The influences of planetary environments on the eruption styles of volcanoes

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Vistas in A3tronomy, Vol. 27, pp. 333-360, 1984 0083-6656/84 $0.00 +.50 Printed in Great Britain. All rights reserved. Copyright © 1985 Pergamon Press Ltd. THE INFLUENCES OF PLANETARY ENVIRONMENTS ON THE ERUPTION STYLES OF VOLCANOES Lionel Wilson Department of Environmental Science, University of Lancaster, Lancaster LA1 4YQ, England ABSTRACT Volcanism appears to have been a major process in the formation of surface materials on each of the silicate-dominated planets and satellites in the solar system at some stage in its development. Observations of the types and extents (in space and time) of volcanic activity on a planetary body can provide important information on the interior structure and chemistry, and on the near-surface environmental conditions. Attempts to develop deter- ministic models of volcanic mechanisms which are sufficiently general to be applicable to eruptions of a given kind on any planet have been a major spur to understanding the geological processes involved. We are currently able to give fairly complete descriptions of some eruption processes, in the forms in which they have operated on several planets. INTRODUCTION One of the many important results of the last one and a half decades of planetary exploration has been the finding (Basaltic volcanism study project, 1981) that volcanic episodes very like some of those which currently occur on Earth have, at various times, played major roles in the formation of many readily identifiable surface features on the Moon (Head, 1976) and Mars (Carr, 1981; Greeley and Spudis, 1981) and may also have been responsible for the emplacement of substantial fractions of those parts of the surfaces of Mercury (Strom et al., 1976) and Venus (Phillips and Malin, 1983; Solomon and Head, 1982) which have been studied in detail so far. In addition, the Voyager missions have revealed that Io, the innermost of Jupiter's four large satellites, is currently volcani- cally active, probably as a result of the strong tidal heating to which it is subjected (Peale et al., 1979): here, the readily detected eruption products are compounds involving sulphur (Smith et al., 1979), but there is every reason to think that more traditional silicate-type volcanism underlies and drives the activity that is seen. Finally, there are various indications that, on the colder, lower-density satellites of Jupiter (Ganymede, Callisto and Europa), and on some of the moons of Saturn, liquid water may at some stage have been "erupted" onto the surfaces as a result of the melting of solid water-ice in the interiors (Parmentier and Head, 1979; Stevenson, 1982). Thus, volcanism in one form or another may be ubiquitous on the larger solar system bodies having solid surfaces. The present review is concerned with the ways in which the planetary environmental conditions of gravity (determined by size and mass), internal chemical composition, atmos- pheric pressure (see Table l), and atmospheric composition combine to influence the nature of volcanic activity observed at the surface. Our current understanding of many of these often complex dependencies is only qualitative, in that we observe certain empirical corre- lations between planetary conditions and eruption styles but do not yet understand in detail how the former control the latter. In other cases, however, we do have a good enough quantitative understanding of the chains of cause and effect to be able to interpret 333

Transcript of The influences of planetary environments on the eruption styles of volcanoes

Page 1: The influences of planetary environments on the eruption styles of volcanoes

Vistas in A3tronomy, Vol. 27, pp. 333-360, 1984 0083-6656/84 $0.00 +.50 Printed in Great Britain. All rights reserved. Copyright © 1985 Pergamon Press Ltd.

THE INFLUENCES OF PLANETARY ENVIRONMENTS ON THE ERUPTION STYLES

OF VOLCANOES

Lionel Wi lson

Department of Environmental Science, University of Lancaster, Lancaster LA1 4YQ, England

ABSTRACT

Volcanism appears to have been a major process in the fo rmat ion of sur face m a t e r i a l s on each o f the s i l i c a t e - d o m i n a t e d p lane ts and s a t e l l i t e s in the so la r system at some stage in i t s development. Observat ions o f the types and ex ten ts ( i n space and t ime) o f vo l can i c a c t i v i t y on a p l a n e t a r y body can p rov ide impor tan t i n f o r m a t i o n on the i n t e r i o r s t r u c t u r e and chemis t ry , and on the near -su r face env i ronmenta l c o n d i t i o n s . Attempts to develop d e t e r - m i n i s t i c models o f vo l can i c mechanisms which are s u f f i c i e n t l y genera l to be a p p l i c a b l e to e rup t i ons o f a g iven kind on any p lane t have been a major spur to unders tanding the g e o l o g i c a l processes i nvo l ved . We are c u r r e n t l y ab le to g ive f a i r l y complete d e s c r i p t i o n s of some e r u p t i o n processes, in the forms in which they have operated on severa l p lane ts .

INTRODUCTION

One o f the many impor tan t r e s u l t s o f the l a s t one and a h a l f decades of p l a n e t a r y

e x p l o r a t i o n has been the f i n d i n g ( B a s a l t i c vo lcanism study p r o j e c t , 1981) t h a t vo l can i c

episodes ve ry l i k e some o f those which c u r r e n t l y occur on Earth have, a t va r ious t imes,

p layed major r o l es in the fo rma t ion of many r e a d i l y i d e n t i f i a b l e sur face fea tu res on the

Moon (Head, 1976) and Mars (Carr , 1981; Gree ley and Spudis, 1981) and may a lso have been

respons ib le f o r the emplacement o f s u b s t a n t i a l f r a c t i o n s of those par ts o f the surfaces

of Mercury (Strom e t a l . , 1976) and Venus ( P h i l l i p s and Mal in , 1983; Solomon and Head,

1982) which have been s tud ied in d e t a i l so f a r . In a d d i t i o n , the Voyager missions have

revea led t h a t Io, the innermost o f J u p i t e r ' s fou r la rge s a t e l l i t e s , is c u r r e n t l y v o l c a n i -

c a l l y a c t i v e , p robab ly as a r e s u l t o f the s t rong t i d a l hea t ing to which i t is sub jected

