Schola Machi a 05 Bull Vol c

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Bull Volcanol (2005) 68: 171–200DOI 10.1007/s00445-005-0430-x

RESEARCH ARTICLE

T. Scolamacchia · J. L. Macıas · M. F. Sheridan ·

S. R. Hughes

Morphology of ash aggregates from wet pyroclastic surgesof the 1982 eruption of El Chich ´ on Volcano, Mexico

Received: 28 May 2004 / Accepted: 5 March 2005 / Published online: 20 October 2005C Springer-Verlag 2005

Abstract The detailed stratigraphic study of the pyroclas-tic surge units S1, IU, and S3 produced during the mostviolent phases of the 1982 eruption of El Chichon vol-

cano, contains a complex succession of hydromagmaticevents triggered by the interaction of different proportionsof magma and external water. Component analyses of thehorizons within single units reveal that almost all wet andcohesive horizons contain ash aggregates. Based on their morphology and internal structure four different types of aggregates were distinguished: (a) accretionary lapilli, (b)armored lapilli, (c) irregular aggregates, and (d) cylindricalaggregates. The first three types have been described in thevolcanological literature (field and experimental studies);cylindrical forms are reported here for the first time. Thesehollow cylindrical aggregates consist of concentric layersof crystals and glass fragments set in a finer-grained ma-

trix. They formed around millimeter-size foliage fragmentsthat are locally preserved in the interior of the aggregatesas scorched or completely carbonized vestiges. SEM anal-yses suggest different mechanisms of formation for thefour types of aggregates. Irregular aggregates and armoredlapilli formed nearly instantaneously, whereas accretionarylapilli and cylindrical aggregates resulted from progressiveaggregation of ash in different regions of theeruptive cloud.

All types of ash aggregates contain fractured particles.This common feature suggests that particles ruptured dur-ing fragmentation prior to the growth of the aggregates.Broken clasts with cracks filled by a fine-grained matrixonly occur inside the cylindrical ash aggregates and to a

lesserdegree in some types of accretionary lapilli. This sug-gests that small thermal contrastsat thecontactof warm par-ticles with the colder fine-grained matrix of the aggregate

T. Scolamacchia () · J. L. MacıasInstituto de Geof ısica, UNAM,Coyoacan 04510, Mexico D. F., Mexicoe-mail: [email protected]: +52-5-5502486

M. F. Sheridan · S. R. HughesGeology Department, University at Buffalo,876 Natural Science Complex, Buffalo, NY, 14260 USA

cause existing small fractures to propagate and open as thealready weakened clasts deform slightly. The occurrence of all four types of aggregates in some horizons indicates that

several mechanisms of aggregation occurred nearly simul-taneously. The pyroclastic clouds therefore were not onlystratified in terms of density but the content of fluid phasesalso were not uniform. A dark-red, Fe-rich amorphous film(locally rich in P and S) envelops the particles and fosterstheir preservation in the deposits by forming a hard shell.The composition of this cement reflects the abundance of these elements in acid fluids of hydrothermal systems thatwere intersected by theconduit duringtheeruption.In distalareas, fallout aggregates were incorporated by dissipatingpyroclastic surges.

Keywords El Chichon . 1982 eruption . Pyroclastic

surges.Cylindrical ash aggregates

.Fe-S rich film

.

Binding forces . Hydrothermal fluids

Introduction

Ash aggregates are common in many types of pyroclas-tic deposits worldwide. Their preservation within ash lay-ers as well as their internal structure and grain size dis-tribution are controlled by factors related to the bindingmechanisms between particles (electrostatic and/or capil-lary forces due to condensation of water or ice close to the

freezing point), particle and pore density, thickness and de-gree of turbulence of the eruptive clouds, grain-size distri-bution and solid-particle concentration within clouds. Sev-eral models have been proposed to explain the formationof roughly spherical ash aggregates or accretionary lapilli(Moore and Peck 1962; Fisher and Waters 1970; Heiken1971; Schmincke et al. 1973; Reimer 1983; Sheridan andWohletz 1983a; Tomita et al. 1985; Kato 1986; Rosi 1992;Gilbert and Lane 1994; Rose et al. 1995; Veitch and Woods2001; Bonadonna et al. 2002; Textor and Ernst 2004; Guoet al. 2004). Experiments using particles of different grain



172

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2 0 0

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Legend

Towns

Rivers

Dome

Sommawalls

Contour (200 m interval)

Stratigraphic sections14

Km1 2 30 4

N

C . A . V . A .

C . V . A .

90˚ 95˚

20˚

15˚

250 km

Gulf of Mexico

Pacific Ocean

M e x i c

o

El Chichón

G u a t e m a l a

63b

A

S1

S3

IU

200

400

200

200

400

1000

Fig. 1 A Sketch map of southern Mexico and Northern Central America. Solid triangles show active volcanoes in Mexico. CAVA=CentralAmerican Volcanic Arc, CVA = Chiapanecan Volcanic Arc. B Revised distribution of pyroclastic surge units S1, IU and S3

sizes (Schumacher and Schmincke 1995) have determinedthe optimum amount of liquid necessary to form ash ag-gregates of different types.

The pyroclastic surge deposits produced during the 1982eruption of El Chichon volcano in southern Mexico pro-vide an opportunity to evaluate various processes of ag-gregation inside eruptive clouds. In this paper we analyzethe occurrence and grain-size distribution of four differenttypes of ash aggregates that exist within the pyroclasticsurge deposits, as well as use their morphological featuresto interpret their formation within the eruptive clouds. Our results provide new insights to better explain the origin of this important eruption.

Summary of the 1982 hydromagmatic surge products

The March 29 to April 4 1982 eruption of El Chichonvolcano (Fig. 1A) occurred in four eruptive phases (I-IV of Macıas et al. 1997) that produced pyroclastic fall, flow, andsurge deposits (Sigurdsson et al. 1984). Pyroclastic surgedeposits S1, S2, IU and S3 formed during the two mostviolent phases of the eruption on April 4 at 01:35 and 11:22GMT (S.E.A.N. 1982). These deposits have a controversialorigin, being considered as either magmatic (Sigurdssonet al. 1984, 1987) or hydromagmatic (Sigurdsson et al.1987; Macıas et al. 1997). Outcrops exhumed by 20 years

of erosion following the eruption provide additional datafor the reappraisal of pyroclastic surge stratigraphy aroundthe volcano. As a result of new fieldwork we recognized a

different distribution of the deposits (Fig. 1B) and a greater complexity in the depositional processes than previouslythought (Sigurdsson et al. 1987; Macıas et al. 1997).

Pyroclastic surge S1 crops out at distances farther than3 km from the crater where it is not buried by subsequentdeposits. It consists of six wet cohesive horizons composedmainlyof fineto mediumash interbedded with twodry hori-zons of medium to coarse lapilli (Fig. 2). The IntermediateUnit (IU) extends up to 1.3 km from the crater. The Lower and Upper IU units respectively consist of successions of two and eight wet and dry pyroclastic surge pairs. Pyro-clastic surge S3 is well exposed on the eastern part of thevolcano. The best exposures are confined to small channels

produced by rain runoff between the few hours separatingthe end of phase III and the beginning of phase IV on April4. S3 consists of eight varicolored cohesive horizons of fineto coarse ash and minor fine lapilli interbedded with four dry horizons of medium to coarse lapilli.

Terminology

In this work we use the term horizon in a broad sense toindicate a bed (s) emplaced during a single eruption unit of



173

S 3

4 A p r i l 0 1 3 5 G M T

4 A p r i l 1 1 2 2 G M

T

F 1 I.U. Lower

I.U. Medium

I.U. Upper

Massive gray to light-green block-and-ashflow consisting of juvenile lithic blocks in a matrix ofcoarse lapilli at the base and medium yellow lapilliwith scattered fine blocks of lithics at the topaccidental

Multiple laminae of light gray and white fine ash.O. L.

O.L.

1

3

5

4

6

7

8b

8a

9

10

2

Yellow pumice lapilli with scattered hydrotermalizedlithic angular blocks.

medium to fine ash with minor whitepumice lapilli

Yellow medium to coarse pumice lapilli with minor juvenile lithics.

Laminar succession of light gray fine ash

Yellow pumice lapilli to fine blocks with scatteredaccidental lithics .

Massive bed of gray fine ash.

Light-gray, lilac and white mm-sized laminae of fine ash.

Yellow fine-medium lapilli withplane parallel beds of white rounded pumice at the base.

Laminar alternation of gray coarseash and yellow fine pumice lapilli.

2 m

Gray to light-greenmassive block-and-ash flow, lithic richat the base.

B

29 March

3 April

S 1

O.L.

O.L.

O.L.

Eruptive Phase

I.U.

O.L.

S 2

F 2

Lithology Description

Pumice

Accessory lithic

Juvenile lithic

Block with cooling joints

Types of ash aggregates

Irregular

Cylindrical

Armored lapilli

Accretionary lapilli

O.L. Ox idized lay er

Normal graded, clast supported layer oflapilli to block-sized hydrothermally alteredaccidental lithics with minor juvenilelithics, pumice and cristals.

Gray

2 m

> 1m

> 10 m

> 15 m

Fig. 2 Composite stratigraphic section of the 1982 pyroclastic deposits according to this work, showing the different types of ash aggregatesfound in the pyroclastic surge horizons. All beds to scale unless thickness is given

pyroclastic surge, whereas eruption unit indicates a depositmade of one or more beds emplaced by a defined eruptivemechanism (Fisher and Schmincke 1984). Each horizon isclearly distinguishable from its neighbor because of over-all color differences and locally it is delimited by a redoxidized mm-sized layer or an erosive surface that marksthe end of a definite eruptive event. Bed thickness withindifferent horizons is defined following Ingram (1954): lam-ina (<1 cm), very thin bed (1–3 cm), thin bed (3–10 cm),medium bed (10–30 cm), thick bed (30–100 cm), very thickbed (>100 cm).

Each pyroclastic surge unit consists of a massive to finelylaminated, varicolored, slightly vesiculated, poorly sortedmixture of fine to medium lapilli rich in fine ash hori-zons. These are intercalated with massive or weakly cross-stratified moderately to well sorted horizons of medium tocoarsepumice lapilli with minor amounts of mediumto fineash (Fig. 3). These horizons have an irregular distributionaround the crater. Much of the variability in these featuresis attributed to the moisture content and temperature of the diluted pyroclastic density currents that are either richin condensing water vapor (wet) or in superheated steam



174

S3-2

S3-3

S3-4

S3-5

S3-6

P.F.2

S3-1

Fallout C

Fig. 3 Close up of S3 horizons in section 1b; the direction of theflow is perpendicular to the plane of the photo. Scale is 5 cm indiameter

(dry) (Walker 1971; Sheridan and Wohletz 1983b; Wohletz1983; Fisher and Schmincke 1984).

