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ELSEVIER Journal of Volcanology and Geothermal Research 69 ( 1995) 217-239

Chronology and dispersal characteristics of recently (last 5000 years) erupted tephra of Cotopaxi (Ecuador): implications for

long- term eruptive forecasting

F. Barberi ‘, M. Coltelli b, A. Frullani ‘, M. Rosi a7*, E. Almeida d a Dipartimento di Science della Terra, Universitri di Pisa, Via S. Maria 53, 56126 Piss, Italy

h CNR Istituto Internazionale di Vulcanologia. Piazza Roma 2, 95123 Catania. Italy

’ Via Leonardo da Vinci 46, 55042 Forte dei Marmi, Lucca, Italy

d Instituto Ecuatoriano de Electrijkacion. Avenida IO de Agosto 5844, Quito, Ecuudar

Received I February 1994; accepted 17 January 1995

Abstract

Cotopaxi, the highest active volcano on earth and one of the most dangerous of Ecuador is constituted by a composite cone made up of lava and tephra erupted from the summit crater. The activity of the present volcano begun with large-volume plinian eruptions followed by a succession of small-volume lava emissions and pyroclastic episodes which led to the edification of a symmetrical cone. The growth of the cone was broken by an episode of slope failure, the scar of which is now obliterated by recent and historical products. Volcanic history, eruptive frequency and characteristics of the activity were investigated by studying the stratigraphy of tephra and carrying out fifteen new 14C dating on paleosols and charcoals. The investigated period is comprised between the slope failure and the present. The deposit of the volcanic landside (dry debris avalanche of Rio Pita), previously believed to be between 13,000 and 25,000 yr B.P., is now considered to have an age slightly older than 5000 yr B.P. The stratigraphy of tephra of the last 2000 years reveals the existence of 22 fallout layers. Seven of them were dated with 14C whereas three were ascribed to the eruptions of 1534, 1768 and 1877 on the basis of comparison with historical information.

Maximum clast size distribution (isopleths) of 9 tephra layers points out that the sustained explosive eruptions of Cotopaxi during the last 2000 years are characterized by very high dispersive power (plinian plumes with column heights between 28 and 39 km) and high intensity (peak mass discharges from 1.1 to 4. I X 10’ kg/s). The magnitude (mass) of tephra fallout deposits calculated from distribution of thickness (isopaches) are, however, moderate (from 0.8 to 7.2 X 10” kg). The limited volume of magma erupted during each explosive episode is consistent with the lack of caldera collapses. Small-volume pyroclastic flows and surges virtually accompanied all identified tephra fallouts. During such an activity large scale snow/ice melting of the summit glacier produced devastating mudflows comparable in scale to those of 1877 eruption. By assuming a 1: 1 correspondence between fallout episodes and generation of large-scale lahar, we have estimated an average recurrence of one explosive, lahar- triggering event every 117 years over the last two millennia. This value compares well with that calculated by considering the period since Spanish Conquest. The probability of having an eruption like this in 100 or 200 years is respectively of 0.57 and 0.82. Such an high probability underscores the need for quick actions aimed at the mitigation of Cotopaxi lahar hazard along all the main valleys which originate from the volcano.

1. Introduction

* Corresponding author

Assessment of volcanic hazard consists in the identification of expected eruptive phenomena and zonation

0377-0273/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved

SSD/O377-0273(95)00017-S

218 F. Barberi et al. /Journal oj Volcanology und Geothermal Research 69 (1995) 217-239

of potentially affected areas. However, at more

advanced stages the quantification of the hazard at each volcano also requires the long-term probabilistic eval-

uation of occurrence of each phenomenon in a pre- determined forthcoming time interval. This target can

be achieved by thorough reconstruction of the past volcanic activity over a suitable period exceeding that cov-

ered by the historical record. From this standpoint

tephrostratigraphic studies, coupled with radiometric age measurements, are very valuable in permitting identification of widespread stratigraphic isochrons.

In this paper we address: ( 1) the eruptive history of

the last 5000 years of Cotopaxi activity with the aim at

assessing the long-term probability of occurrence of

highly hazardous eruptions in the near future; (2) the

clast dispersal characteristic of the main plinian layers

of the last 2000 years to calculate their intensity and

magnitude.

2. Basic background

Cotopaxi volcano is worldwide known for the mag-

nificence of its cone topped by ice and for its attitude

to produce very dangerous lahars (Barberi et al., 1992). Snow fields and glacier extend downward to an altitude

of 4500-4800 m. Although the cone is almost sym-

metrical, the ice cover reaches lower elevation on the eastern side due to the presence of a significant precip- itation gradient across the mountain. The summit crater, 800 m across and 334 m deep, is largely free of snow

and contains a small pyroclastic cone at the bottom.

The Cotopaxi cone is located in the middle portion of the Interandean Depression, a major graben-like structure running between the western and eastern Cor- dilleras of Ecuador, and it is surrounded by several extinct volcanic edifices (Fig. 1). Remnants of deeply

eroded (by glacier) volcanoes (Rumi-ahui, Sinchola- gua and Pasochoa) can be recognized to the north and

northeast, whereas a 12 by 16 km wide caldera, likely generated by the eruption of the Chalupas pumice and ash-flow, lies less than 10 km to the southeast (Hall,

1977; INECEL, 1983). An old dissected volcanic structure (Morurco Peak), crops out on the southern

foot of the Cotopaxi cone. The general evolution of Cotopaxi has been preliminary investigated by Sauer ( 1965), Hradeckaetal. (1974), INECEL (1983),Hall (1987) and only very few radiometric datings have been published so far.

In a recent work, Barberi et al. (in prep.) have out-

lined the entire eruptive history of Cotopaxi volcanic

complex. They found that a more ancient volcano, named PaleoCotopaxi, was initially characterized by large explosive eruptions during which rhyolitic plinian

falls and ash-flows were deposited. Fission-track datings of obsidian fragments from these pyroclasts have

yielded about 0.5 Ma. After the early strong explosive

phase, a mild andesitic explosive and effusive eruptive

activity took place to build up a small stratovolcano slightly to the south of the present cone. The Morurco

Peak, previously interpreted by Sauer ( 1965) as related

to an ancient caldera underlying the recent Cotopaxi,

is recognized to be a sub-volcanic body, settled during

the final activity of the PaleoCotopaxi. In spite of their close spatial association, Paleo-

Cotopaxi and Cotopaxi appear to be separated by a

long-lasting erosive period and by the interposition of

the Chalupas ignimbrite.

Activity resumed in the Cotopaxi area around to

0.010-0.015 Ma ago (Barberi et al., in prep.). In the early phase, at least three large rhyolitic plinian erup-

tions occurred, followed by a cone-building phasechar- acterized by emission of andesitic lava and pyroclastic

material. The growth of the cone was interrupted by an

episode of slope failure, which resulted in the emplacement of a dry debris avalanche in the headwaters of Rio Pita (Smyth and Clapperton, 1986). After that episode

the scar has been filled and the symmetry of the cone completely restored.

3. Historic activity

Cotopaxi is one of the most active volcanoes of Ecua- dor and a large number of reports and chronicles during

the times of the Spanish Conquest refers to it. In the past decades summaries of the historic activity have been compiled by Hantke and Parodi ( 1966), Hra-

F. Burberi et (11. / Jmrnul of‘ Volcunology and Geothermal Research 69 (1995) 217-239

NCHOLAGUA

6 rim of the Chalupas caldera

5 0 5km , -4

I 7035’

219

Fig. I. Topographic sketch map of the area of Cotopaxi. Peaks surrounding Cotopaxi are deeply eroded extinct vo1canoe.s. Elevation in meters.