(Peale et a l . , 1979): here, the r e a d i l y de tec ted e r u p t i o n products are compounds i n v o l v i n g

su lphur (Smith e t a l . , 1979), but t he re is every reason to t h i nk t h a t more t r a d i t i o n a l

s i l i c a t e - t y p e volcanism unde r l i e s and d r i ves the a c t i v i t y t ha t is seen. F i n a l l y , the re

are va r ious i n d i c a t i o n s t h a t , on the co lde r , l o w e r - d e n s i t y s a t e l l i t e s o f J u p i t e r (Ganymede,

C a l l i s t o and Europa), and on some o f the moons o f Saturn, l i q u i d water may at some stage

have been "e rup ted " onto the sur faces as a r e s u l t o f the me l t i ng of s o l i d w a t e r - i c e in

the i n t e r i o r s (Parment ie r and Head, 1979; Stevenson, 1982). Thus, volcanism in one form

or another may be ub iqu i t ous on the l a r g e r s o l a r system bodies having s o l i d sur faces.

The present rev iew is concerned w i th the ways in which the p l a n e t a r y env i ronmenta l

c o n d i t i o n s o f g r a v i t y (determined by s ize and mass), i n t e r n a l chemical compos i t ion , atmos-

pher i c pressure (see Table l ) , and atmospher ic compos i t ion combine to i n f l uence the nature

of v o l c a n i c a c t i v i t y observed a t the sur face. Our c u r r e n t unders tanding of many of these

o f ten complex dependencies is on ly q u a l i t a t i v e , in t h a t we observe c e r t a i n emp i r i ca l co r re -

l a t i o n s between p l a n e t a r y c o n d i t i o n s and e r u p t i o n s t y l es but do not ye t understand in

d e t a i l how the former c o n t r o l the l a t t e r . In o the r cases, however, we do have a good

enough q u a n t i t a t i v e unders tand ing o f the chains o f cause and e f f e c t to be ab le to i n t e r p r e t

333

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Table I . P lanetary environmental f ac to rs i n f l uenc ing vo lcan ic e rup t ions .

Planet : Mercury Venus Earth Moon Mars Io

Grav i t y / (m/s2) : 3.7 8.8 9.8 1.6 3.7 1.8

Atmospheric pressure/Pa: small 5-9 x 106 7-10 x 104 small 2-6 x lO 2 small

Note: atmospheric pressures are given fo r the ranges of e leva t ions at which vo lcan ic features are found; ' s m a l l ' imp l ies t ha t the atmosphere exer ts a n e g l i g i b l e in f luence on the d ispersa l o f vo lcan ic e jec ta .

observat ions of e rup t ion cond i t i ons at the surface in terms of at leas t some aspects of

the nature and or ig in of the magmas invo lved. We can then th ink in terms of using vo lcan ic

erupt ions as remote sensin 8 probes of p lane ta ry i n t e r i o r cond i t i ons . In what f o l l ows ,

an at tempt is made to descr ibe a l l the major types of vo lcan ic a c t i v i t y observed on the

Earth and the o ther p lanets and to i nd i ca te the ex ten t to which a q u a n t i t a t i v e understand-

ing ex i s t s of the processes invo lved.

GLOBAL PERSPECTIVES

The Moon

Analys is of the lunar lava f low samples returned from the j u d i c i o u s l y s i ted Apo l lo

landing po in ts has shown tha t these ma te r i a l s probably a l l belong to the class of h igh-

metal , l o w - s i l i c a content igneous rocks ca l l ed basa l ts (Tay lor , 1975). Most of the impact

basins formed dur ing the f i r s t 500 Ma of lunar h i s t o r y were apparent ly f looded by extens ive

lava f lows dur ing the subsequent 2000 Ma (F ig . l ) . Given tha t the Moon, l i k e the Earth

(and, by i m p l i c a t i o n , the res t of the p lane ts ) formed about 4500 Ma ago, i t seems tha t

s i g n i f i c a n t vo lcan ic a c t i v i t y was conf ined to roughly the f i r s t ha l f of lunar h i s t o r y .

Most of the erup t ions appear to have taken place from f i ssu res - narrow, l i n e a r vents many

hundreds of metres long but on ly a few metres wide - under cond i t i ons which made i t rare

f o r vo lcan ic cones to bu i l d up around the vent (Wilson and Head, 1981).

Mercury

A s i m i l a r v a r i a t i o n of a c t i v i t y w i th t ime probably occurred on Mercury which, l i k e

the Moon, has probably always been devoid of an atmosphere. Various geo log ica l un i ts

cons i s t i ng of r e l a t i v e l y f l a t , smooth p la i ns - f o rm ing ma te r i a l s are commonly assumed to

be the resu l t s of voluminous erupt ions of lava. though the r eso lu t i on of the Mariner I0

images used to make t h i s i d e n t i f i c a t i o n is not good enough to show the vent s t ruc tu res

from which the erup t ions presumably took p lace. S t r a t i g r a p h i c s tud ies (Strom et a l . . 1975)

place the vo lcan ic p la ins in the ea r l y par t o f Mercury's geo log ica l h i s t o r y , but lack of

samples fo r da t ing and chemical study means t ha t only r e l a t i v e ages can be assigned to

the supposed vo lcan ic un i t s . The only evidence r e l a t i n g to the chemistry of these mater ia l~

cons is ts of some low reso lu t i on spect roscop ic measurements which are at l eas t cons is ten t

w i th a b a s a l t i c composi t ion (Hapke et a l . , 1975).

Mars

Both the Moon and Mercury are r e l a t i v e l y small bodies, having 0.012 and 0.055 of

the mass and 0.27 and 0.38 of the d iameter of the Earth, r espec t i ve l y . Mars, however,

is s u b s t a n t i a l l y l a rge r , having 0.107 of the Ear th 's mass and 0.53 of i t s d iameter .

