ELSEVIER Journal of Volcanology and Geothermal Research 69 ( 1995) I 17- I35

An historic subpliniardphreatomagmatic eruption: the 1630 AD eruption of Furnas volcano, S2o Miguel, Azores

P.D. Cole a7*, G. Queiroz b, N. Wallenstein b, J.L. Gaspar b, A.M. Duncan ‘, J.E. Guest a

a University of London Observatory, University College London. Mill Hill, London NW7 4SD, UK b Universidade dos Acores, Ponta De&da, Azores, Portugal

’ Department of Geological and Environmental sciences, University of Luton. Park Square, Luton, LUl 3JlJ, UK

Received 20 April 1994; accepted 22 February 1995

Abstract

The 1630 AD eruption on the island of SIo Miguel in the Azores took place from a vent in the southern part of the 7 X 5 km caldera of Fumas volcano. Precursory seismic activity occurred at least 8 hours before the eruption began and was felt over 30 km away. This seismic activity caused extensive damage destroying almost all buildings within a 10 km radius and probably triggered landslides on the southern coast.

The explosive activity lasted - 3 days and ashfall occurred as far as 550 km away. Published models yield a volume of 0.65 km3 (DRE) for the explosive products. Throughout the course of the eruption more than six discrete airfall lapilli layers, each of subplinian magnitude, were generated by magmatic explosive activity. Dispersal directions initially to the west and finally northeast of the vent indicate a change in wind direction during the eruption. Isopleth maps suggest column heights of up to 14 km and wind speeds varying between < 5 and 30 m/s when compared to published plume models. On steep southern slopes ( > 20”) at least one lapilli layer (L2) shows pinch and swell thickness variations, and rounded pumice clasts suggesting instant remobilisation as grain flows.

Ash-rich layers with abundant accretionary lapilli and vesicular textures are interbedded with the lapilli layers and represent the deposits formed by phreatomagmatic phases that punctuated the purely magmatic activity. The ash-rich layers show lateral thickness variations, as well as cross-bedding and sand-wave structures suggesting that low-concentration, turbulent flows (surges) deposited material on topographic highs. These pyroclastic surges were probably responsible for the 80 people reported burned to death 4 km southwest of the vent. High-particle-concentration, non-turbulent pyroclastic flows were channelled down steep valleys to the southern coast contemporaneously with the low-concentration surges. The massive flow deposits ( - 2 m thick) pass laterally into thin, stratified, accretionary lapilli-rich ashes ( - 20 cm thick) over 100 m horizontally. Lateral transition between thick massive and thin stratified facies occurs on a flat surface unconfined by topography indicating that the flows had an effective yield strength.

Effusive activity followed the explosive activity building a trachytic lava dome with a volume of - 20 x lo6 m3 (0.02 km3 DRE) within the confines of the tuffjpumice cone formed during the explosive phase. Historic records suggest that dome building occurred over a period of at least two months. Calculated durations for eruptive phases and the fluctuation in eruptive style suggest that the eruptioq was pulsatory which may have been controlled by variable magma supply to the surface.

* Corresponding author. Present address: Department of Geology, University of Luton, Park Square, Luton LUl 3JU, UK.

0377-0273/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDfO377-0273(95)00033-X

118 P.D. Cole et al. /Journal of Volcanology and Geothermal Research 69 (1995) 117-135

1. Introduction

Although the products of large Plinian eruptions

have been studied extensively, remarkably few smaller

subplinian eruptions have been described (Francis,

1993). Of those studied, generally only isopach and

isopleth maps have been constructed for whole eruption

tephra sequences (e.g., Baker et al., 1973; Self, 1976;

Boothet al., 1978; Amosetal., 1981). Studies ofsingle

eruption sequences have shown, however, that different

layers had different dispersal directions and maximum

clast sizes owing to changes in wind direction and col-

umn height, respectively, during the eruption (Carey

et al., 1990; Bursik et al., 1992; Bursik, 1993). The 1630 AD eruption of Furnas, on the island of SBo

Miguel in the Azores was, in part, subplinian in mag-

nitude. We describe the deposits formed by this erup-

tion constructing isopleth and isopach maps for several

of the subplinian phases. The eruption apparently fluc-

tuated between magmatic (subplinian) and phreato-

magmatic episodes and explosive activity was followed

by the growth of a lava dome.

Furnas is one of three active composite volcanoes

on the island SHo Miguel in the Azores archipelago

(Forjaz, 1986). The volcano lies in the eastern part of

the island on the western margin of the older, now

inactive, Povoa@o caldera and some 10 km east of

Fogo volcano. Morphologically, Furnas is composed

of at least two calderas. The youngest caldera - 5 km in diameter is nested within an older less distinct struc-

ture some 7 X 5 km (Fig. 1) . Volcanic activity at Fur-

nas spans the last 100,000 years, with much of the

presently exposed construct postdating 48,000 years (Moore, 1991) . Furnas has been the site of at least ten

explosive eruptions in the last 5000 years (Booth et al., 1978) all of which took place from within the caldera.