220 F. Barberi et al. / Joumrrl of Vo/cuno/ogy und tieothennul Reseurck 69 (1995) 217-239

decka et al. (1974), Hall (1977) and Simkin et al.

( 1981). In this paper we have undertaken a critical

review of some of the most significant historical

sources with the aim at reexamining the activity of the

last four centuries and also at pointing out possible

correlation between tephra layers and historical

eruptions. On examining the old documents (Wolf, 1904 and

herein appendix of “10s historiadoresprimititlos de las

Indias y de 10s antiguos archives’ ’ ; Sodiro. 1877; Wolf.

1878) one of the main difficulty arises from the inter-

pretation of the term ‘lava flows’ which in previous

summaries was assumed straightforward with the

meaning assigned by modern volcanology. Actually, it

is quite evident in many reports that past authors used

this term in a broader sense to include any type of

‘flowing incandescent material ” regardless of their

state of aggregation (see i.e. Wolf, 1904). The possi-

bility that nutes ardentes have been misinterpreted as

lava flows is particularly high for those events in which

‘lava flows’ are reported in connection with the for-

mation of large mudflows. It is in fact now well estab-

lished that, while pyroclastic flows/surges are very

efficient in triggering extensive melting of ice/snow

caps, such an ability is much less effective for lava

flows. As a recent example, Major and Newhall ( 1989)

referred to the 1984 eruption of Villarica volcano

(Peru) during which a vigorous lava flow produced

only a very low melting rate of the ice cap.

The first reported eruption of Cotopaxi occurred in

1534. Several Spanish historians (reported in Wolf,

1904 p. 23) relate that Pedro de Alvarado and his party.

on their way to Quito, were affected during several days

by the fallout of hot ash from the volcano of Quito

(Cotopaxi) At the same time, lahar produced by snow

and ice melting destroyed a village called La Contiega

(Zarate in Wolf, 1904). Information about this erup-

tion is not sufficient to unambiguously identify the

related tephra layer; however, a 14C dating of the paleosol underneath layers 1 and 2 (290 &- 80 yr B.P.) may support such a correlation (see description of tephra).

Cotopaxi erupted again in 1742 after more than 200

years without any reported activity (Wolf, 1904) ; two main explosions occurred that year with the production

of ash fallout and destructive lahars. In April, 1743 a new explosive eruption took place: Wolf ( 1904)

reports that the cone was covered everywhere by lava

ilows (nukes ardentes) and ‘immense’ lahars were

formed.

During 1744, Cotopaxi had a major explosive erup-

tion: extensive sand and ash fallout 7 to IO cm thick

occurred about 10 km west of the crater and once again

“corrientes de latla en diferentes direcciones” were

reported by Wolf ( 1904) to have poured out of the

summit crater with consequent snow melting and pro-

duction of lahar greater than the preceding ones. The

eruption was similar to those of 1742 and 1743 and

‘lava flows’ possibly were nukes ardentes as suggested

by the associated fallout activity and the lahar forma-

tion. However, at least one lava effusion might have

occurred towards the end of the eruption. Wolf ( 1904))

interpreting the description of Velasco, refers “...la

gran calle que abrid desde la boca en la cumbre hasta

supie’.. la cual no se ristidde niece en algunosarios...“,

to as a lava flow which remained hot and free of snow

for several years.

After 22 years, a new eruption occurred in 1766;

great lahars and floods affected the town of Latacunga

whereas fallout of coarse pumice west of the volcano

caused the destruction of several farms. Velasco (in

Wolf, 1904) affirms that Cotopaxi remained active all

that year.

On April 1768 an explosive eruption, greater than

all the previous ones, occurred. In the early stage of

activity, bombs fell as far as La Cienega, near Tanicu-

chi, more than 20 km SW of the crater. The scoria

fallout destroyed roofs in Tanicuchi (25 km SW of the

crater), burnt huts and killed 8 people in Mulalo ( 15

km SW of the vent). The fallout of scoriaceous lapilli,

was followed by the emission of white, light pumice,

coarse pumiceous ash and successively by fine ash. The

area affected by the tephra fallout extended over the

western and northern sectors of Cotopaxi, the total

thickness of the deposit being in Tanicuchi about 30

cm, mostly formed by the terminal ash. Diguja (in the

appendix of Wolf, 1904) reported that the volcano opened “muchas y nueuas bocas en circuito, y ha

hecho... un espectaculo luminoso coma fuego de arti-

ficio”. Moreover, Wolf (1904) wrote that“se derra-

maron de1 crater rios de latja incandescente” which

subsequently produced lahars in the valley of Los Chil-

10s 40 km N of the volcano, and of Rio Cutuchi

destroying Latacunga. The area1 distribution of the fallout and the occurrence of mixed pumice and scoria

F. Barberi et al. /Journal of Volcanology and Geotherml Research 69 (1995) 217-239 221

suggest that 1768 deposit corresponds to layer ‘b’ of our stratigraphy.

After 35 years of rest, the activity resumed with the 1803 eruption that seems to have been very short and weak. Very poor information is available on several minor eruptive episodes that occurred in 1843 and 1852.

In 1853, a new eruption lasted three days and produced three ash fallouts. Emission of “luva liquides- cenfe” (nukes ardentes) and production of small-scale lahars are reported by Sodiro ( 1877) ; however, he mis- understood the year with 1854 (Wolf, 1878). This eruption produced also the only documented historical lava flow. The flow was composed by two streams which descended the SW sector of the cone, named by Reiss (in Wolf, 1878) as Manzanahuaicu and Pucahuaicu.

From 1855 to 1866, Cotopaxi had at least four minor and poorly known eruptions with ash fallout and reported emission of lava flows. The examination of the Cotopaxi drawings made by Reiss and Sttibel in the years 1873 and 1874 (Sttibel, 1886) does not confirm the existence of lava flows younger than 1853.

In 1877, a major period of explosive activity (4 main eruptions) took place with the formation of high eruptive columns, and fallout of lapilli and bombs. The dynamics of the eruptive climax of June 26”, described in detail by Sodiro ( 1877) and Wolf ( 1878)) provide the reference example for the so-called ‘boiling over’ mechanism of scoria flow generation. In connection with this activity, highly destructive lahars streamed down the radial main valleys. The thickness of the fallout layer, measured by Wolf ( 1878) on the western slope of the cone, was up to 1 m.

Several small-scale eruptions, with emission of nutes ardentes, lava flows (?) and production of lahars are reported for the period 1878-1885. In 1903-4, lava flows and lahars were reported but, also in this case, lava flows are not visible around the summit crater and on the glacier. Apart from a not verified eruption in 1942, Cotopaxi has remained inactive since then.

With the possible exception of the 1534 event, the last four centuries of activity of Cotopaxi have produced tephra with limited dispersal and thickness whose identification in the field is hard beyond 7-10 km from the summit crater. Only for the 1877 and possibly the 1768 eruptions the identification of the corresponding tephra is reliable.

4. Stratigraphy

The area surrounding Cotopaxi is blanketed by a plane-parallel succession of tephra layers interbedded with paleosols or humified beds. More than 50 sections were measured to reconstruct the stratigraphy of volcanic layers and assess their dispersal. Most of the field work was concentrated on the western side of the cone where the highest number of tephra layers were recognized. Stratigraphic observations carried out on natural and road cuts, E and NE of the mountain, were integrated by seven pits up to 5 m deep dug in selected sites. Three pits were dug on top of hummocks of the debris avalanche of the Rio Pita with the aim at assessing the stratigraphy and chronology of the tephra succession covering this volcanic deposit. A few pits were also dug on top of young lava flows on the northern side of the cone.

