash deposits become more friableß Near the southern margin of the tephra fan, the lower fine-ash layer is absent, and the middle pumice ash rests directly on the middle pumice lapilli layerß At distances _>40 km downwind, the middle pumice ash layer appears to merge with the lower fine-ash layer to form a few millimeters of light-toned relatively fine ash.

ß , .

The change in eruption style from vertical ejection of tephra to ash flows that occurred about 1215 was accompanied by changes in the fundamental processes of tephra formation, the


types and compositions of deposits, inferred gas contents, and seismic character. Ash clouds from the pyroclastic flows produced an elongate ash cloud plume that extended several kilometers north of the vent and convected to altitudes that nearly rivaled the preceeding early Plinian column (Figure 3). I believe that the tonal lightening of the eruption column (ash cloud plume) probably resulted from the very fine grain size of the elutriated ash. Deposits of the lower fine-ash layer in the areas east of Spirit Lake and the Pumice Plain were probably formed by low-altitude, ash clouds transported off the margins' of the flows. The ash clouds probably swept eastward where they deposited a thin mantle over the exposed coarse lapilli of phase II.

Proximal deposits of the ash cloud plume are interpreted to be a widespread, nongraded pumice ash layer. It is conceivable that the plume contained particles introduced directly from the fountains, as well as particles removed from the flows by processes of elutriation and saltation. Most of the coarse materials were likely carried away from the fountain by gravity flow; some may have been lofted into the convecting ash clouds that formed the plume. As the plume drifted downwind, the coarsest components dropped first, and the finer fractions were suspended in the distal plume. I consider the pumiceous ash and rare lapilli of the middle pumice ash layer as the coarsest material convectively removed from the ash fountains and a proximal equivalent to the upper distal tephra deposits (see section on distal tephra). My interpretation of the chronologic and stratigraphic position of the middle pumice ash layer allows the correlation of the change in eruption style about 1215 with the observed characteristics of the proximal and distal tephra deposits.

Although the pyroclastic flow deposits are poorly exposed at this writing, a candidate for a very early flow deposit is exposed in the deep gully on the western part of the Pumice Plain (Figure 6). This flow unit rests on the pisolitic ash layer. It is relatively pumice-rich with banded gray and white pumice clasts; its coarse pumice zones may represent the bright reflector observed in the SLAR image of 1350 [Criswell and Elston, 1984: Rosenfeld, 1980]. Subsequent flows of phase III that erupted from episodic ash fountains are believed to be represented by an accumulation of relatively thin pyroclastic flow units that are poorly exposed beneath the thick deposits of the middle sequence. Although Criswell and Elston [1984] mapped some bright reflector material, which we interpreted as pumiceous pyroclastic flow deposits, at the base of Johnston Ridge, I now believe that the flows of phase III were probably of sufficiently small volume that they formed an aggradational fan in the southern parts of the Pumice Plain; they did not accumulate to any significant thickness at distances as far north as Johnston Ridge (Figure 5).

The appearance of a significant volume of the denser, less evolved, gray pumice in the lower pyroclastic flow sequence suggests that relatively deeper parts of the chamber were affected by the afternoon eruptions. The presence of two types of pumice (see section on chemical variations) suggests concurrent eruption

of magmas from the upper, as well as deeper, parts of the chamber; a wider range of depths appears to have been tapped than during phases I and II. Earthquake studies by Shemeta and Weaver [1986] support this suggestion, earthquakes were from 3 to 4 km depth, but the magnitudes of individual events and the rates of seismic moment release at depths >6 km, the inferred top of the magma chamber, increased about 1300 (Figure 3).

I correlate the stratigraphic appearance of the denser, less evolved, gray pumice with reduced concentrations of magmatic gas (Figure 3) and attribute the upwelling or "boiling over" eruption style to the tapping of deeper, gas-poor parts of the chamber. Stoiber et al. [1981] calculated that leached samples of the upper distal tephra had low S/C1 ratios that ranged from 0.15 to 0.38; a ratio of 0.69 in the time series sample of the afternoon tephra sequence reversed the trend of increasing ratios in the morning tephra sequence (Figure 3). This evidence suggests reduced release of magmatic gases during phase III. Although energy release was probably continuous at depths, the eruption behavior may have been caused by the conduit being locally choked with degassed pyroclasts and lithic debris. Gas pressures periodically exceeded that necessary to transport the clogging debris to the surface; ash fountains then fed pyroclastic avalanches.

Climactic Phase IV C1500-1715)

Observed events. The increased mass flux that characterized the climactic portion of the ?linian eruption appears to have begun about 1500. Observers on the Rosenbaum aircraft observed a significant pyroclastic flow to the east shore of Spirit Lake at 1501 and a mild tonal lightening of the eruption column and intensity increases at 1511 and 1521 (Table 2). From 1525 to 1600, J.G. Rosenbaum (personal communication, 1983) and Rooth [1980] observed and photographed numerous large pyroclastic flows that spilled over the crater rim and engulfed the entire volcano (Figure 15a). Flows on the east, south, and west flanks of MSH descended a few hundred meters and largely resulted in pumiceous lahars in the valley areas [Janda et al., 1981]. A few flows on the east flank left recognizable deposits [Rowley et al., 1981]. Although pyroclastic flows continued in the breach and Toutle Valley areas, J.G. Rosenbaum observed that the eruption intensity decreased slightly from 1605 to 1620. By 1635, the pyroclastic flows appear to have abated. Although visibility was poor, Rosenbaum observed that the ash clouds from the flows in the Toutle Valley became stagnate and diffuse with poorly developed convection cells. Ash supply from the pyroclastic flows was evidently reduced. In contrast, by 1625, a marked vertical acceleration of the central column was apparent (Figure 15b). The Rosenbaum and Christiansen photo sequences (Figure 1) indicate that from 1625 to shortly after 1700, strong vertical ejection of tephra prevailed, termed the late Plinian event in this paper.

The Rosenbaum photos show that during the early part of phase IV, the ash cloud plume maintained the northward elongate morphology of phase III and may have increased its northward


Fig. 15. Photographs by J.G. Rosenbaum of climactic phase IV events at (a) 1548 and (b) 1625. Figure 15a shows pyroclastic flows on flanks of MSH. The late Plinian event in Figure 15b resulted in the column height maximum of 19 km at 1700 (Figure 3).


Fig. 16. Photograph of thickly ponded deposits of the middle pyroclastic flow sequence (location 82, Figure 5). Tan upper flow unit is •10 m thick above a •1 m thick light-toned pumice zone at arrow; unit forms most of the exposures of the western parts of the Pumice Plain. Lighter-toned units (partially covered) contain lithic breccia zones that can be traced south and east to the nested explosion crater. Relief is about 40 m.

extent as the flows to the Toutle Valley increased in volume. Ash clouds from pyroclastic flows on others flanks appear to have been drawn into the central plume. By 1600, the height of the column had increased (Figure 3) probably reflecting the mass flux increase. The last Rosenbaum and first Christiansen photos show a substantial increase in atmospheric haze and clouds around the volcano during the late Plinian event. Local visibility was reduced and atmospheric circulation resembled that of a strong thunderstorm, as cumulus and ash clouds alike were sucked into the central column.

ß

- '. ,•, , ,,--- •-,,- i Deposits attributed t phase IV consi t of (1 several pumiceous ash flow tuffs in the Toutle Valley, informally termed the middle pyroclastic flow sequence in this paper; and (2) proximal tephra deposits of the afternoon tephra sequence that include the upper fine-ash layer attributed to ash clouds, the upper pumice lapilli layer attributed to the late Plinian event, and fine- ash deposits of the upper distal tephra layer [Hooper et al., 1980; Sarna-Wokcicki et al., 1981a, b; Scheidegger et al., 1982; Ikrammudin et al., 1982]. I agree with Waitt and Dzurisin [1981] that the coarse lapilli deposits represent the column height maximum of the late Plinian event but I do not believe that these deposits form much of the distal tephra deposits (see sections on proximal and distal tephra).

ß ,

'' ' ' ' ,• ', ', ': The middle pyroclastic flow sequence is a thick (•35 m) sequence of thick (4-12 m) lapilli and ash flow sheets that conformably and unconformably overlie

the lower and basal pyroclastic flow sequences, respectively (Figure 6). Deposits consist of at least three major flow units that are vertically and laterally graded and are well exposed in the western and north central parts of the Pumice Plain, where flows ponded against the steep bedrock of Johnston Ridge (Figure 5). These provide most of the internal exposures of the Pumice Plain at the current erosion level and determined much of the morphology of the Pumice Plain described by Rowley et al. [1981]. In most areas the basal contacts with the lower pyroclastic flow sequence are defined by widespread lithic breccia zones, which underlie relatively thick flows containing a higher proportion and larger clasts of both pumice and lithic fragments than the lower sequence and contain clasts of pumice breccia (described below) not found in exposures of the lower sequence.