Almost all wet horizons of S1, IU, and S3 contain

vesicules (nearly spherical voids) as evidenced in S1-1, S1-2, S3-3 and S3-4 horizons that occur in topographic lowson the east-southeast flanks of the volcano. In some cases(IU-8b, S3-4, S3-5) small variations in the color hues of beds inside single horizons are caused by mm-scale vari-ations in the degree of vesicularity that characterize thedeposition of material by multiple eruptive pulses.

Generally, the contact between different wet horizons issharp and plane-parallel, but in some cases (S1-1 and S1-2;S3-3 and S3-4) it undulates irregularly without an exchangeof material across the contacts. These undulating contactsare syndepositional structures induced by the load pressureof the overlying bed on the bed beneath that still is plasticbecause of its high water content. This type of structureis recognized in wet pyroclastic surges elsewhere (Lorenz1974; Dellino et al. 1990). The following description of pyroclastic surge horizons is based on new data related tothe features of distinctive ash aggregates.

Granulometric and component analyses

A total of 100 samples representing a variety of horizonswithin units S1, IU, and S3 were analyzed. Three differ-ent methodologies were used: (1) dry, hand sieving, withscreens spaced at 0.5 intervals between fractions -4.0 to

3.5

; (2) wet analyses using a scanning-photo sedimento-graph (Fritsch, Analysette 20) for the fine-grained fractionsfrom 4.0 to 12 (Stein 1985; Kaye 1999); (3) image analy-ses of polished thin sections of ash aggregates by counting400–600 points in several cross sections cut at differentangles, per sample, with a personal computer version for Windows of the public domain Java image processing Im-ageJ (available on the Internet at http://rsb.info.nih.gov/nih-image/).

In natural samples, fine particles commonly adhere onthe surface of larger grains making it difficult to clearlyobserve their shape and surface features. For this reason

all granulometric fractions were washed several times byimmersion in ultrasonic baths with distilled water for ap-proximately 1 min, and then dried at 60◦C. This sample-cleaning method differs from that suggested by Wohletz(1983) and Heiken and Wholetz (1985), who wash their samples with a 10% diluted solution of HCl or alcohol attemperatures of 20–30◦C, rinsing the sample in distilledwater or acetone, or alternatively by immersion in an ultra-

sonic bath during periods of 30 s to several minutes. Cioniet al. (1992) concluded that diluted HCl at any temperaturewould completely remove particles attached to grain sur-faces but would also cause extensive fracturing resemblinghydration cracks and cause the grains to be mistakenlyconsidered as hydromagmatic features (Heiken and Who-letz 1985; Dellino and La Volpe 1995; Buttner et al. 1999).Likewise the use of acetone in the ultrasonic bath for afew minutes could also cause mechanical fractures on thegrains (Cioni et al. 1992).

Component analyses were carried out with the nakedeye or under the binocular microscope, counting between500 and 800 grains in fractions coarser than 4 , dis-

tinguishing between juvenile and non-juvenile fragments.Juvenile fragments were subdivided into three differentclasses: glass fragments (vesiculated or dense particles),crystals (mainly plagioclase, hornblende, augite, and mi-nor sphene), and different types of ash aggregates (con-sisting of glass and crystals). Non-juvenile componentsinclude hydrothermally altered and fractured lithic frag-ments, with colors varying from pale-pink to dark-red.Histograms of the grain-size distributions and componentanalyses (Figs. 4A–E) display the median diameter (Md)and sorting coefficient (σ) parameters of Inman (1952).

Ash aggregates

Based on their morphology, four different types of ash ag-gregates (Fig. 5A–D) are present in wet pyroclastic surgedeposits of the 1982 eruption. To investigate the differ-ences in their accretion processes, the aggregates were an-alyzed with a field emission scanning electron microscope(Hitachi S-4000). Mineral phases were identified with en-ergy dispersive X-ray spectroscopy on clast surfaces. Thedegree of alteration of different types of aggregates wasdetermined on polished sections by elemental mappingfor K, S, P, Fe with an electron probe X-ray microanal-izer (JEOL-JXA8900-R), using an accelerating potential

of 20 kV, a beam current of 2.0×10−8

A, and a resolutionof 1000×1000 pixels per image.

Type A: irregular aggregate

This type of aggregate occurs in almost all wet surge hori-zons in association with other types of ash aggregates. Itis common in size-fractions finer than 3 mm, although insome horizons it also appears in coarser fractions (Table 1).At 2 km to the east, in S3-3 it represents the exclusive com-ponent in fractions coarser than 4 mm (−2) and it is not



175

Fig. 4 Composite stratigraphic sections of pyroclastic surge de-posits accompanied by the granulometric distribution and componentabundance of pyroclastic surge units. The upper right corner of thehistograms displays the Mean (Md) and sorting (σ) parameters

of Inman (1952). A S1 unit to the North, West and South, B S1 unitto the East, C IU unit at the moat, D S3 to the East, E S3 to the Northand South of the crater, and between the crater and the somma walls(moat)



176

Fig. 4 Continued.



177

Fig. 4 Continued.

DC

BA

Fig. 5 Different types of ashaggregates found in pyroclasticwet surge deposits of ElChichon volcano: A irregular type from horizon S3-3 at 2 kmE, B cylindrical type from S3-4at 3.2 km E, C armored lapillifrom S1-6, 4.5 km E, Daccretionary lapilli from S1-1 at7.5 km N. Each subdivision onthe scale represents onemillimeter



178

T a b l e 1

D i s t r i b u t i o n o f t h e d i f f e r

e n t t y p e s o f a s h a g g r e g a t e s i n s i d e t h e p y r o c l a s t i c s u r g e

u n i t s S 1 - I U a n d S 3 o f t h e 1 9 8 2 d e p

o s i t s o f E l C h i c h ´ o n v o l c a n o . T h e t a b l e s u m m a r i z e s t h e

p r e s e n c e o f t h e s e a g g r e g a t e s w i t h r e s p e c t t o t h e d i s t a n c e f r o m t h e c r a t e r , s e c

t o r o f t h e v o l -

c a n o ( a z i m u t h ) a n d f a c i e s o f d e p o s i t i n w h i c h t h e y w e r e f o u n d . T h e d e g r e e o

f a l t e r a t i o n o f

e a c h t y p e i s a l s o i n d i c a t e d . O t h e r c o m p o n e n t s a r e l i s t e d i n o r d e r o

f a b u n d a n c e : J L - j u v e n i l e

l i t h i c s , A L - a c c e s s o r y / a

c c i d e n t a l l i t h i c s , W P - w h i t e p u m i c e , C -

c r y s t a l s ; p . d . s . i n d i c a t e s

p l a s t i c d e f o r m a t i o n s t r u c t u r e s i n t h e d e p o s i t . S e e t e x t f o r d i s c u s s i o n

U n i t

H o r i z o n

F a c i e s

A z i m u t h

D i s t a n c e f r o m

t h e c r a t e r ( k m )

S e c t i o n

T h i c k n e s s o f

d e p o s i t

C o l o r

A s h a g g r e g a t e s

F r a c t i o n ( , l o g 2

m m )

F i l m c o v e r

O t h e r

c o m p o n e n t s

S 1

0

L a m i n a r

S o u t h

5 . 7

1 1

4 c m

R e d

A r m . l a p i l l i ( 4 0 % )

− 2 . 5 t o − 0 . 5

D a r k - r e d

A L - W P - J L - C

I r r e g u l a r ( 3 8 – 1 % )

> 0

S 1

0

M a s s i v e

S o u t h

6 . 3

1 0

7 c m

R e d

I r r e g u l a r ( 5 0 % )

− 1 . 5 t o 1

D a r k r e d

A L - W P - J L - C

S 1

0

M a s s i v e

N o r t h

3 . 5

7 6

4 c m

R e d

A r m l a p i l l i

( 3 0 – 1 0 %

)

− 2 t o 0

D a r k - r e d

A L - W P - J L - C

I r r e g u l a r ( u p t o

5 0 % )

− 1 t o 2 . 5

C y l i n d r i c a

l ( 1 % )

− 0 . 5 t o 0

X

S 1

0

M a s s i v e

N o r t h

3 . 9

5 2

4 c m

R e d

A r m l a p i l l i

( 3 0 – 1 0 %

)

− 2 t o 0

D a r k - r e d

A L - W P - J L - C

I r r e g u l a r ( u p t o

5 0 % )

− 1 t o 2 . 5

O r a n g e

S 1

1

M a s s i v e

( p . d . s . )

E a s t

3 . 7

3 b

6 c m

G r a y

I r r e g u l a r ( 2 8 % ) ,

− 2 t o – 1

X

W P - J L - A L - C

A r m . L a p i l l i ( 3 0 % ) − 2

R e d

S 1

1

M a s s i v e

E a s t

3 . 8

2

2 c m

G r a y

A r m . L a p i l l i

( < 2 % ) ,

− 1 t o 1

X

W P - J L - C - A L

I r r e g u l a r ( 3 6 % t o

1 0 % )

− 2 t o 2

X

S 1

1

M a s s i v e

E a s t

4 . 5

9 0

4 c m

G r a y


( < 2 % ) ,

− 1 t o 1

X

W P - J L - C

I r r e g u l a r ( 1 0 % )

1 t o 3

X

S 1

1

M a s s i v e

E a s t

5 . 1

2 0

1 0 c m

G r a y

C y l i n d r i c a

l ( 3 % )

− 2 . 5 t o 0 . 5

R e d

W P - J L - C

A r m l a p i l l i ( < 1 % )

− 0 . 5

R e d

I r r e g u l a r

( < 5 % )

− 1 . 5 t o

0 . 5

S 1

1

L a m i n a r

( p . d . s . )

S S W

8 . 5

6

2 . 5 c m

G r a y

C y l i n d r i c a

l ( 3 % )

− 3 . 5 , − 2 . 5 t o

− 0 . 5

R e d

W P - J L - C

A r m l a p i l l i ( 1 0 % )

R e d

A c c r . L a p i l l i ( 5 % )

X

I r r e g u l a r ( 4 % t o

3 5 % )

− 0 . 5 t o 2

R e d

S 1

1

L a m i n a r

S S E

5 . 5

1 5

4 c m

G r a y

A r m . L a p i l l i ( 2 0 % ) − 2 t o − 1

R e d

W P - J L - C

I r r e g u l a r ( 1 5 % )

− 1 . 5 t o 2 . 5

A c c r . L a p i l l i ( 1 0 % ) − 1 . 5 t o − 1

X



179

T a b l e 1

C o n t i n u e d .