The reconstruction of the succession of tephra fall during the last 2000 years is almost complete, thanks to the high number of stratigraphic sections and the identification of a few guide horizons. The fallout stratigraphy of the period 2000-5000 yr B.P. was studied only in a limited number of sections, all concentrated in the high valley of Rio Pita. Pyroclastic flows and surges have been encountered sporadically in natural and road cuts. However, the number of identified pyro- elastic flows and surges is probably incomplete due to erosion and remobilization of them from valley bot- toms by lahars.

4.1. Radiocarbon datings

Radiocarbon datings were performed at Teledyne Isotope (USA) on 15 samples respectively of charred vegetation (2) and paleosols ( 13). For paleosols 2-3 kg of dark, humic-rich material were collected from the topmost 2-3 cm in order to obtain ages as close as possible to that of the overlying volcanic deposit. In order to minimize contamination, paleosols were sam- pled where the thickness of the overlying tephra was highest. However, for samples CO90-8 and 9 the thickness of the overlying layers was only 6 and 3 cm. The amount of paleosol was in general sufficient to keep the analytical error low; the error was instead higher for the two charcoals because of the limited amount of material. To calculate the ages a half-life of 5568 yr was used. All radiocarbon ages expressed as yr B.P.

222 F. Burberi et (11. /Journal of Volcanology und Geotherml Research 69 (1995) 217-239

are presented in Table I. In two cases (layers ‘3’ and ‘6’ ) the charred vegetation embedded in the tephra and

the underlying paleosol were both analyzed. Results

point out an older age of the paleosol as compared to

that of the charcoal and a larger error of the latter. On the basis of the smaller analytical error we tend to consider the age of the paleosol as more representative.

All 14C datings are consistent with their stratigraphic

position providing collectively a good constrain on the

timing of the last 5000 years of Cotopaxi activity. Only

one sample (CO90-6)) coming from a paleosol underneath a plinian deposit that preceded the deposition of

the dry debris avalanche of the Rio Pita valley, yielded

an age exceeding the limit of the method ( >40,000

yr).

4.2. The volcanic dry debris avalanche deposit

The volcanic dry debris avalanche deposit has been

produced by the catastrophic failure of the northeastern portion of the edifice as indicated by its distribution in

the upper part of the Rio Pita valley. The deposit covers

a sector of approximately 26 km* between N and E

(Smyth and Clapperton, 1986). No amphitheatre-like

depression, generated by the partial collapse of the

cone, is anymore recognizable, probably as a result of

the filling by products of subsequent activity. The

deposit is characterized by the typical hummocky mor- phology: the closest hummocks (local name ‘Zum- bas’) are a hundred meters high with a sharp conical shape, while the farthest ones are smoother and only a

few meters high showing a continuous decrease in

height moving away from the volcano. Distal outcrops of the deposit present lithological

facies typical of dry transport such as lavas with zig- zag fracture of the single blocks. Lavas of the proximal hummocks show a lower degree of fracturing consis-

tent with a shorter transport. Some outcrops of the debris avalanche are characterized by the presence of pumice flow material, sometimes intimately compe- netrated. On the base of such an observation, Smyth and Clapperton ( 1986) suggested that the emplacement of the debris avalanche was associated to a pyro- elastic flow-forming eruption.

Field observations point out that no pyroclastic flow deposits overlap the debris avalanche near the slope of the Cotopaxi’s cone (on the Zumbas hills), and neither pumice fallout nor blast deposit unquestionably related

to the debris, was found. Moreover, the percent of loose silicic pyroclastic material and the degree of mixing

with andesitic lava blocks, increase moving away from

both the volcano and the debris avalanche deposition

axis. We suggest that a mechanism of downstream mixing

between the flowing volcanic dry debris avalanche,

mainly formed by andesitic lava clasts, and the thick

unconsolidated pyroclastic materials that mantled the upper part of the Rio Pita valley might also account for

the structures observed in the debris avalanche. The age of the debris avalanche formation was con-

sidered to be comprised between 25,000 and 13,000 yr

B.P. on glaciological arguments (Smyth and Clapper-

ton, 1986). However, our radiometric datings suggest

a different age slightly older than 5000 yr B.P.

4.3. The main fallout layers of the recent Cotopaxi {post-debris avalanche)

1877 eruption, the recent most important Cotopaxi eruption, produced radial scoria flows and a limited

fallout of scoriaceous lapilli NW of the crater. The deposit of nukes ardentes (loose scoria flows) occur in

many localities around the cone being sometimes well

preserved in valleys or in gently dipping areas not

affected by the lahars. Lobes of 1877 scoria flow depos-

its have been observed NW of Ingaloma up to 8 km

from the summit crater and on the northern flank of the cone at an elevation of 4300 m. They generally form

elongated and digitate tongues up to a few meters high and some tens of meters across with sharp, steep sides

(Fig. 2a). On the outside the scoria flows are charac-

terized by concentration of pluridecimetric to metric dark, bread-crust bombs whereas the inner part consists of bombs set in a fine matrix.

The 1877 fallout is made up of dark-grey lapilli with a very little amount of lithics. It occurs west of the

crater with thickness up to 20 cm (4.5 km from the crater), thinning out rapidly away from source.

Layer a (between 1768 and 1877) is characterized by whitish pumiceous lapilli mixed up with centimetric to subcentimetric dark lava fragments (20-30 vol.%) ; maximum observed thickness is 5 cm about 3 km NNW of the crater with apparent dispersal toward the northwest.

F. Barberi et al. /Journal of Volcanology and Geothermal Research 69 (1995) 217-239 223

Table 1 14C radiometric datings of Cotopaxi tephra (analyses performed at Teledyne Isotopes - USA) -

Thickness of Stratigraphic position of the paieosols in respect to the fall layers (in parentheses thickness of fallout “C age the paleosol deposits) (yr B.P.)

8cm

6cm

6cm 8cm 7cm 15 cm 37 cm 1.4 m 3m 9cm 4cm

6cm

N.A.

Underneath layer ‘2’ ( 12 cm) 290 f 80 Chamd vegetation collected inside fall ‘3’ (70 cm) 510*200 Underneath layer ‘3’ (85 cm) 820 * 80 Charred vegetation collected inside fall ‘6’ (20 cm) 700* 160 Underneath layer ‘6’ ( 17 cm) 1180*80 Underneath layer ‘9’ ( 3 1 cm) 1210*80 Underneath layer ‘ 12’ ( 18 cm) 1770* 110 Underneath layer ‘16’ (3 cm) 1880f 160 Underneath layer ‘17’ (19 cm) 2050 f 80 Underneath layer ‘17’ ( 15 cm) 217Ok 100 Underneath layer ’ 17’ (6 cm) 2310+90 Underneath a 49-cm-thick fall overlying the dry debris avalanche hummock of the Rio Pita (see Fig. 4) 4460 f 140 Underneath a 15-cm-thick fall and post-date a pumice fall overlying the. dry debris avalanche hummock 4170* 110 of the Rio Pita (see Fig. 4) Underneath a 24-cm-thick fall and post-date a pumice fall overlying the dry debris avalanche hymmock 5010&210 of the Rio Pita (see Fig. 4) Underneath a 1.5-m-thick plinian pumice fall beneath the dry debris avalanche deposit of the Rio Pita > 40,000

Layer b ( 1768 ?) is constituted by dark lapilli, with a subordinate amount of white lapilli and lithics (20- 30 vol.%). Maximum observed thickness (20 cm) occurs about 3 km NNW of the crater with main dispersal toward the northwest. Fallout of both Scotia and pumice reported during the 1768 eruption is consistent with the lithological characteristics of layer b. Although the major destruction was reported by the chroniclers southwest of the volcano, significant accumulation of tephras were also reported to the north and northwest in agreement with observed distribution.

