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Bull Volcanol (1993) 55:523-535 Volc a ology9 Springer-Verlag 1993

A case of no-wind plinian fallout at Pululagua caldera (Ecuador):implications for models of clast dispersalPaolo Papale 1, Mauro Rosi 21 GN V, Universita' di Pisa, Via Santa Maria 53, 1-56126 Pisa, and Istituto Nazion ale di Geofisica, Roma , I-Italy.2 Dipartim ento di Scienze della Terra, V ia Santa M aria 53, 1-56126 Pisa, ItalyReceived May 1, 1991/Accep ted April 2, 1993

Abstract. The ca ldera o f Pu lu lagua is an erup t ive cen-t re o f the Nor thern Volcan ic Zone of the South Ame r-ican vo lcan ic a rc , loca ted ab out 15 km no r th o f Qui to ,Ecuador . Act iv i ty lead ing to fo rmat ion of the ca lderaoccurred about 2450 b.p. as a series of volcanic epi-sodes dur ing which an es t imated 5-6 km 3 (DR E) ofhornb lende-bear ing dac i t ic magma was erup ted . Abasa l pumice- fa l l deposi t covers mo re than 2 .2 x104 km 2 with a volu me of abo ut 1.1 km 3 and repres entsthe principal and best-preserved plinian layer . Circularpatterns of isopachs and pu mice, l i thic and Md iso-p le ths o f the Basa l Fa l lou t (BF) around the ca ldera in -d ica te emplacement in wind- f ree condi t ions . Absenceof wind is conf i rmed by an ub iqu i tous , normal lygraded, thin ash bed at the top of the lapil l i layer whichoriginated from slow sett l ing of f ines af ter cessation ofthe plinian column (co-plinian ash) . The unusual atmo-spher ic condi t ions dur ing deposi t ion make the BF de-posit particularly suitable for the application and eval-ua t ion o f pyroclas t d ispersa l models . Appl ica t ion ofthe Carey and Sparks ' (1986) model shows tha t where-as the 3.2-, 1.6-, and 0.8-cm lithic isopleths predict amodel co lumn heigh t o f about 36 km, the 6 .4-cm iso-pleth yields an estim ate of only 21 km. T he 4.9- and6.4-cm isopteths yield a c olum n height of 28 km usingthe mode l o f Wilson and Walker (1987). The tw o mod-els give the sam e m ass discharge rate of 2 x 108 kg s -1.A simple exponen tial de creas e of thickness with dis-tance, as proposed by Pyle (1989) for plinian falls, f i tswell with the BF. Exponential decrease of size with dis-tance is follow ed by clasts less than abo ut 3 cm, sug-gesting, in agreement with Wilson and Walker (1987),that only a small proportion of large clasts reach thetop of the column. Variations with distance in clast dis-t r ibu t ion pa t te rns imply tha t, in o rder to o b ta in co lumnheights by clast dispersal models, the distr ibutionshould be known f rom both p rox imal and d is ta l zones .Knowledge of only a few isopleths, ir respective of theirdistance from the vent, is not suff icient as seemed justi-f ied by the method of Py le (1989) .Correspondence to: M. Rosi

Key words: Ecuador ian vo lcan ism - exp losive erup-tions - plinian fallout - plinian colum n - clast disper-sa l

IntroductionPl in ian erup t ions are among the most powerfu l mani-festations o f volcanic activity. Such events d epositteph ra across areas as large as 105 km 2, from eruptio ncolumns of 10-55 km height, a nd with discharge ratesof up to 10 9 kg s-1 (Wilson 1976; Wilson et al. 1978;Settle 1978; Walker 198!) . Unlike most explosivesty les , in which products a re expel led b y very unstab leprocesses , pl inian erup t ions are domina ted by sub-s teady d ischarge of gas and f ragme nted magma, w hichal lows a quan ti ta t ive approach to the p rob lem s of theformat ion and evo lu t ion of the resu l t ing erup t ive co-lumn (Wilson 1980). Start ing from a the ory of the as-cent of therma l plum es (M orton et al. 1956), severalauth ors (e. g. Spar ks 1986; Ca rey a nd S parks 1986; Wil-son and Walker 1987) have developed numer ica l mod-e ls tha t a t tempt to p red ic t the behav iour o f vo lcan iceruption columns and[ the characterist ics of tephra falldeposits.Owing to the infrequency of plinian events, only afew erup t ions have been accura te ly observed and doc-umen ted (Walker 1981). Therefore , most o f what weknow about th em der ives f rom analysis o f resu ltan t de-posi ts , i .e . by deduct ive reason ing . Unfor tunate ly th isprocess is very diff icult , because the amount of signif i-can t in format ion tha t can be ob ta ined is smal ler thanthe number o f independent var iab les tha t descr ibe theprocess. This implies that many assumptions are neces-sary, inevitably increasing the uncertainty of the re-su lts. For example , a t p resen t we are ab le to model co-lumn heigh ts wi th an approx imat ion of about 10-20%(Woods 1988; Carey and Sigurdsson 1989).This work is focused on the in it ial e rup t ion of Pu lu-lagua, Ecuador, which led to deposit ion of a plinian falllayer and associa ted products about 2500 years b .p .

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The basal pumice fall shows characteristics that makeit particularly suitable for the application and evalua-tion of clast dispersal models from plinian columns.Isopachs and isopleths show a nearly circular geometrycentred on the vent area, which implies nearly wind-free conditions during the eruption. Th ere are very fewdocumented examples of deposits from such events,making the Pululagua case valuable because the ab-sence of wind removes a variable, the effects of whichare difficult to assess.

G e o l o g y o f t h e P u lu l a g u a c a l d e r aQuilotoa, Gu agua Pichincha, Pululagua, Cuicocha andCerro Negro volcanoes form, from south to north, theactive Andean Volcanic Front of Ecuador (Simkin etal. 1981; Barber i et al. 1988). The i nception of predom-inantly explosive activity at these volcanic centresseems to be coeval and points to a very recent renewalof activity at the vo lcanic front characterized b y therise of volatile-rich magmas (Barberi et al. 1988),

Pululagua, located 15 km north of Quito (Fig. 1), isan irregularly shaped caldera with an area of about19 km 2 and a subrect angula r base at an altitude ofabout 2500 m. The ealdera is surrounded by a group ofolder lava domes (Recent Precaldera Domes, RPD),and the syncaldera products overlie a sequence ofdome rockfalls, rock avalanches, and block-and-ashflows formed in association with extrusion of these lavadomes. Most of the RPD lie within a 9 by 4.5 km areaelongated in a N-S direction. The vents were probablycontrolled by normal faults that bound the Interan-dean Depression to the west, a major graben-likestructure that runs parallel to the volcanic front ofEcuador. The caldera is breached on the west alongthe Rio Blanco valley, but the lack of a pre-existingvolcanic edifice suggests that the notch is probably in-herited from the pre-volcanic topography rather thanthe result of a volcanic avalanche.Due to the irregular topograph y around the caldera,the difference in height between the rim and the flooris hard to estimate; however, where the morphologyseems least uneven, the topographic wall is about

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5253 0 0 m . T w o c o m p o s i t e p o s t c a l d e r a d o m e s o f h e i g h t4 8 0 m a n d 2 0 0 m o c c u p y t h e c e n t r a l p a r t o f t h e d e p re s -s i o n (F ig . 1 ) . Th e y h a v e a n e s t i m a t e d c o m b i n e d v o l -u m e o f a b o u t 0 .6 k m 3. T h e v o l u m e o f t h e c a l d e r a w a se s t i m a t e d a t 5 .3 k m 3, a s s u m i n g a n i n fi ll in g o f a b o u t1 00 m b y s u b s e q u e n t p y ro c l a s t i c p ro d u c t s , p a r t i c u l a r l yt h o s e a s s o c i a t e d w i t h t h e e x t r u s i o n o f t h e p o s t c a l d e r ad o m e s . U s i n g a n e s t i m a t e d l i t h i c c o n t e n t o f 1 0 -3 0 w t %fo r a l l t e p h ra p ro d u c t s , a s d e d u c e d f ro m v i s u a l o b s e r -v a t i o n s , a n d a s s u m i n g e q u a l m a g m a a n d l i t h i c d e n s i t yo f 2 5 00 k g m - 3 , th e v o l u m e o f t h e m a g m a e r u p t e d d u r -i n g c a l d e r a - fo rm i n g e x p l o s i o n s i s i n t h e r a n g e 3 .7 -4 .8 km 3 .B o t h R P D a n d t h e s y n - a n d p o s t c a l d e r a p r o d u c t sa r e m a i n l y d a c i t i c i n c o m p o s i t i o n , w i t h a l o w K z O c o n -t e n t v e ry s i m i l a r t o t h o s e t y p i c a l o f t h o l e i i t i c t r e n d s ;h o w e v e r , t h e y a r e c a l c a l k a l i n e a n d l a c k a n y i r o n e n -r i c h m e n t .