In the central parts of the Pumice Plain (location 135, Figures 5 and 6), a 25-m-thick deposit ponded in the irregular topography of the debris avalanche at the base of Johnston Ridge and flowed westward down the former course of the North Fork Toutle River. The thickly ponded deposits are vertically graded from a basal, 2-3 m thick lithic breccia zone with blocks •80 cm to a massive lapilli and ash zone with abundant scattered lithic fragments •15 cm to an upper zone of discontinuous pumice lenses, •3 m thick each. The surface is composed of fines-poor pumice clasts, which form longitudinal, trans- verse, festoon and eddylike patterns that are locally truncated by fault scarps and collapse structures formed by subsidence of the underlying


•,. .::** .... ;•:..•..:.•:::

Fig. 17. Photograph of lithic breccia zone in the middle pyroclastic flow sequence near location 375 (Figure 5).

debris avalanche deposits and from water withdrawal associated with phreatic explosions pits. Pumice concentrations typically contain expanded white pumice (up to rare 2 m), with local concentrations of subangular blocks of denser, gray pumice and scatttered clasts of the pumice breccia (see section on chemical variations).

On the west side of the Pumice Plain, deposits appear to have been eraplaced as three major flow units. The lower two flow units are vertically graded, from a 2-3 m thick lithic breccia zone to lapilli-rich massive zones containing scattered lithic fragments _<15 cm to a thin (-<1 m) and discontinuous upper zone of pumice concentrations with scattered pumice breccia clasts. These are correlated with the thick central unit described previously. The third flow unit, which forms most of the exposure of the western Pumice Plain, varies from 8 to 13 m thick (tan upper unit in Figure 16) and is laterally graded. In the southern exposures, deposits are relatively massive with abundant (-<20%) ungraded lithic fragments -<40 cm, scattered throughout an ash- rich matrix of comminuted crystals and minor glass shards; discontinuous lithic lenses contain fragments _<1 m. Numerous degassing pipes, up to 30 cm wide and 3 m high, of clast-supported lithic lapilli are typically rooted in the lithic lenses. The northern exposures are more pumice- rich and exhibit a basal pumice zone _<2 m thick (Figure 16) that underlies a massive zone of glass shards, crystals, pumice lapilli, and abundant scattered lithic fragments_<8 cm; degassing pipes are rare.

On the east side of the Pumice Plain (Figure 5), deposits are thinner but are only locally exposed. On the north flanks of MSH, the proximal bedded deposits described by Rowley et al. [1985] contain abundant scattered lithic fragments that locally form lenses of lithic breccia. Northward, the ponded facies appear to

consist of two or more flow units with well- developed basal lithic zones (_<2 m thick) overlain by massive, lapilli-rich deposits containing rare pumice breccia clasts.

On the southern slopes of Johnston Ridge (Figure 5), fine-grained, lapilli-poor, massive deposits up to 8 m thick overlie and intercalate with bedded ash cloud deposits. These deposits are enigmatic as they may represent the distal flow facies of the thickly ponded middle pyroclastic flow sequence, as suggested by Walker [1983] or they may have been deposited from ash clouds that were remobilized downslope (H. Glicken, unpublished data, 1985).

ß ,' , - ' ,,-.: The lithic breccia zones that help to characterize the middle pyroclastic flow sequence are typically _<5 m thick (Figure 17) and consist of poorly sorted and incipiently graded angular to subangular blocks of older MSH rocks and rare fragments of possibly Tertiary bedrock, supported by a crystalline ash matrix; locally, the zones are fines-poor and clast supported. The zones exhibit both gradational and locally sharp contacts with the overlying and underlying deposits, probably indicative of local shearing. The fines-poor exposures appear to represent a ground layer that can be traced locally into matrix supported deposits of the middle pyroclastic flow sequence.

Based on the lithologic classification of Glicken [1986], the lithic zones are composed of the following proportions of fragments.' 20-40% fine-grained dacite (phenocrysts _<1 ram), 25-45% andesite and basalt, and -<15% coarse-grained dacite (phenocrysts -->2 ram). Fragments of basalt, andesite, and the coarse-grained dacite may represent simple rockfalls from the freshly exposed crater walls. The fine-grained dacite, however, is not represented in the crater wall but represents relatively young domes (_<2500


ß x :• ......... '.':%•?.; ,;:. ?. .•!:: ... % ..... .... , ............. .;., ....... .:,11;;* ..,:.:.

. . % •, ...... :• ...................

-.:,.:.:-.• .•,.- ,.. ::.: . .:.**._ ..-.:-: : .-:. :...•: ,..::• .... .. , .:, ..... --,.,...., -:..,.::,•-:--...•.• •...•;,•;.•.,--.--..;.:. '•' ...... ..-.. '"":'"% •.. '• ,.-a½ ;•'::•..:';!.:;i4,;•-•..:; i: .....,:•:...-?:;?,:.?,...u f*%d-'*' '?' ½. ?---:;'**' ':'•

.. *.. '•: .,?'4:-,";::,,-.-..,.• ,½':•::i:i*;':*•';.;•;..:.,.; *" ,.•:-: ............ :.-:.a;. :..,;;-::";:!;.x:.':.?:,i:a .... "•':':;:•¾c.:- •*.;......., ...-.?*;;'"': ..-;.½•

::*•;:::: .,:',;.'. ß ..:::-:--, :.¾:., .;;7%½½/::::;:'::!:-.'77,½•;.::,4,::;;•,..:c."..47:;;;S}:,,-,!.•..,;:::**: ........................ ':., ........ ß ................. ß -.,';¾:..:,. ..... .• .... .,•:.,..-' ;';; 7..:..";'."' ,....,,.. ........ . ....... -i:": ......... ::::::::::::::::::::::::::: "•u/a;:.:: •..-.:,..,.:,;.,:,-,:.:.-,:;:,.,..-. .... ,::,:,:,:--:--.,:., "";•*-,-'•;:,:-:-:. - ...... .:-.': ........... ,-:-,. --.e•,-..,•. ..... .....,.,,•,.;,..,.,.... ........... ;......-..., .......... ** .... , ..... .,.: ........... ......, ............ ?.,.•.,.::....:....,...... q-, ,:'•'•--::-*½xz•;.-:- ' ..... ß ................ .......................... ,...,.. ............. , ...... ,:... ..... ....... Fig. 18. Photograph of bread-crusted, pumice breccia clast from middle pyroclastic flow sequence, western Pumice Plain (50 m east of location 36, Figure 5). Accidental lithic fragments and slightly flattened light-toned pumice are surrounded by a relatively dark gray, scoriaceous matrixß Scale intervals are 10 cm.

years) on the preeruption slopes and summit regions of the volcanoß These rock sources were removed in the massive landslides of phase I, suggesting that their appearance in the stratigraphic record is due to an origin from shallow and unexposed source such as the vent walls.

ß

At a distance of 10-12 km the dispersion axis is shifted about 3 km southward of the axis of the morning tephra sequence (Figure 11). Deposits consist of a clast-supported layer of pumice lapilli (-<10 cm) that is coated with and overlain by the upper fine-ash layer. White pumice

ß ,, - , - -: :: The pumice breccia composes most of the clasts present, but locally clasts are compound pumice clasts found widely in along the dispersion axis, denser, gray pumice the middle pyroclastic flow sequence and locally composes _<25% of the clasts and accidental lithic in the upper pumice lapilli layer of the proximal fragments, mostly andesite, -<20%. At one tephra deposits. Clasts recovered from the flow locality a rare fragment of the pumice breccia deposits are typically -<25 cm but have been observed to be as large as 60 cm (Figure 18). The clast found in the tephra layer was 5 cm, typical of other clasts in that layer. The clast surfaces are partly or wholly bread-crusted, indicating a hot, juvenile origin. Clasts appear to be of two gradational types. The most common type consist of angular clasts of light-toned pumice, sometimes flattened, in a matrix of vesiculated, gray scoria and accidental lithic fragments. Wavy bands of alternating gray scoria and light-toned pumice (see section on chemical variations) pinch and swell irregularly, suggesting the intermingling of diverse magmas during eruption. Less commonly, clasts consist of poorly consolidated, angular fragments of light-toned pumice, ash, and lithic fragments partly encased in light-toned pumice, suggesting the eruption of materials that had been accumulating in the vent of the volcano during phases II and III.