Z_uyers 1 and 2 form a couple of black and white tephra layers (guide horizon) which can be easily traced west and north of the cone (Fig. 2b). The two layers were erupted in close time association as no reworking or oxidized material was observed between them. Isopaches of the deposits show that both layers were dispersed WNW (Fig. 3).

Layer 1 consists of a normally graded bed of black, moderately sorted lapilli with subordinate lithics of grey, andesitic lavas. Maximum thickness (48 cm) was observed about 6 km NW of the crater.

Layer 2 is a non-graded bed of well-sorted pumice lapilli bearing rare lithics of grey andesitic lava. The layer has a maximum observed thickness of 45 cm, 9 km NW of the crater. About 2.5 km W of Ingaloma,

layer 1 ( 10 cm thick) overlies, without interposition of weathered surface, a Scotia flow deposit containing bombs of 70-80 cm in diameter.

The paleosol underneath layer 2 yielded a 14C age of 290f 80 yr B.P. (Table I), which falls between the two historic eruptions of 1534 and 1742. Although attribution of this tephra to one of the two eruptions is not conclusive, we believe that the correlation with 1742 is unlikely because of the limited thickness of the fallout described by the chronicles for this event.

In a stratigraphic pit dug on the northern side of the cone on top of the ‘Yanasasha’ lava flow, layer 2 was observed to directly overlie the blocks of the lava with no interposition of paleosol. Because elsewhere layer 2 always overlies the Quilotoa ash and layer 3 with the interposition of a humic layer, we conclude that the eruption of the Yanasaha lava flow occurred between 1534 and 800 yr B.P.

In a section located 4 km E of the crater in the Q. Chanchunga we have also noticed two fallouts between layer 2 and Quilotoa ash. The upper and lower tephra (respectively, 17 and 6 cm thick) document the occurrence of two explosive events possibly related to the eruption of the Yanasasha lava flow.

Quiloroa ash (guide horizon QA). In the area of Cotopaxi, layer 3 is ubiquitously covered without inter-

224 F. Barberi et al. / Joumul oj Volcanology und Geothermal Research 69 (I 995) 217-239

Fig

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226

‘.-- ------r- -7 r--- --

Fig. 3. Isopachs (in cm) of the main tephra fallout of the past 2000 years of Cotopaxi. Numbers on the up right refer to tephra layers

F. Barberi et al. /Journal of V&analogy and Geothermal Research 69 (I 995) 2 I7-239 227

o”i,md*na

0 5km

Fig. 3. (continued)

228 F. Burberi et al. / Jorrmrrl of Volcunology und Geothermal Research 69 (I 995) 217-239

position of erosive surface or oxidized material by a

massive, IO-30-cm-thick, fallout of whitish ash (Fig. 2b). The ash is fine-grained, mostly vitric with

subordinate amount of sub-millimetric crystal shards

of felsic and mafic minerals including biotite flakes.

The ash bed increases in thickness and grainsize west-

wards away from the Cotopaxi suggesting a prove-

nance from the volcanic centre of Quilotoa located 60 km WSW. The presence of biotite in the ash as well as its close age with layer 3 (820 _I 80 yr B.P.), fits well

the most recent ignimbrite-forming eruption of Quilo- toa (bt-bearing dacite) dated by Mothes and Hall

( 1992) at 800+50 yr B.P. It is worthy of notice that

near Toacazo, about 30 km WSW of Cotopaxi, we have

observed the presence of an incipiently hummed bed

2-3 cm thick interposed between the ash, about 25 cm

thick, and the underlying fallout layer 3. This suggests that a time lapse of years to decades separated the two

volcanic events.

Layer 3 (guide horizon) is the largest and more

characteristic pumice plinian fallout of the last 2000

years of Cotopaxi activity. The deposit is a well sorted.

symmetrically gradatedpumice bed, with its coarse part placed in the middle (Fig. 2b). The topmost 10 cm is

moderately sorted with abundant dark lithic fragments

mostly represented by sub-centimetric chips of lava. In the main plinian bed, lithics are very subordinated and

represented only by andesitic lavas. Cm-sized pumice clasts are whitish in colour, whereas, dm-sized bombs

often show an inner pink to reddish coloured core likely

formed as a result of oxidation during slower cooling.

The isopaches of the pumice fallout are strongly ellip- tical with main axis running westward in the direction of the Iliniza volcano (Fig. 3). Maximum thickness

( 170 cm) was observed about 7 km W of the crater in the high valley of Rio Cutuchi. Along the road to Cam-

pamento Mariscal Sucre (C.M. Sucre, see Fig. I ), near the crossing of Q. Mishagualco (6.5 km SE of the

camp), fallout 3 is directly overlain by a loose pumice flow deposit likely erupted in the late stage of the same eruption.

14C dating of a small charcoal fragment collected from the fallout deposit yielded an age of 5 10 + 200 yr B.P.; the paleosol underneath yielded a significantly older age of 820 k 80 yr B.P. (Table 1) . Considering the large error of the first measurement we assume that the true age is closer to that of the paleosol.

Layer 4 consists of a thin fallout deposit of white pumiceous lapilli distributed NW of the cone (Fig. 2b) ; its maximum observed thickness is 15 cm (Fig. 3). An average 6-cm-thick humic bed is inter-

posed between layer 3 and 4.

Layers 5 and 6 (guide horizon) form another pair of

black scoriaceous lapilli fallouts. Between the two layers, no reworked or weathered material, indicating a significant volcanic inactivity was observed (Fig. 2e). Layer 5 is characterized by a better sorting and abun-

dant lithics of grey lava. Maximum thickness is 60 cm, 3 km W of the crater with a weak dispersal toward

WNW (Fig. 3). Layer 6 consists of a lower part made up of well sorted lapilli and an upper fine moderately

sorted bed. Dispersal of layer 6 is toward NW (Fig. 3).

Radiocarbon ages of 700 f 160 yr B.P. and 1180 + 80 yr B.P. (Table 1) were respectively obtained on

charred vegetation debris from layer 6 and from the

underlying paleosol. Also in this case there is a dis-

crepancy between the charcoal and paleosol dates and

the age of the paleosol seems preferable for its lower

error.

Layer 7 presents a dispersal along a narrow area

toward WSW with a maximum thickness of 10 cm at

15 km WNW from the crater. The deposit is formed by

sub-centimetric pumiceous lapilli with scarce centi-

metric lithics of grey lavas.

Layer 8 probably is related to the underlying eruption

9, as no erosive surface or weathered material have

been observed between the two (Fig. 2~). Layer 8 is formed by grey/white pumiceous lapilli mixed up with

grey and oxidized lava lithics. The deposit is slightly dispersed westward (Fig. 3).