Thermal model l ing of the evo lu t i on of p lane ta ry i n t e r i o r s supports the idea, borne out

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Planetary Environments and Volcanic Eruption Styles 335

Figure I . The south-east edge of the lunar Mare Imbrium. Dark, f l a t mare mater ia ls, consist ing of layers of basa l t ic lava flow deposits, embay o lder , b r igh ter high- land mater ia l . Linear and arcuate fractures and fault-bounded val leys cut both mare and highland un i ts . In the centre of the frame, which is approximately 160 km square, the meandering va l ley of the Hadley sinuous r i l l e is interpreted as marking the path of a turbu lent lava flow which thermally eroded i t s way down- wards in to the underlying st rata. NASA Apollo 15 mission metric camera frame 0586

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336 L. Wilson

Figure 2. The summit area of the Martian volcano Hecates Tholus. The complex of in te r - locking collapse calderas marking the summit measures about I I by 9 km. Small impact craters cover most of the flanks of the volcano but are absent from the west and southwest sectors where they are blanketed by what appears to be an a i r - f a l l ash deposit up to I00 m thick. The deposit width is about 70 km, implying an eruption cloud of about the same height and, consequently, a magma eruption rate of about 3 x lO 7 kg/s. The eruption mu~t have lasted for at least 20 days to produce the estimated ash volume of 65 km ~ (Mouginis-Mark et a l . , 1982). NASA Viking Orbiter frames 651A17-21.

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Planetary Environments and Volcanic Eruption Styles 337

Figure 3. A volcanic eruption cloud on Io seen projected above the limb of the planet. The cloud is about 300 km high and 600 km wide, and i t s edge marks the outer envelope of the paths of small par t ic les ejected from the vent at speeds up to I000 m/s. NASA Voyager photo.

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Planetary Environments and Volcanic Eruption Styles 339

by the study o f the sur face f ea tu res seen in Mar iner 9 and V ik ing 1 and 2 images, t ha t

Mars has had a c t i v e v o l c a n i c cent res over a much l a r g e r f r a c t i o n o f i t s h i s t o r y than the

sma l l e r wor lds (e .g . Young and Schubert, 1974). Again, no samples are a v a i l a b l e f o r

l a b o r a t o r y a n a l y s i s on Earth (un less we accept the proposal t h a t c e r t a i n me teo r i t es were

de r i ved from the mar t ian sur face - Ryder, 1982); however, morpho log ica l ev idence is un-

ambiguous in con f i rm ing the ex i s tence of seven very la rge (and numerous sma l le r ) vo l can i c

mountains (e .g . F ig. 2) o f the k ind r e f e r r e d to as sh ie ld volcanoes (Carr , 1973), t y p i f i e d

on Earth by the p redominan t l y b a s a l t i c volcanoes of the Hawaiian i s land chain. S t r a t i -

g raph ic s tud ies , combined w i th a t tempts a t r e l a t i v e and abso lu te da t i n 8 by count ing the

numbers o f small impact c ra te r s on va r ious geo log i ca l un i t s (Neukum and H i l l e r , 1981),

show t h a t the ages of these fea tu res p robab ly range from the o rder o f 4 Ga to as l i t t l e

as I00 Ma. Mars c u r r e n t l y has a t h i n , carbon d i o x i d e dominated atmosphere w i th a mean

sur face pressure o f about 600 Pa and the re are some i n d i c a t i o n s t ha t the pressure may have

been h igher in the d i s t a n t past (Breed e t a l . , 1982). Var ious fea tu res suggest ive of

vo lcano-atmosphere i n t e r a c t i o n s have been i d e n t i f i e d on the sur face (Reimers and Komar,

1979; Mouginis-Mark e t a l . , 1982).

IO

Since Io has a mean d e n s i t y s i m i l a r to t ha t o f the Moon but is somewhat sma l le r in

s ize , a l l reasonable thermal models o f i t s development would p r e d i c t t h a t any vo l can i c

a c t i v i t y was conf ined to i t s e a r l y h i s t o r y . However, the Voyager images have shown t h a t ,

c u r r e n t l y , about ten vo l can i c vents may be a c t i v e at any one t ime on Io (F ig . 3). Peale

et a l . (1979) have demonstrated t h a t a major heat source f o r t h i s a c t i v i t y is the f l e x i n g

o f the body o f Io caused by the mot ion o f the t i d a l bulge induced in i t by J u p i t e r ; though

o the r heat sources may a lso be requ i red to e x p l a i n the t o t a l amount o f heat which Io appears

to be r a d i a t i n g (S in ton , 1981).

Venus

The same thermal mode l l i ng c a l c u l a t i o n s t h a t are c o n s i s t e n t w i th a long h i s t o r y o f

v o l c a n i c a c t i v i t y on Mars cannot avo id suggest ing t ha t Venus should c u r r e n t l y have a ra te

of heat loss from i t s i n t e r i o r s i m i l a r to t h a t o f the Earth (Solomon and Head, 1982).

We now know t h a t the Earth deals w i th the need to t r a n s p o r t heat through i t s r i g i d , ou te r

l aye rs ( the l i t h o s p h e r e ) by two methods. About 25% of the f l u x moves through the l i t h o -

sphere by conduct ion , w h i l e the res t is t r anspo r t ed by advec t i on , in t h i s case i n v o l v i n g

the phys ica l passage o f batches o f magma (mol ten or semi-mol ten rock) upwards to be

emplaced on the sur face as e x t r u s i v e ma te r i a l or near the sur face as i n t r u s i v e magma bodies.