Like the 1630 AD eruptions, many eruptions of the last 5000 years formed pumice or tuff rings within which

the final activity produced a lava dome (Booth et al.,

1978). The 1630 AD eruption occurred in the southern part

of the caldera, with the probable vent area now marked by a trachytic lava dome (Figs. 1 and 2). The eruption

had an explosive phase that formed a tuff/pumice ring

complex composed of two separate structures. To the

northeast, east and south a semicircular part of a tuff/

pumice cone rises -200 m above the caldera floor

(Fig. 1) . To the northwest is a horseshoe-shaped tuff

ring -0.5 km in diameter. Although uncertain, these

structures could mark a change in vent location as the

eruption progressed. Beyond the tuff/pumice ring the

tephra is composed of alternating ash and lapilli-rich

layers suggesting that there were several eruptive

phases (Figs. 3 and 4). Pyroclastic flows, coarse-clast-

rich and fine-grained debris flows were all generated

during the eruption. The effusive phase that terminated

the eruption produced a lava dome 600-700 m in diam-

eter and 100 m in height (Figs. 1 and 2). All distances

quoted from vent are measured from the centre of the

lava dome (Fig. 1) .

Although not a primary eruptive product, a coarse-

clast-rich debris flow deposit on the beach at Ribeira

Quente, that underlies the 1630 AD tephra (Fig. 4),

may have been generated by collapse of the sea cliff

during intense seismic activity immediately prior to the

eruption.

2. Historical accounts

Based on 14C radiometric dating (Guest et al., 1994)

the last eruption before the 1630 AD event occurred in

about 1445 AD possibly immediately after the first

settlers reached the island. Records suggest that, imme-

diately prior to the 1630 AD eruption, more than one

lake existed within the Fumas caldera. Dias (1936)

reports the existence of three lakes: Grunde (big)

which was probably largely the same as the lake that

exists today; Escuru (dark and mysterious) that had

odorous, dark water; and Barrentu (muddy). The latter two were probably located in the southern part of the caldera and were apparently destroyed by or during the

1630 AD eruption.

Fig. 1. Map of the southern part of the Fumas volcano showing the dome and tufflpumice ring complex formed by the 1630 AD eruption.

RW= area of reworked material (see text). Inset shows the various of volcanoes of the island of &IO Miguel (towns: PD = Ponta Delgada;

L = Lagoa; and the location of Sao Miguel in the Azores Archipelego (dash dotted line = Mid Atlantic Ridge; EAFZ= East Azores Fracture Zone; unlabelled broken line = Terceira rift).

PD. Cole er al. / Jourtd of Volcar~ology und Geothermal Research 69 (I 9951 I 17-135 119

l 37 hcalitv

_ / younger caldera

d22,

- ----- Gaqa LJ L K[lL

120 P.D. Cole et ul. /Journal of’ Volcunolo~y und Geothermcrl Research 69 (I 995) I 17-135

Fig. 2. A view of the 1630 AD vent area from the northeast looking southwest. The flat-topped trachytic lava dome (D) lies within the tuff/

pumice ring (R). The lake (L) is also shown.

Fig. 3. 1630 AD tephra some 2.5 km west of the vent near FL 37 (see Fig. 5). Note alternating lapilli and ash-rich layers and some diffuse contacts between the lapilli and ash. Scale is in 10 cm intervals.

P.D. Cole et al. /Journal of Volcanology and Geothermal Research 69 (1995) I 17-l 35 121

Composite Section

c) ‘/Debris flow .dC deposit _------

Fig. 4. A composite section of the 1630 AD tephm (not to scale). See Fig. 10 for more details on pyroclastic flows associated with ash- rich layers.

2. I. Precursors

Felt precursory earthquakes occurred at least 6 hours before the beginning of the eruption at - 8 p.m. on September 2 (Anonymous, 1880; Jeronimo, 1989). One source states, however, that there were felt earth- quakes between 9 and 10 a.m. (all quoted times are local) on September 2 (Co&a, 1924) up to 18 hours prior to the beginning of the eruption. Earthquakes were detected 30 km away in Ponta Delgada (Anonymous, 1880), where strong or continuous (possibly har- monic) tremor was felt between 9 p.m. and 4 a.m. during the night of September 2 and morning of Sep- tember 3. The varying intensity of the earthquakes could apparently be recognised by the style in which the church bells rang. Much structural damage was caused by these earthquakes and those during the erup- tion. Reports refer to virtually all houses in Ponta Garca and Povoacsio being destroyed (Anonymous, 1880). Furthermore Jer6nimo ( 1989) mentions reports that the earthquakes caused “the hills to move” and “the

land ran to the sea, entering the sea for 200 m”. These reports could be descriptions of the landslide event that produced the debris flow deposit that underlies the 1630 AD tephra on the beach at Ribeira Quente (Figs. 4 and 10). At the time of the 1630 AD eruption Ribeira Quente was apparently not permanently inhabited.

2.2. The explosive phase

The eruption began at 2-3 a.m. on September 3 (Corri?a, 1924; Jeronimo, 1989). Reports describe “a mountain rising through the air” and “clouds of fire from two of the lakes” (Da Purificaclo, 1880) and “fire in the sky that lit the whole island” (Correa, 1924). Descriptions of people being burnt to death in the fields around Ponta Garca (Da Purificacao, 1880; Correa, 1924) and “a burning stream that spread through the woods” (Jer6nimo, 1989) appear to be consistent with pyroclastic surges travelling to the southeast towards Ponta Garca. Accounts suggest between 80 and 115 people (half the population of the village) were killed in Ponta Garca (Anonymous, 1880; Correa, 1924), but whether these people were killed by direct effects of the eruption (i.e., surges) or indirect effects such as houses collapsing due to seismic activity or roof collapse due to the weight of ash is not known.