U n i t

H o r i z o n

F a c i e s

A z i m u t h



S e c t i o n


d e p o s i t

C o l o r



m m )

F i l m c o v e r

O t h e r

c o m p o n e n t s

S 1

1

M a s s i v e

S S E

5 . 8

1 1 2

4 c m

G r a y

C y l i n d r i c a

l

( 1 0 – 3 % )

− 2 t o 0 . 5

X

W P - J L - C


( 1 0 – 2 0 %

)

− 2 t o 0 . 5

R e d

A c c r . L a p i l l i

( 1 0 % - 3 %

)

− 2 t o 0 . 5

X

S 1

1

M a s s i v e

( p . d . s )

S S E

7 . 5

5 6

4 – 6 c m

G r a y

C y l i n d r i c a

l

( 3 – 1 0 % )

− 3 , − 2 t o 0 . 5

R e d

W P - J L - C


( 1 0 – 2 0 %

)

A c c r . l a p i l l i ( 8 – 3 % )

X

I r r e g u l a r ( 1 5 – 3 0 % )

> 1 . 5

R e d

S 1

1

M a s s i v e

( p . d . s )

S E

9 . 3

8 7

6 c m

G r a y


( < 1 0 % )

− 0 . 5 t o 1

R e d

W P - J L - C

I r r e g u l a r ( 1 3 % )

1 t o 2 . 5

R e d

S 1

1

M a s s i v e

N o r t h

3 . 5

7 6

5 c m

G r a y


( 1 0 – 1 2 %

)

− 3 t o 0 . 5

R e d

W P - J L - C

I r r e g u l a r ( 3 % )

> 1

S 1

1

M a s s i v e

( p . d . s . )

N o r t h

5 . 9

4 8

1 t o 2 . 5 c m

G r a y

A r m . L a p i l l i ( 5 % )

− 3 . 5 t o − 1 . 5

R e d

W P - J L - C

I r r e g u l a r ( < 1 0 % )

− 1 . 5 t o − 0 . 5

S 1

1

M a s s i v e

N o r t h

7 . 5

2 8

7 c m

G r a y

A c c r . l a p i l l i

( 1 0 0 % – 5

0 % )

− 3 t o 1 . 5

X

W P - J L - C

S 1

1

M a s s i v e

W e s t

4 . 5

5 4

3 c m

G r a y

A r m . L a p i l l i ( 8 % )

− 1 t o 0

R e d

W P - J L - C

I r r e g u l a r ( 1 0 % )

A c c r . L a p i l l i ( 5 % )

− 0 . 5 t o 0

X

S 1

2

M a s s i v e

( p . d . s . )

E a s t

3 . 7

3 b

3 – 5 c m

B r o w n

I r r e g u l a r ( 1 0 – 4 0 % ) − 2 t o 2 . 5

R e d

W P - J L - C

S 1

2

M a s s i v e

E a s t

4 . 5

9 0

4 c m

B r o w n

A r m . L a p i l l i ( 5 % )

− 1 t o 3

R e d

W P - J L - C

I r r e g u l a r ( 1 0 % )

0 t o 3 . 5

A c c r . L a p i l l i ( 1 0 % ) − 0 . 5 t o 1 . 5

X

S 1

2

M a s s i v e

S E

5 . 5

3 6

3 c m

B r o w n

X

W P - J L - C

S 1

2

L a m i n a r

( p . d . s . )

E S E

4 . 5

1 1 3

2 t o 4 c m

L i g h t -

b r o w n


( 1 0 – 2 5 %

)

− 2 t o 1

R e d

W P - C - J L

I r r e g u l a r ( 1 0 – 6 0 % ) − 1 . 5 t o 2 . 5

S 1

2

M a s s i v e

E S E

7 . 3

8 9

3 c m

B r o w n


( 1 6 – 3 % )

− 1 . 5 t o 1

X

W P - J L - C



181

T a b l e 1

C o n t i n u e d .

U n i t

H o r i z o n

F a c i e s

A z i m u t h



S e c t i o n


d e p o s i t

C o l o r



m m )

F i l m c o v e r

O t h e r

c o m p o n e n t s

I U

1 0

M a s s i v e

N W

0 . 5

1 0 2

2 0 c m

G r a y -

w h i t e

A c c r . L a p i l l i ( 1 5 % ) − 1 . 5 t o - 1

X

W P - J L - C


( 1 0 – 5 % )

− 1 t o 1 . 5

I r r e g u l a r ( ≤ 1 0 % )

0 t o 1 . 5

I U

1 0

L a m i n a r

E a s t

0 . 5

3 0

5 c m

G r a y -

w h i t e


( 1 0 0 – 4 0 % )

− 2 . 5 t o – 1

O r a n g e - r e d

W P - J L - C

I r r e g u l a r ( 4 0 – 1 0 % ) − 1 t o 2 . 5

S 3

1

L a m i n a

E N E

3 . 7

8 3

1 c m

R e d

A r m . L a p i l l i ( < 5 % ) 0 t o 1

R e d

W P - C

I r r e g u l a r ( < 3 % )

1 t o 2

S 3

3

M a s s i v e

E S E

0 . 6

4 6

6 c m

B r o w n


( 8 0 – 1 0 %

)

− 3 t o – 2

X

W P - J L - C


( 4 0 – 2 0 %

)

− 2 t o – 0 . 5

X

I r r e g u l a r ( 3 0 – 5 % )

− 1 t o 1 . 5

S 3

3

M a s s i v e

E a s t

2

1 b

7 c m

P i n k

I r r e g u l a r

( 1 0 0 – 2 0 % )

− 3 t o 2

D a r k - r e d

W P - C

S 3

3

M a s s i v e

E a s t

2 . 5

1 9

3 c m

R e d

A r m . L a p i l l i ( 1 0 % ) − 1 t o 1 . 5

D a r k - r e d

W P - C

I r r e g u l a r ( 2 0 % )

− 0 . 5 t o 2 . 5

S 3

3

M a s s i v e

( p . d . s . )

E a s t

2 . 7

2 4

7 – 1 0 c m

R e d

I r r e g u l a r ( 9 0 – 1 0 % ) − 0 . 5 t o 3

D a r k - r e d

W P - J L - C

S 3

3

M a s s i v e

( p . d . s . )

E a s t

3 . 7

3 b

3 – 5 c m

R e d

A c c r . L a p i l l i ( r i m )

( 5 0 – 5 % )

− 2 . 5 t o 0 . 5

D a r k - r e d

W P - J L - C

A c c r . L a p i l l i ( 1 0 % ) 0 a n d 1

X


( 5 0 – 1 0 %

)

− 0 . 5 t o 2

O r a n g e

I r r e g u l a r ( 2 0 % )

− 1 . 5 t o 2 . 5

R e d

S 3

3

L a m i n a

E a s t

4 . 7

1 8

0 . 5 c m

R e d

I r r e g u l a r ( 8 0 – 2 % )

− 1 t o 2

D a r k - r e d

W P - J L - C

A r m . L a p i l l i ( 1 0 % ) − 1 t o 1

X

S 3

3

L a m i n a

E N E

3 . 6

1 0 3

0 . 7 c m

R e d


( 1 0 0 – 5 %

)

− 2 t o 1

R e d - o r a n g e

W P - J L - C


( 5 0 – 1 0 %

)

− 1 t o 0 . 5

I r r e g u l a r ( 2 0 % )

− 0 . 5 t o 3 . 5

S 3

3

L a m i n a

E N E

3 . 7

8 3

0 . 5 c m

R e d

A r m . L a p i l l i ( 1 0 % ) − 1 . 5 t o 1

X

W P - J L - C


( 1 0 – 5 0 %

)

− 1 . 5 t o 2 . 5

S 3

3

M a s s i v e

N E

4

1 0 5

2 c m

R e d

A r m . L a p i l l i ( 5 % )

0 . 5 t o 1

R e d - o r a n g e

W P - J L - C



182

T a b l e 1

C o n t i n u e d .

U n i t

H o r i z o n

F a c i e s

A z i m u t h



S e c t i o n


d e p o s i t

C o l o r



m m )

F i l m c o v e r

O t h e r

c o m p o n e n t s

I r r e g u l a r ( 5 0 – 5 % )

− 1 t o 2 . 5

S 3

3

M a s s i v e

( p . d . s . )

S o u t h

2 . 3

1 0 0

4 c m

B r o w n

A c c . L a p i l l i ( 1 5 % )

0 . 5

X

W P - J L - C

A r m o r e d l a p i l l i

( ≤ 5 % )

0 . 5

S 3

3

M a s s i v e

S o u t h

3 . 5

7 1

0 . 5 c m

O r a n g e

X

W P - J L - C

S 3

4

L a m i n a r

E a s t

2

1 b

6 c m

D a r k - g r e e n

A r m . L a p i l l i ( 5 % )

1

O r a n g e

W P - J L - C

I r r e g u l a r ( 1 0 – 5 % )

− 1 . 5 t o 2

S 3

4

M a s s i v e

( p . d . s . )

E a s t

3 . 2

3

5 c m

G r e e n


( 3 0 % )

− 3

R e d

W P - J L - C

C y l i n d r i c a

l

( 2 – 1 0 % )

− 3 , - 2 . 5 t o – 1 . 5

O r a n g e

A r m . L a p i l l i ( 5 % )

− 2 . 5 t o – 2

I r r e g u l a r ( < 8 % )

− 1 t o 2 . 5

S 3

4

M a s s i v e

E a s t

3 . 2 + 5 m

4

3 c m

G r e e n

I r r e g u l a r ( 5 0 % )

− 2 t o 2 . 5

O r a n g e

W P - J L - C


( 5 0 % )

− 1 . 5 t o 0

A r m . L a p i l l i ( < 3 % ) − 1 . 5 t o 2

S 3

4

M a s s i v e

( p . d . s . )

E a s t

3 . 7

3 b

4 c m

G r e e n

I r r e g u l a r ( < 8 % )

0 t o 2 . 5

O r a n g e

W P - J L - C

A r m . L a p i l l i ( < 5 % )

S 3

4

L a m i n a

E N E

3 . 6

1 0 3

0 . 7 c m

G r e e n

I r r e g u l a r ( 7 0 % )

− 1 . 5 t o 2 . 5

O r a n g e

W P - J L - C

A c c r . L a p i l l i ( 1 0 % ) − 1 . 5 t o 0 . 5


( 1 0 – 1 5 %

)

− 1 . 5 t o 1

S 3

4

L a m i n a

E N E

3 . 7

8 3

0 . 5 c m

G r e e n

A c c r . L a p i l l i ( 5 % )

− 1 . 5 t o 2

X

W P - J L - C


( 1 0 – 1 5 %

)

− 1 . 5 t o 1 . 5

S 3

4

M a s s i v e

S o u t h

2 . 3

1 0 0

2 c m

G r e e n

X

W P - J L - C

S 3

4

M a s s i v e

S o u t h

3 . 5

7 1

3 c m

G r e e n

X

W P - J L - C

S 3

5

L a m i n a r

E a s t

2

1 b

5 c m

Y e l l o w

A r m . L a p i l l i ( 1 0 % ) 0 t o 1

X

W P - C

S 3

5

M a s s i v e

E a s t

3 . 2

3

3 c m

Y e l l o w

A r m . L a p i l l i ( 5 % )

− 0 . 5 t o 1

X

W P - C

S 3

5

L a m i n a r

E a s t

3 . 7

3 b

3 c m

Y e l l o w

X

X

X

W P - C

S 3

5

L a m i n a

S o u t h

2 . 3

1 0 0

0 . 5 c m

Y e l l o w


( < 1 0 % )

0 . 5

X

W P - C

A c c . L a p i l l i ( < 5 % ) 0 . 5

S 3

5

M a s s i v e

( p . d . s . )

S o u t h

3 . 5

7 1

6 c m

Y e l l o w

X

W P - C



183

T a b l e 1

C o n t i n u e d .