I,,- ,-.,' - ..' ' - - : The upper pumice lapilli layer consists of a reversely graded layer of angular pumice lapilli that widely overlies the middle pumice ash layer and locally overlies deposits of the middle pyroclastic flow sequence on the east flanks of MSH (Figure 8).

was recoveredß The upper lapilli layer is absent near Spirit Lake and appears to be mixed with the upper fine-ash layer in northern areas of the tephra fan. At 50 km, deposits thin to -<1 cm of coarse-pumice ash but are poorly preservedß

,,- ',--.-, . - : The upper fine-ash layer consists of -<2-4 cm of tan fine ash, mostly glass shards, and broken crystals. Deposits are thickest near Spirit Lake, where they overlie the middle pumice ash layerß East and south of the lake, deposits, consistently 1-2 cm thick, overlie and locally enclose the upper lapilli layerß

ß , ß

! I - . el . !e • : el. e •lo ß

The mid to late afternoon events of phase IV are interpreted as the climactic Plinian activity of May 18, 1980. The periods of peak mass flux of pyroClastic flow and strong vertical ejection of tephra were accompanied by peak seismic moment energy release and peak eruption column height, respectively (Figure 3). The period from 1500 to 1520 was probably transitional from intermittent to more sustained fountaining, resulting in more voluminous pyroclastic flows and increased eruption column height (Figure 3). The increase in mass flux during phase IV is recorded by pumice-rich, relatively thick ash flow s•eets of


the middle pyroclastic flow sequence in the upper Toutle Valley and other drainages. The large pyroclastic flows moved across the slightly older fan of the lower pyroclastic flow sequence and ponded to significant depths at the base of John- ston Ridge. The eruption culminated during a short but vigorous interval of vertical ejection of tephra represented by the coarse, upper pumice lapilli layer. As the eruption subsided, a tremendous amount of airborne dust from the central column and the ash flows settled in the proximal areas as the upper fine-ash layer. The eruption had definitely waned by 1720, when eyewitnesses observed a decline in eruption intensity (Table 2); by 1730, the column height over the central vent was diminished from 19 to 6 km (Figure 3).

I attribute the increase in the mass flux of phase IV to the clearing of the central conduit as evidenced by the pumice breccia clasts and the lithic breccia zones. These breccias are interpreted as eroded portions of the throat of the volcano. The lithic breccia zones contain a significant proportion (20-40%) of fine-grained dacite that is not present in the modern crater walls [Glicken, 1986] and provide evidence of vent erosion accompanying phase IV, not simply rockfalls from the unstable crater walls [Rowley et al., 1981]. The correlation diagram of Figure 3 shows a clear relationship between the interpreted period of vent erosion and the period of peak seismic energy release [Shemeta and Weaver, 1986]. The high proportion of white pumice in the pyroclastic deposits suggests that a substantial amount of the more evolved part of the magma body (see section on chemical variations) was tapped, possibly because opening of the vent allowed greater mass flux. The late Plinian event may have resulted in the final partial depressurization of the magma chamber, to the point where lithostatic pressures could control, or at least contain, the residual magma. These events probably exhausted the most silicic part of the compositionally zoned chamber, as Lipman et al. [1981] reported that the most silicic materials appear to have been exhausted after the relatively small eruption of May 25, 1980.

The mechanism responsible for the late Plinian event is not clear. Models of Plinian eruption column behavior [Sparks et al., 1978] indicate that the change from fountain-fed flows to vertically sustained columns requires an increased gas content, decreased vent radius, or decreased mass eruption rate. As the upper pumice lapilli layer from the late Plinian event appears not to have reached the distal areas, no evidence exists in the soluble leachate analyses of Stoiber et al. [1981] that would indicate any late changes in S/C1 ratios or magmatic gas contents. No stratigraphic evidence, such as accretionary lapilli or hydrovolcanic ash, suggests that phreatomagmatic activity increased explosivity by invasion of groundwater. Available evidence suggests that the vent was cleared of rubble, both accidental lithics and previously vesiculated and therefore degassed magma clots, and that a simple, clean condu•t from the magma chamber to the surface provided the conditions necessary for vertical column eruption. A process of simulated vent narrowing could occur if a virtual vent opening [Sparks,

1986] represents the region where atmospheric pressure is achieved by the erupting mixture. It is conceivably that the position of the virtual vent changed during the eruption. During phase I it was probably surficial, as the landslides exposed the cryptodome; during subsequent phases, it was probably at deeper but still shallow depths. The increased mass flux of phase IV, allowed by a cleaned vent, may have accompanied a downward migration of the virtual opening to a region of reduced radius. Hoblitt [1986] discussed such a process in the context of MSH eruptions since May 18.

Late Ash Flow Phase V {1715-181))

Observed events. After 1715, the eruption waned rapidly. The Christiansen (Figure 1) and Krimmel and Post [1981] photo sequences indicate that the vigor of the central column decreased rapidly. Radar measurements (Figure 3) indicate that the height of the column decreased from its maximum (19 km during the late Plinian event) to just a few kilometers over the vent. In addi- tion, the ash clouds and condensation haze began to dissipate, and the atmosphere cleared.

Although the eruption was waning (Figure 19), a short-lived increase in eruption intensity was observed. From 1745 to 1810, Christiansen observed several pyroclastic flows in the upper Toutle Valley and Spirit Lake areas (Table 2). These were followed by a short-lived increase in vigor of the vertical column that appears to have lasted only a few minutes and is not apparent in the column height curve of Figure 3.

Stratigraphy of phase V deposits. Deposits of phase V consist of (1) a sequence of pumiceous ash flow tongues in the Toutle Valley, informally termed the upper pyroclastic flow sequence in this paper; and (2) proximal tephra deposits of the afternoon tephra sequence that probably include parts of the upper fine-ash layer and little of the capping fine-ash layer. Distal deposits are doubtful.

Upper pyroclastic flow sequence: The upper pyroclastic flow sequence is a comparatively thin succession (3-8 m thick) of at least three major and minor lapilli and ash flow tongues that unconformably overlie the middle pyroclastic flow sequence and is well exposed in the western areas of the Pumice Plain (Figure 5), is poorly exposed in the central area, and is not exposed in the eastern areas due to covering by younger deposits [Rowley et al., 1981]. Where best exposed (location 36, Figures 5 and 6), the basal contacts are sharply defined by a lithic breccia zone (Figure 20) that lies in channels cut into the middle pyroclastic flow sequence.

In the southwestern part of the Pumice Plain (location 36, Figure 5) deposits of the upper pyroclastic flow sequence are distinguished by a nested set of erosional channels incised into the underlying middle sequence. The channel margins have characteristically low slope angles (<20); posternplacement slumping of the unconsolidated deposits locally modified the margins and caused secondary flowage. Deposits within the channels consist of stubby, fines-poor pumice lobes that grade down to massive, poorly sorted deposits, rich in comminuted crystals. Scattered acci-

10,258 Criswell: Chronology and Pyroclastic Stratigraphy of Mount St. Melens

Fig. 19. Photograph by R.L. Christiansen of relatively low energy eruption column during late ash flow phase V at 1813. View to east; light-toned ash clouds at low altitude are from small pyroclastic flows.

dental lithic blocks and lapilli, of andesire, dacite and rare basalt, are normally graded to a well-developed, basal lithic layer (Figure 20) containing blocks up to 60 cm across. Ground surge deposits (•10 cm thick) of bedded, lithic and crystal ash occur locally along the basal contact. Remnants of bedded rim deposits of a phreatic explosion pit are locally preserved

along the northern margin of the erosional channels but the pit was filled by the deposits.

Farther north, sheetlike deposits consist of massive, poorly sorted ash and lapilli in sinuous distributary lobes (•3 m thick) with elongate, lenticular profiles, described by Rowley et al. [1981]. Levees are absent; the sheet margins consist of lacy, fines-poor pumice concentrations

Fig. 20. Photograph of lithic layer along planar contact (large arrow) of upper (upf) and middle pyroclastic flow sequences (mpf) near location 36 (Figure 5); view to northwest. Note meter stick at small arrow for scale.


(_<1 m thick). Near the nested explosion crater (Figure 5), deposits consist of 2-3 m of massive pumice and ash that grade down to a lithic zone _<1.5 m thick with lapilli-sized lithic fragments (Figure 6). The terminal parts of this flow were destroyed during crater formation. Pumice zones, with clasts locally up to 30 cm diameter, consist of white pumice that is locally oxidized to tan and buff tones and variable concentrations (_<30%) of gray pumice.

