Journal of Volcanology and Geothermal Research, 59 ( 1993 ) 1-33 1 Elsevier Science Publishers B.V., Amsterdam

Major Holocene block-and-ash fan at the western slope of ice-capped Pico de Orizaba volcano, M6xico: Implications for

future hazards

Claus Siebe a, Michael Abrams b and Michael F. Sheridan c ~lnstituto de Geofisica, Universidad Nacional A ut6norna de Mdxico, Ciudad Universitaria, C.P. 045 I0 Coyoacdn, Mkxico D F.,

Mdxico b Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

C Department of Geology, SUNY at Buffalo, 415 Fronczak Hall, Buffalo, New York, NY, 14260-1550. USA

(Received November 5, 1992; revised version accepted July 2 l, 1993 )

ABSTRACT

A major block-and-ash fan extends more than 14 km westward from the summit of Pico de Orizaba volcano in the eastern part of the Trans-Mexican Volcanic Belt. Radiocarbon dating of charcoal within the fan deposits yielded Holocene ages that range between 4040+ 80 and 4660 +_ 100 y.B.P. Stratigraphical, sedimentological, geochemical, and scanning electron microscope studies indicate that this fan originated within a relatively short time-span by multiple volcanic explo- sions at the summit crater. This activity produced a series of pyroclastic flows (mainly block-and-ash flows) and lahars which were channelized by a glacial cirque and connecting U-shaped valleys as they descended toward the base of the volcano. A recurrence of a similar eruption today would pose severe hazards to the population of more than 50,000 people, who live in a potentially dangerous zone. A detailed reconstruction of the sequence of events that led to the formation of the block-and-ash fan is presented to help mitigate the risk. Special attention is given to the effects of an ice-cap and the role of pre-existing glacial morphology on the distribution of products from such an eruption.

Introduction

Pico de Orizaba or Citlalt6petl ( 1 9 ° 0 4 ' N / 97 ° 15 'W/5700 m a.s.1.), located in the east- ern part of the Transmexican Volcanic Belt (TMVB), is the highest volcano on the North American Continent (Fig. 1 ). The ice-capped summit cone ranks as one of the most perfectly symmetrical volcanoes in the world. It rises 4500 m above the Gulf of Mexico coastal plains situated to the east, and 3000 m above the Mexican Altiplano, to the west.

The summit crater is a relatively small oval with a major NW-SE axis approximately 400 m long. The 300-m-deep crater pit is sur- rounded by nearly vertical wails, exposing an alternation of lavas and pyroclastic units.

The present cone is composed mostly of an- desite and dacite lavas. It was built on top of older volcanic structures that consist of a roughly N-S aligned, deeply-eroded volcanic chain formed by extinct constructs of the Cofre de Perote (4282 m) stratovolcano 50 km to the north, Cerro de las Cumbres (3940 m) com- plex 10 km to the north (Rodriguez-Elizar- rargts and Lozano, 1991), and Sierra Negra (4580 m) 5 km to the south (Fig. 2). It is gen- erally claimed that volcanic activity within the TMVB has migrated toward the south (e.g., Cantagrel and Robin, 1979; Luhr and Carmi- chael, 1985). However, this process does not apply to Pico de Orizaba because the highly eroded Sierra Negra which is clearly older than Pico de Orizaba, is located south of the active volcano (Dannenberg, 1907). According to

0377-0273/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

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110 ° W 100 ° W 9 0 ° W

Fig. 1. General tectonic setting of Mexico and the Trans-Mexican Volcanic Belt. Quaternary volcanoes with known debris avalanche deposits (open triangles) modified from Siebe et al. (1992): CO = Colima; PO= Popocat~petl; DE= Las Der- rumbadas; JO=Jocotitlan. Other Quaternary volcanoes (solid triangles): SJ=San Juan; CE=Ceboruco; TE=Tequila; PA=Paricutin; TA=Tancitaro; JR=Jorullo; TO=Nevado de Toluca; XI=Xitli; IX=Ixtaccihuatl; MA=Malinche; PE= Cofre de Perote; S M T = San Martin Tuxtla; CH= Chichonal; TA = Tacami. Calderas (open circles): P= Primavera; LA = Los Azufres; AM= Amealco; LH= Los Humeros. Cities (solid squares): Tp = Tepic; Ga = Guadalajara; Co = Colima; To= Toluca; Ve= Veracruz; Pu= Puebla; SAT=San AndrOs Tuxtla; VH= Villa Hermosa. Tectonic features: EPR = East Pacific Rise; TFZ=Tamayo Fracture Zone; RFZ=Rivera Fracture Zone.

Cantagrel and Robin (1979), the Sierra Ne- gra-Cofre de Perote chain supposedly lies on a zone of normal extensional faults, which are also oriented in a N-S direction and separate the Altiplano highlands from the plains of the Gulf of Mexico. This hypothesis has not been proven yet. In fact, most faults in the area run either in a NW-SE or NE-SW direction, sug- gesting that the volcanic centers forming the N-S-oriented chain are located at zones of weakness, where these faults intersect.

During the 19th and early 20th century the majestic appearance of this mountain at- tracted many travelers who collected the first scientific data, which include barometric mea- surements and observations of fumarolic ac- tivity at the summit area (e.g., Heller, 1853/ 1857; Pieschel, 1856; Sausurre, 1862; MUller, 1864; Plowes el al., 1877; Ratzel, 1878; Heil- prin, 1890; Angermann, 1904; Waitz, 1910, 1915; Garcfa, 1922; Friedl~inder, 1930/31). Given these early studies, it is surprising that

this volcano has not been investigated more thoroughly in recent decades.

Although the volcano has shown only fu- marolic activity for the last 300 years (Heller, 1857; Angermann, 1904), studies by Robin and Cantagrel ( 1982 ) and Robin et al. ( 1983 ) demonstrate that it had several important eruptions during the late Pleistocene and Holocene.

Torquemada ( 1615 ) reports that the volca- no became active in 1545 for more than twenty years and that the Indians until then believed the volcano to be extinct. Miihlenpfordt (1844) also mentions that the last major his- toric eruption took place between 1545 and 1565; whereas Waitz (1915) cites another eruption that occurred in 1687. Recent studies revealed the existence of debris-avalanche de- posits derived from the partial gravitational collapse of the former cone (Sheridan et al., 1990; H~skuldsson, 1990; Haskuldsson et al., 1990).

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE ORIZABA VOLCANO, M15XICO: HAZARD ASSESSMENT 3

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Fig. 2. Sketch map showing the Cofre de Perote-Las Cumbres-Pico de Orizaba-Sierra Negra volcanic chain as well as other Quaternary volcanic structures located in the basin of Serd~n-Oriental.

Based on prel iminary fieldwork, Hoskulds- son et al. (1990) claimed to have discovered four different debris-avalanche deposits at Pico de Orizaba. Three of them are of the Bezym- ianni-type and traveled toward the east, whereas the fourth is considered by these au- thors to have originated from a landslide that traveled 12 km west from the summit of the volcano, leaving a pronounced horseshoe- shaped scar on the upper part of the cone. The deposit supposedly covers an area of 42 km 2, has a volume of 1.5 km 3, was emplaced with- out accompanying magmatic activity, and is older than 38,000 y.B.P. (Hoskuldsson, 1990; Hoskuldsson et al., 1990).

Preliminary results of our studies were pre- sented in Siebe et al. ( 1991 ). In this report we list our conclusions from investigations of this "landslide-like looking" deposit and present an alternative explanation for its origin, namely that the deposits under discussion represent a block-and-ash fan. Accordingly, this deposit is a composite accumulat ion of many pyroclastic flows, flood deposits, and lahars that were channeled through a glacial cirque and con- necting U-shaped valleys toward the base of the volcano where they were deposited. In this pa- per our model of eruption is discussed in rela- tion to the recent glacial history of the area for possible hazards analysis in the event of re- newed volcanic activity.

4 c. SIEBE ET AL

Distribution, volume, and morphological characteristics

The deposits under consideration form an elongate tongue with a smoothly undulating, relatively fiat surface that is slightly inclined toward the west. They extend between 12 and 14 km westward from a horseshoe-shaped es- carpment located near the summit cone (Fig. 3 ). Little or no material from this unit occurs on the upper slopes of the volcano above 3200 m. Most of the material was deposited near the base of the volcano close to the plains of the Serd~in-Oriental basin, below a major break in slope at a height of 3100 m near the village of San Jos6 Llano Grande (Fig. 4). The deposits reach a maximum thickness of about 8 m; most outcrops have a thickness of 2-4 m. Using a deposit area of 16 km 2 and an average thick- ness of 3 m, this unit has a volume of 0.048 km 3. These figures are in disagreement with those published by Hoskuldsson et al. ( 1990 ),

who give a volume of 1.5 km 3 and a surface area of 42 km 2.

The block-and-ash fan can be easily recog- nized on Landsat Thematic Mapper satellite images because it is vegetated by pine trees which contrast with the surrounding culti- vated areas. Poor soil development and abun- dant blocks on top of these young deposits makes their surface unsuitable for agriculture, hence heavy deforestation associated with farming has not occurred yet (Fig. 5 ).