S t r a t ig r a p h y o f c a l d e r a - f o r m i n g a n d p o s t - c a l d e r av o l c a n i c sE r o s i o n b y t h e R i o B l a n c o h a s e x p o s e d a b o u t 1 00 m o fp y ro c l a s t i c a n d v o l c a n i c l a s t i c d e p o s i t s , m a i n l y r e l a t e dt o e x t r u si o n o f t h e p o s t c a l d e r a d o m e s . T h e s e p r o d u c t sa r e p a r t i a ll y s i n t e r e d b l o c k -a n d -a s h f l o w d e p o s i t s m a d eu p o f n o n -v e s i c u l a t e d l a v a c l as t s w h i c h s h o w o x i d a t i o ni n d ic a t iv e o f a h ig h - t e m p e r a t u r e e m p l a c e m e n t ( W i l-l i ams 1960 ; Crandel l and Mul l ineaux 1973) . Thei r to ta lv o l u m e i s s i m i la r t o t h a t o f t h e d o m e s , i n c r e a s i n g t h ev o l u m e o f la v a e m i t t e d d u r i n g t h e p o s t c a l d e r a s t a g e t oa t l e a s t 1 k m 3. Th e s e b l o c k -a n d -a s h f l o w d e p o s i t s o v e r -l ie t h e c o m p l e x s e q u e n c e o f t h e p y ro c l a s t ic p ro d u c t s o ft h e c a l d e r a - fo rm i n g p h a s e .Th e i n c e p t i o n o f t h e c a l d e r a - fo rm i n g a c t i v i t y i s w e l lc o n s t ra i n e d b y r e c e n t r a d i o m e t r i c d a t a a n d a r c h a e o -l o g ic a l s t u d ie s . P y ro c l a s t i c d e p o s i t s o f th i s p h a s e d i r e c t -l y o v e r l i e a r t i f a c ts o f t h e C o t o c o l l a o a r c h a e o l o g i c a l s i t el o c a t e d i n t h e n o r t h e rn a r e a o f Q u i t o . Ex c a v a t i o n s i n -d i c a t e t h a t t h e s i t e w a s in u s e fo r m o re t h a n 1 0 00 y e a r su n t il 2 45 0 b .p . , a n d w a s t h e r e a f t e r s u d d e n l y a b a n d o n e d(V i l l a l b a 1 9 8 8 ) . F u r t h e r c h ro n o l o g i c a l i n fo rm a t i o nc o m e s f r o m I 4C d a ti n g p e r f o r m e d o n p l a n t r e m a i n s a s -s o c i a t e d w i t h t h e P u l u l a g u a p y ro c l a s t i c s . R a d i o c a rb o nd a t i n g o f p e a t u n d e r l y i n g t h e b a s a l f a ll y i e l d e d a n a g eo f a b o u t 2 6 5 0 b . p . ( G e o t e r m i c a I t a l i a n a - I N E M I N1 98 9) . Tw o y o u n g e r a g e s o f 2 2 85 a n d 2 3 0 5 b.p . w e reo b t a i n e d o n s m a l l b r a n c h e s e m b e d d e d i n p y r o c t a s t i cf l o w a n d s u rg e d e p o s i t s o f u n c e r t a i n s t r a t ig r a p h i c p o s i -t i o n w i t h i n t h e P u l u l a g u a c a l d e r a - fo rm i n g s u c c e s s i o n(Hal l 1977 ; I saacson 1987) .A c o m p l e x s e q u e n c e o f vo l c a n ic e r u p t io n s l e d t oc a l d e ra c o l l a p s e . A t l e a s t t e n e ru p t i v e e p i s o d e s h a v eb e e n i d e n t i f ie d b y t h e p r e s e n c e o f m i n o r w e a t h e r e dl a y e r s a n d / o r e ro s i v e u n c o n fo rm i t i e s . U s i n g t h e r e l a -t i v e i m p o r t a n c e o f t h e s e h o r i z o n s ( i. e . t h e i r t h i c k n e s sa n d i n t e n s it y ) , t h e p y r o c l a s ti c b e d s h a v e b e e n g r o u p e di n to L o w e r , M i d d l e a n d U p p e r U n i t s ( F ig . 2 ). T h e b a s -a l d e p o s it s , t h e fo c u s o f t h is w o rk , m a k e u p t h e f i r st o f

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Fig. 2. Stratigraphy and nomenclature of the products of Pulula-gua caldera-forming eruptions. Location of the section is given inFig. 1. Eruptive units U1 to U10 are separated from each otherby erosive unconformities. BF and F2 to F/ represent plinianand/or subplinian pumice fallout layers. Numbers on the left-hand side of the column indicate the thickness in cm. The sectionis located on a topographic high and lacks most of the valley-ponded pyroclastic flows of the eruptive sequence

t h e t h r e e Lo w e r U n i t s (U 1 i n F i g . 2 ) . Lo w -g ra d e i g n i m -b r i t e a n d s u rg e d e p o s i t s a r e c o n c e n t r a t e d a l o n g v a l l e y sa ro u n d t h e c a l d e r a a n d i n t h e v o l c a n i c l a s t i c a p ro n t h a te x t e n d s e a s t a n d s o u t h e a s t o f t h e c a l d e r a t o w a r d s S .A n t o n i o d e P i c h i n c h a , a t t a i n i n g a m a x i m u m t h i c k n e s so f m o r e t h a n 3 0 m i n t h e R i o L a s M o n j a s v a l le y

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526(F ig . 1 ). F a l l o u t d e p o s i t s c o n s i s t o f s e v e n p l i n i a n p u -m i c e l a y e r s w i t h i n t e r c a l a t e d a s h b e d s , e a c h w i t h at h i c k n e s s l e s s t h a n 1 m . A c c u m u l a t i o n i s t h i c k e s t w e s to f th e c a l d e r a , w h e r e t h e e n t i r e t e p h r a b l a n k e t e x c e e d s1 m i n t hi c k n e s s a t a b o u t 3 5 k m f r o m s o u r c e . A c c o r d -i n g t o I s a a c s o n (1 9 8 7) , t h e i m p a c t o f t h e r e p e a t e dd o w n w i n d f a l l o u t s f r o m P u l u l a g u a o n h u m a n c o m m u -n i t ie s w a s s e v e re a s f a r a s t h e s l o p e s o f t h e W e s t e rnC o r d i l le r a a n d p e r h a p s a s m u c h a s 1 5 0- 2 0 0 k m t o w a r dt h e c o a s t o f E c u a d o r .S t r a t i g r a p h i c c o r r e l a t i o n s w i t h i n t h e p y ro c l a s t i c d e -p o s i t s w e re e s t a b l i s h e d u s i n g t h e s e v e n p u m i c e f a l l s a sm a r k e r b e d s ; th e f a l l u n i ts a r e l a b e l l e d f r o m b o t t o m t ot o p w i th n u m b e r s f r o m 1 ( = B a s a l F a ll o u t, B F ) t o 7 .F a l l o u t l a y e r s 1 -5 c a n b e t r a c e d f a i r l y e a s i l y i n t h e f i e ldt o 2 0 k m f r o m t h e c a l d e r a r i m . I s o p a c h s o f l a ye r s 2 - 5s h o w d i s p e r s a l p a t t e rn s t o w a rd s t h e w e s t , w i t h s l i g h t l yd i f f e r e n t d i s p e r s a l d i r e c t i o n s (F ig . 3 ) .