Upper fine-ash laver: The upper fine-ash layer consists mostly of glass shards and broken crystals with locally rare, scattered pumice lapilli (_<4 ram) in upper parts of the layer. Deposits appear to maintain a thickness of _<3 cm throughout the northern parts of the proximal tephra area (Figure 11) and thin southward (Figure 8). Most exposures have been widely disturbed by bioturbation (plant roots, animal burrows, etc.) and surficial weathering processes, including expansion and formation of bubble vesicles after wetting.

Interpretations and discussions of phase V. _ _

Following the late Plinian event of phase IV nearly all activity waned rapidly but was punctuated by a brief period of fountain-fed pyroclastic flows, followed by increased vigor of the vertical column, as seismicity continued at deeper levels. A few small pyroclastic flows were emplaced in the upper Toutle Valley and are easily mapped by the erosive channel morphology and the distributary lobes resting on the larger underlying sheets. The timing and relative intensity of the short-lived events of phase V correlate with increases in seismicity (Figure 3). In contrast, Shemeta and Weaver [1986] reported that although earthquakes continued at all depths during this time interval (Figure 3), the seismically active area dispersed away from the vent, suggesting adjustment and intrusive processes, as well as eruptive.

Although the pyroclastic flows that form the upper pyroclastic flow sequence are similar in volume and distribution to small flows emplaced during the eruptions later in 1980 [Rowley et al., 1981], they formed elongate sheets rather than the leveed tongue morphology described by Rowley et al. [1981]. These differences are probably reflective of the increased block:ash ratios of the younger flows described by Kuntz et al. [1981]. The May 18 pyroclastic flows appear to have been more erosive, as they carved channels into the middle pyroclastic flow sequence. The abundance of lithic fragments along the basal contacts suggests that the lithic fragments contributed to the erosion. The lack of dacitic clasts and the abundance of andesitic and basaltic clasts in the lithic zones suggest that rockfalls from the crater walls may have been incorporated into the pyroclastic flows, as basalt and andesite form most of the unstable parts of the modern crater wall.

Stratigraphic correlation of tephra deposits with phase V is tentative. I think that most of the upper fine-ash layer is dust that settled during the waning period that followed the eruption climax of phase IV but believe that some of it is due to ash cloud deposition associated with the upper pyroclastic flow sequence. The rare pumice lapilli embedded in the upper parts

of the layer may have originated from the brief intensity increase of the central column, observed by Christiansen.

PosteruDtion Phase VI (1815 to May 19. 1980) -

Observed events. From about 1815 to 1900, the Christiansen photo sequence (Figure 1) shows passive clouds of ash over the Toutle Valley mixed with ash from emissions of the central crater. After 1900, low-energy ash convected from the crater and was carried eastward by low-level winds (Figure 21). Personnel in the USFS aircraft observed small, transient intensity increases in the vigor of the ash emission (Table 2). Photographs taken during the early morning of May 19, 1980 (J.G. Rosenbaum, not tabulated), and later that day [Krimmel and Post, 1981] indicate that ash continued to rise slowly from the crater. Christiansen and Peterson [1981] reported that weak steam and ash emissions continued until May 25, 1980.

Stratigraphy of phase VI deposits. The only primary deposit attributed to phase VI is the capping fine-ash layer. Secondary deposits are numerous and include (1) lahars generated from the dewatering of the voluminous debris avalanche deposit [Cummans, 1981; Janda et al., 1981], and (2) deposits of phreatic explosions [Rowley et al., 1981; Kuntz et al., 1981].

Capping fine-ash layer: The capping fine-ash layer is poorly preserved and has suffered widespread destruction since 1980; I have seen it preserved in only two localities. Waitt and Dzurisin [1981] described it as a well-sorted layer of gray, fine ash. Thicknesses are a few millimeters across the tephra fan, and the layer exhibited an amplified thickness 40 km downwind and thinned rapidly eastward. The isopach contours were mislabeled in the report of Waitt and Dzurisin [1981]; thicknesses were reported in centimeters and should have been millimeters (R.B. Waitt, personal communication, 1985).

Interpretations and disc•ions of phase VI. Phase VI was probably one of weak gas and ash release that accompanied viscous lava movement in the conduit. The gray tone of the capping layer may have been due to an increase in the proportion of gray pumice erupted, but insufficient data exist to evalute this hypothesis. The eruptive activity appears decoupled from the seismic activity, as earthquakes continued after the eruption had visibly terminated. After 1900, earthquakes were mostly from 5 to 12 km deep; the events at 20 km are the deepest ever recorded under MSH and probably represent adjustments to magma withdrawal, as well as intrusions at depths [Shemeta and Weaver, 1986]. Although a dome was not observed, a shallow cryptodome may have formed shortly after the May 18, 1980, eruption and was then ejected as dense gray scoria blocks during the eruption of May 25, 1980 [Rowley et al., 1981; Lipman et al., 1981]. Dome growth during the May 18 eruption would not be unusual, as all the post-May 18 explosive eruptions were accompanied by lava dome extrusions [Moore et al., 1981; Cashman and Tagggart, 1983]. The chemical similarity to pyroclastic rocks suggests that the dome lavas represent degassed magma residues.

10,P60 Criswell: Chronology and Pyroclastic Stratigraphy of Mount St. Helens

Fig, 21, Photograph by R.P. Hoblitt of low-energy ash plume during the posteruption phase VI at 2030.

Revision of Proximal Tephra Stratigraphy

Figure 22 summarizes stratigraphic units and times of origin of proximal tephra layers presented in this paper with those of Waitt and Dzurisin [1981]. I subdivide the proximal deposits (_<40 km) into 10 layers that I correlate with observed eruption phenomena. I group the layers into the morning and afternoon tephra sequences and correlate these with the lower and upper distal tephra layers, respectively.

The morning tephra sequence (Figure 2) forms a relatively simple, graded fan of lapilli and ashfall deposits that thins and fines with increasing distance from MSH (Figure 11). Close to MSH, deposits not described by Waitt and Dzurisin [1981] include a basal lapilli layer that lies below the pisolitic ash layer. I interpret the basal lapilli layer to have resulted from the weak pre-Plinian column that developed immediately after the lateral blast. I concur with Waitt and Dzurisin [1981] that the overlying reversely graded lapilli layers correspond to the vertical column eruption during the mid to late morning. I interpret the lower lithic ash layer to represent a relatively short- lived event in the mid morning in which slight vent erosion or increased rockfall from the crater walls provided for a dark eruption column.

The afternoon tephra sequence (Figure 2) sharply overlies the morning sequence and consists of thin deposits of interbedded fine and coarse ash that form a complex fan with a secondary thickness maximum 325 km from MSH (see section on distal tephra). In the proximal areas the coarse-ash deposits appear twice in the composite stratigraphic column of Waitt and Dzurisin [1981], as the upper lithic layer (b 3) of their Plinian tephra deposits and as the middle pumice sand layer (c 2) of their ash cloud deposits (Figure 22). I consider these

layers as lateral equivalents; they have the same components and occupy the same stratigraphic position, so are termed the middle pumice ash layer in this paper (Figures 2 and 22). Waitt and Dzurisin [1981] interpreted the middle pumice ash layer to have originated from a low-energy, Plinian column during phase II, prior to the first ash flows of phase III. No observed phenomena support their interpretation; instead, the presence of widespread ash cloud deposits beneath and within the middle ash layer indicate it was coeval with the ash flows. I interpret the middle ash layer to represent the coarsest

4 5

__uppe[ lowe! .... J I --distal • -- proximal •

1,4-this paper 2-Waitt and Dzurisin, 1981 :5-Sarna-Wojcicki et al., 1981a

I

t•oJ----•'• capping fine ash c o

upper fine ash o upper pumice lapilli • middle pumice ash ,• lower fine ash

middle pumice lapilli

lower lithic ash

lower pumice lapJill pisolitic ash

o

basal pumice lapJill

Fig. 22. Comparison of stratigraphic units and proximal/distal tephra correlations presented in this paper with those of Waitt and Dzurisin [1981] and Sarna-Wojcicki et al. [1981a]. I consider proximal units b 3 and c 2 to be lateral equivalents, referred to as the middle pumice ash layer. The distal lower and upper units are considered equivalent to the morning Plinian and afternoon pyroclastic flow activity, respectively.