Luyer 9 (guide horizon) is a light-grey lapilli bed containing abundant lithics of oxidized lavas (20-30 vol.%). The presence of oxidized lithics represents a useful lithological feature for its identification in the field. The juvenile fraction is composed of pumiceous and scoriaceous clasts of variable density. The layer (maximum observed thickness 36 cm) is moderately sorted and grades upwards into a normally graded ash bed topped by very fine, grey-brown ash (Fig. 2~). The lowermost l/4 of the bed is coarser and better sorted. The topmost ash bed occurs with almost identical char-


acteristics in sections located tens of km apart being, on average, extensively removed by erosion and occasionally showing lamination due to wind reworking. As indicated by the almost perfectly circular pattern of isopaches and lithic isopleths, the dispersal of layer 9 took place in absence of wind (Figs. 3 and 7). The peculiar atmospheric condition at the moment of the eruption accounts well for the normal grading of the ash due to the late settling of fine particles left in sus- pension after the cessation of the plinian column (co- plinian ash fallout). Isopaches of Fig. 3 only refer to the main lapilli layer leaving aside the fine-grained deposits.

In the stratigraphic section dug aside the Yanasasha lava flow (about 4200 m a.s.l., Fig. 2e), layer 9 is underlain by a 70-cm-thick dune-bedded surge deposit. Dune-bedded surges up to 2 m thick were also observed at the same elevation in the Q. Chanchunga, on the western foot of the cone. In addition scoria flow deposits with an overall thickness up to 10 m and bearing abundant oxidized lavas, were identified on the top of layer 9 in a small gorge W of Ingaloma (3900 m a.s.1.). Some of these flow units are characterized by incipient welding. Another fallout of pumiceous lapilli was observed underneath the surge bed in a few cuts on the upper slope of the cone (Fig. 2e). No evidence suggesting any break in deposition between this fallout and the surge has been found.

Radiocarbon dating of the paleosol underneath layer 9 yielded an age of 1210f80 yr B.P. (Table 1) in good agreement with tephra stratigraphy and other 14C datings.

Layer 10 (guide horizon) is a moderately sorted fallout of black scoriaceous lapilli with sparse oversized decimetric to pluri-decimetric bombs. Lithics are rare and constituted by grey andesitic lavas much smaller than the scoria. On the volcano’s flanks the deposit is incipiently welded as a result of agglutination of the larger spatters (Fig. 2d). Dispersal of layer 10 is relatively wide, with a certain orientation of isopaches toward NW (Fig. 3). Because layers 10 and 9 are largely over- lapped they represent a valuable tephra pair for correlation between the eastern and western sectors of the volcano.

Layer II is a thin fallout bed of yellowish scoriaceous lapilli with relatively abundant lithics of grey and

oxidized lavas. Dispersal is slightly oriented toward the west (Fig. 3).

Layer 12 (guide horizon) is a non graded bed formed by black, moderately vesicular lapilli with scanty bombs and abundant lithics ( IO-20% vol.). Lithics are mainly of grey andesitic lavas with minor oxidized lavas. Dispersal is almost circular around the vent with a small westward elongation of isopaches (Fig. 3). Paleosol underneath this layer yielded a 14C age of 1770* 1lOyr B.P. (Table 1).

Layers 13 and 14 form a pair of tephra without evidence of alteration products in between, but with quite different lithology and dispersal. The lower layer ( 14) is very well sorted, not graded and constituted by grey, poorly vesicularpumice lapilli mixed up with abundant mm-sized andesite lava fragments. Fallout 14 occurs within a very narrow strip oriented WSW of the crater as a result of emplacement under strong wind (Fig. 3). Layer 14 passes upwards to layer 13 without any evident stratigraphic discontinuity. Layer 13 is formed by yellowish Scotia lapilli, coarse ash more abundant at the top, and sometimes oxidized. The deposit is generally massive and poorly sorted. 12 km SW of the crater it has a thickness of 15 cm and contains a centimetric ash bed at 2/3 from the base. As indicated by isopaches distribution (Fig. 3) the dispersal is sub- circular with weak orientation toward the west.

Layer 1.5 is a moderately sorted bed formed by yel- low sub-centimetric pumiceous lapilli. Maximum observed thickness is of 5 cm at 9 km WNW from the crater. Thickness measurements around the cone are insufficient to draw isopaches but suggest that the dispersal of the layer is approximately circular.

Layer 16 (guide horizon) is a moderately sorted bed formed by poorly vesicular scoria mixed up with lithics of grey lavas ( 10-20 vol.%). Maximum size of lithics and scoria is comparable confirming a relatively high density of the juvenile clasts. The dispersal of the bed is sub-circular with a small isopach eiongations towards the west (Fig. 3). Layer 16 lithologically resembles layer 12 showing however a lesser amount of oxidized lithics. Paleosol at the base yielded a 14C age of 1880f 160 yr B.P. (Table 1).

230 F. Barberi et al. /Journal of Volcanology und Geothermal Research 69 (1995) 217-239

36

17

(205o+ao)

CO-56

3460%140\- - - - - _ _-.

O-61 low

17

rl!~ti~,ll

:;.; ; : 7 CIA

-r’ 9

!A 10

Pig. 4. Correlation of stratigraphic pits dug on top of hummocks of the dry debris avalanche of Rio Pita. Pumice Row deposits at the bottom of each pit likely pertain to the same eruption possibly related to the debris avalanche deposit (Rio Pita Formation). Numbers in parenthesis to the right of columns are “‘C datings of paleosols. Numbers from 3 to 17 refers to tephra layers as described in the text; CO are pm2000 yr tephras analysed in this work. Legend: a = fine ash; ca = coarse ash; p = pumice; s = Scotia; I = lithic; pa = paleosol.

Layer 17 is formed by a double bed of yellowish

pumice with similar thickness separated by a few cm

thick grey ash level. The ash bed frequently shows an

unusual undulating pattern without appreciable change in thickness. The origin of such undulation is unclear

as both lapilli and ash beds seem to have been origi-

nated by a normal fallout activity. Tracing of layer 17 from western to eastern sectors is difficult because of

the scarcity of exposures north of the volcano. The

paleosol at the base of fallout in the northeastern quad-

rant, having the same stratigraphic position as layer 17 (Fig. 4)) in three different localities yielded consistent

ages of 21701tlO0, 2310+90, 2050+80 yr B.P. (Table 1).

4.4. Deposits between layer 17 and the dry debris avalanche

Stratigraphic observations and paleosols datings were carried out on tephra layers older than about 2000 yr B.P. in a limited area about 10 kmNNE of the crater. Three representative stratigraphic columns for pits dug on top of the debris avalanche hummocks are presented in Fig. 4. Below layer 17 there is a ca. l-m-thick dark

F. Barberi et al. /Journal of Volcanology and Geothermal Research 69 (I 995) 217-239 231

paleosol with thin fallout intercalations, followed downwards by 2 to 0.5 m of tephra alternating with thin humic-rich layers (Fig. 2f). In all sections the bottom unit is represented by a loose, poorly sorted pumice-rich deposit of the debris avalanche. Humic-rich layer above the debris avalanche yielded i4C ages between 5010+210 and 3460+ 140 yr B.P. (Table I> . Although the three dated layers might not exactly pertain to the same horizon, the span of time seems large in comparison to the stratigraphic uncer- tainty. On the basis of these results we believe that the age of the debris avalanche (Rio Pita formation) is slightly older than 5000 yr B.P. The age of the Coto- paxi’s slope failure is therefore significantly younger than that of 25,000-13,000 yr B.P. suggested by Smyth and Clapperton ( 1986).

4.5. Petrochemical composition of tephra

The composition of tephra has been determined by means of routine petrographic observation and chemical analyses of major elements (22 samples).