Stratigraphic relations

The block-and-ash fan formed an elongated tongue-shaped accumulation of debris from the passage of a series of pyroclastic flows, sheet floods, and lahars. Volcaniclastic material ac- cumulated towards the base of the volcanic structure forming a series of overlapping, dig- itate, coherent flow units with subdued front margins, and a stratigraphy very similar to that described for pyroclastic fans at other volca-

Fig. 3. Photograph of Pico de Orizaba (5700 m) from the west. The picture was taken from the distal edge of the block- and-ash fan. Arrow points towards the horseshoe-shaped escarpment, interpreted in this study as a glacial cirque.

B L O C K - A N D - A S H F A N O F I C E - C A P P E D P I C O D E O R I Z A B A V O L C A N O , M I ~ X I C O : H A Z A R D A S S E S S M E N T 5

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qaNllqc Approximate exlent g[a,21,t[ cirque • 9()16 /<~ 3tl, ,11 of mappablc bllwk & ash flow [,m t~ ~.kanic ,.rater /k ~lllil}~;:l: hut

(da:~hed ~hcre taloned) ,,,' ........ unpaved road A - l{ ] opograFh~c prLffllc

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Fig. 4. Sketch map of the western slopes of Pico de Orizaba showing major morphological features, towns, and sample locations.

Fig. 5. Landsat Thematic Mapper TM showing the summit cone of Pico de Orizaba (S), glacial cirque and U-shaped valleys (C), and block-and-ash flow fan (B). Scale bar is 5 kin. See map in Fig. 4 for details.

6 C. SIEBE ET AL.

5m

P.O. 9004

/////// C.14j P.O. 9126

/ / :

P.O. 9011

P.O. 9010

P.O, 9003 . . - - .

J ~;illi~ill ..... ,ii,iii iiii" : ............ "

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(.141j"]']S>° • Fig. 6. Location of outcrops and stratigraphical relations of different sections measured across the block-and-ash flow fan (see Fig. 7 for descriptions of sections and symbols).

noes ( e.g., Mt Pelre; Fisher et al., 1980 ). Mapping the limits of the fan was difficult

because steep-walled gullies have cut into the deposits and mechanical reworking of uncon- solidated material has partially covered it. The major criterion used for identifying the bound- aries of the fan consisted in examining the lim- its of the forested areas. But the edges of for- ests only delimit a minimum distribution because scattered rocks that occur in corn fields indicate a wider original distribution. Boul- ders removed by farmers from fields on top of the block-and-ash fan are arranged in fences and piles.

Several stratigraphic sections were mea- sured and many other locations were exam- ined on all sides of the block-and-ash fan. Good outcrops occur where small arroyos have in- cised into or through the deposits at their mid-

die and lower reaches. The best outcrops are located along Barranca Paso Buey, Barranca Alpinahua, Barranca Piedra, and Barranca E1 Carnero (Figs. 6 and 7; see also INEGI map E- l 4-B-45, 1984).

The block-and-ash fan principally consists of a succession of breccia deposits with many un- conformities. In most cases, these units are in- tercalated with laharic and fluvial horizons (e.g., sections 9004 and 9015 in Figs. 6 and 7). The whole assemblage comprising the fan can be distinguished by lithological differences, stratigraphical unconformities, and soil hori- zons from underlying volcaniclastic deposits, the origin, distribution, and age of which is mostly unknown. The deposits underlying the fan will first be briefly described before pre- senting detailed description of the fan deposits.

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE ORIZABA VOLCANO, MIfXICO: HAZARD ASSESSMENT 7

P.O. 9010

yello,~ unit

red unil

1.0 m

0.8m

6.0 m

fluviatile reworked block- and- ash flow deposits mixed with soil and gravel

pyroclastic-flow deposit, largest clast is 15 cm, mostl3, gravel-supported in a sandy matrix, contains well-rounded pumice, angular lithics, no bedding, heterolithologic and cuarser towards the bottom. Sample 9010-B was taken from the yellow unit, 9010-A was taken from the reddish unit

present riverbed of Barranca Santa Catarina

P.O. 9011

• • - 0.4 m

t i:i:i:i:i I

~ 2 0 m

block- and- ash flow deposit, coarse, and poorly-sorted, subangular boulders and gravel, mostly monol i thologic (glassy porphyritic hbLandesite) paleosoil

ash-flow deposit, light-brown with occassional pumice, clasts are re)n-welded, friable, abundant charcoal

pumice flow deposit, white-yellowish in color, clast-supported

ash-flow deposit, brown, clast supported

pumice-flow deposit, white-yellowish, unsorted, clast-supported (max. clast = 5 cm). Sample 9011-A taken from this horizon

ash-flow deposit, light brown, with occasional pumice and abundant small ciasts and hollows left by small rootlets

present streambed of Barrmlca Paso Buey

P.O. 9008

1.25 ira

0.35m

0.08 m

2.5 m

0.5 m 0.4m 0.3m 0.2m

3.Ore

reworked fluviatile gravel and sand deposits

block- and- ash flow deposit, coarse, subangular, unsorted

palaeosoil

ash-flow deposit, brown, occasional pumice, non-welded, frequent casts

pumice-flow deposit, whitish, unwelded, clast-supported ash-flow deposit, brownish, partly consolidated

pumice-flow units, white, unwelded, largest clasts are 3-5 cm. locally this horizon is called "Cacahuatillo"

ash-flow deposit, light brown, sandy-silty matrix, poorly sorted, partly consolidated, unwelded

present streambed Barranca Paso Buey

8

Fig. 7. Selected stratigraphic columns of outcrops discussed in the text (for location see Fig. 6).

8 c . SIEBE ET AL

P . 0 . 9 0 1 5

0.6m

0.4m

1.2 m

Reworked block- and- ash flow deposit (fluviatile gravel + sandy and silty soil)

less reworked, more bouldery block- and- ash flow deposit

block- and- ash flow deposit, mostly gravel, large logs (charcoal) on top within the reworked horizon

present streambed

P . 0 . 9 1 2 6

2.0 m

ye t 1.0 m

"~ N i t

block- and- ash flow deposit, coarse and poorly-graded, subangular boulder~ and gravel, mostly monolithologic, glassy porphyritic Hhl-andesite, clast- supported, abundant charcoal

pyroclastic-flow deposit, largest clast is 15 crn, mostly gravel supported in a sandy matrix, well-rounded pumice, angular lithics, no bedding, heterolithologic and COarser towards the bottom.

P . 0 . 9 0 0 4

P . O . 9003

present streambed of Barranca Piedra

block- and- ash flow deposit, coarse and poorly-sorted, subanguiar boulders and gravel, mostly monolithologic (glassy porphyritic ttbl-andesite),

.0 m clast-suppported, few accidental clasts, roughly inversely graded, abundant charcoal at the base, sample 9004-B was taken from this horizon

.15 m .1 m ~ surge-like deposit, rich in charcoal, blast ?

fluviatile-reworked deposit, coarse, clast-supported, sandy matrix .25 m subrounded andesite fragments, normally-graded, erosive lower contact

.7 rn ~ fluviatile and laharic deposits, non-erosive lower contacts, normally graded, cut- and- fill structures, fine-grained upper layers, each layer 10-25 cm thick

.0m ~. block- and- ash flow deposit, subangular clasts, heterolithologic, unsorted,

ungraded

" " " present streambed of Barranca Piedra

0.95 m

22 m

| .0 m

3.0m

fluviatile reworked sands and gravels

block- and- ash flow deposit, coarse, poorly sorted, subangular boulders, mostly monolithologic (glassy porphyritic Hbl-Andesite), t:lasl suppported with few accidental clasts, weak inversely graded, sample 9003 for sieve analysis was taken from this horizon

pyroclastic-flow deposit, largest clast 15 cm, mostly gravel supported in a sandy matrix, also contains well-rounded pumice and angular lithics, no bedding, heterolithologic and coarser towards the bottom.

b Fig. 7. ( c o n t i n u e d ) .

present streambed of Barranca Paso Piedra

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE ORIZABA VOLCANO, M#XICO: HAZARD ASSESSMENT 9

P.O. 9006

Z ///~

yellowish, pumice-rich horiz~on

gradational contact

pyroclastic flow deposit, reversely

i r r e g u l a r lenses of to normally graded, coarse lithic fragments, lapilli sized clast suppor ted , are material supported common in a sandy matrix,

faintly layered

logs (charcoal) are 1-3 m in length

present riverbed of Barranca Escoba

P.O. 9017

.rsaz I 40904 . ~ , - ~ , ~ I 9ov. l l l l l l l l l l l l l

f? !?i?i?i?i?