The explosive phase of the eruption apparently lasted 3 days and from the morning of September 4 onwards falling ash caused darkness over the whole island (Jeronimo, 1989). Ashfall occurred on the island of Santa Maria 90 km to the south and as far as the island of Corvo 550 km to the west-northwest (Fig. 1; Co&a, 1924) which corroborates the easterly winds suggested by the dispersal data. Jer6nimo (1989) described reports of “tongues of fire” (possibly lightning) being seen from the islands of the central group.

Thicknesses of up to 30 palms ( _ 6 m) of ash were recorded in Ponta Garca (Anonymous, 1880; Dias, 1936), probably where ash had banked-up against buildings. Between 10 and 12 palms (2.5-3 m) of ash in Ponta Garca is probably a more reasonable average thickness (Co&a, 1924). In Villa Franca 8 palms ( u 1.5 m) of ash was deposited (Anonymous, 1880; Dias, 1936)) in Lagoa half a leg ( m 30 cm) and in Ponta Delgada (Anonymous, 1880) 5 fingers ( m 10 cm).

122 P.D. Cole et al. /Journal of Volcanology and Geothermal Research 69 (1995) II 7-135

2.3. The lava dome phase

After the main explosive activity had ceased, reports

of smoke, light, noise and small explosions that contin-

ued for the next two months until November 2 (Co&a,

1924) indicate that a lava dome was growing during this period.

The total number of casualties is around 195, of

which 30 were staying in huts near the lakes in the

caldera, 80-l 15 died in Ponta Garsa and most of the

remainder were probably in Furnas and Povoa@o

(Anonymous, 1880; Correa, 1924; Dias, 1936).

3. Field relations

3.1. Lapilli layers

Several lapilli layers, of variable thickness and grain

size occur within the 1630 AD deposits (Figs. 3,4 and 5). Their grading characteristics are distinctive enough

to allow correlation around the volcano (Table 1;

Fig. 5). From the base upwards the lapilli layers are

termed Ll, L2, L3, etc. (Figs.4 and 5); however,

because the total number of lapilli layers is not known,

owing to erosion, incomplete exposure and the highly directional nature of some eruptive phases, the upper-

most lapilli is termed Lf (f = final). The highly angular

nature of the pumice together with the mantling form of these layers indicates that they are fallout deposits. Their coarse grain size and the presence of well-vesic-

ulated pumice suggest they were generated by mag-

matic explosivity. The base of the 1630 AD tephra is

formed by a particularly coarse pumice lapilli layer, Li

(Figs. 4 and 5)) which is 8 m thick at its most proximal

location. An isopach map (Fig. 6a) indicates that Ll was only distributed over a narrow, - 1.5~km-wide

zone to the southwest. Other lapilli layers are finer

grained than Ll, but show a wider distribution, for example L3 can be correlated up to 8 km from the vent (Fig. 6). Initial phases of the eruption were dispersed to the southwest and west (Ll-L3), whereas L4 and L5 have little preferred dispersal indicating very light winds. The final lapilli layer (L,) is, however, strongly dispersed to the northeast. This is evidence that changes in both wind direction and velocity occurred during the eruption, from a northeasterly to an easterly becoming very light and finally a relatively strong southwesterly

(Fig. 6). One isopach map (Fig. 6a, L3) has an irreg-

ular, bilobate shape. There are several possibilities for

the origin of such a bilobate lapilli-fall deposit. One

possibility is that this shape is due to partial erosion by

following pyroclastic surges, as has been suggested for

similar-sized eruptions in New Zealand (Brooker et al.,

1993). Alternatively a rapid shift in wind direction

during this eruptive phase could have dispersed lapilli

in two main directions. However, recent experimental

work (Ernst et al., 1994) has shown that short, bent-

over plumes may naturally bifurcate and thus give rise

to a bilobate deposit. Ernst et al. (1994) cite several

examples of eruptions of similar plume heights to this

lapilli layerL3 ( - 13 km), that show evidenceofbifur-

cation. They suggest that the tropopause may act as a

density interface enhancing any bifurcation. Plume

bifurcation seems a likely contender to have formed

the bilobate nature of this lapilli layer. Grain-size anal-

yses show that although lapilli layers become finer

grained with distance from the vent (from - 3.2 to 1.3

Md,,,), sorting remains approximately constant

(Fig. 7).

At the same stratigraphic horizon as lapilli layer L2,

on the steep ( -20-30’) southern slopes of the vol-

cano, deposits containing rounded pumice and gener-

ally poor in fine ash crop out (localities marked “s”

on Fig. 6a). The layers show thickness variations in

that they pinch and swell independent of topography.

These lensoid lapilli layers locally have convex-up

geometries, giving upper surfaces a positive relief. At

one locality a type of backset bedding is developed

where successive lenses are stacked against preceding

ones. Grain-size analyses of these layers illustrate that

they are similar to typical lapilli layers (Fig. 7). Duf-

field et al. (1979) described pumice lapilli beds that

due to falling on steep slopes underwent flowage and

became reverse graded. Recently Sohn and Chough

(1993) described deposits from the Udo tuff cone,

Cheju island, southern Korea, that they interpreted as

the product of grain flows that were transformed from

fallout of tephra onto the steep slopes of a tuff cone.

The lenses equivalent to lapilli layer L2 show several

features similar to those described by Sohn and Chough

( 1993). It is possible that these features were also

formed by transformation of material from fall to flow

as it fell onto the steep slopes of the volcano.