D e p o s i t s o f t he i n i t ia l e r upt i o nStrat igraphy and l i thologyTh e f i r st s y n c a l d e ra e ru p t i v e u n i t (U 1 i n F ig . 2 ) c o n -s i s t s o f th ree t e phr a l ayers : a th in , s t ra t i f i ed , basa l g reyash fa l l , a p l in ian pumice fa l l , and a f ine-g ra ined , wh i tea s h. T h e t h r e e l a y er s a r e c o n f o r m a b l e a n d a r e b e l i e v e dt o h a v e b e e n e r u p t e d a n d d e p o s i t e d w i t h o u t a n y s i gn if -i c a n t i n t e r ru p t i o n .Th e b a s a l g r e y a s h (B G A ) i s a f a l l u n i t u p t o 1 0 -c mt h i c k w i t h p l a n e -p a ra l l e l s t r a t i f i c a t i o n d e f i n e d b y a l t e r -n a t i o n s o f c o a r s e a n d f i n e a s h . Th e s t r a t i f i c a t i o n i sm o s t e v i d e n t w i t h i n 4 -5 k m o f t h e c a l d e r a r i m (F ig . 4 ) .Th i s l a y e r i s a l w a y s p r e s e n t b e l o w t h e b a s a l p l i n i a n f a l lS E a n d N W o f v e n t t o m o r e t h a n 2 0 k m f r o m s o u r c e ,b u t i t o c c u r s n o fu r t h e r t h a n 1 0 -1 1 k m f ro m v e n t i n t h eN E d i r e c t i o n a n d 6 -7 k m t o w a rd s t h e S . I t c o n si s t sm o s t l y o f j u v e n il e , n o n - v e s i c u l a te d f r a g m e n t s ( a b o u t5 0 w t % ) a n d s i g n if i ca n t a m o u n t s ( 2 5 - 3 0 w t % ) o f f r e ec ry s t a l s ( a m p h i b o l e a n d p l a g i o c l a s e i n n e a r l y e q u a lp r o p o r t i o n ) a n d p u m i c e ( 1 5 - 2 0 w t % ) . L e s s th a n 5 w t %o f t h e e j e c t a a r e l it h ic c l a s t s c o n s i st i n g o f o x i d i z e d l a v a sa n d m e t a s e d i m e n t a r y r o c k s. I n s o m e l o c a li ti e s t h e f in e rb e d s s h o w t h i c k n e s s v a r i a t i o n s t h a t a r e l i k e l y t o h a v eo r i g i n a t e d b y s li g h t s l u m p i n g d u r i n g d e p o s i t i o n , a s w e l la s a v e s i c u l a r s t r u c t u r e , p ro b a b l y d u e t o t h e c o a l e s -c e n c e o f d a m p a c c re t i o n a ry l a p il li (R o s i 1 9 9 2) , s u g g e s t -i n g t h a t t h e a s h w a s n e a r l y s a t u r a t e d w i t h w a t e r a t t h et i m e o f d e p o s i ti o n . T h e h i gh f r a g m e n t a t i o n ( > 8 1 w t %f i n e r t h a n 1 m m i n a ll a n a l y s e d s a m p l e s ) a n d t h e v e s i -c u l a t e d t e x t u r e o f th e a s h s u g g e s t t h a t t h e o n s e t o f a c -t i v i ty w a s c h a ra c t e r i z e d b y s e v e ra l d i s c r e t e p h re a t o -m a g m a t i c e x p l o s i o n s ( c f S e l f a n d S p a rk s 1 9 7 8) .Th e b a s a l p l i n i a n f a l l (B F ) i s a r e l a t i v e l y t h i n(< 1 m ) , w i d e l y d i s p e r s e d , f a l l u ni t . Th e i s o p a c h s a r es h o w n i n F ig . 5 . T h e y s h o w a n e a r l y- c i r cu l a r g e o m e t r yc e n t e r e d o n t h e v e n t a r e a , w h i c h i s i n t e r p r e t e d t o b ed u e t o a n a l m o s t c o m p l e t e a b s e n c e o f w i n d d u ri n g t h ee r u p t i on . T h e l a c k o f d a t a i n t h e N W s e c t o r is d u e t ot h e p r e s e n c e o f t h ic k l y f o r e s te d m o u n t a i n s w i t h n o a c -

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527

Fig. 4. Basal eruption sequence of Pululagua3 km NE of the caldera rim. BGA, Basal GrayAsh; BF, Basal Fall; WA, White Ash; ITD, Ig-nimbrite Type Deposit (see text). Arrows indi-cate finer intervals within the BF. On top ofthe basal sequence is the thinly stratified F2.Bar is 20 cm

, : , ' , . " \ \ \

9 / 1 7. [ ~ f , . 4 O p u l u l . g u a ' ; 5 6 ~ a ~ 9 I I ',~_ 8 }Y \ ~ ~ [ Z_.5ol X z " X ' 9 % A . 7 " ) l : ~ / J /t 7o '1o/ I

. . . . . . . . . : / , . - , / j : 2 4 , ~o/_ "\ .54 37. " t . 0

\ . ."k k k ~ i 2 ~ J / f

? 5 k i n " " - - - - -~ QUITO- '

Fig. 5. Isopach map o f the Basa l Fall (BF ) of Pululagua. Valuesare in cm

cess. BF is usually not graded, except at i ts top where i tgrades from lapil l i to the overlying white ash. North-east of the caldera i t contains two plane-parallel f inerlayers each with gradational boundaries, less than10 cm th ick , and made up of the sam e com ponents(Fig. 4) . These f iner layers increase in thickness towardthe ca ldera , where they are rep laced by surge de-posits.BF contains highly porphyrit ic pumice clasts withnearly equa l quantit ies of plagioclase (18.8 wt % ) andamphibole (23 .7 wt% ) and subo rd ina te amounts o fmag netite (1.54 wt% ). T he glass is whitish, unlike thatof pumice of the overlying fall layers (especially fromF4 to F7)which tends to be ye l lowish to pa le b rown.

The pumice vesicu lar i ty ranges f rom 72 to 80% forclasts in the 32-4 mm range, and is always close to 72%in th e 4-1 mm range. This narro w interval o f vesicu-lar i ty suggests that no signif icant interaction withgroundwater took p lace dur ing the p l in ian phase(Houghton and Wilson 1989) . Li th ics represen t about33 wt% of the deposi t wi th in the 1-cm isopach , becom -ing less abundant with decreasing grainsize, which isalso in agreement with a purely magmatic plinian erup-tion (Barbe ri et al . 1989). Lithics are mo stly lavas, bothf resh and ox id ized , wi th a subord ina te am ount o f wea-thered basement sandstone and s i l t s tone . The la t te rare characterist ic of the BF, being vir tually absent inthe overlying fall layers.The white ash (WA) is a normally graded, f ine-ashlayer up to a bou t 10 cm thick that ubiquit ously sealsthe pu mice lapil l i fallout layer . At 6 km east of thevent, abo ut 97 wt% of the ash is f iner than 1 mm, andmor e than 75 wt% is f iner than 1/16 ram. I t is ma de u palmost entirely ( > 80 wt %) of glass shards, with a low-er p ropor t ion (< 20 wt% ) of c rystals (amphibole andplagioclase in equal p roporti on), and < 1 wt% li thicparticles. The transit i ton between the BF and WA isgradual (Fig. 6) and is characteriz ed by a contin uosgranulometric gradient. This necessarily involves somearb i t ra ry choice about the l imi t be tween BF and WA.The boundary be tween them was p laced a t a leve lwhe re only clasts less than 0.5 mm in size were visiblein the deposi t . We bel ieve tha t WA or ig ina ted by thelate sett l ing of the progressively f iner portion of thematerial originally contained in the plinian column (co-plinian ash) . This origin is supported by i ts grainsizedistr ibution and, in particular , by the relatively goodsorting (crqb ___2), which is consistent with a fallout ori-gin. Since primary ignimbrites are lacking and pyro-clastic surges are very small , we believe that co-ignim-brite ash did not contr ibute signif icantly to the WA. I tsdistr ibution is rather uniform over a circular area thatexceeds the dispersal of the BF itself , with a thicknessof 3 -10 cm with in a rad ius o f abou t 30 km.

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Fig. 6. The Basal Fall laye r (BF) grades upcontinuosly into the Whit e Ash (WA). Bar is20 cm

Deposi ts a t the foo t o f s teep s lopes somet imes in -clude a poor ly sorted (0-4) ___3), decim etre- to metre -th ick , ign imbr i te - l ike layer above the WA ( ITD inFig. 4 ) made up of the same co mpon ents as the un-der ly ing BF, bu t wi th a la rge p ropor t ion of whi te ashmatr ix (a bout 20 wt% of the deposi t be ing f iner than0.063 ram). W her e i t is present, the und erlying W A isreduced to a th ickness o f about 2 -3 cm. Since the de-posi t does no t show any ev idence of a water - suppor tedresed imenta t ion or ig in , we conclude tha t i t o r ig ina tedf rom secondary pyroclas t ic mass f lows resu l t ing f romlocal ized s lumping of the d ry WA and BF. The occur -rence of such secondary pyroclas t ic f lows was p ro-posed for the la te ra l b las t su rge a t Mount St . Helens(Fisher e t al. 1987). In the case of Pululagua, th e phe-nomenon seems to have been s l igh t ly d i f feren t in tha ti t involved cold, freshly accumulated, fallout ash onslopes be ing remobi l ized by a mechanism ak in to snowavalanching. The qui escenc e that followe d the initiale rup t ion is marked by erosional unconformit ies tha tusual ly a f fec t the WA. Af ter a shor t pause the vo lcanoresumed ac t iv i ty wi th the erup t ion of fa l lou t F2 .

thinning law. This model f i ts the data from the BF ofPulu lagua , hence i t was p refer red to o ther methods.Al though the da ta agree very wel l wi th a constan texpone ntial thinning law (Fig. 7) , the distal portio n o fthe deposit shows a sharp change in slope on the ' lnthickness-are a 1/2 ' plot which is not foun d in any of theexamples repor ted by Pyle (1989) . Th is i s possib ly aconseque nce of our a rb i t ra ry separa t ion be t wee n BFand WA, and idea l ly the model should be app l ied tothe whole fa l l deposi t ( i. e . BF + WA ). H owev er , oura t tempt to reconst ruc t the i sopachs o f the whole de-posi t d id no t g ive good resu l ts , p robab ly beca use o f thediff iculty of taking ac count o f erosion and aeolian re-work ing of the WA . Future s tud ies should examine thed is t r ibut ion pa t te rn o f the most d is ta l por t ions o f WA.I t i s wor th no t ing tha t , due to the few measurementson the very distal BF, the 1-cm isopach is reconstructedmainly by analogy with the general trend, and i t is pos-sible that i ts real pattern would indicate a greater dis-persal .The po in t cor responding to 99-cm th ickness inFig. 7 is not a true data point but is an ex trapolation