Criswell: ChrOnology and Pyroclastic Stratigraphy of Mount St. Helens 10,P61

5OO ß

400 ß

300

Km

2OO

I00

P Priest Ropids Dom(,) ,."' W Wanapum Dam (I) H Hanaford (0 PU Pullman(2) C Cheney (3) / / A Aim I r a (4) / ::::::: / Radar Reflectors(5) /

Lower upper ß / : /"•:•:•/

e / / I •:•::• • ::::::[:: / / i /::::::

0900 I100 1300 Time (hrs)

Fig. 23. Summary of tephra plume velocity and depositional history at selected distal sites. Solid diagonal lines are measured wind velocities [Sarna-Wojcicki et al., 1981a]; dashed lines are interpolated from eruption phase boundaries (Table 1 and Figure 2). Radar reflectivity levels are modified from Harris et al. [1981] and are interpreted to represent regions of high particle concentrations. The dark- and light-ash plume fronts were measured on the NOAA satellite images and approximate the measured wind velocities. References are (1) Scheidegger et al. [1982], (2) Hooper et al. [1980], (3) Ikramuddin et al. [1982], (4) Stoiber et al. [1981].

fraction of the material convectively removed from the ash fountains that supplied the pyroclastic flows. I agree with Waitt and Dzurisin [1981] that the coarse upper lapilli layer deposits represent the column height maximum of the late Plinian event, but recognize that these deposits form a unique lobe with a dispersion axis that is shifted south of the morning tephra sequence (Figure 11).

Correlation of Distal Tephra Deposits

Tephra from the morning Plinian and afternoon pyroclastic flow eruptions was transported eastward by high-altitude, tropospheric winds. The plume was tracked by the Seattle and Spokane radar systems [Harris et al., 1981] and NOAA satellite images [Sarna-Wojcicki et al., 1981a; Foxworthy and Hill, 1982]. Quinn [1982] reported that during the morning (phases I and II), consistent westerly winds provided for relatively uniform movements of the ash plume eastward. Figure 23 shows the velocity profile of the leading edge of the dark-ash plume, which I measured from the NOAA satellite images; similar measurements were reported by Sarna-Wojcicki et al. [1981a].

With the change to eruption of pyroclastic flows and the attendant ash clouds of phase III, the ash plume assumed a very light tone, which I attribute to the very fine grained nature of the ash elutriated from the pyroclastic flows. I mapped the eastward progress of the light-ash plume from NOAA satellite images (Figure 23). Eastward transport of the light-toned ash plume occurred under slightly•different atmospheric conditions. Quinn [1982] reported that during the afternoon, the westerly winds were perturbed by the approach of a small Pacific storm. The

high-velocity layer near the tropopause shifted slightly northward, lower level winds increased velocities and assumed more complex flow patterns, and a midlevel secondary velocity maximum directed some of the ash toward the southern margin. The major result was that the fine ash of phases III and IV was accelerated by moister winds into the existing downwind plume, and the tephra dispersion axis was shifted northward [Sarna-Wojcicki et al., 1981a].

Onset of ashfall lagged behind the plume fronts because the leading edges of the plume were probably dilute. Deposition at a given locality generally coincided with the regions of higher particle concentration, interpreted from hourly measurements of radar reflectivity levels (Figure 23, modified from Harris et al. [1981]). Hooper et al. [1980] reported that several hours of massive, dark clouds preceded ashfall at the Pullman site. Ashfall, in general, occurred earlier in the southern areas and was delayed in northern areas [Sarna-Wojcicki et al., 1981a]. The change to deposition of light-toned, upper layer ash also lagged behind the leading edge of that front by several hours but was apparently affected by aggregation [Carey and Sigursson, 1982]. Figure 23 shows that ashfall in the area of the secondary thickness maximum was recorded as an increased intensity radar reflector (modified from Harris et al. [1981]).

The distal tephra deposits have been described as a lower, dark, coarse layer and an upper, light, fine layer [Sarna-Wojcicki et al., 1981a]. At a distance of _<200 km the lower layer is composed of a few millimeters of lithic and crystal fragments and pumice shards that overlies a few millimeters of fine to very fine, gray ash that probably correlates with the pisolitic layer of phase I; at distances >200 km the phase I layer appears absent or merges with the lower layer [Sarna-Wojcicki et al., 1981a]. The upper layer maintains a thinness of several millimeters to centimeters to a distance of 200 km, where the deposits begin to thicken again to form a secondary thickness maximum at 325 km [Sarna-Wojcicki et al., 1981a]. At distances _<200 km the upper layer consist of fine to very fine, gray to tan ash composed of pumice and glass shards, crystals, and minor lithic fragments [Scheidegger et al., 1982]. Beyond 200 km, deposits consist of over 90% fine ash composed of glass shards and broken crystals [Carey and Sigurdsson, 1982].

Based on these data, I correlate the lower and upper distal tephra layers with the morning and afternoon tephra sequences, respectively. I consider the morning tephra sequence as Plinian tephra of phase II; proximal sublayers were simply mixed by atmospheric processes during transport to distal areas. The leading edge of the dark-ash plume may have contained ash from phase I, but significant deposition of the lower distal layer lagged behind the plume front by several hours, and a recognizable phase I deposit exists only at distances _<200 km [Sarna-Wojcicki et al., 1981a]. I interpret the distal deposits of the afternoon tephra sequence as ash cloud tephra from phases III and IV, as originally proposed by Sigurdsson and Carey [1980]. Because the deposits of the upper lapilli layer appear to pinch out at a distance beyond 50 km, I think that tephra from the late Plinian event did not make a significant contribution to the distal tephra deposits. The fine grain size of the


TABLE 3. Representative Chemical Analyses and Clast Density Measurements of Pumice Erupted on May 18, 1980, Arranged by Stratigraphic Units Described in Text

•amDl• _

1 • 3 4 ,5 ,• ? • 9 10 .11 12 Location 133 133 133 133 310 310 135 82 135 135 118 118 Deposit t 3 t 5 t 8 t 8 bpf bpf lpf lpf lpf lpf mpf mpf Phase II II IV IV I I III III III III IV IV

Tone white white white gray gray white gray banded gray white white banded Density** 0.81 0.82 0.77 1.35 1.15 0.69 1.22 1.24 1.08 0.78 0.80 0.84

SiO 2 A1203 Fe203 FeO

MgO CaO

Na20 K20 TiO 2 P205 H20- L.O.I. Total

63.63 63.65* 63.31' 61.98 63.52 63.61 62.42 62.12, 61.81 63.45 63.69* 63.26* 17.49 17.54 17.74 17,85 17.36 17.37 17.52 17.74 17.84 17.29 17.66 17.71

1.76 1.73 1.55 1.81 1.60 1.52 1.71 1.78 1.78 1.80 1.60 1.63 2.80 2.75 2.89 3.14 2.79 2.84 2.85 2.99 3.14 2.81 2.69 2.82 1.97 1.97 1.95 2.23 1.91 1.92 1.91 2.15 2.69 1.93 1.81 2.03 4.86 4.92 4.97 5.29 5.01 4.94 4.97 5.12 5.83 4.81 4.78 5.05 4.74 4.87 4.93 4.82 4.86 4.95 4.89 4.86 4.64 4.84 5.01 4.90 1.31 1.32 1.28 1.22 1.34 1.33 1.26 1.30 1.13 1.34 1.34 1.29 0.64 0.62 0.64 0.67 0.63 0.62 0.62 0.67 0.74 0.57 0.59 0.64 0.13 0.13 0.14 0.14 0.13 0.13 0.12 0.14 0.16 0.13 0.13 0.13 0.18 0.08 0.04 0.04 n.d. n.d. 0.52 0.21 0.04 0.11 0.05 0.02 0.34 0,27 0.27 0.30 n.d. n.d. 1.01 1.71 0.53 1.37 0.47 0.64

99.85 100.57 99.71 99.49 99.15 99.23 99.80 100.64 100.23 100,4• 99.92 100.16 Sample

_

13 14 ,1,.• 1• 17 18 19 20 21 2•, 2-3 24 Location 135 118 142 82 36 36D 36D 135 134 115 134 115 Deposit mpf mpf mpf mpf mpf mpf mpf mpf upf upf upf upf Phase IV IV Iv IV IV IV IV IV V V V V Tone tan banded white gray gray gray matrix gray tan white gray gray Density** 0.69 1.28 0.73 1.24 1.15 n.d. n.d. 1.09 0.70 0.83 1.37 1.29

SiO 2 A1203 Fe203 FeO

MgO CaO

Na20 K20 TiO 2 P205 H20- L.O.I. Total

63.05 62.86 62.82* 62.34* 62.39 62,36 60.95* 60.47* 63.59* 63.26* 61.74' 61.73' 17.57 17.86 17.53 17.53 17.83 17.86 i8.31 18.08 17.42 17.59 17.94 17.88