P.D. Cole et al. /Journal of Volcanology and Geothermal Research 69 (1995) 117-135 123

(a) FL.51 FL 10 FL 56130 \ I

1.75 km SW 2.5 km NE

A3

-

L3 -

A2 \

FL 39

- \\

L2 \’ \

(b) NE FL 22

4km FL129 4.75km

&$@j I 50cm

FL 133

z

FL 147 6km

FL 38 ) 2.5km lm*-.&w

Fig. 5. Measured sections of the 1630 AD tephra. Distance and direction from vent are given above each section. (a) Proximal to medial sections. (b) Medial to distal sections northeast of the vent. (c) Medial to distal sections northwest of the vent. Ornamentation is as for Fig. 4.

124 P.D. Cole et al. /Journal of Volcanology and Geothermal Research 69 (1995) I1 7-135

(a -_ / FN”Z

-1 - isopach , ’

/ *

~3 - lsopach

L4 - isopach

_f - isopach (Lwr R. graded unit)

P.D. Cole et al. / Journul tf Vrkmology and Georhertnul Research 69 (1995) I 17-135 125

L2 - pumice isopleih, ./:

‘P : F”llUW

/n

L5 - pumice isopleth

L2 - Lithic isopleth / (

‘A

L3 - pumice isopleth

- -I--

Lf - pumice isopleth (Main pa

Fig. 6. Isopach and isopleth maps for the various lapilli layers of the 1630 AD eruption

126 P.D. Cole et (11. /Journal of Volcanology and Geothermal Research 69 (1995) I 17-135

Table I Field characteristics and grading styles of the various lapilli layers of the 1630 AD eruption

Lapilli layer Grading Distinctive characteristics

L,

L5

L4

L3

L2

LI

Lower layer reverse-multiple.Main part reverse-multiple

(weak)

Variable symmetric (W) normal-reverseSymmetric (E)

reverse-normal

Rare weak reverse grading

Reverse graded pumice lithics often show well developed

normal grading

Normally graded pumice and rarely lithics

Symmetric, finer at bae and top

Thicker and more homogeneous than other lapilli layers. Thin

basal layer separated from main part by fine ash layer 0.5-S cm

thick in distal areas. Banded pumice in proximal locations

Distinctive “double” layer in distal western localities. Grading

varies between different regions

Typically stratified due to interdigitation with fine ash layers

suggesting contemporaneous phreatomagmatic pyroclastic

surges

Well developed normal grading of pumice

Some stratification in proximal regions. Evidence of

transformation into grain flows on steep southern slopes

Some obsidian, banded and bread-crusted pumice

3.2. Ash-rich layers

Grey, ash-rich layers are interbedded with the lapilli

beds (Figs. 3,4 and 5). Accretionary lapilli, randomly

scattered or strongly concentrated in horizons, are

abundant. Vesicular ash horizons are widespread but appear to have a localised distribution and are thus

impossible to correlate between exposures (Fig. 5). Furthermore, the vesicular textures suggest that the ash

was quite wet on deposition. Accretionary lapilli are

typical of, but not exclusive to, phreatomagmatic erup-

tions where moisture causes aggregation of ash. Nev-

ertheless, the fine comminution of these ash-rich layers

(Figs. 7 and 8) suggests that water/magma interaction occurred (cf. Sheridan and Wohletz, 1983). The local-

ised distribution of accretionary lapilli and vesicular

ash-rich layers suggests that moisture within the eruption cloud had a heterogeneous distribution, and

may have been influenced by local rainstorms. The contact between ash-rich and lapilli layers is

variable. Some boundaries are sharp whereas others are

diffuse and gradational (see Fig. 3). In some cases the upper or lower parts of an ash-rich layers contain abun- dant pumice clasts. Uncompacted ash-rich layers would be easily deformed and impacted by the pumice lapilli, and might generate diffuse, gradational upper contacts. It is possible that the transition between magmatic and phreatomagmatic activity was sometimes very rapid. Another possibility is that phreatomagmatic and mag- matic eruptive activity may have occurred simultane- ously from two closely spaced vents as has been

observed at Surtsey in Iceland (Thorarinsson et al.,

1964; Kienle et al., 1980) and has been inferred for some strombolian and phreatomagmatic deposits in

Germany (Houghton and Schmincke, 1986). One or a combination of these processes might cause mixing of

the pumice and ash to produce a diffuse gradational

contact between the two.

Within proximal exposures, <2 km from the vent

coarser pumice-rich lenses, pinch and swell layers, low- angle cross stratification and sandwave structures are

abundant within several ash-rich layers (Figs. 4,5 and

8). These features are all typical of deposition from

relatively low concentration, turbulent flows (surges),

When these ash-rich layers are traced to progressively

more distal exposures ( > 2 km) evidence for a tlow

origin becomes less abundant, with only rare discontin-

uous pumice stringers set in a fine-grained grey ash

(Fig. 5). Some ash-rich layers, especially in more dis-

tal exposures are often completely massive, showing

no visible stratification and displaying only rare thick-

ness variations. The lack of evidence for flow at local-

ities > 2 km from the vent may have been caused by

rapid deceleration and “lofting” of the flow toward distal regions. This reduction in velocity would reduce lateral movement and traction processes within the bed-

load, as a result the formation of cross-bedding and pinch and swell structures would be inhibited and more

continuous, structureless deposits would form. Indeed features of surge deposits in Korea (Sohn and Chough, 1989) suggest a similar mechanism where the surge cloud, if it remains hot enough, loses most of its sedi-

P.D. Cole et a[. /Journal of Vokanology and Geothermal Research 69 (I 995) I 17-135 127

6 l-

I

1 ,

c

l . 0. . . ..---

_.c-------..___ --......I....--

*4--r ._.--- ,*--

-4 -3 -2 -I 0 1 2 3 4 5 6

M&J

Fig. 7. Plot of Md, vs. setting for the various facies of the 1630 AD tephra. Tie lines connect massive pyroclastic how deposits with laterally equivalent thin ash-rich layers that occur on the beach at Ribeira Quente (shown in Fig. 10). The thick broken and dash dotted lines mark the fields of fall and flow of Walker ( 1980).