1.2 in

0.2 m

1.7m

0.4 m

2 .0m

block- and- ash flow deposit, partly reworked, monolithologic

surge deposits, grey, lithic-rich, sand-sized

ash-rich pyroclastic flow deposit, yellowish with about 50 % pumice

pyroclastic flow deposit, rich in gravel (lapilli-siz, c&

ash- flow deposit, pumice poor, sandy', yellow

present streambed of Barranca El Camero

P.O. 9016

0.4 in

0.2m

2.0 in

block- and- ash flow deposit, partly reworked

surge deposits, grey, lithic rich, sand-sized

ash-rich pyroclastic flow deposit, yellowish with about 50 % pmmcc

5 8 m ashfall deposit, dark grey (probably from a nearby cinder cone)

C

Fig. 7. (continued).

present streambed of Barranca El Camero

10 o SIEBE ET AL

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BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE ORIZABA VOLCANO, MI~XICO: HAZARD ASSESSMENT 1 1

Older deposits

To the north the block-and-ash fan is under- lain by a relatively thick sequence ( > 7 m) of pumice-flow and ash-flow deposits (sections 9008 and 9011 in Figs. 6 and 7). The best ex- posures occur along Barranca Paso Buey. Ap-

parently, the inclined surface on which San Miguel Zoapan is located overlies these depos- its. The deposits are mostly light brown to yel- lowish-white, non-welded, poorly sorted (see also histogram in Fig. 8 ), and topped by a thin palaeosoil that locally was eroded during em- placement of flows that formed the fan.

Fig. 9. Location 9006 within Barranca Escoba showing reversely to normally graded pyroclastic flow deposit consisting mostly of lapilli-sized material supported in a sandy matrix. Person at the base of outcrop is pointing at carbonized log which was dated at 8470 + 160 y.B.P.

12

TABLE 1

Pico de Orizaba radiocarbon dates

Sample location

9006 9004 9015 9017

Stratigraphic unit

C. SIEBE ET AL.

pyroclastic-flow deposit block-and-ash flow deposit fluviatile reworked horizon block-and-ash flow deposit

(~ 13 C Reservoir-corrected (%o) age (years B.P. )

-25.0%0 8470± 160 -25.0%0 4660± 100 -25.0 %0 4040± 80 -25.0 0/~ 4090± 90

In the central and distal parts of the block- and-ash fan (sections 9003, 90t0, 9126), the underlying units consist of a coarse-grained pyroclastic-flow deposit. This deposit is heter- olithologic, clast-supported, and contains an- gular lithics and rounded pumice set in a sandy matrix. It can be divided into a reddish col- ored, very thick (> 6 m) horizon at the bot- tom and a 1-m-thick yellow horizon at the top. The change in color from red to yellow is the most conspicuous and characteristic feature of this deposit in the whole area, especially be- cause the transition is not gradual, but sharp. The color difference does not correlate with any other drastic change in structure, bedding, or composition, and therefore its origin is un- clear. It might represent a thermal (oxidation) boundary.

To the southwest the fan is underlain by a thick sequence (5-8 m) of dark gray, sandy ash that probably originated from the nearby cin- der cone Ahuatepec (section 9016). Upslope from this section along Barranca Escoba (a tributary of Barranca E1 Carnero), a thick se- quence of non-welded ash-flow deposits con- taining large carbonized tree trunks crops out in the deeper parts of the riverbed (section 9006 and Fig. 9 ). These deposits are typically matrix-supported in contrast to the deposits of the fan. They were produced by a major explo- sive event that surpassed in magnitude the eruption that produced the block-and-ash fan. Charcoal from the carbonized logs within these older ash-flow deposits yielded an age of 8470+ 160y. B.P. (Fig. 9 and Table 1 ).

Block-and-ash flow deposits

Most of the deposits forming the block-and- ash fan consist of coarse breccia units that originated from passage of block-and-ash flows, and particulate-rich flows consisting of var- ious mixtures of debris and water, such as la- hars. It is difficult to distinguish, correlate, or map individual units because of their similar- ity in appearance. The texture of some lahar deposits resemble that of coarse fluvial sand- and-gravel deposits with which lahars are typ- ically interbedded (e.g. Crandell, 1971 ).

Breccia layers at the bottom of sections are heterolithologic, but towards the top of the se- quence they tend to be monolithologic with angular to subangular lithic clasts that mostly consist of dark porphyritic, glassy, banded an- desite and dacite lavas. Accessory and acci- dental lithic fragments from the walls of the vent and from the basement are usually small in size and subordinate in amount. A weak in- ternal stratification locally exists but the bed- ding is generally irregular and laterally discon- tinuous. These layers are extremely coarse, unconsolidated, poorly sorted, and seldom have specific marker horizons useful for cor- relation. Upper and lower contacts of breccias are generally sharp but locally they are irregu- lar. Because such flows apparently move mostly in laminar fashion, inverse grading is common due to different shear rates within the laminar flow (e.g. Fisher, 1973). Inverse grading is best developed in poorly-sorted layers that locally rest on underlying material with surprisingly little contact erosion.

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE ORIZABA VOLCANO, MI~XICO: HAZARD ASSESSMENT 1 3

Some large blocks show breadcrust surfaces, expansion cracks, and exfoliation structures. Most blocks are dense, without vesicles, and with a smooth glassy surface that is reddish in color. Most of the material appears to have so- lidified rapidly or been very viscous before the time of eruption, as indicated by frozen flow banding. Carbonized branches and vegetation lie subhorizontally, mostly within the lower half of the deposits, aligned in the direction of flow. No carbonized trees are in growth posi- tions; all were broken and incorporated into the flow pattern. The pink surface of clasts and the burnt wood indicate that hot gases were possi- bly present during emplacement.

Individual units appear to have lobate fronts and are normally 1-2 m thick. The surface of the fan is dotted with large blocks up to 5 m, which are separated by distances of several de- cameters from each other. These blocks are rarely exposed in eroded scarps. The ability of the flows to transport such large clasts more than 14 km from the source reflects the high yield strength of the dense underflows. Be- cause the diameter of the largest blocks (5 m) is grater than the average thickness of a breccia layer (3 m) , large blocks typically project above the surface of the deposit.

Because there are no ash-cloud deposits (e.g., Fisher, 1979), contacts between individual block-and-ash flows are difficult to recognize in outcrops. The fine-grained dust of the cloud either was not deposited on top of each flow, or if it was, it was subsequently removed by av- alanches, heavy rain fall, or erosive lahars. Rowley et al. ( 1981 ) report ash-cloud depos- its, several cm thick, adjacent and on top of the June 12, 1980 pyroclastic-flow deposits at Mt. St. Helens. These fine beds were completely eroded and did not last more than a few weeks. In contrast, the ground beyond the margins of the breccia fan is mostly composed of uncon- solidated silt and sand of light greyish color. This material might have been deposited from ash clouds that extended beyond the limits of the dense underflows. Because these areas un-

derlain by silt and sand are used for agricul- ture, the original stratigraphy of the upper- most layers has been destroyed.

Lahar and fluvial deposits

These deposits are very similar to block-and- ash deposits because they are composed of the same materials that have been remobilized. In this study "lahar" is defined as: "a rapidly flowing mixture of rock debris and water (other than normal stream flow) from a volcano. A lahar is an event; it can refer to one or more discrete processes (such as debris flow and hy- perconcentrated streamflow) but does not re- fer to a deposit" (Smith and Fritz, 1989; Smith and Lowe, 1991 ). Deposits produced by lahars are divided into debris-flow deposits and hy- perconcentrated flow deposits depending on the solid/water ratio of the flow that trans- ported them. Debris flows have water contents ranging from as little as 10 wt% up to but no more than about 25 wt%. They are non-New- tonian fluids that move as fairly coherent masses in predominantly laminar fashion. Hy- perconcentrated streamflows contain 25 to about 40 wt% water, they posess a much lower yield strength than debris flows, and are char- acteristically turbulent (Pierson, 1985; Pier- son and Costa, 1987 ).

Although we did not study the laharic depos- its in sufficient detail to allow us to make a clear distinction between debris flows and hyper- concentrated flows (Smith and Lowe, 1991 ), a wide spectrum of solid/water ratios should be expected in such a sedimentary environ- ment. We believe that both types of flows oc- curred and significantly contributed to the for- mation of the fan. A wide range of facies may be generated from a single flow that trans- forms both spatially and temporally (e.g., Pierson and Scott, 1985; Best, 1992) to leave sediments that display significant vertical, lat- eral, and downstream variations. Pulsing and multiple flows can produce a complex series of deposits that record the changing fluid and

14 c. SIEBE ET AL.

Fig. 10. Location 9004 showing block-and-ash flow deposit (A) lying on top of laharic and fluviatile reworked deposits (B). Arrow is pointing towards the base of the block-and-ash flow, where abundant charcoal dated at 4660_+ 100 y.B.P. was obtained.

sediment dynamics of the mass flow. Several studies have demonst ra ted how mass flows may t ransform from non-Newtonian, viscosity dominated, cohesive or cohesionless flows to Newtonian, iner t ia-dominated flows through downstream flow-dilution and various types of t ransformation (Fisher, 1973; Pierson and Scott, 1985 ). Such transformations can occur

in many environments and are dependent upon grain size and composition, changes in flow viscosity and velocity and the type of di lut ion/ t ransformation mechanisms. These changes within the deposits were observed at many places of the fan.