V o l u m eKnow ledge of the to ta l vo lume of a tephra deposi t i sfundamenta l fo r def in ing the magni tude of an erup-tion. For plinian falls a major obstacle is the diff icultyof ob ta in ing d ista l th ickness in format ion . To overcom eth is p rob lem, var ious methods have been proposed inthe literature (e.g. Rose et al. 1973; Suzuki et al. 1973;Vucetich and Pullar 1973; Howorth 1975; Carey and Si-gurdsson 1980; Walker 1980). Different methods arebased on var ious assumpt ions and mathemat ica l mod-e ls , and when app l ied to the same deposi t they of tenyield quite different results (Froggatt 1982). More re-cen t ly , Py le (1989) p resen ted a method for comput ingthe vo lume of a fa l l deposi t assuming an exponent ia l

"~ 100 0 r = 0.9 98o p > 0.99 9100 - -o - -~" 10-_o( " 1 -

0.1 " , ~ ~ J I0 10 20 30 40 50

s q u a r e r o o t o f is o p a c h a r e a { k m )Fig . 7. 'ln Thickness vs square root of isopach area' diagram forthe BF (after Pyle 1989). r, correlation coefficient;p, coefficientof probability of correlation; both refer to the straight line withlower slope. Explanation in the text

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WA

BF

BGA

[]o,

O\O[]

I I-2 0 2Md

n nqn ~ ~

~ [ ][]I|& 2'0

~a / , t

I I I !2 4 6 8 10a=ML A=MP

Fig. 8. Representative vertical grainsizevariations of the basal sequence. BGA,Basal Gray Ash; BF, Basal Fallout; WA,White Ash; s, surge levels. Samples ofBGA and BF come from a section lo-cated on the northeastern rim of thecaldera (about 2 km from the probableposition of the vent), with total(BGA + BF) thickness of 90 cm. Sam-ples of WA (total thickness 6 cm) comefrom a section located about 6 km eastof the caldera rim. MP and ML valuesare in cm

\ 05~, 0.4\

0.9 ~.8

h 0.77_~/7"' I O '

i " " ' , _

I.i?o. /.u,. 6. .91.7 1.3 .4~ '1

0 5km p//~OuITOI I

Fig. 9. Isopleth map of the median diamete r of grainsize of theBF. Values are in mm,

for the only northern proximal section to a circular iso-pach in accordance wi th the genera l t rend .The vo lume of the BF, as ob ta ined f rom the ' lnth ickness-area m' p lo t in Fig. 7 , i s 0 .75 km 3 ( D R E =0.20 kin3). Assuming tha t the whole deposi t (BF +WA) cont inues to fo l low a constan t exponent ia l th in-ning trend until i t is vanishingly thin, the n th e total cal-cu la ted v o lu m e r ise s t o ab o u t 1 .1 k m 3 ( D R E =0.34 km3), with the 1-mm isopach being at a mean dis-tance of about 70 km f rom the ven t .

Grainsize characteristicsGrainsize analyses were car r ied ou t on more than 40samples of BF. Sampling for grainsize distr ibution wascar r ied ou t on the en t i re th ickness o f BF, bu t d id no tinc lude WA.As a l ready po in ted ou t , in media l to d is ta l ou tcropsNE of the ven t the BF has a t least two f iner subuni ts in

its middle portion (Fig. 4) . In more proximal locationsless than 3- 4 km from the calde ra r im, these subunitsbecome more prominent and are rep laced by surge de-posits (Fig. 8) . The surge horizons are ma rked by anincrease in the M d+ value , a decrease in the maxim umdiameter o f pumice (M P) and l ith ics (ML) , and by awide range of sorting. These are all characterist ics thatref lec t the i r d i f feren t emplacemen t mo des wi th respectto the fall layer . They are possibly due to partial col-lapses that involved material from the sides of the as-cending column (of Carey et al . 1988).The WA appears to be a ra ther un i form layer up tolocali t ies 6-7 km away from the caldera. Due to i ts f inemode, the ash was analysed down to 4-2 microns usir iga 'Coul ter Counter ' T A II inst rument . The overa l l nor -mal grading of the BF continues in the WA (Fig. 8) . Asystemat ic s ize s tudy of the WA is a necessary fu turestep for gaining knowledge of the total grainsize distr i-bu t ion as well as fo r be t te r understanding of the t rans-por t and de posi t ion processes o f the p l in ian ash .The p resen ce of f iner layers interf ingering with thenorm al lapill i fall northeas t of the vent strongly affectsthe t rend of i sop le ths fo r the me dian d iameter o f c las ts .These show an asymmetr ica l pa t te rn (Fig . 9 ) tha tagrees well with a process of contamination by f ines.This e f fec t becomes less ev iden t away f rom the ven t ,unti l i t vanishes at abo ut 15 km, as shown by the regu-lar circular pattern of the 0.4-ram isopleth, suggestingthat at this distance the influence o f the f iner materialwas negligible.The grainsize characterist ics of the BF are similar tothose repor te d in the f i te rature for the p l in ian deposi ts(Figs. 10 and 11), with the exc epti on of the tre nd fo rmedian d iameter , which is par t ly due to the above-men tione d c ontam ination by f ines. Points in Fig. 10ader ived f rom proximal i sop le ths tend to be sh i f tedf rom the p l in ian f ie ld by th is con taminat ion , whereasthe 0,4- , 0 .5- and 0.6-mm isopleths, corresponding togrea ter d is tances f rom the ven t , where contaminat ionby f ines is absen t or we ak, l ie within the plinian f ield. Ifthe i sop le ths o f the median d iameter a re rem odel led ascircles with a radius equal to their radius in a southwes-ter ly direction (full squares in Fig. 10a) , then the dis-crepancy is g rea t ly reduced . In contrast to the mediangra insize , the MP and ML values are unaf fec ted by

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530A (km2)

1000

100

10

\

~ \ \ \ \ \ \ \\

It, I I1 lOmedian grainsize mm)

\

\\\\\ ~X \' \\\\\\ , %

b \lb 100maximum diameterof pumice (ram)

\ \ \x x x ~ \ X \ \

' \ '\ \ \ \ N \ ~\e \I ,, ,",10 100

maximum diameterof lithic s (ram)

Fig. 10. Diagram 'lnA (area enclosed by e achisopleth) vs a lnMedian diameter; b lnMP; ehaML' (all in ram). The dashed lines indicatethe field of plinian fall deposits (after Walker1981). Open squares refer to the BF layer.Dark squares in Fig. 10a refer to the area o fisopleths as computed by considering circularpatterns with radius equal to that in the SWdirection

3

0 ~ I I I I ~(- --~ I j- 5 0 M d ~ 5

F2{. 30, , ) j }

i I 1 I I 1 1 1 i i 00 F 1 1 0 0

Fig. 11. Comparison betweengrainsize characteristics of theBF (open squares) and thefiled of plinian fall layers, t71,wt% less than i mm, and F2,wt% less than 63 p,m. Con-tours after Walker (1983)

c o n t a m i n a t i o n b y f i ne s a n d d e p e n d o n l y o n t he m a x i -m u m h e i g h t o f th e c o l u m n ; a s a r e s u l t , th e r e l e v a n tpo in t s l i e en t i re l y wi th in the p l in ian f i e ld (F ig . 10b , c ).On t h e ' M d + -o -+ ' d i a g ra m (F i g. 1 1 ) B F s a m p l e s l i ei n t h e f i n e r g r a i n e d p o r t i o n o f t h e f a l l f ie l d ; t h i s is p a r t -l y d u e t o a s c a r c i t y o f p ro x i m a l d a t a p o i n t s , b u t i t p ro b -a b l y a l so r e f l e c t s a n a b u n d a n c e o f f i n es in t h e c o l l e c t e ds a m p l e s . T h e v a l u e o f o -+ d e c re a s e s r e g u l a r l y w i t h i n -c r e a s i n g M d + , d i s p l a y i n g r a t h e r h i g h v a l u e s fo r p ro x i -m a l s i t e s ; t h i s f e a t u re i s o n c e a g a i n a t t r i b u t e d t o t h ea b s e n c e o f w i nd . T h e p o s i ti v e c o r r e l a t i o n b e t w e e nwi n d v e l o c i t y a n d s o r t i n g o f f al l d e p o s i t s wa s a l r e a d yp o i n t e d o u t b y W a l k e r ( 1 97 1 ). M o r e o v e r , t h e f i n e r t o pwh i c h wa s i n c l u d e d i n t h e s a m p l i n g o f t h e B F (u p t o ac l as t s iz e o f 0 .5 m m ) r e d u c e s t h e m e d i a n d i a m e t e r a n di n c re a s e s t h e v a l u e o f o-+, p a r t i c u l a r l y i n p ro x i m a l s e c -t i o n s wh e re t h e B F i s c o a r s e .