Mineralogical characteristics of juvenile fraction, dispersal axis, and age of tephra of the last 5000 years are summarized in Table 2. Mineralogy is characterized by a rather uniform phenocryst assemblage with plagioclase, clino- and ortho-pyroxenes and Ti-magnetite as fundamental minerals. Olivine and hornblende are occasionally present as accessory minerals, only in layers 12, 13 and 16 olivine occurs as fundamental. Porphyricity (volume %, void free) is moderate ranging between 8 and 21%, except one sample with 30%. Major-element analyses of the juvenile fraction (Table 3) confirm a fairly narrow compositional range of tephra of the last 2000 years with silica values comprised between 56.1 and 62.6%. Except for the 1877 tephra characterized by the occurrence of rare crystals of amphibole and a few more olivine bearing andesites (layers 12, 13 and 16), the large majority of tephra are very similar and cannot be distinguished on the basis of their major-element composition.

Compositional data strongly suggest that the whole last 5000 years of Cotopaxi activity has been dominated

Table 2 Phenocryst abundance, porphyric index, density (g/cm3), direction of dispersal and age of tephra layers of Cotopaxi. PI: plagioclase, Cpx: clinopyroxene, Opx: orthopyroxene, Mt: magnetite, 01: olivine. Stratigraphic position of ‘CO’ samples in column ‘36’ of Fig. 4

Description of deposit Tephra Phenocryst abundance in vol.% Porphyric Density Dispersal Age ( A.D.)

layer index axis PI Cpx Opx Mt 01

Fallout of dark-grey lapilli 1877 12.31 2.26 1.42 0.92 0.00 Fallout of pomiceous lapilli a 20.49 3.89 4.29 1.13 0.00 Fallout of dark and banded lapilli b 15.31 1.48 2.90 1.08 0.00 Fallout of black scoria 1 11.72 1.56 2.69 0.87 0.00 Fallout of white pumice 2 14.91 2.34 1.86 1.51 0.00 Fallout of lapilli Fallout of lapilli Fallout of white pumice 3 8.29 0.50 0.22 0.29 0.00 Fallout of white pomiceous lapilli 4 9.56 0.20 0.33 0.33 0.00 Fallout of black scoria 5 13.65 0.81 1.13 0.32 0.00 Fallout of black scoria 6 11.73 1.51 3.23 0.43 0.00 Fallout of pomiceous lapilli 7 Fallout of light-grey pumice 8 9.28 1.56 1.76 0.61 0.00 Fallout of grey pumice 9 8.88 1.48 0.35 0.63 0.00 Fallout of black scoria IO 12.98 1.92 1.15 0.58 0.00 Fallout of yellowish pumice II 10.25 1.79 1.79 1.22 0.00 Fallout of black scoria 12 6.40 4.25 0.93 0.23 0.47 Fallout of yellowish pumice 13 6.24 1.02 0.20 0.31 0.10 Fallout of grey pumice 14 11.57 0.00 2.03 1.94 0.00 Fallout of pomiceous lapilli 15 Fallout of black scoria 16 6.65 0.53 0.35 0.18 0.00 Fallout of yellowish pumice 17 13.09 4.24 0.86 0.65 0.00

16.91 0.77 29.80 0.88 20.77 0.81 16.84 0.83 20.62 0.89

9.29 0.56 10.41 0.55 15.91 1.15 16.90 0.92

13.21 1.03 11.35 0.93 16.63 0.79 15.05 0.97 12.28 0.94 7.87 0.65 15.54 0.60

7.71 0.63 18.85 0.65

NW? NW NW NW W? W? WNW NW WNW NW WNW? radial/W radial radial/NW WNW W W WSW? WNW? W/radial

1877

1768 (?) 1534 (?)

1130 (‘4C)

770 ( ?T)

740 ( “V)

180 (‘T)

70 ( ‘V) 227 B.C.(Y)

232 F. Barberi et al. /Journal of Volconolo~y und Geothermal Research 69 (1995) 217-239

Table 3

Selected chemical analyses of the juvenile clasts of Cotopaxi tephm of the past 5000 years

Description of deposit Tephra layer Chemical composition in wt.%

SO2 TiO, AlzO, Fe,O, Fe0 MnO MgO CaO Na,O K20 P20s LOl

Fallout of dark-grey lapilli I877 56.58 0.90 17.11 3.21 4.51 0.12 3.56 6.85 3.92 1.40 0.18 I .66

Fallout of pomiceous lapilli a S7.19 0.86 17.60 2.90 4.63 0.12 3.89 6.70 3.98 1.35 0.19 058

Fallout of dark and banded lapilli b 56.76 0.95 17.44 4.13 3.82 0.12 3.69 6.93 4.03 1.32 0.22 0.59

Fallout of black scoria I 56.30 0.97 17.48 4.94 3.66 0.13 3.69 7.10 3.77 1.28 0.21 0.47

Fallout of white pumice 2 59.78 0.73 17.28 3.07 3.92 0.13 2.98 6.20 3.89 1.39 0.20 0.44

Fallout of white pumice 3 61.86 0.73 17.84 2.98 2.58 0.1 1.42 5.64 4.15 1.78 0.21 0.72

Fallout of white pomiceous lapilli 4 61.82 0.74 17.92 I .77 3.46 0.09 1.68 S.78 3.9 1.69 0.20 0.95

Fallout of black scoria s 57.41 0.92 17.83 3.71 4.25 0.12 2.86 6.79 3.78 1.45 0.17 0.62

Fallout of black scoria 6 56.78 0.95 17.55 3.68 4.76 0.13 3.28 7.11 3.65 150 0.17 0.43

Fallout of light-grey pumice 8 57.77 0.88 17.00 3.30 4.36 0.12 3.32 6.38 3.81 1.53 0.16 I .37

Fallout of pumice grey 9 59.60 0.81 17.64 3.25 3.50 0.10 2.47 6.26 4.15 1.64 0.17 0.40

Fallout of black scoria IO S6.25 1.00 17.39 4.00 4.61 0.13 3.26 7.31 3.85 IJO 0.18 0.53

Fallout of yellowish pumice II 99.1s 0.86 17.44 3.05 4.04 0.11 3.00 6.08 4.02 I.55 0.18 0.52

Fallout of black scoria I? 56.1 0.96 16.96 3.18 5.54 0.13 4.42 7.47 3.36 1.29 0.16 0.43

Fallout of yellowish pumice 13 S7.38 0.94 17.04 3.78 4.62 0.13 3.22 6.99 3.87 1.48 0.16 0.39

Fallout of pumice grey 14 60.09 0.78 16.92 2.69 3.96 0.10 2.88 5.87 3.83 1.59 0.16 1.12

Fallout of black scoria I6 56.50 0.95 17.73 4.64 3.97 0.13 3.24 7.00 3.93 1.23 0.16 0.52

Fallout of yellowish pumice 17 60.39 0.86 17.1 I 2.02 4.26 0.10 2.56 6.00 4.12 1.63 0.20 0.75

by andesitic magmas with limited range of variation

(Fig. 5).

5. Eruptive frequency

Data on historical periods and the tephra stratigraphy of Cotopaxi were used to assess the eruptive frequency

SiO2 o/(1

Fig. 5. Plot of the composition of Cotopaxi tephra in the normal

andesite field of the talc-alcalinerock classification grid of Peccerillo

and Taylor ( 1976 1.

of explosive eruptions and calculate the probability of

having an explosive eruption in a forthcoming time

interval. In our approach we have arbitrarily considered

an ‘eruption’ not a single explosive outburst but a

period of activity ranging from one to a series of explo-

sive events which take place in a relatively short time

interval (days to years) and are separated one from the

other by a fairly long repose period (tens to hundreds

years). This definition is convenient from the hazard

point of view because various lahars generated within

days or years basically count as one event in that people

tend to reoccupy dangerous areas only after several years or decades of volcanic quiescence. For the pre-

historic activity of Cotopaxi we have counted the erup-

tions using as stratigraphic boundaries paleosols or

weathered surfaces (volcanic quiescence lasting tens

to hundreds of years).