On the western slopes of Pico de Orizaba la- har deposits are abundant within the lower part

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE ORIZABA VOLCANO, MI~XICO: HAZARD .ASSESSMENT 15

of the depositional sequence and extend be- yond the margins of block-and-ash deposits. The deposits of unchannelized debris flows (lahars) typically have non-erosional bases, poor-sorting, relatively minor silt content, and only traces of clay-sized ash. They are either matrix or clast supported, and commonly con- tain very large clasts at their tops. The most obvious characteristics of the deposits are their generally massive or weakly-stratified nature and their clast-supported texture, reflecting rapid and uninterrupted deposition from the flood waves (Fig. 10). These lahars probably formed from the distal margins of pyroclastic flows, as described by Aramaki ( 1957 ) for the 1783 eruptions of Asama volcano.

Radiocarbon dating

Numerous charred logs and branches are embedded in the deposits of the fan. Whether the vegetation was first uprooted and broken by a strong blast blowing down the slope and the logs later incorporated by flowing material, or whether it was directly incorporated by the block-and-ash flows and lahars, is difficult to determine. Three of the best carbon samples were collected and dated by the C-14 method. Two were collected within block-and-ash lay- ers at locations 9004 and 9017, whereas the third sample at location 9015 was recovered from a layer that appears to be a fluviatile re- worked block-and-ash deposit (see Figs. 7, 9, 10). The ages obtained range between 4660_+ 100 and 4040_+80 y.B.P. (Table 1). The short time-span represented by these dates is in good agreeement with the lack of soil de- velopment within the fan. Our work implies that all of these deposits were formed within a time-span of ca. 600 years. It is noteworthy that previous workers (Robin et al., 1982; Robin and Cantagrel, 1983) dated a pyroclastic de- posit located to the NE of the present cone at 4060 _+ 120 y.B.P. This date lies within the pe- riod of time in which the block-and-ash fan was formed and therefore must be related to the

same eruptive episode. We suspect that more young lavas and pyroclastic deposits of the same age will be found in the future at other localities surrounding the volcano.

Mineralogy and chemistry of juvenile material

Four of the six samples analyzed in this study were collected from the upper block-and-ash layers of the fan at an elevation between 2800 and 3000 m a.s.l.; the other two samples were collected from the front of a stubby, morphol- ogically young lava flow at an elevation of 4200 m a.s.1. (Fig. 5). This lava flow extends from the summit crater into the pronounced hor- seshoe-shaped scar that opens to the west on the upper parts of the summit cone. The flow has a length of about 3.2 km and a steep front up to 30 m in height. It appears to be related to the same eruptive event that produced the block-and-ash flows.

Macroscopically, all of the samples studied are typically dark porphyritic andesites with abundant plagioclase phenocrysts (1-3 mm) and minor hornblende. The samples from the block-and-ash deposit are dense and have a glassy luster, whereas those from the lava flow are opaque and slightly vesicular. Modal anal- yses were determined by point counting thin sections and are given in Table 2. All samples were analyzed for major elements by DC Plasma and trace element analyses were made by instrumental neutron activation (INA) analyses (Table 3). Phenocrysts and intersti- tial glass in these samples were extensively analyzed at Arizona State University using a wavelength-dispersive JEOL JXA-8600 fully automated electron-microprobe analyzer (EMPA) equipped with Tracor Northern TN 5500/5600 stage automation. The analytical procedure and results are given in Siebe (1992).

The mineralogical composition of all sam- ples is strikingly similar: They contain pheno- crysts and microphenocrysts of plagioclase, hornblende, clinopyroxene, orthopyroxene and

16

TABLE 2

Modal analyses (vol.%), ( > 500 points counted) of Plco de Orlzaba andesites

c . SIEBE ET AL

Sample -~ plag hbl cpx opx ol mt cc vesc grndm

9120" 20.9 5.1 2.4 1.5 1.2 0.9 0.0 6.0 67.0 9121" 26.5 6.6 2.1 1.5 0.0 0.8 0.0 2.0 59.5 9002 24.4 5.6 1.1 1.1 1.2 0.7 0.0 0.0 65.0 9007-B 27.7 5.3 1.9 0.8 0.0 0.4 0.0 0.0 63~9 9011-B 24.0 5.9 2.1 0.6 0.0 0.2 0.3 0.0 66.9 9028 21.2 4.0 2.5 1.7 0.0 0.8 0.3 0.0 69.5

plag = plagioclase; hbl = hornblende; cpx = augite; opx = hypersthene; ol = olivine; mt = magnetite; cc = calcite; vesc = vesicle grndm = groundmass. *= lava flow within cirque.

TABLE 3

Whole-rock chemical analyses

P.O.: 9120 9121 9002 9007 9011 9028

SiOz 63.50 62.80 62.90 63.60 62.70 63.00 TiO2 0.61 0.61 0.61 0.59 0.59 0.58 A1203 15.90 16.10 16.30 16.30 16.00 16.30 FezO3 4.67 4.45 4.61 4.48 4.47 4.40 MnO 0.08 0.07 0.07 0.08 0.07 0.07 MgO 2.46 2.70 2.41 2.39 2.36 2.32 CaO 4.75 4.77 4.63 4.62 4.58 4.62 Na20 4.52 4.45 4.51 4.57 4.49 4.52 KzO 2.38 2.01 2.37 2.36 2.33 2.35 P205 0.30 0.34 0.23 0.28 0.23 0.2(!

LOI 0.10 0.15 0.15 0.20 0.24 0.15 Total 99.26 98.45 98.79 99.47 98.06 98.5I

La 29 24 28 30 31 2" Ce 44 44 45 43 43 41 Pr <25 <25 <25 <25 <25 <25 Nd 21 21 23 21 17 23 Sm 4.3 4.1 4.4 4.6 4.6 4.4 Eu 1 1 1 1 1 ! Gd < 100 < 100 < 100 < 100 < 100 < 10~ Tb 1 1 1 <1 1 ~ i Dy 3 2 2 2 3 3 Ho 1 1 1 1 1 ~-i Er < 50 < 50 < 50 < 50 < 50 < 50 Tm <2 <2 <2 <2 <2 <2 Yb 2.0 1.6 1.5 1.8 1.4 I9 Lu 0.3 0.2 0.3 0.3 0.3 ,~ 3

BaO 0.08 0.061 0.078 0.079 0.078 0.079 Cr203 0.03 0.02 0.02 0.02 0.02 (I.02 S 0.02 <0.02 0.02 <0.02 <0.02 0.02

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE OR1ZABA VOLCANO, MEXICO: HAZARD .ASSESSMENT 17

titano-magnetite set in a groundmass ofmicro- crystallites and pale brown rhyolite glass. Groundmass textures are hyalopilitic in which augite is subordinate to orthopyroxene. The main difference between the samples basically resides in minor differences in the relative amounts of each type ofphenocryst present but not in the type of phases contained in the rocks. Two of the samples surprisingly did contain rare crystals of olivine (samples 9120 and 9002), which are Fo-rich. Two other samples (9011-B and 9028) contained rare minute grains of calcite. Because calcite does not fill any voids or cracks, it was probably incorpo- rated by wallrock assimilation of the conduit from the Cretaceous limestone basement dur- ing the ascent of the magma. Taking into ac- count both the mineralogical and chemical data, all samples can probably be best classi- fied as high-silica hornblende andesites that were contaminated by incorporation of minor amounts of Mg-rich olivines and limestone.

Plagioclase

Plagioclase is the most abundant phase in all samples. The largest grains are generally greater than 3 mm in length. Glass inclusions within plagioclase are present but not abundant. As typical for calc-alkaline andesites, the plagio- clase in these samples displays complex oscil- latory zoning as also described at other Mexi- can stratovolcanoes (e.g., Luhr and Carmichael 1980; Nixon 1988). Some phenocrysts have developed sieve textures containing inclusions of glass. Asymmetric resorption is common. Microprobe analyses of rims, cores, and mi- crophenocrysts are strikingly uniform. The norm was calculated for these analyses and the resulting composition plotted near the ande- sine/labradorite (Anso) boundary in the pla- gioclase triangle.

Hornblende

The most abundant mafic mineral is strongly pleochroic hornblende, which occurs as crys-

tals up to 2 mm in length in all samples. Crys- tals are generally idiomorphic or asymmetri- cally corroded and surrounded by a thin reaction rim of opaque oxides (opacite), that apparently formed due to oxidation during eruptive quenching. Inclusions of idiomorphic Ti-magnetite as well as plagioclase are wide- spread.

Pyroxenes

In contrast to hornblende, pyroxenes are mostly present as small grains in the ground- mass. Clinopyroxene crystals are generally subhedral and weakly zoned whereas orthopy- roxene occurs as colorless to weakly pleochroic phenocrysts. Crystal clots of clinopyrox- ene + orthopyroxene _+ plagioclase _+ Ti-magne- tite are common. The composition of the py- roxenes is typical of calc-alkaline rock series. Microprobe analyses revealed that clinopyrox- enes have a typical augite composition while orthopyroxenes plot in the bronzite and hyper- sthene fields.