1 9 8 7) , f o r wh i c h M P a n d M L a re o n l y c ru d e e s t i m a t e s .T o e v a l u a t e t h e d e g r e e o f a p p r o x i m a t i o n w e r e p e a t e dm e a s u r e m e n t s o f M P a n d M L a t s e v e n p o i n ts a t o n el o c a l i t y 10 k m s o u t h o f t h e c a l d e ra r i m . T h i s l o c a l i t yw a s c h o s e n b e c a u s e o f t he p a r t i c u l a rl y g o o d e x p o s u r ea n d b e c a u s e i t a p p e a rs t o h a v e b e e n u n a f f e c t e d b y b al -l i s ti c c las t s. In eac h o f the s even 0 .5 m 2 fa l lou t a reas wea l so m e a s u r e d t h e l e n g t h o f t h e t h r e e m a i n a x e s o f th ef i v e l a rg e s t c l as ts ; t h e g r e a t e s t v a l u e o f t h e a v e ra g e o ft h e t h r e e a x e s i s r e f e r r e d t o a s A (AP f o r p u m i c e , ALfo r l i th i c s) , wh i le M re f e r s t o t h e a v e ra g e o f t h e m a x i -m u m l e n g t h o f t h e f i v e l a rg e s t c la s ts (M P fo r p u m i c e ,M L fo r l i t h i c s ) (T a b l e 1 , F i g . 1 2 ) . T h e r e l e v a n t r e s u l t sa re :1 . t h e v a l u e o f A i s a lwa y s s m a l l e r t h a n t h e c o r r e -s p o n d i n g M ( i n o n e c a s e t h e y c o i n c i d e ) ;

Dispersa l o f pumic e and l ith ic c las tsM a x i m u m g r a in s iz e d a t a f o r t h e B F w e r e o b t a i n e d a ta l m o s t 5 0 s e c t i o n s a ro u n d t h e c a l d e ra , d o w n t o a t hi c k -n e s s o f 1 c m . E x c e p t i o n a l p r e s e r v a t i o n i s f a v o u r e d b yt h e d r y c l i m a t e o f t h e I n t e r a n d e a n D e p r e s s i o n a n d b yt h e s e a l in g e f f e c t o f t h e o v e r l y i n g W A . M a x i m u m d i-a m e t e r s o f p u m i c e ( M P ) a n d l i th ic s ( M L ) a t e a c h l o ca -t i o n w e r e o b t a i n e d b y a v e r a g i n g t h e m a x i m u m l e n g t hof the f ive l a rges t c las t s co l le c ted on a 0 .5 m 2 su rface .T h i s p r o c e d u r e f o l l o w s t h a t d e s c r i b e d e l s e w h e r e ( e . g .C a re y a n d S p a rk s 1 9 8 6 ; C a re y a n d S i g u rd s s o n 1 9 8 7 ) .W h e n c o m p a r i n g s u c h d a t a w i t h m o d e l s o f c la s t d i s pe r -s a l , a n a p p ro x i m a t i o n i s i n t ro d u c e d s i n c e e x i s t i n g m o d -e l s r e f e r t o t h e a v e r a g e o f t h e t h r e e m a i n a x e s o f e a c hc l a s t (W i l s o n a n d Hu a n g 1 9 7 9 ; W i l s o n a n d W a l k e r

Table L M and A values obtained on seven different 0.5 m 2 sur-faces of the BF, at about 10 km southeast of the caldera rim. Val-ues a re in cm (except for ~r//x). ~r: standard deviation. /x: me anvalueSection M A

MP ML AP AL1 7.1 4.5 6.1 3.92 7.6 3.9 5.7 3.33 8.0 4.7 8.0 4.14 7.5 5.0 6.5 4.55 8.8 4.5 8.3 4.16 7.1 4.0 6.5 3.27 9.6 4.2 7.1 3.60.9 0.4 1.0 0.57.96 4.40 6.89 3.81~ 0.11 0.09 0.15 0.13

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531

EO

iiiiiiii iiiiiiiii iiiiiiiiiiiii

1 2 3 4 5 6 7Fig , 12 . Bar d iagram compar isons be tween M (average of the f ivela rges t d iamete r s ) and A ( la rger va lue of the average of the threepr inc ipa l axes ) for pumice (P) and l i th ics (L) . Measurementswere ca r r ied out a t a loca l i ty about 10 km SE of the ca ldera r im

2. the standard deviations nearly coincide for M andA;3 . the ra t io o f s tandard dev ia t ion of M to the meanvalue of M nearly coincides for pumice and l i thics;4 . the h ighest va lues o f AP and A L am ong the sevensec t ions c lose ly resemble the average va lues o f M P andML respective ly .Point 2 suggests that the use of A instead of M d oesnot signif icantly smooth f luctuations in the measure-ments . Po in t 3 ind ica tes tha t the more regu lar pa t te rnof pumice i sop le ths co mpare d to the l i th ic i sop le ths o fthe BF (Fig. 13) is not related to greater variabil i ty ofthe l i thic data. Point 4 is the most interesting result : i t

suggests that the collection of M values is a suff icientlyre l iab le p rocedure when a la rge number o f s t ra t ig ra-ph ic sec t ions are avai lab le , whereas when they arescarce the collection of A values over a larger (e.g.about 4 m=) area o f the deposi t i s p referab le . B ecauseof the genera l ly good exposure o f the BF, we adoptedthe collection of M values. I t is once again evidentfrom the circular pattern of the isopleths (Fig. 13) thatthe erup t ion occur red in a near abs ence of wind .

Pyroclast dispersal modelsGeneral characteristics of modelsThe d is t r ibu t ion of pyroclas ts c lose to the ven t de-pends on the charac teris t ics and dynamics o f the erup t-ing mix ture a t the base o f the co lumn, whereas the d is-tr ibution at great distance is a function o f both colum nheight and .wind veloc ity (Wilson 1972, 1976; C areyand Sparks 1986; Wilson and Walker 1987). Carey andSparks (1986) developed a co lumn model (here named'CS' ) which makes i t possib le to d is t ingu ish be tweenthese last two factors. This model takes into accountbo th the d is to r t ion of the e las t suppor t envelope bywind and the expansion of the p lume a t the neu tra lbuoya ncy level . Fur thermore , c last t ra jec tories a re ad-justed to take in to account the add i t ional rad ia l expan-s ion of the umbrel la reg ion due to the la te ra l fo rcedin t rusion of the p lume in to the a tmosphere . Two pa-rameters o f each isop le th must be considered : the ha l fmaximum wid th perpendicu lar to the ax is o f d ispers ion( ' c rosswind range ' ) and the maximum d is tance f romthe ven t ( 'downwind range ' ) . The model wind ve loc i tydepends on the vertical profile of the wind i tself , which

_ M P

1.6 / .- " " < " 129 " / .,. .. 6 . 5 3 5

[ [ /4.2 ! ~ v u l u l a g u a "~,x\'"_._7" \ o .~ \3.8"" /7.3, "11.~ 6.6 "4.8 2.0\ " ~7 . 4 / 7.3 " 4.2\ ~ k-- \ ~ 10.' ~k"X13 .~/7'~" ;.::!"2. 2"6) )

"21 9 4 / V f l 3 8 1 I\X'\ \ t;.'4.a 0 5 km ~ ~ O U I T O

ML

" X ,_ - ' ; 2 - - . 0 7 __.

~.1.J/!241 ,, g \k"~ ~/o"19 / \5..~q I i s /

9 4 9 9 a[J i3

~~ '2 " - ~r r Q U I TOb ? 5km ~ F / AFig . 13a , b . MP and M L isople th map s of the BF. Values ( in cm) were ob ta ined by averaging the ma ximu m diam ete r of the f ive la rgestclasts collected on a 0.5 m ~ surface