U n i t

H o r i z o n

F a c i e s

A z i m u t h



S e c t i o n


d e p o s i t

C o l o r



m m )

F i l m c o v e r

O t h e r

c o m p o n e n t s

S 3

8

M a s s i v e

E a s t

3 . 2

3

2 c m

G r e e n

A r m . L a p i l l i ( 2 5 % ) − 1 . 5 t o 0 . 5

R e d - o r a n g e

W P - J L - C

C y l i n d r i c a

l ( 5 % )

1

I r r e g u l a r ( 1 0 – 2 0 % ) − 1

S 3

9

L a m i n a r

E a s t

3 . 2

3

2 c m

Y e l l o w

I r r e g u l a r ( 1 7 – 3 6 % ) − 0 . 5 t o 1

X

W P - J L - C

S 3

1 0

M a s s i v e

E a s t

2

1 b

3 0 c m

P i n k


( 1 0 – 8 % )

0 . 5 t o 2 . 5

X

W P - J L - C

S 3

1 0

M a s s i v e

E a s t

3 . 5

3

7 c m

P i n k

A r m . L a p i l l i ( < 4 % ) 0 . 5 t o 1

X

W P - J L - C


( < 5 % )

1

associated with other types of ash aggregates. At this loca-tion it has a maximum dimension of 9 mm. It is irregular in shape (Fig. 5A) and consists mainly of crystals, juvenilelithics, and mm-sized white rounded pumice immersed ina fine grained matrix of the same composition. Some smallarmored lapilli also occur as accreted particles (Fig. 6A).

A dark-red to orange film cements the accreted particlesin many S1 horizons to the E-SE, to the N, and to the W

at variable distances from the crater (Table 1); in Upper IU horizons to the ESE and ENE; in S3-3, S3-4 and S3-8 horizons between 2 and 4.7 km E and NE. Betweenthe crater and the somma walls (IU-2, IU-4, IU-8a; S3-3,Table 1) this type of aggregate consists of crystals <200µm in maximum diameter (in one case they reach 1 mm)set in a matrix of glass shards arranged in a closely-packedstructure.

Internally, these aggregates donotshow anydefinite grainsize variation from the interior to the margins. The size of the accreted fragments is extremely variable. In the pres-ence of the red film white pumice up to 3 mm in size areaccreted (Fig. 5A), although a great percentage of parti-

cles <40 µm (50–60 wt%) can also be present (Fig. 6B).Grain-sizes finer than 16 µm (50–85 wt%) predominate inash aggregates free of a red-orange film, in this case ac-creted particles have a maximum grain-size between 100and 300 µm (Fig. 6C). The red-orange film limited thedetermination of the composition of the particles and therecognition of their surface features. When the red film ismissing the main components are crystals of plagioclase,hornblende, and augite, and angular glass fragments some-times with stepped fractures (Heiken and Wholetz 1985;Buttner et al. 1999).

Type B: cylindrical aggregate

This type of aggregate has not been described previouslyin the literature. Its occurrence is limited to four horizons:S1-0 at 3.5 km N; S1-1 at 8.5 km SSW, between 5.8 and7.5 km SSE, and at 5.1 km E; S3-4 and S3-8 both at 3.2 kmE of the crater.

Generally, these aggregates have a hollow cylindricalshape with a nearly constant central void diameter of 1 mm(±0.2), surrounded by concentric layers of crystals, andglass fragments (tens to hundreds microns in dimension),and mm-sized pumice; a thin red film coats their exte-rior (Fig. 7A, B). They have an almost uniform overall

diameter (4–6 mm), and length (8–10 mm), although ag-gregates up to 12 mm occur in horizons S3-4 at 3.2 kmE, and S1-1 at 8.5 km SSW. The presence of mm-sizedscorched and non-carbonized leaves observed floating inthe water during the sample cleaning process, suggests thatthis organic material might represent the original core of cylindrical aggregates. This hypothesis was supported in afew cases where mm-sized organic fragments, consisting of non-charred, scorched or completely carbonized leaves or stem fragments, were found inside some aggregates in S1-1(Figs. 7B, 8A) and in S1-0 at 3.5 km N. Layers representingthe first stages of the accretion process were observed in



184

AFig. 6 A Detail of an irregular ash aggregate from S1-1horizon at 5.1 km East. Arrowpoints to an armored lapilliaround a juvenile lithicfragment, inside the aggregate.Scale in millimeters. B SEMimage of an irregular ashaggregate from S1-5 at 3.5 kmto the East of the crater. Notice

the abundance of fine materialin which coarser clasts areembedded. A micrometric-sizedfilm homogeneously covers theaggregate. C Detail of anirregular ash aggregate fromS1-1 at 3.7 km East. Coarser clasts are fragments of plagioclase (Pl) sometimes witha glass cover (Gc), hornblende(Hbl), and glass shards (Gs)with elongate vesicles

A B

C

Fig. 7 A View along the axisof a cylindrical aggregate foundin S3–4 horizon at section 3,3.2 km East of the crater. Theblack bar for scale is 5 mmlong. Note the alteration of theexternal (darker ) portion. BView along the axis of acylindrical ash aggregate fromS1-1 horizon found at section20, 5.5 km E of the crater. Theblack bar for scale is 1 mmlong. The white arrow points toa leaf inside the aggregate. CTransversal view of acylindrical aggregate from S1-1

horizon at section 56, 7.5 kmSSE of the crater. Scale at thebottom in millimeters

few cases (section 20 and 52) between -0.5 and 0 . Whenthe nucleus consists of a carbonized leaf, the internal sec-tion is asymmetric and varies along its main axis between2 (Fig. 7B) and 1.5 mm (±0.001) (Fig. 8A, B).

The specific identity of the organic material inside theaggregates was difficult to determine because of its smallsize and rare occurrence. Therefore, five different typesof leaves (chichonal - after which the volcano is named,quelele, guarumbo, cacate, and palm) as well as wood frag-

ments from the present vegetation, corresponding to thevegetation that existed prior to the eruption were sampled.All of the leaves are longer than 15 cm, but the queleleleaves vary from 2 to 6 cm. They were heated in a fur-nace to determine their approximate charring temperature(incomplete combustion of the organic material when theleaves have lost their moisture and became dark-brown incolor). The results of these experiments show that leavesof chichonal and guarumbo char at ∼140◦C, the leaves of cacate and palm char at ∼125◦C, and the quelele leaves at∼110◦C.

Several samples of cylindrical aggregate from S1-1, S3-4 and S3-8 were immersed in epoxy resin, sectioned atdifferent angles with respect to the main axis (Figs. 8Band 9A, B), and viewed under the Scanning Electron Mi-croscope. The aggregates contain, in order of abundance,crystals of plagioclase, hornblende, augite, scattered Ti-magnetite (Fig. 8C) and minor vesiculated or dense glassclasts ranging in size from tens to hundreds of microns.Rounded white pumice 0.5–1.5 (±0.01) mm in maximum

diameter are also present (Fig. 8A, B). All larger grainsare immersed in a fine grained matrix of the same com-position. The aggregates have a main size mode between7 and 8 , with a secondary mode between 3 and 4 .The percentage of material finer than 63 µm varies from62 to 85 wt%, most of which is represented by fragments<16 µm (45–61 wt%).

There are small differences in grain-size distributions be-tween the innermost and outermost parts of aggregates inabout one quarter of the samples analyzed. In sections par-allel to the axis of hollow samples (Fig. 9A) the innermost



185

A

phi

Upper right

0

10

20

30

40

50

-2 -1 0 1 2 3 4 5 6 7 8

% w e

i g h t

phi

Upper left

0

10

20

30

40

50

-2 -1 0 1 2 3 4 5 6 7 8

% w e

i g h t

B

C

60 m 500X µ

phi

Bottom left

0

10

20

30

40

50

-2 -1 0 1 2 3 4 5 6 7 8

% w e

i g h t

Fig. 8 A Transversalview of ashaggregate from S1-1 horizon (sam-ple 199) in Fig. 7B. The black bar is 5 mm long. White arrow pointsto a carbonized leaf inside the aggregate. B Backscattered imagesof ash aggregate of Fig. 8A. White arrows point to bubbles insidethe matrix. White ellipse on right bottom indicates a broken augite

filled with matrix. The white square box indicates a detail shown inFig. 8C. The granulometric distributions of different parts of the ag-gregate are also shown by dotted white arrows. C Detail of quenchedtitanomagnetite in Fig. 8A

part is coarser grained, with the main mode occurring at2 and a secondary mode at 5 . In contrast, the mainbody of the aggregatecontains 81–83 wt%of particles finer than 4, with the principal mode at 6–7 and a secondarymode at 2–3 . All samples exhibit scattered rounded tosub-rounded bubbles, 0.07–0.44 (±0.01) mm in diameter (Figs. 8B and 9A, B), inside the body of the aggregates.The zone with bubbles contains many fractured (Fig. 8C)or completely broken crystals (Fig. 9C, D). A fine-grainedmatrix infills the cracks of the broken crystals. A smallamount of deformation causes displacements to open frac-

tures as much as 20 µm (Fig. 9C). The original shape of the fractured and slightly deformed crystals is preserved.

The outermost part of the aggregates (Fig. 9B), witha maximum thickness ranging from 0.153 mm (Fig. 9A)to 0.200 (±0.01) mm, is characterized by a high-densitycontrast with respect to the other zones (Fig. 9A, B).However, there is no granulometric difference with respectto the body of the aggregate. Particles finer than 4

are abundant (85–89 wt%), mostly represented by sizes<16 µm (74–79 wt%). Compositional X-ray mapping of Fe, K, P, S (Figs. 9E–F), indicates that the outermost layer



187

Pl

C D

F

K 1mm

1mmS 1 mmP 1mmFe

Fig. 9 Continued.



188

is characterized by a high Fe-content, and in some cases(Fig. 9E) by a high S-content.

Type C: armored lapilli

Armored lapilli are typically associated with deposits fromturbulent pyroclastic surge clouds where fine and sticky

particles adhere onto coarser fragments of different na-ture (Waters and Fisher 1971; Lorenz 1974; Fisher andSchmincke 1984; Schumacher and Schmincke 1995). Ar-mored lapilli in wet surge deposits at El Chichon consist of crystals and glass fragments (a few to hundreds of micronsin size) packed around a juvenile fragment (Fig. 5C). Ag-gregates with accidental lithic cores, only occur in S1-0 at5.7 kmS, and 3.5 kmN, and inS1-1 atdistances<4.5 kmE.