Olivine

Olivine crystals are rare and occur either as equant crystals with euhedral outline or as an- hedral-subhedral crystals, the margins of which are scalloped by fne-grained reaction prod- ucts (Fig. 11 ). The compositions of measured crystals range between Fo84 and F082.

Fe-ti oxides

Titanomagnetite is the most common opaque mineral and occurs as subhedral-an- hedral microphenocrysts in the groundmass as well as inclusions in other phenocrysts.

Glass

Matrix glass is remarkably homogeneous and high in silica (71-76%) with low CaO and high alkalis.

18 C. S1EBE ET AL.

Fig. 11. Back Scattered Electron Microscope (BSE) image of an olivine phenocryst in Pico de Orizaba andesite sample 9002. Corroded rims as well as formation of opacite (Fe-Ti oxides) at the edge can be recognized. Scale bar = 100 ~m.

Discussion

The samples considered in this study are all homogeneous in composition. Even the rela- tive amounts of rare earth elements are almost identical, suggesting that the lava flow and the upper block-and-ash flow both might have been produced during the same eruption. The mor- phologically young appearance of the lava flow is compatible with the Holocene C- 14 ages ob- tained for the block-and-ash fan. The probable near contemporaneous emplacement of the lava flow and block-and-ash flows is another strong argument favoring a glacial origin for the horseshoe-shaped scar during the last major glaciation dated by Heine (1988) at 8500 y.B.P, rather than as a volcanic collapse feature.

The chemical compositions and petro- graphic characteristics of the phenocrysts and rhyolite glass in samples analyzed in this study suggest that they constitute disequilibrium as- semblages, and imply a complex origin for the

host magma. Many of the minerals are ob- viously from a source separate from the rhyo- lite melt in which they were included. The glassy matrix (SIO2>71 wt.%) is too high in silica to be in equilibrium with the minerals, especially forsteritic olivine and Mg-rich py- roxene. Therefore, the grain of slightly cor- roded but still idiomorphic olivine shown in Figure 11 is most likely a xenocryst. A com- mon interpretation for disequilibrium textures and compositions invokes magma mixing and assimilation of wall rocks (e.g., Eichelberger, 1975, 1980; Grove et al., 1982; Gerlach and Grove, 1982; Nixon, 1988a,b). This mecha- nism has already been proposed for generating young andesites at Pico de Orizaba (Robin and Cantagrel, 1982; Robin et al., 1983; Kudo et al., 1985).

Although the evidence for mixing in the Pico de Orizaba dacite is not as clear as described at other places, it appears that after the injec- tion of basaltic magma from a mantle source

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE ORIZABA VOLCANO, MI~XICO: HAZARD ASSESSMENT l 9

into a silicic chamber, both magmas mingled to such an extent that the groundmass homog- enized. The original basaltic lavas had under- gone fractional crystallization during ascent and were carrying crystals of olivine, clinopy- roxene, and plagioclase prior to mixing in a highly silicic crustal chamber containing mafic crystals of hornblende and hypersthene. After contamination with smaller amounts of basalt the silicic groundmass glass retained its rhyo- lite composition. The absence ofquarz and K- feldspar suggests high temperatures of the magma.

The presence of forsteritic olivines that have been only slightly resorbed and which show only a thin corona of opaques has strong im- plications for magma chamber processes. Tsu- chiyama ( 1986 ) demonstrated experimentally that crystals of forsteritic olivine immersed in silicic melts may react to form bronzite co- ronas in as short a time as 1.5 hours. The lack of orthopyroxene reaction rims and the reten- tion of quasi-idiomorphic shapes displayed by the olivines in samples from Pico de Orizaba may therefore indicate eruptive quenching in the order of minutes to hours after these phenocrysts were incorporated in mixed magmas.

From the preceding discussion, it appears that the interval between mixing and eruption may be vanishingly small on a geologic time scale, implying that magma chamber replen- ishment may have triggered the volcanic erup- tion (Sparks et al., 1977; Eichelberger, 1980). Injection of basic magma into an acid magma chamber could have resulted in superheating and induced convection of the acid magma as well as physical mixing. An accompanying in- crease in pressure might have been sufficient to fracture the volcanic edifice. Sudden de- compression of an acid magma supersaturated in volatile phases should lead to extensive ves- iculation. The scarcity of large vesicles in rocks of the block-and-ash deposits can be explained by rapid quenching prior to eruption due to groundwater intrusion into the magma body.

This could result in magma-water interaction that would add even more volatiles in a super- critical state to the system. The lithic-rich blast- like deposit at the base of some of the block- and-ash flows is compatible with such a deep- seated hydromagmatic eruption (e.g., Barberi, 1985; Wohletz, 1986).

Another important characteristic feature of these rocks is the existence of rare rounded grains of calcite within the glassy matrix. Their most obvious explanation is incorporation of calcareous wall rocks of Cretaceous age that form the basement in this area. It seems possi- ble that the silicic magma chamber might be located within the Cretaceous limestones, which are known to be excellent aquifers ca- pable of providing sufficient groundwater to produce hydromagmatic explosions.

From the chemical and petrographical data it can be concluded that a relatively shallow, chemically stratified magma chamber was being periodically replenished by juvenile ba- saltic magma. Fractional crystallization, magma mixing, and wall-rock assimilation combined to affect the liquid line of descent of an ultimately basaltic parent magma. Eruption soon after the magmas were mixed is consis- tent with models in which mixing of hot dry basalt and cooler wet silicic magmas causes boiling and triggers the eruption (Sparks et al., 1977). In addition magma-groundwater in- teraction aided rapid quenching and hindered major vesiculation of the magma saturated in volatiles leading to an enhanced explosive eruption.

Scanning electron microscopy (SEM) observations

Samples from the uppermost block-and-ash deposit were examined with the scanning elec- tron microscope. After samples were sieved, the 1 o fraction was isolated and cleaned using the method described in Komorowski ( 1991 ). In- spection with reflected light under the binocu- lar microscope revealed that fragments of this

20 c. SIEBE ET AL

-,,-, r . ' ~ O ~

- ~ . ~ ~ . ~

~ ~= .,.., "~ ,",

~ , . ~ ~ . ,~ ,,, '.~ .~

. ~ ~ ~ . ~ £ ~

~ , ~ . ~ ,~ "~ ~ ~ 8 ~ ' ~ ~ ' ~

. ~ ; = ~ ~ ~

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE OR1ZABA VOLCANO MI~XICO: HAZARD ASSESSMENT 21

size in the deposit are microscopically monol- ithologic since all grains had the same miner- alogical composition as the macroscopically visible gravel and boulders. Chemical analyses of grains were made with an energy-dispersive Spectrum X-ray analysis system (EDS) (Fig. 12).

All composite grains are hyalocrystalline ju- venile hornblende dacite. Most are cleanly fractured; the forms of grains range from splin- ter-shaped shards to blocky, angular, subangu- lar to subrounded fragments with slightly modified edges. Some clasts show fresh con- choidal fracture surfaces as well as zones of abrasion. The most important observation made with the SEM is the presence of small vesicles ( < 5% by volume) implying exsolu- tion of some gases before solidification.

The clasts studied are very similar to those

found in block-and-ash flow deposits of the 1902 eruption of Mont Pel6e described by Hei- ken and Wohletz (1985). These eruptions of Mont Pel6e produced devastating block-and- ash flows after periods of growth and mechan- ical disruption of andesitic lava domes. Most tephra in these devastating block-and-ash flows consisted of dense juvenile fragments from these domes. Explosive disruption of the domes apparently resulted through a combination of several processes including: (1) overpressure within small vesicles scattered throughout the dome during cooling and crystallization and (2) phreatomagmatic interaction of meteoric water and hot dome rocks. At Pico de Orizaba melting of the ice-cap probably was a sufficient source of groundwater for magma-melt inter- action leading to phreatomagmatic explosions at a summit dome.

Fig. 13. View of the glacial cirque at the western flank of Pico de Orizaba. The steep front of the young stubby lava flow discussed in this study is also shown. Circles denote sample locations 9120 and 9121 from right to left. Arrow points towards topographic peak known as "El Sarc6fago". The bowl-shaped cirque lies below and to the right. Rockfall from the cirque walls is frequent and rock debris accumulated at the base of the walls can be recognized on the lower picture.

22 C. SIEBE ET AL

Glacial origin of the horseshoe-shaped escarpment

The horseshoe-shaped escarpment at the up- per west slope of the present cone (see Figs. 13 and 14) has been interpreted by Heskuldsson et al. (1990) to have been produced more than 38 000 y. ago by a major landslide that led to the emplacement of a debris avalanche de- posit. In contrast, in an earlier paper, Robin and Cantagrel (1982 ) claimed that the most recent eruptive period at Pico de Orizaba "started 13000 y.B.P, with the eruption of a dacite ash-flow containing pumice and scoria bombs. This was such an intense event that products were found 30 krn SE of the summit, erasing the top of the former volcano and cre- ating a large crater (4-5 km wide). The pres- ent cone, of 1,400-1,500 m elevation, grew in this crater". It is difficult to understand why

97 ° 20'

19°05

after such a major eruption 13,000 y.B.P., that supposedly left a crater at least 4000 m in di- ameter, there should still today exist the clearly visible remnants of a landslide scar that formed already before 38,000 y.B.P. This is especially perplexing, because the horseshoe-shaped scar is located within the limits of this supposed caldera.