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53 2m u s t b e a r b i t r a r i l y a s s u m e d o r b a s e d o n c u r r e n t - d a ya t m o s p h e r i c p r o f i l e s fo r u n o b s e r v e d e r u p t io n s . I n c o n -t r as t , t h e m o d e l c o l u m n h e i g h t d e p e n d s o n l y o n t h ec ro s s wi n d r a n g e , a n d i s n o t a fu n c t i o n o f t h e a s s u m e dwi n d p ro f i l e . S i n c e p u m i c e c l a s t s c a u s e s o m e c o m p l i c a -t i o n s d u e t o d e n s i t y v a r i a t i o n s w i t h s i z e o r p o s s i b l eb r e a k a g e u p o n i m p a c t , t h e C S m o d e l p r e f e r e n t i a l l yuses l i th ic i sop le ths .A s o m e w h a t s i m p l e r a n d y e t m o r e c o m p l e t e u s e o ft h e c l a s t - si z e d a t a c a n b e a c h i e v e d i f i s o p l e t h s a r e p l o t -ted in a ' ln d i am ete r -a rea 1 /2 ' ( ' ln D -A 1 /2 ') d iag ram . In -d e e d , n o t o n l y t h e t h ic k n e s s , b u t a l s o t h e t h e o re t i c a lg ra i n s i z e d i s t r i b u t i o n o f a f a l l d e p o s i t a s o b t a i n e d b yC a re y a n d S p a rk s (1 9 8 6 ) , t e n d s t o fo l l o w a s i m p l e e x -p o n e n t i a l d e c re a s e l a w (P y l e 1 9 8 9 ) . P o i n t s o n t h e ' l nD - A r e ' ) d i a g r a m t h e r e f o r e a p p r o x i m a t e t o a s tr a ig h tl in e t h e s l o p e o f w h i c h d e p e n d s o n c o l u m n h e i g h t a n dwi n d v e l o c i t y d u r i n g t h e e ru p t i o n . I n r e a l i ty , t h e i n t e r -p re t a t i o n o f p o i n t s r e l a t i v e t o p u m i c e i s o p l e t h s is m o rec o m p l e x d u e t o t h e a b o v e - m e n t i o n e d c o m p l i c a t i o n s .W i l s o n a n d W a l k e r ( 1 9 8 7 ) p r o p o s e d a m o d e l ( h e r en a m e d ' W W ' ) w h i c h r e l i e s o n d i f f e r e n t a s s u m p t i o n sa b o u t t h e v e l o c i t y d i s t r i b u t i o n i n s i d e th e c o l u m n , a n dwh i c h d o e s n o t t a k e i n t o a c c o u n t fu r t h e r r a d i a l e x p a n -s i o n i n t h e u m b re l l a r e g i o n . T h e i r t h e o re t i c a l c u rv e sa p p l y o n ly t o t h e m a t e r i a l r e l e a s e d b y t h e l o w e r p a r t o ft h e c o l u m n , w h i c h d o e s n o t e n t e r t h e u m b r e l l a r e g i o n .T h e a g r e e m e n t b e t w e e n t h e e s t i m a t e s o f m a s s f lu x o b -t a i n e d b y W W a n d C S i s g o o d f o r a ll th e m o d e l e de r u p t io n s ; h o w e v e r , W W m o d e l y i e ld s a c o lu m n h e i g h twh i c h i s s y s t e m a t i c a l l y l o we r b y a b o u t 1 5 % t h a n t h a to b t a i n e d b y t h e C S m o d e l . T h i s d i f f e r e n c e i s a t t r i b u t e db y W i l s o n a n d W a l k e r (1 9 8 7) t o t h e f a c t t h a t t h e C Sm o d e l c o r r e l a t e s t h e m a s s f l u x t o t h e h e i g h t o f t h e c o -l u m n b y a s s u m i n g a g r e a t e r v a l u e fo r t h e f r a c t i o n o fp a r t i c l e s r e t a i n e d i n t h e c o l u m n a t a g i v e n h e i g h t , a n dh e n c e a s s ig n i n g a h i g h e r e f f i c i e n c y t o t h e c o n v e r s i o n o ft h e rm a l i n t o m e c h a n i c a l e n e rg y .

App lication to the Pululagua basal fallou tF ro m F i g. 1 4 i t is p o s s i b l e t o o b t a i n , f o r e a c h m a s s f l u x ,t h e li m i ti n g v a l u e o f t h e p ro d u c t ' d i a m e t e r x d e n s i t y 'o f c l a s ts wh i c h , a c c o rd i n g t o W W , e n t e r t h e u m b re l l ar e g i o n a n d h e n c e u n d e rg o fu r t h e r r a d i a l e x p a n s i o n i na c c o r d a n c e w i t h t h e m o d e l o f f o r c e d i n tr u s io n . T h i sl i m i t i n g v a l u e c o r r e s p o n d s t o t h e i n t e r c e p t b e t w e e n ag i v e n m a s s d i s c h a rg e r a t e c u rv e a n d t h e d i a g o n a ld a s h e d l i ne o n F i g . 1 4. F o r c l a s ts w i t h a ' d i a m e t e r xd e n s i t y ' v a l u e l o we r t h a n t h e l i m i t i n g v a l u e , C S t r e n d sd o a p p l y , a n d t h e s e c l a s t s t h e re fo re w i l l f o l l o w a n e x -p o n e n t i a l d e c re a s e l a w a s s h o wn b y P y l e (1 9 8 9 ) . I n t h er a n g e o f m a s s f l u xe s r e l a te d t o c o l u m n h e i g ht s b e t w e e n3 0 a n d 3 6 k m (a b o u t 1 .4 -5 x 1 08 k g s -1 ) _ _ t h e p ro b a -b l e r a n g e fo r t h e f i r st p l i n i a n e ru p t i o n o f P u l u l a g u a - -a n d a s s u m in g a c o n s t a n t d e n s i t y o f 2 50 0 k g m - 3 f o rl it h ic c l a st s , t h e m a x i m u m d i m e n s i o n o f c l as t s wh i c hr e a c h t h e l e v e l o f n e u tr a l b u o y a n c y ra n g e s b e t w e e n 2a n d 3 c m i n d i a m e t e r . I f we n o w re f e r t o F i g . 1 5 b , wef ind tha t on ly the l as t fou r i sop le ths (2 .4 - , 1 .6 - , 0 .8 - and

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8 -12 16 20 24crosswind range ( km )F i g . 1 4 . C r o s s w i n d r a n g e v s t h e p r o d u c t ' d i a m e t e r x d e n s i t y ' f o rM L i s o p le t h s o f t h e B F ( a f te r W i l s o n a n d W a l k e r 1 9 8 7)

1 0

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F i g . 1 5 . ' S q u a r e r o o t o f i s o p l e t h a r e a v s a l n M P ; b l n M L ; c l n M e -d i a n d i a m e t e r ' d i a g r a m f o r t h e B F ( a f te r P y l e 1 9 89 )

0 .4 -cm ) l i e o n a s t r a i g h t l i ne , i n v e ry g o o d a g re e m e n twi t h b o t h W i l s o n a n d W a l k e r ' s ( 1 9 8 7 ) a n d P y l e ' s(1989) a rgumen ts . The sh i f t f rom th i s l ine i s smal l fo rt h e 3 .2 - c m i s o p l e t h , b u t i t b e c o m e s s i g n i f i c a n t fo rg re a t e r d i a m e t e r s . A s i m i l a r a l t h o u g h l e s s s h a rp t r e n di s s h o wn b y p u m i c e i s o p l e t h s (F i g . 1 5 a ) a s we l l a s t h ei s o p l e t h s fo r t h e m e d i a n d i a m e t e r o f g r a in s (F i g . 1 5 c ).F i g u re 1 5 c h a s b e e n o b t a i n e d b y c o n s i d e r i n g c i r c u la rc u rv e s w i t h a r a d i u s e q u a l t o t h e i s o p l e t h r a d i u s i n t h eS W d i r e c t i o n , i n a n a t t e m p t t o e l i m i n a t e t h e a b o v e -

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53330

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maximum downwind range (km)Fig. 16. 'Crosswind range-maximum downwind range' diagramfor ML isopleths of the BF (after Carey and Sparks 1986)

ment ioned complex i t ies caused by the process o f con-taminat ion by f ines . The most p rox imal po in t inFig. 15a, b represents an isopleth which is constructedon the basis o f on ly one measurement by assuming ageneral circular pattern. I t relates to the northeasternsection which was sheltered from ball ist ic dasts; evenin this case, the agreement with the general trend isvery good . The s lope g iven by the l inear t rend inFig. 15b is equal to 0.13, which corresponds to a 'clasthalf distance' bc (as defined by Pyle 1989) equal to3.01. By f i t t ing an empirical function between pointson theoretical dispersal curves from CS, Pyle (1989)def ines a re la t ionsh ip b e twe en bc and the he igh t Hb ofthe leve l o f neu tra l bu oyancy , which g ives Hb =

20.5 km. Fro m the re la t ion maximum column heigh t ,Ht = Hb/0 .7 (Sparks 1986) we then ob ta in Ht =29.3 kin. In Fig. 16 lithic data are plotted on the 'cross-wind range - dow nwind range ' d iagram of CS. In ac-cordance with the above discussion, the 6.4-cm isoplethyields a signif icantly lower column height (about21 km) as com pare d to th at o btain ed by the 3.2-, 1 .6-and 0.8-cm isopleths, which all give a column height of36 km. The d iscrepancy be twee n th is he igh t and th eone (H t = 29 km) prev iously ob ta ined is p robably dueto a numb er of causes. In th i sc ase Ht has been ob-ta ined d i rec t ly , whereas p rev iously i t was ob ta inedf rom Hb by means of a re la t ionsh ip tha t con ta ins amarg in of approximat ion . Fur therm ore , the d iagram inFig. 16 must also be constrain ed by clast density,whereas Fig. 15 lacks this dependence. In any case themodel es t imates o f co lumn heigh t a re p robably on lycor rec t to wi th in 10-20% due to the grea t number ofvariables which play a role in the process (Woods 1988;Carey and Sigurdsson 1989) . Thus the most p robablevalue for the maximum CS model co lumn heigh t canbe hypothesized to be about 32-33 km.If we now derive a mass f lux from this value by us-ing the eff iciency of transformation of thermal into me-chanical energy proposed by Sparks (1986), we ob-t a ined M =2 x l 0 8 k g s - 1. Th i s i s o b ta in ed f r o m th edistal distr ibution of ctasts, which is in good agreementwi th an exponent ia l decrease law.The distr ibution of clasts which did not reach neu-t ra l buoyancy he igh t bu t were re leased f rom lower re-g ions of the erup t ion co lumn, i s be t te r descr ibed by theWW model . The agreement wi th CS is very good forestimation of the mass discharge rate, the value ofwhich must be ob ta ined using on ly po in ts above thedashed l ine in Fig. 14. WW, however, assume a less ef-f ic ien t t ransformat ion of thermal in to mechanica l ener -gy and f rom the ir equat ion w e ob ta in Ht = 28 km.The va lues thus ob ta ined for the maximum columnheight , Ht = 32-33 km f rom CS and Ht = 28 km f romWW, should be considered as upper and lower l imi ts ,respect ive ly . The CS model g ives h igher co lumnheights since i t does not take into account heat lostfrom clasts in the lower regions of the column, whilethe WW model g ives lower model he igh ts s ince i t im-plies a greater entra inme nt of air which tends to atte-n u a te t h e u p war d m o m en tu m ( W i l so n an d W alk e r1987).