1.74 1.86 1.63 1.67 2.05 1.79 2.01 2.07 1.57 1.53 1.77 1.82 2.72 2.90 2.82 2.76 2.79 2.94 3.38 3.37 2.82 2.73 3.24 3.14 1.88 2.08 1.95 1.95 2.20 2.15 2.78 2.68 1.92 1.89 2.22 2.29 4.98 5.14 4.95 5.14 5.26 5.28 5.89 5.87 4.79 4.81 5.23 5.33 4.92 4.93 4.82 4.82 4.89 4.75 4.69 4.64 4.82 5.03 4.81 4.92 1.31 1.29 1.30 1.35 1.27 1.23 1.18 1.12 1.33 1.37 1.24 1.25 0.63 0.66 0.62 0.62 0.68 0.65 0.77 0.74 0.62 0.60 0.67 0,70 0.13 0.13 0.13 0.13 0.14 0.13 0.15 0.15 0.13 0.13 0.14 0.15 0.02 0.02 0.08 0,21 0.14 0.19 0.18 0.06 0.01 0.33 0.03 0.01 0.54 1.10 0.83 1.54 0.57 0.74 0.60 0.79 1.05 0.33 0.30 0.33

99.49 100.83 99.12 100.15 100.21 100.74 100.66 100.14 100.06 99.60 99.33 99.54

Symbols are the same as in Figure 2; sample locations are shown in Figures 6 and 8. X ray fluorescence analyses are by author, S. Seaman, and J. Husler. FeO is measured by standard titration methods. Samples 1-4 are single large tephra clasts (Figure 8), others from pyroclastic flow deposits (Figure 6). Samples 11, 12, and 14 are from surficial pumice lobes (Figure 5). Sample 20 is bread-crusted, subangular block from pumice zone (Figure 6). Eruption phases from Table 1 and Figure 2. H20- weight loss, 110 C for 1 hour. L.O.I. is loss on ignition, 1000 C for 1 hour.

n.d., no data. * SiO 2 checked by gravimetric, wet chemistry by author and J. Husler. • pumice breccia: sample 18, single clast inclusion; sample 19, separation of dark, wide band. ** g/cm 3.

upper distal tephra layer [Scheidegger et al., the Aleira and Hanaford sites is attributed to 1982; Sarna-Wojcicki et al., 1981a] suggests that their positions off the dispersion axis; timing the fine ash tended to remain in suspension and at the Almira site is consistent with other that aggregation processes artificially hastened northern areas [Sarna-Wojcicki et al., 1981a3. deposition in the area of the secondary thickness maximum [Carey and Sigurdsson, 1982]. Depositio•n Chemical Variations in the May 18, 1980, Deposits in that area (which includes the Cheney site, Figure 23), was probably from ash clouds of Compositional variations of the May 18, 1980, phases III and IV. Upper layer ashfall at the pyroclastic suite are summarized in Table 3 and sites of Scheidegger et al. [1982] may have been illustrated in Figure 24. The analyses represent due to the high concentrations of ash from the a sampling of single, large lapilli and blocks pyroclastic flows of the climactic phase IV. The from the stratigraphic sequences of pyroclastic late change to deposition of the upper tephra at flow and tephra deposits described in this


explanation deposit phase tephra Ate IV

a t5 II •t3 II flows

o upf V 0 mpf IV 0 bre½½io a bpf I

V _

CaO

5'

ß

FeOi

.

MgO 2

61 62 6• 64 wt.% Si02

Fig. 24. Chemical variation diagrams of selected oxides from Table 3, recalculated to 100% volatile-free; deposits and eruption phases from Figure 2. Weight percent FeO represents measured values, not total Fe. Dashed lines are from Lipman et al. [1981] and Cashman and Taggart [1983] for all 1980-1982 eruptive products and are not the regression lines for these samples; division of silicic andesite/mafic dacite is from Ewart [1979]. Shaded symbols represent augite- bearing gray pumice, open symbols represent the volumetrically dominant white pumice (and gray clasts of phase I). The overlap between the pumice types suggests a continual zonation. The upper diagram relates variation of SiO 2 with time and is interpreted to indicate that compositions became more diverse.

paper. These data indicate small, but significant compositional changes accompanied changes in eruption style and appear consistent with the progressive tapping of a zoned magma body.

The volumetrically dominant magma is a white pumice of relatively low density (0.69-0.85 g/cm3): high SiO 2 and K20 and low CaO, MgO, and FeO. Simple thin section point counts suggest 20-24% phenocrysts by volume of plagio- clase, hornblende, hypersthene, and Fe-Ti oxides. It is a mafic dacite [after Ewart, 1979] that forms the "typical" May 18 pumice recognized by previous workers [Lipman et al., 1981; Melson, 1983; Rutherford et al•, 1985]. Surficial heat coloration or oxidation turned many of the clasts within the middle and upper pyroclastic flow sequences to tan, buff and pink tones; clasts in the basal pyroclastic flow sequence are light gray to white. The augite-free dacite was the principal component of the May 18, 1980, eruption, including the lateral blast, but was accompanied by increasing proportions of augite- bearing, gray pumice as the eruption progressed.

The volumetrically lesser magma type is a gray silicic-andesite JEwart, 1979] with densities that range from 1.1 to 1.4 g/cm 3. It is

consider•'less"•evolved than the white pumice, as it is less silicic, and contains more FeO, MgO, and CaO (Figure 24), although overlaps exist. It contains 29-35% phenocrysts by volume of plagio- clase, hornblende, hypersthene, augite, and Fe-Ti oxides. It is most similar chemically to the dome lavas that erupted after the 1980 explosive sequence. The dashed lines in Figure 24 are not the regression lines of these samples but represent magmatic trends defined by Cashman and Taggatt [1983] and Lipman et al. [1981] for the 1980-1982 eruption sequence. The gray pumice appears only as a small fraction (_<2%) of the middle pumice lapilli layer of the morning tephra sequence (Figure 8) but composes many of the pumice concentration zones in the lower and middle pyroclastic flow sequences (Figure 6), indicating that a substantial increase in the proportions and volumes of the gray pumice accompanied the change in eruption style from tephra ejection to ash flows.

Banded and streaky gray and white clasts are present in many of the pumice concentration zones of the lower and middle pyroclastic flow sequences. Some of the banded clasts exhibit only small but possibly significant compositional variations. Samples 12 and 14 (Table 3) represent analyses of light and dark banded clasts, respectively, from the middle pyroclastic flow sequence. These compositions grade toward the gray pumice (sample 14 is augite bearing) and mirror the general compositional changes. Mineralogies within individual bands have not been examined in detail. Many of the banded pumices exhibit larger chemical variations. Samples 19 and 20 (Table 3) represent the streaky inclusions of gray scoria from the pumice breccia clasts and a single, bread-crusted block. The scoria contains more total Fe and more FeO, MgO, and CaO and less SiO 2 than the gray pumice. It contains up to 40% phenocrysts by volume of plag- ioclase, hornblende, augite, hypersthene, and Fe-Ti oxides. Sample 20 is similar to the least evolved samples reported by Lipman et al. [1981].

Summary and Conclusions

This paper addresses a comprehensive account of the May 18, 1980, eruption of Mount St. Helens and the major stratigraphic characteristics of the pyroclastic deposits. Figure 25 graphically depicts the estimated eruption rate curve, based on eyewitness accounts. The type and compositions of deposits are summarized from Figures 2 and 24. The following conclusions are based on the chronologic and stratigraphic correlations.

1. The chronology of the Plinian eruption proceeded from a sluggish column of ash that followed an initial sequence of phreatomagmatic explosions (phase I, 0832-0900). Mass eruption rates appear to have generally increased, with fluctuations, until the peak magma discharge at 1525-1700. Four main phases of eruptive activity are distinguished after the paroxysmal phase I: early Plinian phase II (0900-1215) of vertical tephra ejection; early ash flow phase III (1215- 1500) of intermittent pyroclastic flows and ash cloud tephra production; climactic phase IV (1500-1715) of peak mass flux of pyroclastic flow followed by strong vertical ejection of tephra during the late Plinian event; late ash flow


I " I "' !,v Iv Iv, •1ower -'upper

t5 T6 '• •.•--

g" / •----vento' •- t3 / A •,// erosion

t•/•', t . •o..•... • •t t•j :1•7 •pf •lpf...-' • o ."' o mpt .'*'.• X . sio .. ß ..... , . -': , ,

0900 I100 1300 1500 1700 1900 Time (hours-Pacific Daylight Time)

proximal tephra • pyroclastic flows ?ash clouds distal • debris-avalanche

Fig. 25. Schematic mass flux model of the May 18, 1980, eruption of Mount St. Helens. Heavy dashed line depicts total mass eruption rate; in detail rates fluctuated over short time intervals. Deposit symbols same as Figure 2. Compositional variations (dotted lines) depicted as SiO 2, summarized from Table 3. Eruption of significant volumes of gray pumice (62% SiO 2) corresponds to start of harmonic tremor [Malone et al., 1981]. A vertical line designates eruption style, deposits, and general chemistry of that eruption phase. Correlations with other data sets are shown in Figure 3.

phase V (1715-1815) of small pyroclastic flows during waning eruption intensity. During the posteruption phase VI, low-energy ash was emitted as a cryptodome (or conduit plug) was formed.