Based on our studies we favor an alternative explanation for the origin of this major mor- phological feature, namely that it was pro- duced by glacial erosion and represents a gla- cial cirque. This interpretation already has been proposed by Lorenzo (1964) and Heine ( 1975 and 1988). Glaciomorphological and palaeoclimatic studies at Mexican stratovol- canoes have shown that several glaciations ocurred in the Late Pleistocene. A wide variety of morphological features such as lateral and terminal moraines, U-shaped valleys, glacial

97015 '

19005 '

190 00' 9000' 97 ° 201 97015

Fig. 14. Sketch map showing the present distribution of ice at Pico de Orizaba volcano (modified after Lorenzo, 1964 ), and two major eirques.

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE ORIZABA VOLCANO, MI~XICO: HAZARD ASSESSMENT 23

cirques, rock-surface striations, indicating a glacial origin have been reported (e.g., White, 1951, 1962; Heine, 1975, 1988; White et al., 1990).

Although there appears to be little agree- ment among investigators about the exact tim- ing of the several advances and retreats of the ice masses at the flanks of the highest peaks in Mexico, there is no doubt that the last major glaciation occurred well before the formation of the block-and-ash fan dated by us between 4040 and 4660 y.B.P. According to White et al. (1990) the last major glaciation on Mexican volcanoes ended about l l ,000 y.B.P., while Heine (1988) asserts that it terminated about 8500 y.B.P. Both ages are substantially older than the age of the block-and-ash fan. The lava flow with a chemical and mineralogical com- position identical to juvenile material from the block-and-ash fan flowed into the already formed bowl-shaped escarpment. Within the bottom of this escarpment and also in U- shaped valleys that originate at its base we were able to identify abraded surfaces showing striations.

We conclude that the geological features of the bowl-shaped escarpment could not have formed 38,000 y.B.P., as suggested by Hoskuldsson et al. (1990). Our interpretation is that the bowl-shaped scarp was formed dur- ing repeated glaciations and it did not change much since the last major glacial advance that ended between 11,000 and 8500 y.B.P. There- fore, the escarpment at the west wall of Pico de Orizaba represents the headwall of a formerly existing cirque glacier. Likewise we doubt that the large eruption, dated by Robin and Canta- grel (1982) at 13,000 y.B.P., left a crater 4000- 5000 m in diameter. Lava flows and pyroclas- tic rocks exposed within the cirque walls dip in the opposite direction of the main angle of slope of the present cone attesting to the exis- tence of an older volcanic edifice (Fig. 13). This implies that a large cone must have been formed at least twice since that major eruption.

Reconstruction of the chronology of eruptive events

We can speculate regarding the chronology of eruptive events leading to the formation of the block-and-ash fan at the west base of Pico de Orizaba. A working model could assume a situation similar to the present at Pico de Ori- zaba. The volcano had a summit cone extend- ing more than 5600 m a.s.l., a relatively small, 300-m-deep summit crater with nearly vertical walls, and a dormancy of several centuries. The only apparent signs of activity were fumaroles in the crater area and the summit flanks were covered with an ice-cap similar in size to the present one, because climatic conditions also were similar.

Juvenile magma, andesitic in composition, is assumed to have intruded the volcano and formed a viscous dome within the crater. This lava was derived from the mixing of basaltic and silicic magma in an upper crustal chamber with minor incorporation of wall-rock (e.g., limestone) fragments. Continuous extrusion accompanied by occasional explosions might have led to the partial filling of the crater depression by a dome. Steady pressure that built up in the magma beneath the plug caused explosions. The pressure rise could have been due to exsolution of magmatic gas or partial vaporization of groundwater or both. Nairn and Self (1978) have postulated that as new magma rises in a conduit after a previous ex- plosion, its upper surface cools, forming a con- gealed cap, which can be blasted apart by suc- ceeding explosions. This broken cap rock then becomes entrained in the eruption column generating the first block-and-ash flows. About half of the material in the block-and-ash flows was juvenile, the other part was lithic debris from preexisting rocks as evidenced by the predominance of heterolithologic breccias at the base of the block-and-ash fan. Fragmenta- tion of the erupted material was primarily due to volcanic explosions within the vent; the sub- sequent transportation was gravity driven.

24 C. SIEBE ET AL.

Whether pyroclastic flows were accom- panied by a preceding, ground-hugging, hurri- cane-like surge cloud (Sparks et al., 1973; Sheridan, 1979) is not certain, but this is prob- able because in several historical eruptions, such as those at Santiaguito (Rose et al., 1977) and May6n (Moore and Melson, 1969), pyr- oclastic flows were preceded by hot surges which extended beyond the margin of the flows themselves.

In any case, the glacial cirque and connect- ing U-shaped valleys acted as an enormous re- ceptacle on the west flank of the volcano through which the material was funneled prior to deposition at its base. Because the valleys opened onto a broad plain at lower altitude, the distal portions of the pyroclasic flows were not confined by valley walls, but spread out. The flow deposits accumulated at the foot of the cone forming an overlapping digitate series of coherent deposits with lobate frontal margins. Due to the funneling effect of the preexisting topography, the pyroclastic flows probably maintained their energy and possessed a strong erosive power in their upper courses carrying away fragments of older rocks and vegetation as described by Aramaki (1956).

At a late stage in this eruption the juvenile lava appears to have entirely filled the crater and a dome formed at the summit. This is in- ferred from the fact that the block-and-ash de- posits within the upper part of the fan are al- most entirely monolithologic, composed of juvenile andesite clasts. Whether the pyroclas- tic flows that formed during these later erup- tive phases originated mostly by directed low- angle blasts from the base of the dome which was occupying the crater (Lacroix, 1904), or by repeated gravitational partial collapse of a dome carapace (Roobol and Smith, 1975; Rodriguez-Elizarrarits et al., 1991 ) is difficult to say. A question remains as to whether pyro- clastic flows descended all sides of the volcano or if they were confined to only one sector. It would seem that the largest eruptions pro- duced pyroclastic flows that moved down all

sides of the volcano because Robin and Can- tagrel (1982) and Robin et al. (1983) report a pyroclastic deposit with an age of 4060 _+ 120 y.B.P, located to the northeast of the present cone. Unfortunately, their description of the outcrop where the dated material was found is so vague that we were not able to find the lo- cation in the field in order to establish a cor- relation with the deposits on the western slopes. Despite the present lack of confirmation, we believe that it is very likely that more deposits related to this eruptive event will be found by future investigators at other places around the volcano.

It is very probable that most of the flows rapidly segregated by gravity into dense under- flows strongly confined by topography and ov- erriding, turbulent ash clouds that swept be- yond the limits of the pyroclastic flows and that were able to surmount topographic barriers (e.g., Sheridan, 1979; Fisher and Heiken, 1982; Mellors et al., 1988). As the ash-cloud surges moved down the mountain and continued to expand outward they probably maintained sufficient energy to devastate the landscape beyond the limits reached by the dense under- flows as occurred at Mont Pele6 in 1902 (Lac- roix, 1903) and Hibok-Hibok in 1951 (Mac Donald and Alcaraz, 1956). Because ash-cloud material typically has the consistency of flour and an extremely low yield strength just after deposition, they are easily removed by erosion within a few weeks after formation (Rowley et al., 1981; Rodriguez-Elizarrar~ls et al., 1991 ).

Field evidence indicates that lahars were generated episodically from the beginning of the eruption until the end, because such units are intercalated throughout the entire strati- graphic sequence of the depositional fan. It is likely that the initial sequence of pyroclastic flows and surges melted snow and ice from the summit ice cap to trigger lahars as observed at the recent eruptions of Hudson (Best, 1992 ), Nevado del Ruiz (Pierson et al., 1990), and Mount St Helens (Waitt, 1989). The transfer of heat from the eruptive products to the

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE ORIZABA VOLCANO, MI~XICO: HAZARD ASSESSMENT 2 5

snowpack, combined with seismic shaking probably released large volumes of meltwater within a short period of time. Downslope in- corporation of unconsolidated volcanic depos- its, rock-debris, and valley-fill sediments gen- erated lahars that extended tens of km from the summit. Sparse vegetation in the source re- gions of the lahars was probably an important contributory factor to runout distance.