Dis cu s s i onCompar ison b e twe en models o f tephra d ispersa l f romeruptive columns (Sparks 1986; Carey and Sparks1986; Wilson and Walker 1987; Pyle 1989) and dataf rom the B F of Pu lu lagua shows tha t a s imple expon-en t ia l law seems su i tab le to descr ibe the th ickness o fthe deposit , whereas for clast dispersal an exponentialdecrease only occurs below a cr i t ical particle diameter .This f i ts with Wilson and Walker 's (1987) model andimplies som e restr ictions in the us e of the ' ln D- A 1/2'p lo ts to ob ta in maximum column heigh t . Indeed , we

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534emphasize the necessity of obtaining the exact slope onsuch a diagram, implying that the geometry of iso-pleths must be well established in proximal and distalzones. The use of only a few or just two isopleths is notsufficient as seemed in itially justified by Pyle's (1989)work . The same can be said about the use of crosswindand maximum downwind range diagrams proposed byCarey and Sparks (1986).It is not clear why some eruptions seem to show alinear trend from proximal to distal zones. It is possiblethat the presence of wind tends to modify the true dis-tribution of clasts, by acting with greater efficiency onsmaller clasts as a consequence of both their lowerinertia and longer time spent in the atmosphere. Insuch a case the value of the horizontal axis in a 'lnD-A 1/2' plot would increase progressively for distalpoints, and hence points tend to approximate to astraight line, the slope of which is less than that Of acurve for the same column height under no-wind con-ditions. This would produce an overestimation of co-lumn height. This idea is supported by the clast distri-bution pattern of the Taupo ultraplinian deposit. Hereclasts less than 5 cm in dia meter were transport ed tomore than 150 km from the vent (Walker 1980), whilesubjected to a wind with an estimated velocity of al-most 30 m s -1 (Carey and Sparks 1986), On a 'lnD-A v2' diagram the 4-cm isopleth (the most well con-strained distal one) is strongly displaced from the lin-ear trend in favour of a larger dispersion (Fig. 17). TheFogo A deposit is also of particular interest since itrepresents a nothe r case of no-wind conditions during aplinian eruption, and was used by Carey and Sparks(1986) to test their model. Unfor tuna tely the island ofSao Miguel (Azores) on which Fogo A is located hasan elongate f orm which provides only a partial pictureof the distribution pattern. The well-constrained lithicisopleths are on ly those greater t han 10 cm, and we canarrive only by extrapolation at the 5-cm isopleth(Walker and Croasdale 1971). The use of a diagramlike Fig. 14 is more suitable to furnish correct informa-tion on column energetics in such cases.The good aggrement between the CS and WWmodels for the determination of mass flux is con-firmed, and Fig. 15b suggests that a significant propor-tion of coarse clasts are released before reaching thetop of the column. This implies that the conversion of

100

E E" 0 5 10

~ JE

i i

20 40 60 8 0 100 120 140Fig. 17. 'lnML vs square root of isopleth area' diagram for Taupofall layer; data from Walker 1980

thermal into mechanical energy is better described byWW than by the equation proposed by Sparks(1986).

S u m m a r y a n d c o n c l u s i o n sThis work focused on the plinian Basal Fall (BF) ofPululagua. Its dispersal indicates a near absence ofwind during eruption; this leads to the possibility ofobtaining especially reliable data not complicated bythe effect of wind. The fall deposit is characterized ev-erywhere by a finer top passing gradationally into afine ash layer, which is interpreted to comprise the fi-nest portion of the material originally contained in thecolumn; this was favoured by the unusual no-wind con-ditions existing during the eruption. Grainsize charac-teristics and the dispersal pattern of the basal fall ofPululagua were investigated, and the results were ap-plied to models of clast dispersal. The major resultsare:1. the characteristics of the deposit concur to suggestnearly no-wind conditions during its emplacement;2. grainsize characteristics of the lapilli fall layer donot differ significantly from other plinian deposits. Theonly departure is shown by the trend of the mediandiameter of clasts, and is explained by contaminationby ash in the region northeast of the vent, possibly dueto partial collapses of the eruptive column whichformed surges;3. a simple exponent ial decrease of thickness with dis-tance as proposed by Pyle (1989) for plinian falls fitswell with the BF;4. the same law is followed by maximum lithic clastsbut only below a critical size of about 3 cm, suggestingthat only a small proportion of large clasts were able toreach the top of the column;5. point 4 implies that, in order to obtain columnheights by clast dispersal models, the clast distributionmust be well established in proximal and distal zones.Knowledge of o nly a few or just two isopleths, regard-less of their distance from vent, is not sufficient.Acknowledgements. The authors are grateful to INEMIN (Insti-tuto Nacional Ecuadoriano de Mineria) for having provided log-istical support and assistance during the fieldwork. Steven Carey,David Pyle and Judy Fierstein provided many helpful commentson a previous version of the paper. We also thank G La Volpefor having kindly provided fine-grained granulometric data witha Coulter Counter TAII instrument.

R e f e r e n c e sBarberi F, Cioni R, Rosi M, Santacroce R, Sbrana A, Vecci R(1989) Magmatic and phreatomagmatic phases in explosiveeruptions of Vesuvius as deduced by grain-size and compo-nent analysis of the pyroclasticdeposit. J Volcanol GeothermRes 38: 287-307Barberi F, Coltelli M, Ferrara G, Innocenti F, Navarro JM, San-tacroce R (1988) Plio-Quaternaryvolcanism n Ecuador. GeolMag 125 : 1-14