Armored lapilli range in size from 1 to 9 mm in diame-ter and most form around sub-rounded to rounded mm-sizewhitepumice.Less commonly they formaround angular ju-venile lithics or crystals of hornblende or plagioclase (oneto a few millimeters in length). Smaller armored lapilli

(1 mm to 250 µm) only occur in S3-5, and they alwaysconsist of few tens to a hundred-microns glass shards sur-rounding fragments of plagioclase.

Backscattered images of red-cemented armored lapillithat coat rounded pumice (Fig. 10A) show a loosely packedstructure, lacking any granulometric sorting from core torim (Fig. 10B). Accreted particles have a unimodal grain-size distribution with a mode at 3 . The particle shape isalways angular to subangular. A variable degree of fractur-ing characterizes coarser (0.5–1 mm) crystals. Few smallparticles occur between larger grains as also demonstratedby the low percentage of clasts in the fractions <63 µm(10–11.5 wt%).

The particles are cemented by a few microns thick filmsurrounding different shaped bubbles that are widely dis-persed throughout the aggregate (Figs. 10C, D). In detail,low (dark portions) and high (white portions) density bandsalternate within the film (Fig. 10E, F). Microbeam analy-ses of these bands indicate that they are composed of highamounts of Fe > S > P with minor amounts of Si, Al,Na, Mg, K, and Ca. Fe is more abundant in the white bandsthan in the darker ones. There is a slight increase in Na,Mg,K, and Ca near the crystals to which they adhere (mainlyplagioclase, pyroxenes or amphiboles). Microprobe analy-sis of this material shows a large margin of error (>3%),suggesting that this phase is strongly hydrated. Qualitative

determinations of variations in the film composition wereobtained by EDS spectra measured along a transversal line(Fig. 10G). Compositional maps for different elements in-side the aggregates (Fig. 10H) only show the relativelylarge contents of Fe and P, indicating a large concentrationof Fe in the external portions.

Type D: accretionary lapilli

Accretionary lapilli are associated with several types of deposits: ashfall (Moore and Peck 1962; Rosi 1992), pyro-clastic flow and co-pyroclastic flow ash fall (Schumacher

and Schmincke 1991, 1995; Bonadonna et al. 2002), pyro-clastic surge produced by phreatomagmatic activity andco-pyroclastic surge ash fall (Fisher and Waters 1970; Mooreet al. 1966; Schmincke et al. 1973; Sheridan and Wholetz1983a; Schumacher and Schmincke 1991, 1995; Ritchieet al. 2002). Two types of accretionary lapilli occur in sev-eral horizons of S1, IU and S3 units.

The first type of accretionary lapilli (Fig. 11A), consists

of concentric layers of glass shards and crystals (rim typeof Schumacher and Schmincke 1991) cemented by a redoxidized film and exists only on the eastern flanks of thevolcano in horizons S1–6, IU-8a, S3-3, and S3-4 (Table 1).It generally ranges in diameter from 1.6 to 6 mm althoughlarger ones (up to 11 mm) occur in 3–5 cm thick beds of S3-3 and S3-4 horizons (Table 1).

The internal structure of samples from S3-3 and S3-4show the presence of different accreted zones. Grain-sizeanalyses of the inner portions of these aggregates (bot-tom of Fig. 11B) show a bimodal distribution with a mainmode between 7 and 8 and a secondary mode between3 and 4 . Particles <63 µm (69–72 wt%), mostly due to

fragments finer than 16 µm (65–69 wt%), are abundant.This fine material engulfs larger crystals of plagioclaseand hornblende, some with glass coatings, and irregular to subrounded glass fragments. Rounded, subrounded, or elongated bubbles with maximum diameters ranging froma few microns to 0.7 (±0.02) mm are widely dispersedinside the inner portion. A few broken clasts are partiallyfilled with a fine-grained matrix (Fig. 11C).

The external portions (upper part of Fig. 11B) consistof coarser particles with a unimodal grain-size distribu-tion at 3 , with minor particles <63 µm (20–30 wt%).Angular to subangular crystals (mainly plagioclase, horn-blende, secondary augite, and scattered Ti-magnetite), and

glass fragments are in close contact with each other. Nogranulometric or density distinction exists. The apparentlyloosely packed external part is due to the presence of irreg-ular void spaces. Broken clasts (Fig. 11D) are only presentinside this zone of the aggregate. The outermost layer isdarker than the inner part, and has an irregular rim. Itsdarker color is due to the high abundance of particles finer than 63 µm (73–75 wt%), most of which are finer than16 µm (52–54 wt%). It contains scattered coarser (up to140µm) angular and sub-angular crystals and sub-roundedglass fragments. This part has a bimodal distribution witha main mode at 3 and a secondary mode at 7 .

The second type of accretionary lapilli (Fig. 5D) is gen-

erally gray in color, with a pale orange film covering itssurface. It coexists with other types of ash aggregates, near the distal edge of horizons in several directions aroundthe volcano (Table 1). This type of accretionary lapilli hasdiameters varying from 250 µm to 6 mm, the size being di-rectly related to the thickness of the deposit. In other words,thegreater the thickness the larger theaggregates. Themax-imum lapilli diameter of 11.2 mm occurs inside the S1-1horizon (Table 1). This type of accretionary lapilli lacksinternal structure (Fig. 12A). Grain-size analyses displaya bimodal distribution (main mode at 7 and secondarymodeat3). Thepercentage ofgrains<63µm rangesfrom



189

60 to 68 wt% with most of the variation due to the pres-ence of fragments <16 µm (55–61 wt%). Particles up to500 µm (<3 wt%) and 900 µm (<1 wt%) are also present.Coarser clasts have variable shapes, ranging from angu-lar to subangular, subrounded and rounded (upper rightin Fig. 12A). However, the mineral phases are the sameas those of other types of ash aggregates. The main com-

ponents of the coarse fraction are crystals of plagioclase,hornblende and minor augite; Ti-magnetite occurs as either small crystals (up 30 µm) locally with apatite included,or as inclusion in plagioclase (Fig. 12B). Some crystalsare deeply fractured but without any filling by fine-grainedmaterial. Glass appears as non-vesiculated rounded frag-ments (up to 700 µm) or as a vesiculated layer surrounding

A

E

C D

B

Pumice

0

10

20

30

40

50

-2 -1 0 1 2 3 4 5 6 7 8 9

phi

%

w e i g h t

Pumice

C

E

D

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Fig. 10 A Polished transversal section of an armored lapillus witha pumice core in S1-1 horizon (sample 8018) at 3.7 km from thecrater. B Backscattered image of armored lapillus in Fig. 10A. Thewhite dashed line marks the limit between the core and the aggregateparticles. Note the porous structure and the absence of granulometricselection from the inner to the outer part. Square boxes labeled C, D,E are details of the aggregate shown in subfigures C, D and E. C De-tail of Fig. 10B showing features of the contact between the pumiceand the first accreted particles. White arrows point to bubbles with

different shapes. D Detail of Fig. 10B showing the contact betweenthe accreted particles far from the pumice core. White arrows point tobubbles with different shapes. E Detail of film cementing a crystal of plagioclase (Pl) to the pumice core. F Close up of Fig. 10E showingthe banded nature of the film. White line, AA, shows the profile of EDS analysis in Fig. 10G. G EDS spectra showing the compositionof the cementing film across a line traced from the pumice core tothe cemented plagioclase crystal in Fig. 10F. H Compositional mapof Fe, P, K, and S for the armored lapilli of Fig. 10B



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crystals. It represents a large percentage of the fine-grainedfraction (matrix). Irregular fractures (up to 25 µm wide)cut across the matrix without transecting crystals.

Discussion: formation of ash aggregates

This study reports four different types of ash aggregatesin the wet pyroclastic surges S1, IU, and S3 of El Chichon

volcano; these aggregates are absent in dry surge horizons.Considering that morphology alone is insufficient toinfer the genesis of the aggregates, we investigated other parameters: (a) the composition and morphology of accreted particles, (b) the variation of dimension andgrain-size of ash aggregates with respect to the grain-sizeof the sample, the facies of the deposit in which they occur,and the distance from the eruptive center, (c) the nature



191

Fig. 10 Continued.

and distribution of the red film coating the aggregates, and(d) their internal structure.

Composition and morphology of the accreted particles

The nature of accreted particles depends on the originalgrain-size population inside the eruptive clouds (Sheridan

and Wohletz 1983a; Reimer 1983; Gilbert and Lane 1994).All accreted particles in the four types of ash aggregatesat El Chichon are juvenile material. Accessory lithic frag-ments from the old dome destroyed during the eruption,occur only as aggregation nuclei in the S1-0 horizon, andless frequently in S1-1 in medial zones to the east. Bothhorizons were emplaced during the beginning of eruptivephase III, on April 4. Accreted particles in all types of ashaggregates reflect the type and abundance of componentscharacteristic of the deposits in which they were immersed,except for horizon S1-0 that is rich in accessory lithic frag-ments that were never observed as accreted particles. Theabsence of accreted accessory/accidental lithic fragments

in aggregates from S1-0 and S1-1 samples indicates thatnon-juvenile material was poorly fragmented during theeruption. This observation suggests that aggregate forma-tion was not a mineral selective process, but rather de-pended on clast density. The larger dimensions (>3 ) of the lithic clasts with respect to juvenile fragments causedthem to concentrate in the basal portions of the density cur-rents rather than being uplifted into the upper more diluteportions.

Most accreted particles consist of phenocrysts of pla-gioclase and hornblende, secondary pyroxene (augite), and

scattered Ti-magnetite, some withinclusions of apatite, andsub-millimeter to millimeter-sized white pumice. Thesecomponents are generally embedded in a fine-grained ma-trix (mostly <16 µm) of glass shards and microcrystals.Types and abundance of mineral phases correspond to thatof the fresh pumice (Luhr et al. 1984).

Phenocrysts (tens to hundreds of microns) in all types of aggregates are generally covered with glass. Glass coatings

are common on free crystals occurring in the same or finer fractions of wet pyroclastic surge horizons,but do notoccur on free crystals within dry surge horizons, where crystalsurfaces are clean and glass-free. Sheridan and Wohletz(1983b) and Heiken and Wholetz (1985), explained thisfeature as the result of less energetic fragmentation due toan excess of water in the hydrovolcanic rupturing of themelt.

Phenocrysts and glass with cracked surfaces occur inall types of ash aggregates suggesting that the fracturingprocess was previous to their aggregation.

Fractures on the surface of glass fragments (hydrationcracks) occurred under both natural (Dellino and La Volpe

1995) and experimental conditions (Wohletz 1983; Buttner et al. 1999; Buttner et al. 2002). They are interpreted asthe results of a rapid contraction of a particle surface thatoccur when the already fragmented, but still hot melt entersagain in contact with liquid water. This mechanism couldbe responsible for forming similar cracks that pierce thesurface of glass particles inside the aggregates.