I f Venus p a r t i t i o n s i t s heat f l u x between conduct ion and advec t ion in a s i m i l a r way to

the Earth then c l e a r l y a high l eve l o f vo l can i c a c t i v i t y is to be expected the re . Un fo r tun-

a t e l y , the presence o f an ex tens i ve carbon d iox ide -domina ted atmosphere, which has a sur face

pressure o f 5 to 9 MPa and is opaque to v i s i b l e wavelengths (Avduevsk iy e t a l . , 1983),

p revents a d i r e c t search f o r v o l c a n i c f ea tu res in spacecra f t images. Our on ly o p t i c a l

images of the sur face come from fou r o f the Sov ie t Union 's Venera s o f t - l a n d i n g probes,

and these i n e v i t a b l y show r e l a t i v e l y small reg ions - up to about a km 2 in area. No major

v o l c a n i c cons t ruc t s are seen at any of these s i t e s ; however, the composi t ion of the sur face

m a t e r i a l s , measured in some d e t a i l a t two o f the s i t e s (Veneras 13 and 14), is very s i m i l a r

to t h a t o f two types of t e r r e s t r i a l basa l t (Surkov et a l . , 1984), and i t is poss ib le t ha t

c e r t a i n s m a l l - s c a l e f ea tu res , such as d i s t i n c t l aye rs o f f i n e - g r a i n e d ma te r i a l w i th t h i c k -

nesses o f tens to hundreds of mm, may be of v o l c a n i c o r i g i n .

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340 L. Wilson

At present , the on ly method o f seeking l a r g e - s c a l e vo l can i c f ea tu res on Venus

i nvo l ves examinat ion of the p lane t by radar techniques (Masursky et a l . , 1980) which g i ve

data on topograph ic he igh t , sur face roughness and d i e l e c t r i c constant ( the l a t t e r being

determined in a f a i r l y complex way by the sur face chemis t r y ) . Most o f the sur face has

been mapped at a r e s o l u t i o n of about I00 km by the Pioneer Venus O r b i t e r spacecra f t , w h i l e

much sma l le r areas are being examined a t a r e s o l u t i o n of about 3 km using the Arec ibo

radar f a c i l i t y (Campbell e t a l . , 1984). The Venera 15 and 16 probes have r e c e n t l y mapped

about one t h i r d o f the sur face ( c h i e f l y the reg ion centred on the nor th po le ) a t a r e s o l u -

t i o n b e t t e r than I0 km. At the moment t he re is general agreement t ha t c e r t a i n major topo-

graph ic f ea tu res are very p robab ly vo l can i c sh ie lds , m o r p h o l o g i c a l l y s i m i l a r to , but

commonly l a r g e r than, those found on Mars and the Earth. However, the re is much less

agreement on the i d e n t i f i c a t i o n on Venus o f the ex tens i ve l i n e a r systems of r idges , small

vo l can i c mountains and t renches found on Earth in assoc ia t i on wi th the edges of the l i t h o -

spher ic reg ions ca l l ed p la tes .

Tecton ic environments on the Earth

The study o f t e c t o n i c processes - the causes and consequences of s t resses w i t h i n

the p l a n e t a r y l i t h o s p h e r e - has led to a p i c t u r e of the Ear th ' s l i t h o s p h e r e in which p l a te

t e c t o n i c s dominate. The r e l a t i v e l y r i g i d p la tes migra te sideways away from the l i n e a r

r idges, g e n e r a l l y conf ined to the cent res o f ocean basins, where new m a t e r i a l , predomi-

nan t l y b a s a l t i c in composi t ion, is added to t h e i r edges by vo l can i c ac t i on . To conserve

the p l a n e t a r y sur face area the re e x i s t a rcuate zones where one p l a t e is subducted beneath

another and recyc led back i n t o the i n t e r i o r . At these boundar ies, l i n e s of volcanoes

form as p a r t i a l me l t i ng of some of the subducted ma te r i a l occurs to generate magmas of

more s i l i c a - r i c h , meta l -poor composi t ion than basa l t s , descr ibed as andes i tes . The con-

t i n e n t a l areas of the Earth represen t r e l a t i v e l y l ow -dens i t y pa r ts o f the l i t h o s p h e r e

which, very rough ly speaking, r i de on the tops of moving p la tes and take l i t t l e pa r t in

subduct ion processes. Vo lcan ic cent res are found in numerous l o c a t i o n s on the Earth

besides p l a t e boundar ies, i nc lud ing places where p l a t e i n t e r i o r s are cut by f r a c t u r e zones,

and reg ions of s t ress or i n c i p i e n t r i f t i n g in c o n t i n e n t a l b locks. A wide v a r i e t y of magma

composi t ions, spanning the range between basa l t s and andes i tes , is found in these reg ions;

fu r the rmore , the p r o t r a c t e d chemical and phys ica l e v o l u t i o n of batches of magma in rese r -

v o i r s in the upper pa r t o f the l i t h o s p h e r e can produce melts ca l l ed r h y o l i t e s , which are

more s i l i c a - and v o l a t i l e - r i c h than andes i tes , in most o f the above t e c t o n i c envi ronments.

Tecton ic envi ronments on o the r p lane ts

We are c u r r e n t l y not c e r t a i n whether the apparent absence of E a r t h - l i k e p l a t e

t e c t o n i c f ea tu res on Venus is a consequence o f the inadequate r e s o l u t i o n of the a v a i l a b l e

data sets or is a t r ue r e f l e c t i o n o f some fundamental d i f f e r e n c e between the s t r u c t u r e s

of the l i t h o s p h e r e s of Venus and the Earth. Our i n a b i l i t y to p r e d i c t the d e t a i l s o f

l i t h o s p h e r i c t e c t o n i c processes on Venus, even though we appear to have an adequate under-

s tanding o f p l a t e t e c t o n i c processes on the Earth, is a good i n d i c a t i o n of the lack of

a t r u l y genera l model o f p l a n e t a r y l i t h o s p h e r e s . E m p i r i c a l l y , we can note t h a t the

presence or absence o f p l a t e t e c t o n i c processes does not c o r r e l a t e in a s imple way w i th

p l a n e t a r y heat f l u x : p l a t e t e c t o n i c processes are not on ly absent from the Moon, Mars

and Mercury ( a l l o f which have had sma l le r heat f l uxes than the Ear th) but a lso from Io,

even though i t is es t imated t h a t the heat f l u x produced in l o ' s i n t e r i o r by t i d a l d i s s i -

pa t i on (and p o s s i b l y o the r ) e f f e c t s is a t l e a s t an o rder o f magnitude g r e a t e r than t h a t