Regardless of other factors at volcanoes such as Pico de Orizaba located in humid regions, lahars can be triggered exclusively by torren- tial rainfall that occurs both during and long after an eruption. Given the meteorologic set- ting and slope characteristics at Pico de Ori- zaba, intense rainfall and the resulting runoff on the volcano's slopes probably triggered "rain-lahars", which included not only hot flows that occurred during eruptions but also cold flows at ambient temperatures during re- pose periods, months or maybe even years after the eruption, as described by Arguden and Ro- dolfo (1990), and Rodolfo and Arguden (1991) for Mayon. Repeated floods of hot water and hot lahars were also one of the dom- inant features before the 1902 eruption of Mont Pel6e on Martinique (Lacroix, 1903 ). In addition torrential seasonal rains at Mont Pe- 16e eroded pyroclastic deposits and produced lahars, which underwent a downstream tran- sition into stream deposits (e.g., Roobol and Smith, 1975).

Although only lahar deposits on the west flank were considered in this study, it is almost certain that lahars developed on all sides of the volcano. The largest lahars probably evolved along the drainage of the Jamapa River where they changed into distal lahar-runout flows that reached the Gulf of Mexico, more than 100 km to the east near Veracruz. These flows in par- ticular must have undergone significant tem- poral and spatial transformation as they evolved eastward within the constrained Ja- mapa valley. Because other major valleys feed eastward, the mass flow was probably diluted significantly by tributary input along their path

as has been described in other volcaniclastic flows (e.g., Scott, 1988; Pierson et al., 1990). The hyperconcentrated streamflows/ lahars probably progressed some 50 km downvalley before attenuation due to simple lowering of river banks and broadening of the valley.

After block-and-ash flows and lahars had swept down the slopes of the mountain, and lava extrusion had formed a dome at the sum- mit, explosive activity apparently ceased and a thick blocky flow decended the west slopes of the cone and flowed into the glacial cirque. It is not known whether or not any of the other morphologically young flows that can be rec- ognized at other locations around the present crater were emplaced during this eruption, but it is possible. One may speculate from the se- quence of the activity that the volatile content of the magma decreased gradually as the erup- tion proceeded.

To summarize, the magma that issued from the crater was relatively homogeneous in chemical composition throughout the whole period of the activity as inferred from the chemical and petrographic character of the rocks. The corresponding late lava at Pico de Orizaba stands out from the adjoining depos- its by its dark color, fresh appearance and rug- ged surface.

Although we did not find any significant ash- fall deposits, we believe that at least intermit- tent fallout must have occurred, especially at the beginning of the eruption. This loose ma- terial must have been continuously stripped by subsequent rain, lahars, and block-and-ash flows.

An unanswered question regards the dura- tion of the entire eruption. Was the bulk of the fan formed during the course of 600 years, bounded by the present radiometric ages? We believe that the eruptive period was much shorter (maybe a few years or decades) as sug- gested by several lines of evidence:

(1) Chemical and mineralogical data are very homogeneous. If the eruption took sev-

26 C. SIEBE ETAL.

eral centuries, some compositional variation would be expected.

(2) Charcoal within lahar deposits and block-and-ash flow deposits is only abundant within the lower layers of the sequence, indi- cating that the time interval between eruptions was insufficient for vegetation to recover.

(3) No unconformities due to long-time weathering and erosion, such as the formation of deep gullies or streambeds with steep walls, were observed within deposits of the fan. Perhaps the date 4660_ + 100 y.B.P, was ob- tained from charcoal of dead wood picked up by a block-and-ash flow. All other dates, in- cluding the date reported by Robin and Can- tagrel (1982) for a deposit northeast of the present summit, cluster between 4000 and 4100 y.B.P.

Future hazards and risk evaluation

Previous studies (Robin and Cantagrel, 1982; Robin et al., 1983; H~skuldsson, 1990; Siebe et al., 1991 ) have revealed the existence of a wide variety of pyroclastic deposits sur- rounding Pico de Orizaba. These deposits in- clude the products of major catastrophic events that reshaped the entire landscape including Plinian pumice fall, pumice-and-ash flows, and debris avalanches. It is beyond the scope of this study to re-examine the entire spectrum of eruptive scenarios that might occur at Pico de Orizaba in the future. Instead, we have made an attempt to anticipate the possible sequence of events that might be expected in case of re- newed "Pel6ean-type" activity as described for the block-and-ash fan of this paper. For our hazards analysis we prefer to consider the west flank of the volcano. In doing so we are far from describing a worst case scenario. On the other hand, eruptions of this type have occurred fre- quently at the volcano and therefore are more likely to occur again in the future.

With information obtained from INEGI maps (1984) and the 1990 National Census we estimate that at least 55 000 people living in 45

TABLE 4

Census data for towns at risk at the western flank of Pico de Ori- zaba assuming a similar eruption to that which produced the block- and-ash fan discussed in the text (data from the 1991 National Census)

A. Towns buried by dense underflows Manuel E. Avatos 695 San Jos6 Llano Grande 357 Santa Maria Aserradero 293

1345 B. Towns in the direct path of overriding ash clouds Ahuatepec del Camino [ ] 74 Rafael Avila Camacho 670 San Diego Texmelucan 586 San Fco. Independencia ! 777 San Miguel Ocotenco 1574 Tlachichuca 6682

12463 C. Towns located 0-5 km from the lateral margins of dense underflows Arcos de Ojo de Agua Miguel Hidalgo San Fco. Cuautlaltzingo San M. Ojo de Agua San Miguel Zoapan

D. Towns possibly affected by overriding ash clouds Agua Escondida Aljojuca Jalapaseo El Orande Jos6 Maria Morelos La Gloria l_Azaro C~irdenas San Antonio Jalapasco Ciudad Serdain San Antonio Cuchilla San Felipe San Miguel Tecuitlapa Sta. ln~s Borbolla Sta. Inds Varela Zimatec San Juan Atenco

E. Towns possibly affected by lahars Alamos Tepetitl~in Ahuatepec del Camino Aljibes Buenavista Candelaria La Mata Miguel Hidalgo Paso Puente Sta. Ana Rafael Avila Camacho San Diego Texmelucan San Jos~ Guerrero San Martin Rinconada Sta. Cruz Coyotepec Sta. ln6s Borbolla Tepetitlfin Venustiano Carranza

Total inhabitants at risk

424 2533

669 J714 6536

560 3994

424 1192 3222 1068 261

12824 464

8 ~363 29O

~038 12

3369 30095

938 i174

130 i40 ~32 509 325 789 670 586 547 102 598 290 853 419

9202

56921

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE ORIZABA VOLCANO, MI~XICO: HAZARD ASSESSMENT 27

towns and settlements distributed over an area of about 1000 km 2 in 6 municipios (counties) would be directly impacted in some way (Ta- ble 4) by an event similar to the 4000 y.B.P. eruption. A preliminary hazard map that takes into account the areas possibly affected by block-and-ash flows, overriding ash-clouds and lahars on the west part of Pico de Orizaba is shown in Figure 15. From the hazard map it is clear that an adequate contingency plan is ab- solutely necessary in order to avoid a disaster in case of renewed activity. In the following paragraphs the potential impact of the most dangerous phenomena, namely block-and-ash flows, overriding ash-clouds, and lahars will be discussed briefly.

In case of emplacement of block-and-ash flows of the same type and magnitude as those which occurred more than 4000 years ago an area of about 16 km 2 would be buried under the hot deposits. Three settlements with more

than 1300 people, mostly devoted to agricul- ture, are currently living within this area. These flows would extend more than 14 km from the summit. Historic flows of this type at Mount St. Helens have been observed to travel at ve- locities as high as 60 m/ s (Rowley et al., 1981 ). Considering this speed, a pyroclastic flow orig- inating at the summit of Pico de Orizaba could reach Avalos, which is located 14 km from the source, in only 4 minutes. Figure 16 shows a profile, using the energy line model of Sheri- dan (1979), applied to the actual flows em- placed about 4000 years ago as well as to a hy- pothetical stronger eruption. The area possibly affected by overriding ash-clouds that might travel beyond the limits of the block-and-ash flow deposits is more difficult to estimate, be- cause no unequivocal deposits of this type are still preserved due to their ephemeral nature. We speculate that at least 6 towns with more than 12,000 habitants could be seriously dam-

PRELIMINARY VOLCANIC RISK MAP: WEST SLOPE PICO DE ORIZABA

/ \ 19 10 97 15

PICO DE ¢ " ORIZABA

~ (~) Tlachichuca

San Nicolas ~ / ~

I ~ ASHCLOUD E~Z] LAZAR i m OLOCKANOA HFLOW I POTENTIAL LAHAR AREAS _L ~ REC':NT':',..OCK-ANO-ASR

Fig. 15. Simplified preliminary risk map of the area showing towns and settlements that would be endangered in case of recurrence of block-and-ash flows and lahars similar to those that formed the presently existing fan.

2 8 C. SIEBE ET AL.

Pico de Orizaba Summit Crater f 5,7(~0 m a s.I.)

A 1 B Meters ~ ~,leters

Q Energy line for Avalos block and ash flow

Energy line for hypothetical block and ash flow

6,000 6,(YO( 1

5,000 5,0(gl

4,000 4,000

3,000 3,000

2,0(30 2,000

Fig. 16. Profile showing energy lines for actual and hypothetical block-and-ash flows with the same H/L ratio of 0.23. Where the topographic slope is greater than the energy line the flows accelerate and erode. Where the topographic slope is lower than the energy line the flows decelerate and deposition occurs.

aged. For these towns the clouds would be le- thal. Another 20 towns located in more distal areas with more than 36,000 people might be affected to a lesser extent by blowing dust.