In contrast, phenocrysts represent about 24% of the meltby volume (Luhr et al. 1984). Therefore, theyhad less prob-ability of entering into contact with liquid water at highvelocities, and fracturing due to a thermal shock, during



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Fig. 11 A Transversal section of an accretionary lapilli from S3-3horizon at 3.7 km E. A second process of aggregation is markedby the dotted line. B Backscattered image of transversal section of Fig. 11A. Notice the variation in the grain size between the inner and outer portions of the aggregate. See text for discussion. C and

D are details shown in Fig. 11C, D. C Broken glass fragment insidethe first aggregate near a bubble (center ). Plagioclase (Pl) coveredby glass (Gc) (bottom left ). D Detail of a fractured (upper part ) andbroken (bottom part ) plagioclase in Fig. 11B. The arrows point tofine matrix inside the fracture



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Fig. 11 Continued.

fragmentation. The presence of a vesicular melt coating ob-served both on free crystals and those inside of aggregatesrules out the possibility that their surface fractures werecaused by mutual impacts inside the expanding magma of the venting system as suggested by Fisher (1963).

Best and Christiansen (1997) reported fractured and bro-ken felsic phenocrysts in several cooling units of Tertiaryash flow tuffs in the Great Basin (USA). They pointed outthat despite the role of the relatively large elastic moduli of the host crystal in preventing a significant decompressionof the gas inside melt inclusion, stress within the crystalin the neighborhood of the inclusion may grow to exceedits tensile strength, causing the crystal to crack as the sur-

rounding magma decompresses. Once the crystal cracks,the melt within the inclusion decompresses, volatiles ex-olve, bubbles nucleate and the crystal eventually breaksapart. Crystals containing many inclusions separated byrelatively thin walls are especially prone to breakage, be-cause the cracking threshold decreases with an increase of inclusion size. Inclusions of pink to brown glass up to 80µm in diameter are frequent in phenocrysts of plagioclase,augite,hornblende and Ti-magnetitewithin fresh pumiceof fallouts deposits at El Chichon.They occur either randomlyscattered throughout the crystal cores, or more commonlyarranged in bands parallel to growth surfaces (Luhr et al.

1984). The vesiculation of melt entrapped in the inclusionsat high pressure, and rapidly decompressed during magmaascent is the most likelymechanism to explain the fracturesobserved in crystals.

Grain size and dimensions of ash aggregates

In this work we were not able to statistically evaluatethe variations between the types of ash aggregates, thehost horizons, and the distance from the crater, as it wasdone elsewhere (Schumacher and Schmincke 1991, 1995;Ritchie et al. 2002; Bonadonna et al. 2002). The reasonsfor this include: (1) the lack of proximal exposures of theS1 unit, which shows the widest dispersion, (2) the burialor erosion of most of the S3 horizons, and (3) the limiteddistribution of the IU unit.

Accretionary lapilli are visible at outcrop scale in onlya few horizons (IU-8b at 0.6 km to the SE, S3-3 andS3-4 at 3.2 km to the E). No other types of ash ag-gregates were directly observed in the field. Most ash

aggregates became noticeable during hand-sieving, al-though they were covered with fine ash until they werecleaned.

The only horizon that can be traced almost continuouslyfrom proximal (0.6 km) to distal zones (4.7 km) on theeastern flank of the volcano is S3-3. Therefore, it can beused as a reference horizon to evaluate lateral variationsof ash aggregates with distance from the crater and withthickness of the deposits. The red color of S3-3 is due toits highly oxidized character. Only close to the crater it islight brown in color.

Between the crater and the somma walls, three types of ash aggregates coexist within S3-3. These are in order of

abundance: armored lapilli, accretionary lapilli, and irregu-lar ash aggregates (Table 1). The size of the armored lapilli(8 mm) depends on the diameter of its core (5–6 mm).The accreted material (2 mm in thickness) is made of glassand crystals <30 µm (80–90 wt%). Irregular aggregatesconsist mainly of glass fragments and crystals <90 µm(70 wt%) and between 64 and 30 µm (10–20 wt%). Ac-cretionary lapilli are structureless and contain mainly glassfragments (64–30 µm) that reach up to 80 wt% of the total.The larger ratio of core/accreted zone in armored lapilliresponsible for their larger dimension in proximal zonescompared to armored lapilli (3 mm) at further distances,was already noted by Schumacher and Schmincke (1995)

for accretionary lapilli (core type) in pyroclastic surge de-posits of Laacher See volcano. They concluded that closeto the source where a minimum of condensation occurs,water condenses preferentially on small particles resultingin liquid films capable of binding ash particles <350 µm.This mechanism can explain the smaller size of the ac-creted particles around coarse nuclei in the armored lapilliof S3-3 near the vent. In these locations a red film does notcement the accreted particles, and crystals and glass frag-ments are arranged in a densely packed structure aroundthe core. No aggregates exist in S3-3 up to a distance of 2 km, but accidental lithics fragments from the old dome



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Fig. 12 A Backscattered image of accretionary lapilli of Fig. 5D.Fractures are distributed almost uniformly inside the aggregate, af-fecting only the fine-grained matrix, but not larger crystals. The

granulometric distribution of the aggregate is shown in the bottompart of the photograph. B Detail showing Ti-magnetite inclusionsinside a plagioclase crystal in an accretionary lapilli

are present. Irregular ash aggregates, armored lapilli, andaccretionary lapilli (with a concentric structure) occur invarious proportions between 2 and 3.5 km E from the vent.This difference in distribution is probably due to the factthat flows were not turbulent enough for aggregates to beformed or they were accelerating down the flanks of thecone and caused the abrasion of the aggregates.

There is not a clear decrease or increase in the dimen-sion of the ash aggregates with either increasing distancefrom the crater or thickness of the different horizons con-trasting with previous studies (Lorenz 1974; Schumacher and Schmincke 1991, 1995; Sisson 1995; Ritchie et al.2002; Bonadonna et al. 2002). An exception is repre-sented by the gray slightly cemented, structureless, accre-tionary lapilli that appear at the outer margin of these hori-zons in different directions (Table 1). Only in this case,the maximum dimension of the aggregates increases withincreasing distance from the crater and thickness of thedeposits.

Most aggregates have a bimodal size distribution withcoarse modes between 2 and 4 , and fine modes between6 and 8 . The coarse mode in the interval 250–63 µm re-flects thegranulometricdistribution of most wethorizons of pyroclastic surge units S1, IU and S3, where a pronouncedmode occurr in the same size range (Figs. 4A–E). In some

S1 horizons and in many S3 horizons finer modes between8 and 4 µm, also occur. Thus, the grain-size distribution of the ash aggregates reflects the grain-size distribution of thehost deposits and is independent of their maximum dimen-sion as observed by other investigators (Bonadonna et al.2002; James et al. 2003).

Occurrence and nature of the red-orange film

The occurrence of a red-orange film coating the four typesofaggregatesat El Chichon variesdependingon thehorizonconsidered, the distance from the vent, and the azimuth(Table 1). Ash aggregates formed during the beginningof phases III and IV (S1-0 and S3-3 respectively) consistmainly of armored lapilli and irregular aggregates. Theseclusters are characterized by the presence of a dark-red filmalong their main dispersal axis (N-S for S1-0 and E-SE for S3-3).

The orange-red film occurs discontinuously on all typesof aggregates in S1-1, the most widely dispersed horizon,to the E-SE, is scarce to the W, and occurs as far as 5.9 kmto the N (Table 1). Other horizons showa variable degree of red-orange covering (S1-2, S1-5, S1-6), or are free of anysurface coating (S3-5, S3-9, S3-10). Ash aggregates to the



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South (S3-3,S3-4, S3-5) are freeof surface coating. Amongthehorizons of IU deposited during phase III, characterizedby a distribution limited to the first 1 km, an orange-red filmis only present in the last-emplaced wet horizons (IU-8A,8B and 10).

EDS and EPMA analyses performed on the cementingfilm of irregular, cylindrical, and accretionary lapilli re-vealed that it consists of abundant Fe with minor P and S,

while the composition of the film on armored lapilli sur-rounding a pumice core, show unusual enrichments in Sand P, besides the high-Fe content, with variable amountsof Na, Mg, K, and Ca. Although the composition of thisfilm might appear anomalous, it is not unexpected, sincelarge amounts of sulfur (2.2×1013 g) were emitted duringthe 1982 eruption (Luhr and Logan 2002). Part of this sul-fur was released as an oxidized vapor phase in the eruptiveclouds (Varekamp et al. 1984; Luhret al. 1984). In addition,Luhr etal. (1984) pointed out that the El Chichon trachyan-desite melt was not only extremely rich in SO3 (1.24 wt%),but also rich in P2O5. The unusual high values of 34δ (5.8‰) of the bulk magma were explained by a loss of H2S

present either as gas or a fluid phase at depth. This is con-sidered consistent with the development of a small high-Thydrothermal system beneath El Chichon(Rye et al. 1984).

Clusters of particles with different sizes produced poly-modal distributions in fallout deposits, and leachate anal-yses on fresh ash indicated high concentrations of SO4,Cl, and F (Varekamp et al. 1984). In addition, samples of stratospheric clouds taken in late May-July 1982 (MacK-innon et al. 1984) showed that a significant portion of theclouds contained angular fragments (2–40 µm), many of which belong to larger irregular clusters (10–50 µm). SEManalyses of these clusters indicated that they consist mainlyof glass shards, with minor plagioclase, Ca-pyroxene, Ca-

sulfate, K-feldspar and Fe-Ti oxides. These particles werecoated by sulfuric acid droplets, and larger (0.5–1 µm) sul-fategel droplets with various amounts of Na, Mg, K, CaandFe, depending on the mineral phase to which they adhered.The sulfate gel droplets were more abundant on samplescollected in May with respect to those taken in July, sug-gesting that the formation of this gel was related to theinitial conditions of the eruptive clouds. MacKinnon et al.(1984) proposed the formation of the sulfate gel dropletsas a result of a reaction between the solid phase and a largeconcentration of SO2 plus water vapor at relatively hightemperature. Sulfuric acid aerosols, instead of a gel phase,would form either by the dilution and progressive cooling

of the plume or alternatively in particular regions of theclouds due to its heterogeneity (MacKinnon et al. 1984).Luhr et al. (1984) indicated that the mafic minerals at

El Chichon were unusual for their high FeO content. Thehigh Fe content of the amorphous film in different typesof aggregates (Figs. 9E, F, 10E–G), suggests that at leastpart of this element was already dissolved in the fluid phasethat cemented the aggregates. The high iron-concentrationon surfaces of different types of ash aggregates was likelyenhanced during post-depositional alteration.