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produced in the Earth by rad iogen i c processes (Peale et a l . , 1979; S inton, 1981). On

Io, severa l hundred small v o l c a n i c cent res are sca t te red across the sur face (Schaber,

1982) and one speaks of a pa t t e rn o f ho t - spo t vo lcanism. On Mars, the re are two major

vo l can i c p rov inces w i t h i n which g e o l o g i c a l l y recent a c t i v i t y has concent ra ted (Carr , 1981)

and, wh i l e these areas show signs o f f r a c t u r i n g and updoming of the l i t h o s p h e r e , l i t t l e

or no l a t e r a l movement o f sec t ions o f the c rus t is apparent , On the Moon, volcanism

appears to have been almost e n t i r e l y assoc ia ted w i th the deep-seated f r a c t u r e s around

the margins of major impact basins and c r a t e r s , and the lunar l i t h o s p h e r e seems to have

been p a r t i c u l a r l y s tab le over most o f lunar h i s t o r y . As noted e a r l i e r , we cannot ye t

be sure o f the s p a t i a l arrangement o f the anc ien t vo l can i c cent res on Mercury; but again,

t he re is no sign o f recogn isab le fea tu res of p l a t e t e c t o n i c processes.

VOLCANIC ERUPTION STYLES

D e f i n i t i o n s o f e rup t i on s t y l e s (see, f o r example, Macdonald, 1972; Walker, 1973a;

Se l f and Sparks, 1978; Wi l l iams and McBirney, 1979) are based almost e n t i r e l y on obser -

va t i ons o f e rup t i ons and e rup t i on products on the Ear th. This is not s u r p r i s i n g : not

on ly are events on Earth v a s t l y eas i e r to s tudy than elsewhere ( i t w i l l be r e c a l l e d t h a t

the on ly o the r body a c t u a l l y known to be c u r r e n t l y producing e rup t i ons is Io ) , but a lso

the Earth may have a w ider range o f common magma chemis t r i es , and hence o f d i f f e r e n t s t y l es

of a c t i v i t y , than is or has been the case f o r any o the r body. This l a t t e r p o i n t cannot

be proved u n t i l such t ime as we have very much more i n f o rma t i on ( v i a sur face sampling

missions or o r b i t a l - s a t e l l i t e remote sensing methods) about the sur face chemis t r i es o f

Mars, Venus and Mercury; but i t is s i g n i f i c a n t t h a t the commonest vo l can i c landforms

detec ted on the Moon and Mars (and by assumption Mercury) are b u i l t up ma in ly from long

lava f lows assoc ia ted w i th a s i ng le , major form of a c t i v i t y - the r e l a t i v e l y high e f f us i on

ra te e rup t i on o f b a s a l t i c magma ( B a s a l t i c volcanism study p r o j e c t , 1981).

The complete l i s t o f f a c t o r s i n f l u e n c i n g e r u p t i o n s t y l e inc ludes such items as:

the rheo logy o f the magma ( i . e . , the way in which the magma responds to shear ing s t resses

- a f u n c t i o n of composi t ion and tempera tu re ) ; the f ree , i . e . , exso lved, gas con ten t o f

the magma (a f unc t i on o f i t s composi t ion and the ex te rna l pressure ac t i ng on i t , the l a t t e r

being in tu rn dependent on the p l a n e t a r y atmospher ic pressure and, to a l esse r ex ten t ,

on the p l a n e t a r y g r a v i t y ) ; the presence or absence, on the sur face near the vent , o f v o l a -

t i l e compounds ( the l i s t c o n s i s t i n g e s s e n t i a l l y o f l i q u i d water on Earth, water ice on

Earth and Mars, carbon d i o x i d e ice on Mars, su lphur and su lphur d i o x i d e on Io) w i th which

the magma can i n t e r a c t ; and the v a r i a t i o n of atmospher ic pressure and d e n s i t y w i th he igh t .

The ways in which these f a c t o r s combine to con t ro l e rup t i on s t y l es is becoming reasonably

we l l - unde rs tood f o r the Ear th.

I t i s , o f course, poss ib le t h a t the combinat ions of these f a c t o r s occu r r i ng on some

o the r p l a n e t a r y sur face may lead to an e rup t i on s t y l e fundamenta l l y d i f f e r e n t from any

on Ear th; however, no evidence f o r t h i s s t a te o f a f f a i r s has ye t been found from observa-

t i ons or t h e o r e t i c a l s tud ies . What is c l e a r is t h a t i t w i l l be dangerous to t r y to app ly

e r u p t i o n s t y l e c l a s s i f i c a t i o n s developed s p e c i f i c a l l y f o r the Earth to e rup t i ons elsewhere

w i t h o u t some m o d i f i c a t i o n s . In p a r t i c u l a r , i t has been found usefu l to u t i l i s e f a c t o r s

such as the g ra in s ize d i s t r i b u t i o n o f e jec ted fragments of rock and the ex ten t o f t h e i r

d i spe rsa l to c h a r a c t e r i s e va r ious types o f e x p l o s i v e e rup t i on on Earth (Walker, 1973a);

we now know enough about the i n f l u e n c e o f the presence or absence of an atmosphere on

these d i spe rsa l processes to be c e r t a i n t h a t such c l a s s i f i c a t i o n schemes w i l l need major

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342 L. Wilson

m o d i f i c a t i o n f o r use elsewhere. A much sa fe r approach is to de f i ne the va r ious e rup t i on

s t y l e s , using our t e r r e s t r i a l exper ience, in terms o f the basic physics of the processes

i nvo l ved . An a t tempt is made to do t h i s in what f o l l o w s .