For the period from 1534 to present it is possible to conclude that major explosive events or clusters of events have occurred in years 1534, 1744, 1768 and

1877. Stratigraphic analysis confirms the occurrence of

three ‘eruptions’ leaving uncertain the possibility of a

fourth one because our reconstruction is not very accu-

rate in the most proximal area and in the southern

sector.


The stratigraphic analysis of the prehistoric fallout ometric datings available for eruptive episodes 3,5-6, succession around Cotopaxi over the last 2000 years 8-9, 12, 16 and 17 permit a precise location of these presented here, permits the identification and counting eruptions in the chronogram of Fig. 6 (full circles). of all the major eruptive episodes. In Fig. 6 the historic The undated explosive events (open circles) were arbi- and pre-historic chronogram of activity of Cotopaxi is trary placed assuming repose intervals obtained by presented. In the time interval 230 b.C.-1534 A.D. 24 dividing the time between two subsequent dated erup- tephra layers have been identified. Of these layers, pairs tions by the number of field identified repose intervals. l-2,5-6,8-9 and 13-14 have, however, been consid- This is roughly in agreement with the fairly uniform ered as single ‘eruptions’ as no intervening reworked thickness of humified beds between tephra layers or weathered material was observed in between. Radi- belonging to different eruptions.

yrs a.C.

1900

1600

1700

1600

1500

1400

1300

1200

1100

1000

900

800

700

600

500

400

300

200

100

0

100

200

8

8

0

0

0

8

3

b

0

0

8

0

0 -

dated tephra field identified deposits

erupt. layers

1077 fallout, p. flow

1768 1744

fallout, p. flow, lava flow tefra, p. flow

1534 (l-2) fallout. p. flow

fallout Yanasasha lava

fallout flow

820 b.p. (3) fallout, p. flow

(4) fallout

1180 b.p. (5-6) fallout

1210 b.p. I 7) 8-9) fallout, surge, p. flow

(10) fallout

(11) fallout

1770 b.p.

1

,2) fallout ; ;- 14) double tephra fallout

1880 b.p. 16 1 fallout

fallout

2177 b.p. (17) fallout

Fig. 6. Chronogram of the activity of Cotopaxi during the past 2000 years. Full circles represent historical and 14C dated eruptions (ages greater than 1534 are radiocarbon values referred to 1950). The age of the fallout 17 is the average of three “C dating% Open circles are. undated explosive eruptions arbitrarily placed at regular intervals between two dated events.

If we now consider all the repose intervals over the pre-historic period the duration of volcanic quiescence ranges between 15 to 187 years. These values compare well with the historical repose periods which range from 24 to 210 years. Using data of repose lenght of the past 2000 years, the average time interval between two eruptions is 117 f 70 years. As the last eruption occurred in 1877 the length of the present repose is right about this average. Assuming a Poisson distribution of the length of reposes (Wickmann, 1976) and the average length of repose of 117 years, the probability of having an explosive eruption similar or greater than that of 1877 in 50, 100 and 200 years is respectively of 0.35,0.57 and 0.82.

6. Characteristics of explosive eruptions

Eruptive activity of Cotopaxi during the last 5000 years has displayed a wide range of styles from mild, cone-building lava emissions to violent, plinian-type eruptions. The constructive activity seems to have been important in the recent history of the volcano as it led to the complete filling of the scar left by the debris avalanche episode. The sustained explosive eruptions have also been common even if they apparently did not result in the formation of large craters or calderas.

Basic parameters of column height and magma discharge rate (intensity) and total mass (magnitude) of the sustained explosive eruption of the last 2000 years, were established from data on maximum clast size (isopleths) and thickness (isopaches), using theoretical models of Sparks ( 1986)) Carey and Sparks ( 1986), and Pyle (1989). The maximum clast size of lithics (ML) and pumice (MP) were measured using two different techniques. For all studied deposits we obtained ML/MP data by averaging the maximum length of the three largest clasts recovered from a 2-m-

234 F. Barberi et al. /Journal of Volcunology und Geothermal Research 69 (1995) 217-239

r

iniza

l MACHACHI

ML

r- . MACHACHI

3

ML

0.8 .

03 iniza l

2 l MACHACHI

Sincholaguz .

SACIUISILI l

MP ’ MACHACHI 3

3.6 Rumitiahui 47.: 4.5- A

l MACHACHI

liniza L

F. Barberi et al. / Joumal of Volcanology and Geothermal Research 69 (1995) 217-239 235

ML l MACHACHI

0 5km -

l MACHACHI

MP 0 MACHACH I

0 5km

0 MACHACHl

llinlra

Fig. 7. Isopleths (in cm) of the main tephra fatlouts of the past 2000 years of Cotopaxi. Dotted lines refer to measurements obtained by averaging the maximum length of the three largest lithic clasts (ML) collected from 2 m long exposure and by excavating 5 cm of deposit. Solid lines (eruptions 3 and 9) refer to the average maximum length of the five largest lithic (ML) and pumice ( MP) clasts recovered from the deposit accumulated over a 0.5 mz area. Numbers on the up right refer to tephra layers.

length exposure and by excavating about 5 cm of deposit. For the two most important layers (‘3’ and ‘9’) we carried out the MP/ML measurements by averaging the maximum length of the five largest clasts recovered from the deposit accumulated over a 0.5-m’ area. The last technique provide more conservative data which better compare with the theoretical models on clast dispersal (Papale and Rosi, 1993). Field measurements and ML isopleth contours of layers 1,2,3,5, 6, 9, 12 and 16 are presented in Fig. 7 along with MP isopleths of layers 3 and 9. For layers ‘3’ the isopleths were obtained with both three and five clasts tech-

niques. By comparing isopleths obtained with the two techniques (see Fig. 8) we assessed that the three clast average provides cross-wind ranges underestimated by 20-40%.

Data on cross-wind range, column height, and mass discharge rate for different fallout layers are presented in Table 4. Column heights and peak mass discharges of the different eruptions were obtained from cross- wind range of ML 3.2 cm isopleth. Moreover data of cross-wind range of all studied deposit but ‘3’ and ‘9’ were increased by a factor of 1.3 to compensate for the ML isopleths based on three clasts. For layers ‘3’ and

236 F. Barberi et al. /Journal of Volcanology and Georhermal Research 69 (I 995) 217-239

ML 0 MACHACHI 3

Rumitiahui

0 MP MACHACHI

3

/’ Y COT0 PAXI

0 5km

Fig, 8. Comparison between lithic and pumice isopleths (in cm) of the tephra layers 3 measured averaging the three largest clasts (dotted lines)

and the five largest clasts (solid lines). Three-clast isopleths are underestimated of about 2040%.