2. Tephra deposits are informally grouped into the morning (phases I and II) and afternoon (phases III, IV, and V) tephra sequences derived from vertically directed eruptions and ash clouds from the pyroclastic flows, respectively. The morning tephra sequence occurs as a relatively simple graded fan of lapilli and ash deposits that thins and fines downwind. The afternoon tephra sequence occurs as a relatively broad fan of complexly interbedded fine ash from the ash flows, coarse ash from the convective portions of the ash fountains, and coarse lapilli from the late Plinian event that climaxed the eruption. The dual subdivision of the proximal tephra deposits correlates with similar subdivisions of the distal deposits.

3. Pyroclastic flow deposits are informally subdivided into: (1) basal sequence (phase I) consisting of lithic-rich and pumice-poor deposits intercalated with, and widely overlying the debris avalanche deposits; (2) lower sequence (phase III), consisting of a _<20? m thick accumulation of thin (1-2 m) pumice and ash flows; (3) middle sequence (phase IV), consisting of a 8-35 m thick accumulation of thick (4-12 m) flows; and (iv) upper sequence (phase V) consisting of thin, areally restricted, sheetlike tongues. Basal contacts of these units are correlated with observed events at about 0832, 1215, 1500, and 1745, respectively (Figure 2). The middle sequence is interpreted to have resulted from the eruption intensity maximum of phase IV and contains an assemblage of lithic breccia zones and pumice breccia clasts, indicative of vent erosion. The time period deduced for the

eruption of the breccias correlates with the period of peak seismic energy release.

4. Whole rock chemical analyses of pumice clasts from the pyroclastic suite indicate that compositions generally became slightly more mafic with time, consistent with compositional varia-' tions reported by Lipman et al. [1981] for the entire 1980 eruptive sequence. In general, compositions of the May 18 eruption became more diverse as the eruption proceeded. The most evolved, most silicic pumice (mafic dacite of 63-64% SiO 2) remained the major component of the eruption but was accompanied by concurrent eruption of more mafic, less evolved silicic andesite (61-62% SiO 2) during the latter and climactic parts. This suggests that eruption of deeper parts of a zoned magma body occurred without complete evacuation of the silicic top. The climactic phase IV eruptions may have exhausted the most evolved (most silicic) part of the magma body.

5. Speculations of eruption mechanisms suggest that the initial phreatomagmatic explosions (phase I) were followed by a sustained, vertical column (phase II) that resulted from discharge of the evolved, gas-rich parts of the magma chamber, evidenced by the lack of silicic andesite and the high S/C1 ratios from leachate analyses of tephra deposits [Stoiber et al., 1981]. I attribute the "boiling over" eruption style of the pyroclastic flows (phase III) to the introduction of a significant volume of less evolved magma from gas-poor parts of the chamber, evidenced by the denser, silicic andesite and reduced S/C1 ratios of the ash leachate [Stoiber et al., 1981]. The climactic (phase IV) eruptions are attributed to enlargement of the conduit vent, evidenced by the abundant breccias, that allowed a greater mass flux; a simulated vent narrowing may account for the vertical column eruption during the late Plinian event. A cryptodome may have been eraplaced during the latter parts of the eruption as low-energy ash emissions accompanied continued seismic activity. Dome remnants may then have been ejected during the small eruption of May 25, 1980. The dome building eruptions, in general, may represent degassed, magma residues from the explosive eruptions.

Acknowledgments. This study benefited by the support, encouragement and criticism of members of the USGS David A. Johnston Cascades Volcano Observatory, particularly D.W. Peterson, D.A. Swanson, and W. Kinoshita. I also thank H. Glicken, R.P. Hoblitt, W.E. Elston, R.V. Fisher, N.S. MacLead, R.B. Waitt, D.A. Swanson, P.D. Rowley, C.J.N. Wilson, G.P.L. Walker, S. Self, T.H. Druitt, and D.R. Mullineaux for many discussions in the field. I thank C.D. Miller for the copy of the USFS radio log of May 18, 1980; A.M. Sarna-Wojcicki for the copies of the NOAA satellite images; J. Husler and S. Seaman for help with chemical analyses; T. Servilla for the polished, thin sections; and R.Y. Anderson for computer use. The manuscript was critically reviewed and improved by W.E. Elston, J.G. Rosenbaum, D.A. Swanson, and an anonymous reviewer. Financial support was provided by a McGetchin Volcano Fund Award (1982), the USGS David A. Johnston Cascades Volcano Observatory,


University of New Mexico, Department of Geology, and NASA grant NGR 32-004-062, National Science Foundation grants EAR-8417143 and EAR-8507028 (W.E. Elston, Principal Investigator).

References

Carey, S.N., and H. S igurdsson, Influence of particle aggregation on deposition of distal tephra from the May 18, 1980 eruption of Mount St. Helens volcano, J. GeoDhvs. Res., 87,

_

7061-7072, 1982. Cashman, K.V., and J.E. Taggart, Petrologic

monitoring of the 1981 and 1982 eruptive products from Mount St. Helens, •_•, 221, 1385-1387, 1983.

Christiansen, R.L., and D.W. Peterson, Chronology of the 1980 eruptive activity, U.S. Geol. Surv. •rof. Pap.,...1250, 17-30, 1981.

Criswell, C.W., and W.E. Elston, Intra-eruption geologic map from an x-band radar image during the May 18, 1980 eruption of Mount St. Helens, Washington, Reports of the Planetary Geology and Geophysics Program-1984, NASA Tech. Memo. 87563, 467-469, 1984.

Cummans, J., Chronology of mudflows in the South Fork and North Fork Toutle River following the May 18 eruption, U.S. Geol. Surv. Prof. Pap., 1250, 479-488, 1981.

Ewart, A., A review of the mineralogy and chemistry of Tertiary-Recent dacitic, latitic, rhyolitic, and related salic volcanic rocks, in Trondh•emites, dacites, and related rocks,

_

edited by F. Barker, pp. 13-121, Elsevier, New York, 1979.

Foxworthy, B.L., and M. Hill, Volcanic eruptions at Mount St. Helens, The first 100 days, U.S, Geol. Surv. Prof, PAD., 1249, 56, 1982.

Glicken, H., Rockslide-debris avalanche of May 18, 1980, Mount St. Helens volcano, Washington, Ph.D dissertation, 337 pp., Univ. of Calif., Santa Barbara, 1986.

Harris, D.M., W.I. Rose, R. Roe, and M.R. Thompson, Radar observations of ash eruptions, U.S. Geol, Surv, Prof. PAD., 1250, 323-334,

_

1981. Hickson, C.J., and W.C. Barnes, The initial

pyroclastic surge at Mount St. Helens and its deposits, in Proceedings, Mount St. Helens: One ]fear Later, edited by S.A.C. Keller, pp.13-20, Eastern Washington University Press, Cheney, 1982.

Hoblitt, R.P., Observations on the eruptions of July 22 and August 7, 1980 at Mount St. Helens, Washington, U.S. Geol. Surv. Spec. PAD., 1335, 1-44, 1986.

_

Hoblitt, R.P., C.D. Miller, and J.W. Vallance, Origin and stratigraphy of the deposit produced by the directed blast, U,S, Geol. Surv. Prof. PAD., 1250, 401-420, 1981.

_

Hooper, P.R., I.W. Herrick, E.R. Laskowski, and C.R. Knowles, Composition of the Mount St. Helens ashfall in the Moscow-Pullman area on 18 May 1980, Science, 209, 1125-1126, 1980.

Ikramuddin, M., S.J.M. Digby, and A.R. Buehler, Chemical composition of the Mount St. Helens ash from the 18 May eruption, in Proceedings, Mount St. Helens: One Year Later, edited by S.A.C. Keller, pp.27=32, Eastern Washington University Press, Cheney, 1982.