Bigham et al. (1996) described a poorly crystalline andmetastable phase that represents a common Fe-precipitate

from acid sulfate waters in a pHrangebetween 2 and 4.Thisschwertmannite, ideal formula is Fe8O8(OH)6SO4*nH2O,is yellow in color and usually occurs in mixtures with oth-ers minerals that range from amorphous (ferrihydrite) towell crystalline (jarosite). The Fe/S ratio in schwertman-nite ranges from 8 to 4.6 (Bigham et al. 1996), and itmay be partly or fully substituted by anions such as arse-nate, nitrate and phosphate. Bigham et al. (1996) indicates

the existence of a paragenetic relationship between schw-ertmannite and associated minerals (ferrihydrite, goethite,

jarosite) over a wide range of pH (2.8–6.5); changes inpH and solution, concentrations of Fe, and SO4 controlthe gradual hydrolysis of schwertmannite and its conver-sion into goethite. Solubility and stability relations for Fe-hydroxysulfate minerals over a reasonable range of sulfuricacid concentrations indicate their formation at a tempera-ture around 60◦C (Merwin and Posnjac 1937; Bigham et al.1996; Bigham and Nordstrom 2000). Fe-rich precipitateshavealso been recognized in hydrothermalsubmarinevents(Murray 1979). Studies on hydrothermal fluids in seawater at shallow depths (Pichler and Veizer 1999; Savelli et al.

1999) indicated thepresence of amorphous or slightly crys-tallized phases (ferrihydrite and proto-ferrihydrite respec-tively). Direct precipitation from solution can occur either by slow hydrolysis of Fe3+ or due to oxidation of a Fe2+

bearing solution (Murray 1979). Pichler and Veizer (1999)report that an increase in Eh and pH and a decrease intemperature at the contact of cold, oxygenated, alkalineseawater and hydrothermal fluids, controls the oxidation of Fe2+ and causes a rapid (1 cm/year) precipitation ofan Fe3+

amorphous phase over a range of temperatures between 60and 93◦C.

The particular composition of the film binding ash ag-gregates in wet surge horizons at El Chichon may be thus

attributed to different amounts of interplay between groundwater, magmatic and hydrothermal fluids during varioushydromagmatic events. In fact, a variable degree of al-teration, strongly related to the color of wet pyroclasticsurge deposits, was observed at La Fossa di Vulcano, Italy(Dellino et al. 1990; Capaccioni and Coniglio 1995). Thedifferences in color wereattributed to syn-depositional pro-cesses related to the reactivity of acid condensed solutionson fresh highly fragmented pyroclasts. The first stage of alteration consisted in the precipitation of Fe-Al hydrox-ides followed by a regular increase in pH, due to cationsexchange between acidic water solutions and silicate glass.At the last stage authigenic smectites with a different de-

gree of crystallinity were produced. A comparison betweenred and gray samples revealed that a higher degree of al-teration (supported by a high C.I. of smectites) occurredon red samples. Therefore, the occurrence and abundanceof the red film of ash aggregates at El Chichon can be re-lated to the amount of fluids that were present in the erup-tive clouds or in portions of them, during specific eruptiveevents. The greater variety of cemented aggregates in me-dial and distal zones of the volcano reflects the enhancedbinding action of hygroscopic acid fluids (mainly H2SO4)because of increase in relative humidity due to a higher degree of condensation of water vapor.



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Internal Structure

The internal structure of the aggregates provides a key toreconstruct the accretion process. SEM analyses of the ag-gregates revealed that armored lapilli and irregular aggre-gates are structureless, while cylindrical aggregates showdistinctive accreted layers. Accretionary lapilli can be ei-ther structureless or composed of different accreted layers.

These variations are related to the specific aggregation pro-cess.

The absence of granulometric and/or density sorting in-side armored lapilli and irregular ash aggregates, suggeststhat they formed almost instantaneously, but along differentpaths.

While the shape of the accreted particles provides evi-dence of the main fragmentation mechanism (magmatic or hydromagmatic, Wohletz 1983; Heiken and Wholetz 1985;Buttner et al. 2002), the relationship between the modifica-tion structures of particles’ shape or surface features mayfurnish some clues to the post-fragmentation processes.In the irregular ash aggregates, most of the particles are

blocky. In some cases, stepped structures were observedon glass particles. This feature as described in both natu-ral and experimental situations (Heiken and Wholetz 1985;Buttner et al. 1999; Dellino et al. 2001; Buttner et al. 2002)was interpreted as due to the brittle fragmentation of themelt that occursduringhighlyenergeticphasesof explosivemagma-water interaction.

Pyroclastic surge deposits at El Chichon were producedby an alternation of multiple highly energetic explosions(responsible for the deposition of dry horizons) and lessenergetic pulsating explosions (responsible for the deposi-tion of wet horizons), as a consequence of different pro-portions of water entering in contact with magma. Thus

particles formedduringhighlyenergeticepisodes (dryhori-zons) were available and could have been likely incorpo-rated in ash aggregates formed during less energetic erup-tive pulses (wet horizons). Irregular aggregates consistingof particles ranging in size between 350 µm and 2 mm,have been obtained in experimental studies (Schumacher and Schmincke 1995) for water contents greater than 50%in volume. The intensity of spraying and the size of wa-ter droplets represented an important factor in controllingthe size of accreted particles. Coarser particles (1–2 mm)were mainly accreted when the water was quickly sprayedwithout intervals, in larger droplets. A spontaneous andrapid aggregation (clotting) of particles of different sizes

and densities, due to an excess of the liquid phase insidethe eruptive clouds, can be thus envisaged as the maincause of accretion in irregular ash aggregates. Consideringthat hygroscopic compounds (such as sulfuric acid) inducecondensation in unsaturated air, particles inside pyroclas-tic clouds were likely already coated by layers of a liquidphase at lower humidity (Gilbert and Lane 1994) with re-spect to those suggested by Schumacher and Schmincke(1995). The size of acid liquid droplets increased due to anincrease in the relative humidity inside the clouds, whenfurther condensation of water vapor occurred (at greater distances from the vent, see above). Thicker liquid layers

around particles determined the accretion of bigger clastsat greater distances.

An accretion process due to the electrostatic attractionbetween charged particles in a fluid free phase medium(James et al. 2002) is excluded, because of the large dimen-sions of the aggregates found in the deposits (up to 10 mm),with respect to those obtained in experiments (maximum800 µm). The large dimensions of this type of ash aggre-

gate (up to 8 mm) at El Chichon, with respect to thoseobtained experimentally, is not surprising considering thatthey occur in medial distal exposures of S3-3, in conditionsof a large concentration of acid fluids, and that irregular clusters up to 50 µm were still present in the stratospherea few months after the eruption (MacKinnon et al. 1984).Thepresence of mm-sized armored lapilli found as accretedparticles in some irregular aggregates, suggests that accre-tion processes occurred inside the eruptive flow, at differentscales, for different liquid contents, and at different times,prior to the final aggregation process. Scattered secondarymineral phases as Ca-sulphate were observed in irregular aggregates without a red film. This is not surprising consid-

ering the presence of primary anhydrite crystals in pumicesof this eruption (Luhr et al. 1984), which can explain thebinding forces needed between particles in the absence of ared film, as suggested elsewhere (Gilbert and Lane 1994).

However, Zimbelman et al. (2005) recently pointed outthat Al-Fe hydroxysulfate minerals, as well as gypsumand anhydrite can be subjected to many cycles of solutionand re-deposition in low pH environments at the surfaceof active volcanoes, as the minerals go through superfi-cial wet and dry periods related to seasonal climate andto variations in the rate of degassing. Soluble Fe-sulfateminerals may precipitate directly from acid surface wa-ters at the interface between saturated and unsaturated

zones (BighamandNordstrom2000) where theevaporationtends to an accumulation of dissolved species. This con-sidered both Ca-sulfates and Fe-hydroxysulfates found inash aggregates could have resulted from post-depositionalprocesses. These processes, however, do not explain thedifferent degree of alteration observed in closely spacedhorizons, with a similar degree of permeability, as well asthe occurrence of these phases being limited to wet surgedeposits.

The most common type of armored lapilli surroundswhite pumice cores. Angular and subangular particles areheld in contact through a micrometric-thick film that sur-rounds bubbles with different shapes. The presence of bub-

bles widely dispersed within the aggregate, suggests aninstantaneous vaporization of a liquid phase, and a rapidaccretion of particles. The aggregation in armored lapilliwas therefore “triggered” by the movement of coarser hotfragments (pumice) uplifted into portions of the cloudsrich in condensing water vapor where particles of differ-ent types, already coated with thick layers of acid dropletswere dispersed. This contact stimulated the instantaneousvaporization of water, and a contemporaneous drop in lo-cal pH conditions which determined the precipitation of an amorphous phase rich in Fe>S>P which “froze” thebubbles guaranteeing their preservation. This liquid phase



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covering the particles likely results from a combination of hydrothermal and magmatic fluids (see above).

In contrast, the concentric zones observed in cylindri-cal aggregates and some types of accretionary lapilli sug-gest that accretion of particles occurred in several stagesduring turbulent transport in the eruptive flows, or in dif-ferent regions of a plume that contains diverse grain-size populations as suggested elsewhere (Gilbert and Lane

1994).Concentric accretionary lapilli (rim type lapilli of Schu-

macherand Schmincke 1991) mainlyoccur in medial zones(3.2–4.5 km) on the E-SE slopes of the volcano (Table 1).Their finer-grained inner portion suggests that aggregationstarted in a region of the surge cloud rich in fine frag-ments (matrix) where coarser ones randomly fluctuated.The presence of vesicles inside the matrix indicates thatcondensation of water vapor took place during accretionas suggested by Sheridan and Wohletz (1983a). A post-depositional re-vaporization of binding fluids inside a hotdeposit (Schumacher and Schmincke 1991) did not likelyoccur considering the nature of the deposits that contain

ash aggregates (cohesive and vesiculated with plastic de-formation structures). In some cases, already coherent ac-cretionary lapilli were set in motion again inside the flow,or picked up by subsequent surge flow, as indicated bythe lack of evidence for particles exchange between thefirst-formed accretionary lapilli and the second accretedpart (Fig. 11B). The process of aggregation may have con-tinued in portions of the cloud (horizontally or vertically)richer in coarser particles, since the second accreted por-tion lacks fine-grained material. The external rim, similar to the outer portion of the first accretionary lapilli, suggeststhat the aggregates moved in portions of the eruptive cloudwhere finer particles were abundant.

Aggregates with spherical shapes, consisting of particlesranging in size from 350 µm to 2 mm, formed in experi-ments with liquid contents between 30 and 50% in volume(Schumacher and Schmincke 1995), therefore it is reason-able to assume that similar or lower concentrations of aliquid phase were present during the formation of this typeof ash aggregate at El Chichon.

Concentric zones inside cylindrical ash aggregates, sug-gest a similar mechanism of formation to that of rim typeaccretionary lapilli. However, several differences exist be-tween the two types of aggregates such as: their shape, their distribution around the vent and their abundance inside sin-gle horizons.

Cylindrical aggregates have a similar overall diameter (3–6 mm) and length (8–12 mm). Millimeter-sized organicfragments represent the nucleus around which the accre-tion began. The shape of the core influenced the overallmorphology of these aggregates. The accretion occurreduniformly around small rigid wood fragments, determin-ing a well-developed cylindrical form with a uniform voidsection 0.9–1 mm in diameter (Figs. 7A, 9A) that waspreserved even after the fragment was removed. Insteadthe accretion took place asymmetrically around irregular shaped and softer leaves, determining a roughly cylindricalshape with an elongated void section which varies up to

500µm (±0.02) in diameter along the main axis (Figs. 7B,8A, B).