‘9’ the higher number of field data also allowed the

constraining of more than one isopleth (Fig. 7). In these two cases the height of the column provided by

isopleths of 3.2 and 1.6 cm are fairly consistent whereas the value given by the 6.4-isopleths of layer 9 is 5 km

less. Heights of the eruptive columns of Cotopaxi are

always in excess to 28 km with a maximum of 39 km (eruption ‘9’). By using the model of Sparks ( 1986)

ho5

lE+lO lE+l I II?+14 It:+15

Log Plinian Mass (kg)

Fig. 9. Eruption column height versus the Log of the plinian fall

deposit mass. Full squares = plinian eruption of Cotopaxi. Open cir-

cles and solid line are respectively: literature data and best-fit straight

line, from Carey and Sigurdsson ( 1989).

these column heights correspond to mass discharge

rates ranging from 1.1 X 10’ to 4.1 X 10s kg/s

(Table 4). It is interesting to note that all studied

explosive eruptions of Cotopaxi of the last two millen-

nia have to be considered as plinian in terms of disper-

sive power and column height even if many layers are

made up of dark scoria. No systematic relationship

exists between the type of juvenile products (pumice

or scoria) with eruptive dynamics and in any case the

highest peak rates surprisingly pertain to fallouts 5, 9

and 12 which are all composed of dark lapilli.

The volume of fall deposits was estimated using the

method of Pyle (1989) based on the exponential best

fit of the thickness versus square root of the area of

isopaches. For eruptions ‘3’ and ‘9’ where more than

two isopaches were available, the plotting in the Pyle’s

diagram shows good fitting with the exponential law.

For eruptions 5, 8, 12, 13 and 16 the volume estimate was obtained from extrapolation of only two isopaches,

and results are therefore less precise. Volumes of all

layers are small, mostly ranging between 0.1 and 0.3

km3; only eruption ‘3’ shows an higher volume of 0.6

km3 (Table 4). The limited volume of the eruptions is

consistent with the apparent lack of major crater or

caldera structures.


Table 4 Eruptive parameters of plinian eruptions of Cotopaxi during the past 2000 years

Eruption Age A.D. hew 6.4 hew 3.2 hew 1.6 CH MDR Volume Mass (km) (km) (km) (km) (kg/s) (km’) (kg)

I N.A. N.A. 13’ N.A. 30 1.4x IO8 N.A. N.A. 2 1534 N.A. N.A. 13’ 28 I.1 x 10” N.A. N.A. 3 1130 5 9 12 28 1.1 x IOX 0.65 7.2x IO” 5 N.A. N.A. 15’ N.A. 34 2.4 X 10’ 0.26 1.8x IO” 8 N.A. N.A. N.A. N.A. N.A. N.A. 0. I 0.7x IO” 9 740 8 15 N.A. 39 4.1 x 10s 0.23 1.6x 10” 12 180 8’ 17’ N.A. 38 3.7x IOX 0.13 0.9 x IO” 13 N.A. N.A. N.A. N.A. N.A. N.A. 0.12 0.8x 10” 16 70 N.A. 12’ N.A. 30 1.4x IOU 0.18 0.9x IO”

hew: half cross wind range of 6.4, 3.2, and 1.6 cm lithic isopleths (Carey and Sparks, 1986), data with asterisk were augmented by a factor of I .3 to compensate for the three-clast measurement; CH: maximum column height (Ht) as deduced from hew of 3.2 or 1.6 isopieths; MDR: mass discharge rate calculated as MDR = (H/0.274)“; Volume of the deposits calculated with the method of Pyle ( 1989); Mass: mass of magma of plinian fallout assuming a standard lithic content of 30 wt.%; N.A.: not available.

Carey and Sigurdsson (1989) have reviewed liter-

ature data of plinian eruptions pointing out the exis-

tence of a gross positive correlation of total erupted

mass (and mass of the plinian phase) with peak dis-

charge rate. According to them, this behaviour could

reflect a positive correlation between magma chamber

size (eruption volume) and mass eruption rate. The

plinian fallout 3 of Cotopaxi (7.2X 10” kg, 28 km

column height) compares well with literature data,

plotting very near to the regression line of Carey and Sigurdsson ( 1989, see Fig. 9). This behaviour is not

followed by the eruptions of dark lapilli which all

exhibit much higher mass fluxes as compared to their

masses. The apparent inconsistency of these eruptions

with the general pattern pointed out by Carey and

Sigurdsson ( 1989) is difficult to interpret. Worldwide

column height estimations for low-magnitude plinian eruptions are very scarce so that it is presently even

difficult to assess if the Cotopaxi’s behaviour has to be

regarded as anomalous or not. Nevertheless, the data draw attention on the fact that for small-volume plinian events some additional factors besides the total volume

of erupted magma might become important in control- ling the dynamics of magma discharge during sustained

explosive phases.

One remarkable feature of Cotopaxi’s sustained plinian eruptions is the apparent scarcity of pyroclastic flows resulting from plinian column collapse. This

behaviour might be related to the short duration of the eruptions during which vent/conduit erosion is insuf-

ficient to cause collapse as predicted by the model of Wilson et al. ( 1980). The 1877 Scotia flow produced

by ‘boiling over’ of moderately to poorly fragmented

magma (bombs and lapilli) was not preceded by the

formation of a sustained column and seems to represent

the emission of a magma-batch with a small volatile

content. This eruptive style appears to be recurrent for

the Cotopaxi eruptions.

7. Summary and conclusions

The study of the tephrostratigraphy of Cotopaxi dur-

ing the last 5000 years has been used as a base for a

quantitative reconstruction of the past activity of the

volcano, assessment of eruptive frequency and eruptive dynamics of explosive eruptions. The onset of this

period is considered to coincide with an important slope

failure of the cone. Subsequent to the debris avalanche, the activity of the volcano became dominantly constructive (lava flow from central vent) but it was also

punctuated by several episodes of sustained explosive eruptions.

The eruptive dynamics appears dominated by the emission of both volatile-rich and volatile poor magmas without a simple correlation with the overall chemical composition and crystal content of magmas. The sustained explosive eruptions which led to the deposition of tephra layers during the last 2000 years were in most of the cases of plinian type on the basis of clast

238 F. Barber; et ul. / Joumul oJ Vol~~~olo~y und Geothermul Reseurch 69 (1995) 217-239

dispersal. However, total masses (magnitude) of sustained eruptions (0.8 to 7.2 X 10” kg) were modest in

comparison to height of eruptive columns (30 to 42 km) and peak mass fluxes ( 1.3 to 5.5 X 10’ kg/s).

Virtually all explosive events producing tephra fallouts

have been associated to emplacement of pyroclastic flows, but only during the most voluminous fallout

events (3 and 9) highly fragmented pyroclastic flows

(pumice or ash flows) and surges were produced by column collapse. In most cases, the pyroclastic flows

are scoria flows similar to those of the 1877 eruption

resulting from boiling over activity. Explosiveness

related to magma/water interaction seems of minor

importance, as deposit with phreatomagmatic signature

were not found anywhere. Volcanic hazard deduced from the study of historic

and prehistoric activity of Cotopaxi is represented

mainly by tephra fallout, for the villages located on the

western side of the mountain, and above all by the

formation of destructive lahars triggered by pyroclastic

flow/surge activity. Our data suggest that all eruptions

which produced tephra fallouts were virtually associated to pyroclastic flow/surge activity, extensive

snow/ice melting and formation of large scale lahars.

The average recurrence period of these events calcu-

lated over the last 2000 years is one event every 117

years. Assuming a Poisson distribution of the repose time lengths, the probability of having an eruption in

100 years is 0.57 whereas such probability rises to 0.82

for 200 years. This estimation emphasizes the lahar hazard in the valleys which originate from the volcano (Barberi et al., 1992) and urges the importance

undertaking adequate measures for the mitigation

the risk.

oi of

Acknowledgements

Authors are indebted with INECEL for kindly giving the permission of publishing j4C datings of Cotopaxi. Work carried out with a financial contribution of MURST (40% )

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