Sub-surface magma genera t i on

The l e a s t we l l - unde rs tood aspect o f vo lcanism on Earth concerns the exact c i rcum-

stances under which p a r t i a l me l t i ng o f rocks takes p lace at depth and the melted f r a c t i o n

migra tes upwards to accumulate in s u b s t a n t i a l magma bodies. However, i t seems l i k e l y

t h a t magma segregat ion takes p lace on l y a f t e r some minimum amount o f p a r t i a l me l t i ng has

occurred (Tu rco t t e and Ahem, 1978). The buoyant r i s e of the r e s u l t i n g pods of accumu-

la ted l i q u i d is b e t t e r understood ( e . g . , Marsh and Kantha, 1978) and i nvo l ves a d e l i c a t e

balance between heat losses from the r i s i n g body i n t o the surroundings, heat p roduc t ion

as a r e s u l t o f i n t e r n a l convect ion in the v iscous l i q u i d , and d e n s i t y changes as minera l

g ra ins form and s ink or f l o a t through the c o o l i n g l i q u i d . I t appears t ha t the upward

r i se speed of magma bodies e v e n t u a l l y becomes so small t ha t they can be considered s t a t i o n -

ary in the upper pa r t o f the l i t h o s p h e r e . They are then ca l l ed magma r e s e r v o i r s or magma

chambers, and t h e i r subsequent h i s t o r y depends e n t i r e l y on t h e i r chemical and phys ica l

e v o l u t i o n . In p a r t i c u l a r , the f l o t a t i o n or s e t t l i n g of c r y s t a l s formed as the l i q u i d

cools represents a chemical zonat ion o f the magma chamber, and v o l a t i l e species such as

H20 and CO 2 can migra te through the l i q u i d in an a t tempt to reach chemical e q u i l i b r i u m .

An e rup t i on can occur whenever a s u f f i c i e n t l y wide pathway opens up between such

a magma chamber and the sur face. The pathway may open due to the b r i t t l e f r a c t u r i n g of

the o v e r l y i n g rocks as some accumulat ing s t ress is r e l i e v e d . The s t ress may be produced

at l eas t in pa r t by reg iona l t e c t o n i c forces ( r e l a t e d to p l a te motions on Ear th) . A l t e r n a -

t i v e l y , the s t ress may r e s u l t from some process i n v o l v i n g pressure bu i l d -up in the magma

chamber i t s e l f : a v o l a t i l e phase may become supersa tura ted and begin to exso lve to form

compressed gas bubbles in the upper pa r t o f the magma; new magma may be i n j e c t e d at the

base of the chamber; or the re may be volume changes ins ide the chamber as a r e s u l t o f

the growth o f c r y s t a l s . Whatever the reason, magma sets out towards the sur face through

the f resh ly -opened cracks and f i s su res , aided by the f ac t t ha t i t is g e n e r a l l y less dense

than the sur rounding, co lder , s o l i d c rus ta l rocks. I f the magma r i s e speed is too slow,

or the f i s s u r e is too narrow, the magma w i l l coo l , becoming e x c e s s i v e l y v iscous, be fore

i t can reach the sur face and an i n t r u s i v e , r a t h e r than e x t r u s i v e , even% w i l l have occurred.

Stress accumulat ion w i l l then resume from a l eve l c lose to t h a t present before the

f r a c t u r e took p lace. I f a l eng thy d ischarge of magma to the sur face occurs, however,

most o f the s t ress w i l l p robab ly be re laxed and a new cyc le of magma accumulat ion or evo l u -

t i o n w i l l begin.

Near-vent e r u p t i v e processes

Basa l t i c magmas, which appear to occur commonly on the p lane ts , have g e n e r a l l y low

v i s c o s i t i e s and are able to r i s e at r e l a t i v e l y high speeds through the c rus t in response

to the pressures or d e n s i t y d i f f e r e n c e s d r i v i n g them to the sur face. When the magma con-

t a i ns l i t t l e gas, and emerges smoothly and q u i e t l y a t the vent , the e rup t i on is descr ibed

as e f f u s i v e and a lava f low is formed. The f low moves away from the vent down the s teepest

a v a i l a b l e s lope at a speed determined by the va lue of the s lope and the r h e o l o g i c a l p roper -

t i e s o f the lava . In some cases the wid th of the f low is c o n t r o l l e d by the p r e - e x i s t i n g

topography; but even i f t he re are no na tu ra l obs tac les , the non-Newtonian shear ing p roper -

t i e s o f t y p i c a l lava p revent the f low from spreading sideways i n d e f i n i t e l y .

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Planetary Environments and Volcanic Eruption Styles 343

The f low cools by conduct ion i n t o the unde r l y i ng ground at i t s base, by convect ion

i n t o the sur round ing atmosphere (on p lane ts w i th atmospheres) from i t s s ides and top,

and by r a d i a t i o n from a l l i t s sur faces. I t s forward motion is e v e n t u a l l y ha l ted when

waves o f coo l i ng have propagated i n t o the i n t e r i o r to a s u f f i c i e n t ex ten t to produce a

major change in the r h e o l o g i c a l p r o p e r t i e s at the cen t re . Normal ly , the heat l o s t by

the f low to the unde r l y i ng ground does not have a dramat ic e f f e c t on the subs t ra te - though

i t may evapora te ice or snow or bo i l groundwater to the ex ten t o f causing l o c a l i s e d exp lo -

s ions. However, i f the e f f us i on ra te of the lava is high enough and i t s res i s tance to

shear is low enough, i t s motion may be t u r b u l e n t , thus enhancing the ra te o f heat t r a n s f e r

to the ground. I f the t r a n s f e r ra te is high enough, the temperature of the ground sur face

may reach the p o i n t where p a r t i a l me l t i ng begins (Hulme, 1973) and the f low w i l l then

begin to erode the subs t ra te to form a channel i n t o which the f low p r o g r e s s i v e l y subsides.

The e ros ion process is very slow, on the o rder o f microns per second, but i f the e rup t i on

cont inues f o r a long t ime a deep channel can be formed. Indeed, g iven a long enough dura-

t i o n , the process can even occur i f the mot ion of the lava is not t u r b u l e n t (Carr , 1974).