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C a r e y S , S i g u r d s s 0 n H ( 19 8 0 ) T h e R o s e a u A s h : d e e p - s e a t e p h r ad e p o s i ts f ro m a m a j o r e r u p t i o n o n D o m i n i c a , L e s s e r A n t i l l e sa r c. J V o l c a n o l G e o t h e r m R e s 7 : 6 7 - 8 6C a r e y S , S i g u r d s s o n H ( 19 8 7 ) T e m p o r a l v a r i a t io n s o f c o l u m nh e i g h t a n d m a g m a d i s c h a r g e r a t e d u r i n g t h e 7 9 A D e r u p t i o no f V e s u vi u s. G e o l S o c A m B u l l 9 9 : 3 0 3- 3 1 4C a r e y S , S ig u r d s s o n H ( 19 8 9 ) T h e i n t e n s i t y o f p l i n i a n e r u p t i o n s .B u l l V o l c a n o l 5 1 : 2 8 - 4 0C a r e y S , S p a r k s R S J ( 1 9 8 6) Q u a n t i t a t i v e m o d e l s o f t h e f a l l o u ta n d d i s p e r sa l o f t e p h r a f r o m v o l c a n ic e r u p t i o n c o l u m n s . B u l lV 0 1 c a n o l 4 8 : 1 0 9 - 1 2 5C a r e y S , S i g u r d s s o n H , S p a r k s R S J ( 1 9 8 8 ) E x p e r i m e n t a l s t u d i e so f p a r ti c l e l a d e n p l u m e s . J G e o p h y s R e s 9 3, B 1 2 : 1 5 3 1 4 -15 328C r a n d e l l D R , M u l l i n e a u x D R ( 19 7 3) P i n e C r e e k v o l c a n i c a s se m -b l a g e a t M o u n t S t . H e l e n s , W a s h i n g t o n . U S G e o l S u r v B u l l1 3 8 3 - A : 1 - 2 3F i s h e r R V , G l i c k e n H X , H o b l i t t R P ( 19 87 ) M a y 1 8 , 19 8 0 , M o u n tS t H e l e n s d e p o s i t s in S o u t h C o l d w a t e r C r e e k , W a s h i n g to n . JG e o p h y s R e s 9 2 , B 1 0 : 1 0 2 6 7 -1 0 2 8 3F r o g g a t t P C ( 1 9 82 ) R e v i e w o f m e t h o d s o f e s t im a t i n g r h y o l i t i ct e p h r a v o l u m e s ; a p p l i c a ti o n s t o t h e T a u p o V o l c a n i c Z o n e ,N e w Z e a l a n d . J V o l c a n o l G e o t h e r m R e s 1 4 : 3 0 1 - 3 1 8G e o t e r m i c a I t a l i a n a - I N E M I N ( 19 89 ) M i t ig a c i o n d e l R i e s g o V o l -c a n i c o e n e l A r e a M e t r o p o l i t a n a d e Q u i t o . M i n i s t e r o d e g l iA f f a r i E s t e r i- D G C S . P i s aH a l l M L (1 9 77 ) E 1 v o l c a n i sm o e n e l E c u a d o r . I P G M S e c c i on N a -c i o n a l d e l E c u a d o r , Q u i t oH o u g h t o n B F , W i l s o n C J N ( 19 8 9 ) A v e s i c u l a r i t y i n d e x f o r p y r o -c l a s t ic d e p o s i t s. B u l l V o l c a n o l 5 1 : 4 5 1 - 4 6 2H o w o r t h R ( 1 9 7 5) N e w f o r m a t i o n s o f L a t e P l e i s t o c e n e t e p h r a sf r o m t h e O k a t a i n a V o l c a n i c C e n t r e , N e w Z e a l a n d . N Z JG e o l G e o p h y s 1 8 (5 ) : 6 8 3 -7 1 2I s a a c s o n J S ( 19 8 7) V o l c a n i c a c t i v i t y a n d h u m a n o c c u p a t i o n o f th eN o r t h e r n A n d e s : t h e a p p l i c a t io n o f t e p h r o s t ra t i g r a p h i c t e c h -n i q u e s t o t h e p r o b l e m o f h u m a n s e t t l e m e n t i n t h e W e s t e r nM o n t a f i a d u r i n g th e E c u a d o r i a n F o r m a t i v e . P h D t h e si s, U n i -v e r s i t y o f I l l in o i sM o r t o n B R , T a y l o r G , T u r n e r J S ( 19 56 ) T u r b u l e n t g r a v i t a t i o n a lc o n v e c t i o n f ro m m a i n t a i n e d a n d i n s ta n t a n e o u s s o u r c e s . P h ilT r a n s R S o c L o n d o n S e r A 2 3 4 : 1 - 2 3P y l e D M ( 19 8 9) T h e t h i c k n e s s , v o l u m e a n d g r a i n s i z e o f t e p h r af a l l d e p o s i t s . B u l l V o l c a n o l 5 1 : 1 - 1 5R o s e W I , B o n i s S , S t o i b e r R E , K e l l e r M , B i c k f o r d T ( 1 97 3 ) S t u d -i e s o f v o l c a n i c a s h f r o m t w o r e c e n t C e n t r a l A m e r i c a e r u p -t i o n s . B u l l V o l c a n o l 3 7 : 3 3 8 - 3 6 4R o s i M ( 19 9 2) A m o d e l f o r t h e f o r m a t i o n o f v e s i c u l a t e d tu f f b yc o a l e s c e n c e o f a c c r e t i o n a r y l a p il l i. B u l l V o l c a n o l 5 4 : 4 2 9 - 4 3 4S e l f S , S p a r k s R S J ( 19 7 8 ) C h a r a c t e r i s t ic s o f w i d e s p r e a d p y r o c l a s -t i c d e p o s i t s f o r m e d b y t h e i n t e r a c t i o n o f s i li c ic m a g m a a n dw a t e r . B u l l V o l c a n o l 4 1 : 1 9 6 - 2 1 2

53 5S e t t l e M ( 1 97 8 ) V o l c a n i c e r u p t i o n c l o u d s a n d t h e t h e r m a l o u t p u to f e x p l o s i v e e r u p t i o n s . J V o l c a n o l G e o t h e r m R e s 3 : 3 0 9 - 3 2 4S i m k i n T , S i e b e r t L , M c C l e l l L , B r id g e D , N e w h a l l C , L a t t e r J H( 1 9 8 1 ) V o l c a n o e s o f t h e w o r l d . S t r o u d s b u r g , P e n n s y l v a n i a :H u t c h i n s o n R o s s 2 3 2 p pS p a r k s R S J ( 19 8 6) T h e d i m e n s i o n s a n d d y n a m i c s o f v o l c a n i ce r u p t i o n c o l u m n s . B u l l V o l c a n o l 4 8 : 3 - 1 5S u z u k i T , K a t s u i Y , N a k a m u r a T ( 19 7 3 ) S i z e d i s tr i b u t i o n o f t h eT a r u m a i T a - b p u m i c e - f a l l d e p o s i t, B u l l V o l c a n o l S o c J a p a n18: 47-64V i l l a l b a M ( 19 8 8) C o t o c o l l a o : u n a a l d e a f o r m a t i v a d e l v a l l e d eQ u i t o . M u s e o d e l B a n c o C e n t r a l d e l E c u a d o r , Q u i t oV u c e t i c h C G , P u l l a r W A (1 9 73 ) H o l o c e n e t e p h r a f o r m a t i o n se r u p t e d i n th e T a u p o a r e a , a n d i n t e r b e d d e d t e p h r a s f ro m o t h -e r v o l c a n i c s o u r c e s . N Z J G e o l G e o p h y s 1 6 : 7 4 5 - 7 8 0W a l k e r G P L ( 19 7 1) G r a i n s i z e c h a r a c t e r i s t i c s o f p y r o c l a s t i c d e -p o s i t s. J G e o l 7 9 : 6 9 6 - 7 1 4W a l k e r G P L ( 1 9 8 0 ) T h e T a u p o p u m i c e : p r o d u c t o f t h e m o s tp o w e r f u l k n o w n ( u l t r a p l i n i a n ) e r u p t i o n ? J V o l c a n o l G e o -t h e r m R e s 8 : 69-94W a l k e r G P L ( 1 9 8 1 ) P l i n i a n e r u p t i o n s a n d t h e i r p r o d u c t s . B u l lV o l c a n o l 4 4 - 2 : 2 2 3 - 2 4 0W a l k e r G P L ( 19 8 3) I g n i m b r i t e t y p e s a n d i g n i m b r i te p r o b l e m s . JV o l c a n o l G e o t h e r m R e s 1 7 : 6 5 - 8 8W a l k e r G P L , C r o a s d a l e R ( 1 9 71 ) T w o P l i n i a n - t y p e e r u p t i o n s i nt h e A z o r e s . J G e o l S o c L o n d o n 1 2 7 : 1 7 -5 6W i l l i a m s H ( 19 6 0 ) V o l c a u l c h i st o r y o f t h e G u a t e m a l a n H i g h -l a n d s . U n i v C a l i f P u b l G e o l S c i 3 8 : 1 - 8 7W i l s o n L ( 1 9 72 ) E x p l o s i v e v o l c a n i c e r u p t i o n s - - I I . T h e a t m o s -p h e r i c t r a j e c t o r i e s o f p y r o c l a s t s . G e o p h y s J R a s t r S o c30: 381-392W i l s o n L ( 1 9 7 6) E x p l o s i v e v o l c a n i c e r u p t i o n s - - I I I . P l i n i a ne r u p t i o n c o l u m n s . G e o p h y s J R a s t r S o c 4 5 : 5 4 3 - 5 5 6W i l s o n L ( 1 9 8 0) R e l a t i o n s h i p s b e t w e e n p r e s s u r e , v o l a t i l e c o n t e n ta n d e j e c t a v e l o c i t y i n t h r e e t y p e s o f v o l c a n i c e x p l o s i o n s . JV o l c a n o l G e o t h e r m R e s 8 : 2 9 7 - 3 1 3W i l s o n L , H u a n g T C ( 19 7 9) T h e i n fl u e n c e o f s h a p e o n t h e a t m o -s p h e r i c s e t t l i n g v e l o c i t y o f v o l c a n i c a s h p a r t i c l e s . E a r t h P l a n e tS c i L e t t 4 4 : 3 1 1 - 3 2 4W i l s o n L , W a l k e r G P L ( 19 87 ) E x p l o s i v e v o l c a n ic e r u p t i o n s - -V I . E j e c t a d i s p e r s a l i n p l i n i a n e r u p t i o n s : t h e c o n t r o l o f e r u p -t i o n c o n d i t i o n s a n d a t m o s p h e r i c p r o p e r t i e s . G e o p h y s J R a s t rS o c 8 9 : 6 5 7 -6 7 9W i l s o n L , S p a r k s R S J , H u a n g T C , W a t k i n s N D ( 19 78 ) T h e c o n -t r o l o f v o l c a n ic c o l u m n h e i g h t b y e r u p t i o n e n e r g e t i c s a n d d y -n a m i c s . J G e o p h y s R e s 8 3 : 1 8 2 9 - 1 8 3 5W o o d s A W (1 9 88 ) T h e f l u i d d y n a m i c s a n d t h e rm o d y n a m i c s o fe r u p t i o n c o l u m n s . B u l l V o l c a n o l 5 0 : 1 6 9 - 1 9 3E d i t o r i a l r e sp o n s i b i l i t y : B . F . H o u g h t o n