The temperature range between 110 and 140◦C at whichdifferent types of organic fragments are charred, is onlyindicative for the conditions existing inside the eruptiveclouds. In fact, considering the erosive capacity of pyro-clastic clouds up to 3.5 km from the crater (Sigurdssonet al. 1984, 1987), thecarbonized material found embedded

in some cylindrical aggregates of the S1-1 horizon couldhave been picked up from the underlying fallout depositsA1-A2. Quelele leaves represent the most common organicmaterial found inside fallouts A1-A2 and considering their small dimensions, they could represent those around whichcylindrical aggregatesbegan their formation.Therefore, thecharring temperature of the quelele leaves (≈110◦C) mightbe considered as a good approximation of the formationtemperatures of the cylindrical aggregates.

Cylindrical aggregates are absent in proximal zones.Their occurrence is limited to four horizons (S1-0, S1-1, S3-4 and S3-8), where they represent only 3–10% of the total components abundance of the sample and lack of

lateral continuity. These evidences indicate that their for-mation was determined by the presence of millimeter-sizedorganic fragments set in motion when the clouds becamebuoyant and highly turbulent in medial-distal zones of thevolcano and was strictly related to the availability of or-ganic debris on the ground, susceptible to remobilizationor disintegration.

Horizontal rolling or saltation near the substrate in thedirection of the flow can be excluded as a dominant accre-tion process. If we consider that during a windy day, largeleaves and other debris are uplifted and remain suspendedin the air for several seconds, it is logical to extrapolate thisprocess to diluted density currents, where sticky particles

dispersed in the cloud adhered around mm-sized leaves inturbulent movement during the entire process of accretion,until reaching a maximum dimension of 12 mm and/or amaximum weight of 0.23 g, after which they fallout and de-posit.This explains whythedimensions of these aggregatesare not related to the distance from the crater. Aggregatesof the same dimensions were found both at 8.5 km SSWin S1-1 and at 3.2 km E in S3-4. A progressive accretionin the basal part of a density current, a higher-concentratedand coarser-grained zone, with respect to the upper dilutepart (Cole and Scarpati 1993; Sohn 1997) should have de-stroyed these mm-size cylinders by the interaction withlarger particles.

Similarly to rim type accretionary lapilli, vesicles (fewto tens of microns in diameter) are widely dispersed withincylindrical aggregates. Coarser clasts are often fractured asthey are in other types of ash aggregates, but only in thesetwo types of ash aggregates the clasts are broken. The pres-ence of matrix that fills some of the broken crystals, sug-gests that fractures opened during the process of accretion.

Fractures and micro-cracks in crystals and glass frag-ments can be both mechanically (Komorowski et al. 1991)and thermally induced (Buttner et al. 1999). The occur-rence of mechanical fracturing supposes a close-packingfabric in the moving debris, where grains collide with each



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other. This type of micro-crack shows no or small lateraldisplacements (0.5–5 µm wide) between broken pieces(Komorowski et al. 1991; Bonadonna et al. 2002). In thiscase, sub-angular submicrometer particles chipped fromthe edges of the microblocks were sometimes observedwithin the microcracks (Komorowski et al. 1991). In con-trast, in cylindrical aggregates and concentric accretionarylapilli, a fine-grained matrix of variable nature intrudes the

cracks displacing both sides away up to 20 µm (in cylin-drical aggregates), but preserving the original shape of theclast. Broken clasts are found in the intermediate portionof the aggregates and they are never in touch with eachother, a reason that stands against a mechanical origin or apost-depositional origin.

Considering that both crystals and glass fragments werealready weakened by primary fracturing during fragmenta-tion (see above), we believe that small thermal stresses aris-ing at the contact with a colder fine-grained medium (ma-trix), during the accretion in different parts of the clouds,caused a further opening of fractures, followed in somecases by the intrusion of the viscous medium. The pres-

ence of vesicles in both types of ash aggregates supportthis hypothesis indicating that condensation of water vapor took place during accretion. In fact, turbulent fluid motionis an irregular condition of flows in which various quan-tities show a random variation with time and space, thusnot only velocity, but also other flow parameters such astemperature, pressure, sediment concentration at all pointsalong a streamline, would vary instantaneously (Garde1994).

A different genesis is proposed for the structureless grayaccretionary lapilli that predominate in distal facies of thedeposits (Table 1). Their distribution suggests that theywere deposited from a co-surge ash fall, although no dis-

tinctive bed deposited by fallout was recognized in anyof the horizons. The sparse orange film on their surfacethat favored their preservation inside the deposit however is missing in the internal part of the aggregate, indicat-ing that the accretion process occurred in portions of theclouds with very low concentrations of acid fluids. Theshape of coarser particles (hundreds of microns in size)is extremely variable, indicating that they were probablytransported to other portions of the clouds, before beinguplifted and accreted. Fractured clasts are present but theyare not broken and no matrix penetrates them. Vesicles areabsent, suggesting that no condensation of water vapor oc-curred during accretion and the clasts were already cold

enough in higher portions of the clouds, to prevent any fur-ther thermal stress responsible for the opening of fractures.The fractures in this type of aggregate divert around coarser clasts and mainly affect the fine-grained material. Becausethese fractures resemble desiccation cracks, their genesiscould be ascribed to the subsequent emplacement of hotter deposits (i.e. fallout) directly above them (asS1-1 at 7.5 kmto the N, underlying fallout B). Alternatively, consideringthat in most cases they have an elongated shape, the loadof overlying deposits could have caused their mechanicalfracturing.

Conclusions

A complex succession of wet and dry pyroclastic surgeserupted during the final phases of the 1982 eruption of ElChichon that occurred on April 4, 1982 at 0135 GMT (III)and 1122 GMT (IV). Considering its stratigraphic posi-tion, pyroclastic surge S1, emplaced at the start of phaseIII, represents the most violent episode. The eight eruptiveevents that occurred during this period involved differentproportions of ground water and magma. Ash aggregatesare present in almost all of the wet S1 horizons. They ap-pear as irregular-shaped agglomerates associated with ar-mored and accretionary lapilli and locally with cylindricalaggregates (S1-1 horizon). The distribution and internalstructures of these four types of aggregates suggest thatthey were formed by different mechanisms. The variableamounts and composition of the fluid phase in the surgecloud as well as different grain sizes of the constituentparticles mainly controlled the process of aggregation. Py-roclastic surge S1 contained variable amounts and types of acid fluids from an active hydrothermal system during its

deposition. The presence of these fluids is consistent withthe existence of a hot water hydrothermal system at the timeof theeruption (Casadevall et al. 1984; Rye et al. 1984).Thetype and abundance of aggregates in the deposit show thatfalling ash was contemporaneous with the horizontal pyro-clastic density currents in medial zones, and became a pre-dominant component of the surge deposits in distal zones.

Smaller hydromagmatic events produced the pyroclasticsurge deposits of IU, as evidenced by the confined dis-tribution of these deposits inside the somma crater. Theabsence of altered ash aggregates and juvenile componentswithin the Basal IU and the first Upper-IU wet and dryhorizons indicate that no acid fluids were available during

these hydromagmatic episodes. Rather acid-bearing cloudsdeveloped during the final hydromagmatic events of Upper IU (IU-8a, IU-8b, IU-9 and IU-10).

Phase III continued with the establishment of a pliniancolumn that emplaced a lithic-rich horizon (B). Its subse-quent collapse formed more pyroclastic flows (F2). After apause of approximately 5 h, indicated by widespread ero-sion of the F2 surface, phase IV began with a plinian col-umn rich in pumice that deposited fall layer C. The eruptioncontinued with a succession of hydromagmatic events thatemplaced nine wet and four dry pyroclastic surge horizons(S3 unit). Most of the surges were smaller events mainlydirected to the east. The third of these deposits (S3-3) re-

sulted from the most violent episode, based upon its widedistribution. The strong degree of alteration of ash aggre-gates within the S3-3 horizon indicates an extremely highcontent of acid fluids in these eruptive clouds. The removalof acid species progressively increased in medial to distalzones towards the east due to the cooling of eruptive clouds.Particles of different grain sizes and densities clotted be-cause of their thick coating of acid fluids. They fell in theform of “mud rain” (Gilbert and Lane 1994), as indicatedby the common occurrence of irregular-shaped ash aggre-gates.



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The presence of four different types of ash aggregatesin the S3-4 horizon indicates that these eruptive cloudswere not only stratified in density (Valentine 1987) but alsocontained variable amounts of condensing water vapor andacid fluids that originated from the rupture of the activehydrothermal system. The rare occurrence of smaller unal-tered armored lapilli in the S3–5 horizon suggests that thisevent produced highly fragmented material that was car-

ried within dilute eruptive clouds and that no fluids fromthe hydrothermal system were involved. The last depositthat gives evidence for the incorporation of waters from thehydrothermal system is the S3–8 horizon.

The occurrence of such a variety of ash aggregates in thewet surge deposits of El Chichon is not surprising, consid-ering that previous studies of distal fallout deposits and ashdispersed in the stratosphere (Varekamp et al. 1984; Ryeet al. 1984; MacKinnon et al. 1984) reported the commonoccurrence of clustered particles due to condensation of acid aerosols (mainly of sulfuric acid) on suspended parti-cles. That condensing water vapor is the main cause for thebinding action of acid fluids is supported by the fact that

the different types of aggregates occur only in wet surgehorizons. Dry surge horizons contain only slight evidencefor alteration of juvenile components. In fact loss of vol-canic sulfur from eruptive clouds may occur in both dryand wet conditions. Fujita et al. (2003) pointed out the im-portance of condensing water vapor and/or liquid dropletsfor removal of H2SO4 from volcanic clouds.

The strong polymodality and poor sorting of most sam-ples from S1 and S3 is likely due to aggregation pro-cesses that efficiently removed fine-grained material fromthe clouds, as suggested for other deposits (Laneet al. 1993;Gilbert and Lane 1994; Schumacher and Schmincke 1991;Bonadonna et al. 2002).

The presence of cylindrical aggregates, within pyroclas-tic surge sequences in other areas may indicate an older vegetation cover existed in places where none had beendetected before.

Acknowledgments This work wassupported by CONACYT grants(27993-T and 38586-T to JLM), a CONACYT-CNR bilateral projectto JLM. NSF grant (EAR-0087665) supported the work of MFSon this project. The help of several people is gratefully acknowl-edged: M. Reyes (Instituto de Geologıa) and C. Linares (L.U.P.) of UNAM, and P. Bush, Director of the U.B. Instrumentation Center of SUNY at Buffalo, for their support with the SEM analyses. Dis-cussions with C. Siebe, and earlier revisions of the manuscript byW.A., Duffield, and R.I. Tilling were useful to improve this work. S.Lane and C. Bonadonna gave us some very penetrating and helpful

reviews.

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