Janda, R.J., K.M. Scott, K.M. Nolan, and H.A. Martinson, Lahar movement, effects and deposits, U.S. Geol, Surv. Prof. Pap., 1250, 461-478, 1981.

Krimmel, R.M., and A. Post, Oblique aerial photography, March-October 1980 U.S. Geol. Surv, Prof, Pap., 1250, 32-52, 1981.

Kuntz, M.A., P.D. Rowley, N.S. MacLeod, R.L. Reynolds, L.A. McBroome, A.M. Kaplan, and D.J. Dike, Petrography and particle-size distribution of pyroclastic-flow, ash-cloud and surge deposits, U.S. Geol, Surv. Prof, PAD., 1250, 525-540, 1981.

_

Lipman, P.W., D.R. Norton, J.E Taggart, E.L. Brandt, and E.E. Engleman, Compositional variations in 1980 magmatic deposits, U.S. Geol. Surv. Prof. PAD., 1250, 631-640, 1981.

_

Malone, S.D., E.T. Endo, C.S. Weaver, and J.W. Ramey, Seismic monitoring for eruption prediction, U.S. Geol. Surv, Prof. Pap., 1250, 803-814, 1981.

Melson, W.G., Monitoring the 1980-1982 eruptions of Mount St. Helens: Compositions and abundance of glass, Science, 221, 1387-1391, 1983.

Moore, J.G., and W.C. Albee, Topographic and structural changes, March-July 1980- Photogrammetric data, U.S. Geol, Surv. Prof. PAD., 1250, 123-134, 1981.

_

Moore, J.G., and C.J. Rice, Chronology and character of the May 18, 1980 explosive eruptions of Mount St. Helens, in Explosive Volcanism: Inception, Evolution and Hazards,

_

pp. 133-142, National Academy Press, Washington, D.C., 1984.

Moore, J.G., and T.W. Sisson, Deposits and effects of the May 18 pyroclastic surge, U.S. Geol. Surv. Prof. Pap., 1250, 421-438, 1981.

Moore, J.G., P.W. Lipman, D.A. Swanson, and T.R. Alpha, Growth of lava domes in the crater June 1980-January 1981, U.S. Geol. Surv. Prof. PAD., 1250, 541-548, 1981. _

Quinn, R.R., Variations in atmospheric circulation, Mount St. Helens eruptions, 1980, in Proceedings, Mount St. Helens: One Year Late•, edited by S.A.C. Keller, pp.71-18, Eastern Washington University Press, Cheney, 1982.

Rooth, G.H., A personal account of a nuee ardente on Mount St. Helens during the eruptions of May 18, 1980, Oreg. Geol., 42(8), 141-144, 1980.

Rose, W.I., Scavenging of volcanic aerosol by ash: Atmospheric and volcanologic implications, •, 5 (10), 621-624, 1977.

Rosenbaum, J.G., and R.B. Waitt, Jr., Summary of eyewitness accounts of the May 18 eruption, U.S. Geol. Surv. Prof, PAD., 1250, 53-68, 1981.

Rosenfeld, C.L., Observations on the Mount St. Helens eruption, A•., 68, 494-509, 1980.

Rowley, P.D., M.A. Kuntz, and N.S. MacLeod, Pyroclastic-flow deposits, U.S. Geol, Surv. Prof. PAD., 1250, 489-512, 1981.

_

Rowley, P.D, N.S. MacLeod, M.A. Kuntz, and A.M. Kaplan, Proximal bedded deposits related to pyroclastic flows of May 18, 1980, Mount St. Helen, Washington, Geol. $oc, Am. Bull., 96, 1373-1383, 1985.

Rutherford, M.J., H. Sigurdsson., S. Carey, and A. Davis, The May 18, 1980 eruption of Mount St. Helens, 1, Melt composition and experimental phase equilibria, J. Geophys. Res., 90, 2929-2947, 1985.


Sarna-Wojcicki, A.M., S. Shipley, R.B. Waitt, Jr., D. Dzurisin, and S.H. Wood, Areal distribution, thickness, mass, volume and grain size of air-fall ash from the six major eruptions of 1980, U.S. Geol, Surv. Prof. PAD., 1250, 577-600, 1981a. _

Sarna-Wojcicki, A.M., C.E. Meyer, M.J. Woodard, and P.J. Lamothe, Composition of air-fall ash erupted on May 18, May 25, June 12, July 22 and August 7, U.S, Geol, Surv. Prof. PAD.,

_

1250, 667-681, 1981b. Scandone, R., and S.D. Malone, Magma supply,

magma discharge and readjustments of the feeding system of Mount St. Helens during 1980, J. Volcanol. Geotherm. Res., 23, 239-262, 1985.

Scheidegger, K.F., A.N. Federman, and A.M. Tallman, Compositional heterogeneity of tephras from the 1980 eruptions of Mount St. Helens, J. Geophys. Res., 87, 10,861-10,881, 1982.

Schmid, R., Descriptive nomenclature and classification of pyroclastic deposits and fragments: Recommendations of the IUGS Subcommission on the Systematics of Igneous Rocks, Geology, 9, 41-43, 1981.

__

Shemeta, J.E. and C.S. Weaver, Seismicity accompanying the May 18, 1980 eruption of Mount St. Helens, Washington, in Proceedings, Mount St. Helens Five Years Later, edited by S.A.C. Keller, pp.44-58, Eastern Washington University Press, Cheney, 1986.

Sigurdsson, H., and S.N. Carey, Stratigraphy of air-fall tephra from the May 18 explosive eruption of Mount St. Helens (abstract), Eos, Trans. AGU, 61, 1136, 1980.

Sparks, R.S.J., The dimensions and dynamics of volcanic eruption columns, Bull. Volcanol., 48, 3, 1986.

Sparks, R.S.J., and G.P.L. Walker, The signifi- cance of vitric-enriched air-fall ashes associated with crystal-enriched ignimbrite, J, Volcanol, Geotherm. Res., 2, 329-341, 1977.

Sparks, R.S.J., S. Self, and G.P.L. Walker, Products of ignimbrite eruptions, Geology, 1, 115-118, 1973.

Sparks, R.J.S., L. Wilson, and G. Hulme, Theoretical modeling of the generation, movememt, and emplacement of pyroclastic flows by column collapse, J, Geophys. Res., 83, 1727-1739, 1978.

Sparks, R.S.J., J.G. Moore, and C.J. Rice, The initial giant umbrella cloud of the May 18, 1980, explosive eruption of Mount St. Helens, J. Volcanol, Geotherm. Res., 28, 257-274, 1986.

Stoiber, R.E., and W.I. Rose, The geochemistry of Central American volcanic gas condensates, Geol. Soc. Am. Bull., 81, 2891-2912, 1970.

Stoiber, R.E., S.N. Williams, L.L. Malinconino, D.A. Johnston, and T.J. Casederail, Mount St. Helens: Evidence of increased magmatic gas component, J. Volcanol. Geotherm. Res., 11, 203-212, 1981.

Voight, B., Time scale for the first moments of the May 18 eruption, U.S. Geol. Surv. Prof. PAD., 250, 69-86, 1981.

_

Voight, B., H. Glicken, R.J. Janda, and P.M. Douglass, Catastrophic rockslide avalanche of May 18, U.S. Geol. Surv. Prof. Pap., 1250, 347-376, 1981.

Waitt, .R.B., Jr., Devastating pyroclastic density flow and attendant air fall of May 18-Strati- graphy and sedimentology of deposits, U.S. Geol. Sur¾, prof. Pap., 1250, 439-460, 1981.

Waitt, R.B., Jr., and D. Dzurisin, Proximal air-fall deposits from the May 18 eruption -Stratigraphy and field sedimentology, U.S. Geol. Surv. Prof. PAD., 1250, 601-616, 1981.

.

Walker, G.P.L., Ignimbrite types and ignimbrite problems, J, Volcanol. Geotherm. Res., 17, 65-88, 1983.

Wilson, L., R.S.J. Sparks, T.C. Huang, and N.D. Watkins, The control of volcanic column heights by eruption energetics and dynamics, J. GeoDhvs. Res., 83, 1829-1836, 1978.

_

Wilson, L., R.S.J. Sparks, and G.P.L. Walker, Explosive volcanic eruptions, IV, The control of magma properties and conduit geometry on eruption column behavior, Geophys. J, R. Astron. Soc., 63, 117-148, 1980.

C.W. Criswell, Department of Geology, Univer- sity of New Mexico, Albuquerque, NM 87131.

(Received January 22, 1986; revised July 7, 1986;

accepted March 30, 1987.)

journal of geophysical research, vol. 92, no. b10,...

Documents