(a) FL 37

2 km WNW a

& 3

:

Lli

I

09

I

2m

0

FL 35

Fig. 8. (a) A section illustrating the variation in median diameter (MC&,) and sorting (cr) between Iapilli and ash-rich layers through the 1630 AD tephra. Solid ornament = lithics; open = pumice. (b) Measured section of the upper part of the uppermost lapilli layer L/at its most proximal locality 0.5 km from the vent. Note the decrease in grainsize with height as more, slightly finer grained, dense juvenile material is erupted. Solid ornament is dense juvenile fraction not lithics.

128 P.D. Cole et al. /Journal of Volcanology and Geothermal Research 69 (1995) I1 7-135

Fig. 9. The uppermost part of lapilli layer L,at locality 35, - 0.5 km from the vent (see Fig. I and Fig. 8b). Note the predominantly dark, dense juvenile lapilli with thin layers and locally larger clasts of white vesicular pumice. Scale is in IO cm intervals.

ment load, eventually becomes lighter than the ambient

air, and rises up into the atmosphere and ash-fall depos- its are produced. This interpretation is similar to that of

Scarpati et al. ( 1993, p. 354) where field relations show

the same trend, but on a larger scale, in the lower part (member A) of the Neapolitan Yellow Tuff, Campi

Flegrei, southern Italy.

3.3. The jinal explosive phase (L+

The final explosive phase of the eruption is repre-

sented by a lapilli layer (Lr) that is more homogeneous, poorer in lithics and thicker (reaching 11 m on the

cone) than any other lapilli layer within the 1630 AD tephra (Table 1; Figs. 4 and 5). In addition it is quite

distinctive in that it has a finer-grained, reverse-graded and more lithic-rich basal part which is separated from the upper part of the layer by a thin, <OS cm, fine- grained, grey ash (see Figs. 4 and 5b). An isopach map indicates a NE-directed dispersal for this final phase (Fig. 6a) and explains the particularly high rim of the

1630 AD pumice ring on its east side (Figs. 1 and 2). At the most proximal exposures, - 0.5 km southwest

of the vent in the pumice ring, the uppermost part of Lr

is composed of a sequence, up to - 5 m thick, of dense,

dark grey, juvenile lapilli (Figs. 8b and 9). The tran-

sition from the underlying vesicular white pumice to

the dense, dark grey lapilli is gradational. The dense,

grey juvenile lapilli first appears as thin bands, 3-5 cm

thick, within otherwise white pumice. This passes

upwards into predominantly finer, dense, dark lapilli

with subordinate beds of coarse pumice, until it is com-

posed almost entirely of dense lapilli with only rare

coarse white pumice clasts (Figs. 8b and 9). The lim-

ited distribution, probably < 1 km from vent, suggests

that the dense, dark juvenile material was derived from

a low column as the explosive phase waned. One or

two thin ( < 10 cm) ash layers rich in accretionary

lapilli with occasional vesicular textures occur at the

very top of the 1630 AD tephra. These layers probably result from small phreatomagmatic explosions that

occurred during growth of the lava dome. Dark, poorly vesicular juvenile material is also seen

forming banded pumice within proximal exposures of

the main part of L,. The banded pumice is particularly

abundant within coarser horizons of L, that occur within the most proximal exposures on the northwest side of the pumice ring. As well as banded pumice an

P.D. Cole et al. /Journal of Volcanology and Geothermal Research 69 (1995) 117-135 129

Table 2 Geochemical analyses of juvenile material from the 1630 AD tephfa from FL 35

White Grey

SiO, 61.85 62.43

Al203 15.3 16.91

Fe@, 5.84 4.82

CaO 1.63 1.41

MgO 0.18 0.15

TiOz 0.36 0.32

P*O, 0.08 0.08

MnO 0.06 0.05

K,O 6.28 5.83

Na,O 5.12 5.94

LOI 1.71 1.15

Total 99.01 99.09

Zn 175 185

V 58 57

CU <to < 10

Ni <5 <5 Ba 450 435

Cr 98 118 Zr 980 930 Nb 48 45 Rb 242 235 Pb 35 28

Both samples are from the top of L, at site FL 35 (see Fig. 8b). The grey sample is dense, from the top of the sequence while the white pumice is from lower in the sequence. Samples were analysed by XRF at the University of Luton.

unusual type of pumice breccia is locally formed by juvenile material with different vesicularities. Individ- ual pumice blocks, locally up to 1 m in size, are com- posed of fragments of white, vesicular pumice, variable in both size and form, set in a matrix of dense, dark glass, some of which is obsidian. The white vesicular pumice ranges from subcentimetre- to centimetre- sized, typically rounded fragments to large decimetre- scale angular clasts. Local folding of both juvenile types, due to plastic deformation, is also observed. Some fragments of breccia occur set within individual pumice blocks suggesting more than one phase of frag- mentation associated with the formation of these brec- cias. These various textures suggest that the darker, more gas poor magma was less viscous than the white vesicular magma which behaved in a more brittle man- ner. Nevertheless, both juvenile types were locally plas- tically deformed. Chemical analyses of the two pumice types in the upper part of Lf show that they have a very

similar composition (Table 2), although they were clearly quite different in their gas contents. A similar type of pumice breccia was described by Criswell ( 1987) from the May 18,198O pyroclastic flow depos- its of Mt. St. Helens, where the different pumice colours correspond to changes in chemical composition. At Mt. St. Helens there is, however, apparently no variation in vesicularity.