This mechanism appears to have been respons ib le f o r the fo rmat ion of the sinuous r i l l e

channels on the Moon (Hulme, 1973) and has r e c e n t l y been suggested as the cause of the

development o f n i cke l su lph ide ore depos i t s in assoc ia t i on w i th f l u i d k o m a t i i t e lavas

dur ing the e a r l y h i s t o r y o f the Earth (Huppert e t a l . , 1984).

In genera l , b a s a l t i c magmas con ta in enough gas (ma in ly H20 and CO 2 - Moore, 1979)

to ensure t h a t they are v e s i c u l a r ( i . e . , con ta in t rapped gas bubbles) and t h a t the re is

a t l e a s t a small c o n t r i b u t i o n to the e r u p t i o n speed of the magma from the expansion of

the gas. The v i s c o s i t y o f such magmas is o f ten low enough t h a t the gas bubbles can r i se

through the l i q u i d a t an app rec iab le ra te r e l a t i v e to t h a t a t which the l i q u i d is i t s e l f

r i s i n 8 through the c rus t . Bubbles may coalesce and, as they become b igger , they r i s e

ever f a s t e r , o v e r t a k i n g more small bubbles. The gas and l i q u i d components o f the magma

may become segregated in t h i s way, w i th la rge gas bubbles emerging e p i s o d i c a l l y a t the

sur face to cause t r a n s i e n t exp los ions on a t ime sca le of seconds to tens of minutes

(Blackburn e t a l . , 1976). Explos ions of t h i s type are descr ibed as S t rombo l ian . In c i rcum-

stances where the gas bubbles and the l i q u i d magma are not ab le to segregate e f f i c i e n t l y ,

the m ix tu re may emerge f a i r l y s t e a d i l y a t the sur face at a speed which can be as high

as I00 m/s; the r e s u l t is a f i r e - f o u n t a i n o f incandescent c l o t s o f d i s rup ted magma ex tend-

ing to a he igh t o f severa l hundred metres above the vent in an e rup t i on of the type known

as Hawaiian. Depending on the d ischarge ra te and the s izes of the magma c l o t s , they may

cool in f l i g h t and c o l l e c t around the vent to form a c inde r cone or sco r ia cone; a l t e r n a -

t i v e l y the f i r e f oun ta i n may be o p t i c a l l y t h i c k , in the sense t h a t heat r ad ia ted from

a magma c l o t w i t h i n the assemblage is blocked from escaping by the presence o f the sur -

rounding c l o t s (Wilson and Head, 1981), and in t h i s case most o f the c l o t s w i l l reach

the ground hot enough to s t i c k to one another , forming a welded sco r ia cone, or even to

coalesce i n t o a cont inuous sheet which may move away from the vent as a lava f low.

The more s i l i c a - r i c h magmas (andes i tes and r h y o l i t e s ) have h igher v i s c o s i t i e s than

basa l t s and r i se much more s l ow l y through the c rus t , commonly forming t h i c k , s l ow l y spread-

ing lava f lows or bulbous s t r u c t u r e s over the vent ca l l ed domes. Gas bubbles forming

in such magmas cannot m ig ra te r e l a t i v e to the l i q u i d a t any app rec iab le ra te (Sparks,

1978) and so, i f the magmas are s u f f i c i e n t l y gas - r i ch , they e v e n t u a l l y form a foam of

t rapped gas bubbles which becomes mechan ica l l y uns tab le as the bubbles expand, d i s i n t e -

g r a t i n g at some sha l low depth i n t o a m ix tu re of re leased gas, f i n e - g r a i n e d p a r t i c l e s (ash)

and pieces of v e s i c u l a r magma ca l l ed pumice. The process of gas re lease acts to increase

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344 L. Wilson

the v i s c o s i t y o f the l i q u i d (Sparks and P inker ton , 1978) to the po in t where i t becomes

e f f e c t i v e l y r i g i d , so t ha t the bubble s t r u c t u r e is preserved in the pumice as i t coo ls .

The gas expansion causes the ash and pumice fragments (known c o l l e c t i v e l y as p y r o c l a s t i c

fragments (F i she r and Schmincke, 1984) or j u s t p y r o c l a s t s ) to emerge at the sur face at

high speeds in a ve ry exp los i ve e rup t i on of the type termed P l i n i a n (Walker and Croasdale,

1971) a f t e r the d e s c r i p t i o n by P l i n y the Elder o f the AD79 e rup t i on of Vesuvius.

When h i g h - v i s c o s i t y magmas which are not ve ry gas - r i ch t r y to e rup t , they are o f ten

not ab le to d i s i n t e g r a t e con t i nuous l y in the P l i n i an e rup t i on s t y l e . Indeed, i f they

are e x c e s s i v e l y c h i l l e d dur ing t h e i r slow r i s e to the sur face they may f reeze in the upper

pa r t o f the vo l can i c plumbing system, so t h a t the near -sur face magma becomes an obs tac le

to the emergence o f the somewhat h o t t e r ma te r i a l beneath. In such cases, gas may accumu-

l a t e in the reg ion beneath the c h i l l e d cap and a high enough pressure may be generated

to cause a sudden, v i o l e n t exp los ion which acce le ra tes the o v e r l y i n g cap - and much of

the surrounding rock - outwards a t a high speed in a cannon - l i ke b l a s t o f the type termed

Vulcanian ( S e l f e t a l . , 1979).

Any of the above modes o f e rup t i on can be rendered more e x p l o s i v e than i t might

o therw ise have been by a l l ow ing the magma at , or near, the sur face to i n t e r a c t w i th v o l a -

t i l e m a t e r i a l s (water or water ice on Earth, water ice or carbon d i o x i d e ice on Mars,

su lphur or s o l i d su lphur d i o x i d e on Io ) : the magmatic heat evaporates the l i q u i d or s o l i d

and produces an expanding vapour phase which conver ts thermal energy to k i n e t i c energy.