Lahars should be expected along the chan- nels of Barranca Paso Buey and Barranca Honda. The area flooded in a worst case see- nario is shown in Figure 15. A total area of 155 km 2 divided into a southwestern (92 km 2) and a northwestern sector (63 km 2) would be flooded producing damage in 16 towns inhab- ited by more than 9000 people. The possible paths of future lahars deserve special attention because they will probably accompany any type of volcanic activity. In addition, hyperconcen- trated flows might also be triggered by non- volcanic phenomena such as earthquakes and torrential storms.

The glacial cirque and U-shaped valleys high on the volcano acted as a prominent ravine that served as the most important trap and sedi- ment-delivery system for volcaniclastic flows of several types. At the mouth of this delivery system a major block-and-ash fan formed more than 4000 y.B.P. Since then the major hydro- graphic network draining the glacial cirque has developed a pair of major channels (Barranca Paso Buey to the north and Barranca Honda to the south) that run along the lateral margins of

the fan (see Fig. 15 ). These barrancas and re- lated channels draining the glacial cirque and its vicinity have widths that range from 15 to 50 m and depths of 3 to 20 m. Along most of their lengths, excepting small portions of their upper reaches where old lava flows are ex- posed, the channels are incised into coarse, sorted deposits of pyroclastic flows and lahars. Channel walls which are frequently modified by slumping and lateral undercutting are steep and commonly vertical, despite the friable na- ture of the volcaniclastic sediments. The chan- nels are dry most of the year except during and shortly after intense rain storms and in the late afternoons due to melting of snow and ice. The generally dry character of the channels is largely because the sediments are very porous and permeable.

In the future Barranca Paso Buey and Bar- ranca Honda are probably going to be the most active lahar conduits for the western slopes of Pico de Orizaba. The farthest areas likely to be affected by hyperconcentrated streamflows and flood flows within waning stages are near Te- petitlan to the northwest and San Salvador El Seco to the west-southwest. In fact, in the late 1930s a debris flow apparently destroyed the train station at Guerrero, west of Ciudad Ser- dan. This event was triggered by a small land-

BLO('K-AND-ASH FAN OF ICE-CAPPED P1CO DE OR|ZABA VOLCANO, MI~XICO: HAZARD ASSESSMENT 29

4~ .~roz, hic nel,,,,ork sdrrourd;nc Pioo de Crizcoc~ volzono

Le ~ r i d :

• V:,ICGrIC cone

Scc;e 0 ~ '0 :3 i r~

9 7 °15 ' 9 6 ° 4 5 ' 9 0 o 1 5 ,

I J I i ~ \ .

_19oi{ " % . -

, " )

/ ~ ~ ~ ' t k ' \ ,O-,".Zz2-, .---/ I

, ....... T(\ /- ....... - ,,r.,:"o:

;9o15 '

; ~ o d

; 8 o 4 5 '

9 7 o l 5 ' 9 6 o q 5 ' 9 G o l b '

Fig. 17. Hydrographic network around Pico de Orizaba showing the course of the Jamapa river along which major lahars might occur in the future.

slide that occurred within the inner southern walls of the glacial cirque at about 5000 m a.s.1. (Sr. Herminio Garcia, pers. commun. , 1990). In the summer of 1985 the author observed during and after heavy rainfall a flood that came down Barranca Paso Buey and totally obstructed the unpaved road connecting Te- petitl~in and Paso Nacional.

The catastrophic effects of the relatively small eruption of November 13, 1985 at Nev- ado del Ruiz (5390 m a.s.1.), Colombia have focused special attention on the potential haz- ards related to glaciers on active volcanoes. At Nevado del Ruiz, a sequence of pyroclastic flows and surges interacted with snow and ice on the summit ice cap to trigger the most cat- astrophic lahars in recorded history. These la- hars killed more than 23,000 people living at the base of the volcano (e.g., Williams, 1990; Voight, 1990). The transfer of heat from the eruptive products to about 10 km 2 of snow- pack, combined with seismic shaking released anomalously large volumes of meltwater within a short period of time. Downslope incorpora- tion of new volcanic deposits, rock debris, and valley-fill sediments resulted in the generation of lahars that reached as far as 104 km from

the summit, destroying several settlements in- cluding a large part of the town of Armero lo- cated 72 km from the summit (Pierson et al., 1990; Thouret, 1990). Although Pico de Ori- zaba's glaciers cover a much smaller area than those ofNevado del Ruiz, it cannot be stressed enough that a similar catastrophe is extremely likely there, because many other factors are very similar.

It is interesting to note that along the Rio Ja- mapa, which is the most important river orig- inating at the volcano and draining towards the east from the volcano, there are no major cities or towns until Soledad de Doblado located 80 km from the shore of the Gulf of Mexico ( Fig. 17 ), despite the fact that the entire area is rich in agricultural land and water is abundant. Erosion has kept the upper part of the Jamapa River relatively deep. All major towns near the river are located at relatively safe places on topographically higher places (e.g., Atlapahua, Calcahualco, Ixhuatl~ln and Coscomatepec), (see Fig. 17). Only smaller groups of farm- houses are located directly at the river, sug- gesting that catastrophes might have occurred in Prehispanic times and induced the earlier inhabitants to locate their towns to safe areas.

30 C SIEBE ET AL.

Soledad de Doblado, a small city of 12,000 people located 87 km to the east, where the Ja- mapa finally reaches the plains, might be one of the towns most endangered by lahars. Here, 2 km west of the town the channel of the Ja- mapa widens so that lahars or hyperconcen- trated flows might spread out and affect Sole- dad de Doblado.

One of the strongest recent earthquakes in the area occurred January 3, 1920 causing se- vere damage in the counties of Jalapa, Coate- pec, Huatusco, C6rdoba, Orizaba, Jalacingo and Ciudad Serd~tn. Its epicenter was located only 40 km north of Pico de Orizaba. The earthquake itself destroyed several buildings, but it was the numerous landslides that oc- curred in the headwaters of major rivers drain- ing the Cofre de Perote-Pico de Orizaba range towards the east, which later converted into mudflows that were generated seconds or min- utes after the earthquake and flowed down the drainages of the Pescados, Jamapa and Huit- zilapa rivers, that produced the biggest losses of human life and property. The towns of Pet- lacuac~in, Acuatlipa, and Barranca Grande where at least 700 people were living, were to- tally swept away by flows that left water-marks as high as 15 m above the former riverbed (Comisi6n del Instituto Geol6gico de M6xico, 1922).

Conclusions

Between 4000 and 4100 y.B.P. Pico de Ori- zaba experienced a major explosive eruption during which hornblende-andesite block-and- ash flows, and lahars were produced. On the western flank of the volcano these flows were channelized through preexisting topographic features such as a prominent horseshoe-shaped glacial cirque and U-shaped valleys, before coming to rest at the base of the volcano. Re- peated deposition of these flows formed a tongue-shaped fan of volcaniclastic deposits that easily can be recognized on satellite im- ages. The area of the present fan should be con-

sidered of high risk in case of renewed activity, because it is very likely that future flows will use the same pathways. The present ice-cap at Pico de Orizaba poses additional hazards to nearby living population.

We reject the proposition of Hoskuldsson et al. (1990) who interpreted these deposits as the product of a landslide that originated at the horseshoe-shaped escarpment. Although land- slides are also very dangerous phenomena, the risks presented by a landslide are significantly different from those of a series of block-and- ash flows and lahars.

Acknowledgements

Microprobe analyses were performed with the help of Jim Clark and Jean-Christophe Ko- morowski at Arizona State University on equipment supported by NSF grant EAR- 8408163. Granulometric analyses and SEM studies were performed with the help of Pablo Ramirez and Margarita Reyes at Instituto de Geologia, UNAM. Work by M. Abrams was done at the Jet Propulsion Laboratory / Cali- fornia Institute of Technology under contract with the National Aeronautics and Space Administration. An award from the Associ- ated Students of Arizona State University to C. Siebe covered part of the expenses of this investigation. A National Science Foundation U.S. - Latin America Cooperative Science project on " Volcanic risk evaluation: Colima and Popocat6petl volcanoes" provided funds for M.F. Sheridan. Logistic support and the purchase of laboratory equipment were made possible by a grant from the Consejo Nacional de Ciencia y Tecnologia (CONACYT) to C. Siebe. This work benefitted from discussions with Sergio Rodriguez, Armann H~skuldsson, and Gerardo Carrasco. We specially want to thank Jonathan Fink, Stanley Williams, Don Burt, John Holloway, Phil Christensen, and J.C. Komorowski for reviewing an earlier draft of this manuscript. The comments of G.A.

BLOCK-AND-ASH FAN OF ICE-CAPPED PICO DE ORIZABA VOLCANO, Mt~XICO: HAZARD ASSESSMENT 31

Smith and an anonimous reviewer also helped to improve the manuscript.

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