3.4. Pyroclasticjow deposits

Although widespread evidence for low-particle-con- centration, turbulent flows is commonplace within the ash-rich layers, deposits resulting from non-turbulent, high-concentration flows have only been found at a few localities on the southern coast. The best exposure occurs on the beach at Ribeira Quente (Figs. 1,4 and 10). The massive, poorly sorted deposits of pumice, lithics and ash occur in units up to _ 2 m in thickness (Fig. 10). The units have finer-grained basal regions and some have concentrations of coarse pumice toward their upper surface. Thus they may be defined as “flow units” and are interpreted as the deposits of laminar pyroclastic flows. Partially carbonised wood is abun- dant within these deposits, indicating emplacement temperatures of around 300°C. The long axes of logs and branches are aligned parallel to the unit surface and the axis of the valley. On the beach at Ribeira Quente the massive flow deposits do not pond into a topo- graphic depression but pass laterally, at the same ele- vation, into thinner, finer-grained ash-rich layers. They cannot therefore be termed “overbank” facies of pyro- elastic flows. The ash layers are -20-30 cm thick, show some stratification on a centimetre to metre scale and are rich in accretionary lapilli (Fig. 10). As a con- sequence the massive flow deposits are lobate, having a convex-up morphology, suggesting the flows had a yield strength. The pyroclastic flows were contempo- raneous at least with ash-rich layers Al, A2 and A5 (Fig. 10). Grain-size data (Fig. 7) show that the lat- erally equivalent ash layers are finer grained and better sorted than the massive deposits probably owing to the absence of coarse material in the ash layers. The mas- sive flow deposits occur on the beach immediately in front of a steep-sided “ravine-like” valley that leads in NNW direction toward the eruption site. This sug- gests that this valley first channelled then directed the pyroclastic flows as material drained off the strong

130 P.D. Cole et al. /Journal of Volcanology and Geothermal Research 69 (1995) I 17-135

c deposit I ,.1 _I

Fig. 10. Two measured sections of 1630 AD tephra on the beach at Ribeira Quente. The transition between thin stratified ashes and thick massive

layers occurs over continuous exposure with thickening to massive, coarser deposits to the left (towards the axis of the ravine-shown by the

path of the arrow in Fig. 1). This thicknening occurs on the same horizon and is not due to ponding into a depression. Grain-size histograms

illustrate the difference in grain size between the ashes and the coarser massive flow deposits (see also Fig. 7).

topography, contemporaneous with more widespread low-concentration surges. The general absence of this

type of flow deposit could be explained by the strong

relief of the area, as the very steep slopes are likely to

hinder emplacement of laminar pyroclastic flows.

Some pyroclastic flows may, therefore, have swept off

the volcano into the sea leaving little evidence for their existence.

3.5. Proximal reworked deposits

On the southern caldera rim, immediately south of the 1630 AD vent area a sequence of deposits, N 30 m

in thickness, occurs within a small depression u 300 m

wide (marked RW on Fig. 1). This depression is prob- ably related to an irregularity, of unknown origin, in the caldera rim which was present prior to the eruption. The uppermost N 5 m of this sequence are deposits of fluvially reworked 1630 AD tephra (Fig. 11). They consist of many small scours and discontinuous lenses rich in dense fragments (lithics and juvenile). The lay- ers of dense clasts show strong normal grading and rapid lateral variations in grain size. Local thin pumi- ceous lenses are also present. The reworking by water

has apparently separated the dense material from the lighter pumice. Interbedded within this sequence are primary unreworked ash and lapilli layers. Below this

at least 3 m of massive vesicular ash crops out, within

which are blocks, up to 75 cm across, of bedded lapilli

and ash of primary 1630 AD tephra. This massive

vesicular ash is interpreted as a debris flow deposit and the blocks of undisturbed tephra are considered to have been transported within the Mar. At lower levels

within this sequence of 1630 AD tephra primary depos- its of ash and lapilli occur.

A possible explanation is that this reworked sequence originated from the remobilisation of tephra

at higher levels on the caldera rim during the eruption.

The reworking of tephra during the eruption was prob-

ably promoted by rainstorms.

4. Physical parameters of the eruption - column heights, wind speeds, volumes, durations

The isopleth maps constructed for the various airfall lapilli layers can be combined with existing plume models (Carey and Sparks, 1986; Sparks, 1986; Wilson

P.D. Cole et al. /Journal of Volcanology and Geothermal Research 69 (1995) 117-135 131

- load siructures

\ - lenses & layers

d coarse dense ’ material

- lenses of sorted - pumice lapilli

. load structures

b primary ash-fall layers

debris flow deposit with fragile tephra blocks and pumiceous lenses

Fig. 11. A measured section through the upper part of the reworked

material at locality 27 (see Fig. 1 for location).

Table 3

Column height calculations for the lapilli layers of the 1630 AD tephra

and Walker, 1987) to calculate the mass discharge/ eruption rate and eruption column heights. The results are given in Table 3 and show that the model of Wilson and Walker ( 1987) yields consistently lower eruption column heights than that of Carey and Sparks ( 1986). As the column height decreases the difference between the two models calculations increases, probably as a result of the data being close to the lower limit of the Wilson and Walker (1987) model. However, accord- ing to both models the eruption column increased in height, reaching a peak (L3) at - 14 km and then decreased with later lapilli layers being derived from lower columns. When integrated with the model of Sparks ( 1986) these column heights indicate mass dis- charge rates up to -8X lo6 kg/s (Table 3). Plume models (Carey and Sparks, 1986) also allow estimates of wind speeds and indicate that winds were initially light ( - 5 m/s or less) but increased (25-30 m/s) during the final explosive phase (L,) (Table 4).

The volumes of the various lapilli layers were cal- culated using the method of Fierstein and Nathenson ( 1992). Calculations for each of the lapilli layers, the total ash and the lava dome yield a total volume of 2.52 km3 and a DRE volume of 0.675 km3 (Table 4). This volume can only be considered a minimum estimate as the volume of pyroclastic flows which entered the sea is not taken into account. Of the total erupted volume 68.4% were phreatomagmatic products, 28.6% mag- matic products and 3% was the effusive extrusion of the lava dome.

As the volume of the various parts of the eruption are known by using the magma density the mass of each layer could then be derived. Combining this with the mass discharge rate (MDR) , calculated from the column heights using the model of Carey and Sparks

Lapilli layer Wilson and Walker (1987) Carey and Sparks ( 1986) Sparks

(1986) model MER Column height Column height (P) Column height (L) Wind speed MDR

(X 106) (km) (km) (km) (m/s) ( x lo6 kg/s)

L2 34 10.6 11 S-12.8 12-13 5 4-6 L3 56 11.7 11.5-12.8 13.3-14 5 5-8 LS -2 8.9 11.2-13 5 4-6

L, -0.2 5 8-11 _ 25-35 24

Cakxlations made using the models of both Carey and Sparks ( 1986) and Wilson and Walker ( 1987). Column height (P) = calculation using

pumice clasts; (L) = calculation using lithic lava clasts.

132 P.D. Cole et al. /Journal of Volcanology and Geothermal Research 69 (1995) 117-135

Table 4 5. Eruption narrative Volume calculations for different parts of the 1630 AD eruptive products

Deposit Volume Density DRJZVol Mass MDR Duration (Xl06 (g/cm) ( X lo6 (X109 (X106 (min) m’) m3) kg) kg/s)

Ll 179 0.5 37.9 90.96 nd _

L2 67 0.75 20.8 49.92 4-6 166.4 L3 76 1.04 32.7 78.48 5-8 186.9 IA 10 0.73 3 7.2 nd _

L5 28 0.73 8.4 20.16 4-6 67.2

L, 503 0.44 90.5 217.2 4 905 Total ash 1230 0.9 461.3 - nd - Dome 20 2.4 20 _ nd _

Total 2093 654.6 Vol.

After at least 6 hours of felt precursory activity the 1630 AD eruption commenced with magmatic activity

(Fig. 12). Although abundant water, in the form of two

lakes, was located on and/or around the vent, the onset

of phreatomagmatic activity occurred only after a sig-

nificant period of magmatic activity. The eruption then fluctuated between magmatic and phreatomagmatic

activity for a large part of the explosive phase. During magmatic phases tephra was deposited almost exclu- sively by airfall, except where material fell on to the

(2113) (674.6)

The volume of each layer (Ll, L2 etc..) was calculated using the technique of Fierstein and Nathenson ( 1992) using a thickness ver- sus isopach (area) “2 plot. Densities of the different lapilli layers and ash-rich layers were calculated by pouring the weighed sample into a graduated container and tapping until a constant volume was acheived. DRE (dense rock equivalent) was calculated assuming an original magma density of 2.4 g/cm. The mass of each layer was calculated by multiplying the density and volume (both DRE). The volume calculations ignore the lithic contribution. as many lapilli layers are extremely lithic poor ( < 1 wt.%). The total volume of products ( pyroclastic + dome) is given in parentheses.

I , txlt precurhory earthquakes occurred up 10

IX hours pnor to the surt of the eruplmn These

exrthquakes ma) haw Induced landslides /debris

flows on the wulbem coat (arrow) Prior to the

cruptmn three lakes existed wilhin the caldera.

21 The eruptmn commenced beween 2 and

?a,,~ or, ,hc i,d Sept The ,n,l,al phase ~b

n~agmatic w$th the development of a \hori

crupt~on column probably of ctrombol!an

magmmde. Tb,a ,,,,t,al phax produced a cvitrse

lap,,,, layer d,\tnbuted to the south west.

( 1986), gives an estimation of the eruption duration of

a number of lapilli layers (Table 4). The results indi-

cate that lapilli layers L2-L5 were each erupted over a 1-3-hour period. Results for the lapilli layer Ln which

has a volume several times greater than any other lapilli

layer, indicate that this final explosive phase had a

duration of approximately 15 hours.

.\I Phreiltomagmdt,c act,v,ty produced

wIdespread pyroclashc surges (killing -80 people

I” f,elds near Ponta Gar$a PC). DHl%

p~n,c,d\t~ flous ,,‘cre channelled down sleep

r,,\,nes Bg ,he mormng of the 5th Sepl. ash

,I,,\cu,cI, ,hc \ ,,,, ~,,un~,,,~ ,he whole ,\l.wd 1n1”

d.,rhnc\\

Woods ( 1993) theoretically modelled the effects of

moisture on an ash-laden volcanic plume and con- cluded that plumes in humid atmospheres would reach

greater heights than those in drier atmospheres. The island of So Miguel has a humid subtropical climate,

where humidity often reaches 100%. The abundant atmospheric moisture, therefore, may have played a role in increasing the height of the column and as a consequence dispersion of tephra. The mass discharge rates calculated may therefore be overestimates as the

column heights may have been elevated due to atmos- pheric conditions which the models of Carey and Sparks (1986) and Wilson and Walker (1987) do not take into consideration.

Fig. 12. Cartoons showing the different phases of the 1630 AD eruption. The views are looking north from the sea.

P.D. Cole et al. /Joumal of Volcanology and Geothermal Research 69 (199.5) 117-135 133

steep slopes and was transformed into grain flows. Apart from the initial magmatic phase which was prob- ably strombolian in magnitude, eruption column heights, calculated from dispersal of the lapilli layers, vary between 8 and 14 km (Table 2), consistent with a subplinian magnitude (Walker, 1980; Pyle, 1989). Abundant evidence indicates that pyroclastic density flows were produced during the phreatomagmatic phases. Cross-stratification and pinch and swell struc- tures on topographic highs indicate that these flows were largely low-particle-concentration turbulent pyro- elastic surges. Subordinate denser flows, contempora- neous with the surges, were channelled down ravines giving rise to massive deposits at the coast at the mouths of valleys (Fig. 12). Moisture within the phreatomag- matic surge cloud and dilute ash suspension was appar- ently heterogeneous as accretionary lapilli and vesicular layers have a localised area1 distribution. Ash- fall that was reported on the other islands of the Azores archipelago, as far as 550 km away could have been derived from a number of mechanisms. These mecha- nisms include magmatic or phreatomagmatic activity and co-ignimbrite clouds.

The final explosive phase (L,) was a sustained mag- matic phase. Although this phase was derived from a relatively low column height of - 9 km (compared to 13-14 km during the peak of the eruption) it must have been longer lived than the other magmatic phases as it produced a volume of tephra nearly three times larger than several other lapilli layers (see Table 4). Explo- sive activity apparently waned gradually, erupting poorly dispersed, dense juvenile material near the end of the eruption. This was followed by the extrusion of a trachyte lava dome (Fig. 12) over at least the next two months. Historical records indicate that by the end of the eruption only one caldera lake remained.

6. Discussion

The alternation between lapilli and ash-rich layers, in relatively small volume pyroclastic deposits, is an exceedingly common phenomenon. At Furnas it is per- haps the most typical style of pyroclastic deposit and is common at other volcanoes, e.g., Roccamonfina (Cave unit) (Cole et al., 1992)) and Campi Flegrei in Italy (e.g., Lirer et al., 1990). The Fogo 1563 AD Plinian eruption was also of this character, composed

of alternating ash and lapilli layers (Walker and Croas- dale, 1971).

It is also interesting to note that the penultimate erup- tion, Furnas I ( N 1445 AD), and the 1630 AD event were similar in many respects. The Furnas I tephra is also formed by an alternation between ash and lapilli beds, the uppermost lapilli bed is a notably thick, homo- geneous, lithic-poor deposit and final explosive prod- ucts are dense, poorly vesicular and fine grained. Furthermore, the final phase of the eruption produced a trachytic lava dome. The volume and magnitude of the two eruptions also appear similar (Booth et al., 1978).

Several workers have recognised or inferred that subplinian eruptive events are pulsatory or episodic (Self, 1976; Scandone and Malone, 1985; Bursik, 1993). Both Scandone and Malone ( 1985) and Bursik (1993) attributed the episodic nature of subplinian eruptions to the disparity between the magma supply (influx) and magma discharge (eruption). At Mt. St. Helens, the volume of products and eruption rate for a particular phase decreased with time (Scandone and Malone, 1985). However, Bursik (1993) found that, for the north Mono deposits, USA, eruption rate increased with time, which he explained by an increase in the conduit cross-sectional area for each phase. For the 1630 AD eruption the volume of magma erupted from distinct phases shows an irregular trend, although the last explosive phase (L,) is much larger in volume than any of the other phases (Table 4). The gradual decrease in column height toward the end of the erup- tion (Table 3) and the transition from explosive to effusive activity indicate that the eruption rate decreased with time. The different phases are, however, defined by the change from magmatic phreatomag- matic activity (lapilli to ash), which might suggest a pulsatory supply of magma to the surface. Indeed a waning supply rate of magma and lowering of the magma column has been invoked to allow sufficient water penetration into the vent for extensive phreato- magmatic activity (Decker and Christiansen, 1984; Mastrolorenzo et al., 1993). As historical accounts state that the eruption lasted a total of three days it seems unlikely that explosive activity was continuous over this time, and pauses between the various phases probably occurred. Variations in magma supply may have been important factors in the eruptive mechanism. Although phreatomagmatic activity was typical during

134 P.D. Cole et al. /Journal of Volcanology and Geothermal Research 69 (1995) 117-135

the eruption (68% by volume of the eruptive products) the final phase of the eruption was entirely magmatic and relatively voluminous (forming 47% by volume of all magmatic products). This suggests that either the water necessary for water/magma interaction had been used up, as in the case of caldera lakes for example, or water was prevented from accessing the vent, possibly due to the buildup of pyroclastic material around the vent.

Acknowledgements

This study forms part of a project on styles of vol- canic activity at Furnas volcano, funded by the EEC environment programme. The whole international pro- ject was coordinated by Victor Hugo Forjaz whom we thank for logistic support and hospitality. We also would like to thank Zeta Pacheco and Teresa Ferreira for assistance in the field. Michael Sheridan and Clau- dia Principe are thanked for providing constructive reviews.

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