The thermal shock o f con tac t w i th the co ld v o l a t i l e s may lead to a more thorough f r a g -

mentat ion o f the magma than would o therw ise have been the case, and the r e s u l t i n g fragment~

are then c a r r i e d away in the expanding vapour stream. Whenever exp los ions i n v o l v e near -

sur face v o l a t i l e s in t h i s way they are ca l l ed p h r e a t i c ; i f an apprec iab le amount o f f resh

magma reaches the sur face dur ing the event the term phreato-magmatic is used. T rans ien t

exp los ions occu r r i ng when magma erupts under a sha l low l aye r o f water have been ca l l ed

Sur tseyan (Wright e t a l . , 1981), a f t e r the s t y l e o f a c t i v i t y seen dur ing the fo rmat ion

of the i s land of Surtsey o f f Ice land in 1963. The more ene rge t i c and, p robab ly , more

nea r l y cont inuous exp los i ve events caused by P l i n i a n a c t i v i t y in sha l low water are des-

c r ibed as p h r e a t o - P l i n i a n ( S e l f and Sparks, 1978).

I t may have been noted t h a t phrases l i k e " t r a n s i e n t " and "more nea r l y cont inuous"

have been used in the d e f i n i t i o n s of severa l exp los ive e rup t i on s t y l es in the above d iscus-

s ion. This h i g h l i g h t s the f ac t t h a t the term " e x p l o s i v e " is used in vo l cano logy f o r both

extremes o f the process whereby the re lease of gas pressure causes ma te r i a l to be erupted

at high speed. In the s h o r t e s t - l i v e d exp los ions , pressure is re leased by the r up tu r i ng

of some r e t a i n i n g l a y e r - o f c h i l l e d , v iscous magma or coherent , und is turbed coun t ry rock

( e . g . , Bennet t , 1974; Se l f e t a l . , 1979; E i che lbe rge r and Hayes, 1982; K i e f f e r , 1982)

- and the c h a r a c t e r i s t i c t ime sca le o f the exp los ion is equal to the s ize of the magma

body in which the excess pressure was present d i v i d e d by the speed of sound in the t rapped

gas ( t h i s being the maximum speed at which any d is tu rbances can propagate in i t ) . As

soon as the accumulated excess pressure is re leased, the exp los ion is a t an end. At the

o the r extreme, Hawai ian and P l i n i a n e rup t i ons , which can con t inue f o r tens of minutes

to tens o f hours, a lso i n v o l v e the expansion o f a gas phase, but t h i s occurs much more

nea r l y u n i f o r m l y over the l a s t severa l tens to severa l hundreds of metres of r i s e of the

magma to the sur face vent and the re may never be an excess ive pressure b u i l d - u p anywhere

in the system. The speed of magma r i s e is l i m i t e d ma in ly by f r i c t i o n e f f e c t s along the

wa l l s o f the condu i t or f i s s u r e through which the magma moves, and the du ra t i on of the

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Planetary Environments and Volcanic Eruption Styles 345

e rup t i on is equal to the mass o f a v a i l a b l e magma d i v i ded by the average ra te a t which

i t is erupted.

Pyroc las t -a tmosphere i n t e r a c t i o n s

The p y r o c l a s t i c ma te r i a l produced in any exp los ion may i n t e r a c t w i th the atmosphere

( i f one e x i s t s ) in one or more of t h ree ways. F i r s t , r e l a t i v e l y coarse fragments w i l l

be acce le ra ted by the expanding gas to on ly r e l a t i v e l y low v e l o c i t i e s due to t h e i r l a rge

masses and w i l l t r a v e l away from the vent on paths t h a t are c lose to being b a l l i s t i c ,

i . e . , c o n t r o l l e d on l y by g r a v i t y . These p y r o c l a s t s w i l l , however, be sub jec t to drag

fo rces in the atmosphere which w i l l reduce t h e i r ranges (Wilson, 1972). They w i l l c o l l e c t

around the vent t o form a cone or a p r o n - l i k e depos i t . The second mode of atmospher ic

i n t e r a c t i o n occurs at the o the r end of the s ize spectrum, where very small ash p a r t i c l e s

w i l l have t h e i r motions dominated by the drag fo rces exer ted on them. They w i l l be

acce le ra ted to nea r l y the same speeds as the expanding gases in exp los ions and w i l l con-

t i nue to f o l l o w the mot ions o f these gases as they mix w i th the surrounding atmosphere.

Heat w i l l be l o s t very e f f i c i e n t l y from the small f ragments because of t h e i r la rge su r face -

area to volume r a t i o s and t h i s heat may decrease the d e n s i t y o f the in-mixed atmospher ic

gas to the p o i n t where an in tense convec t ion c loud can form over the vent , c a r r y i n g f i n e

m a t e r i a l up to a g rea t he igh t in the form of a P l i n i a n e rup t i on c loud. The cloud spreads

sideways as i t r i ses and is e v e n t u a l l y pushed downwind. The s o l i d ma te r i a l t h a t i t con-

t a i n s is re leased p r o g r e s s i v e l y , the sma l l es t p a r t i c l e s being c a r r i e d to the g r e a t e s t

he igh ts and d is tances , and reaches the ground to form an a i r - f a l l p y r o c l a s t i c depos i t .

The s p a t i a l d i s t r i b u t i o n of the s izes o f the py roc l as t s in such a depos i t p rov ides v i t a l

i n f o rma t i on f o r c l a s s i f y i n g the type and v i o l e n c e of the e rup t i on .

The t h i r d mode of i n t e r a c t i o n between exp los ion products and an atmosphere is one

in which a la rge p r o p o r t i o n of small p y r o c l a s t s is formed, but the e rup t i on cond i t i ons

are such t h a t the m ix tu re of erupted gas and p a r t i c l e s is much denser than the surrounding

atmosphere and cannot e n t r a i n and heat the sur rounding atmospher ic gas f a s t enough to

ensure t ha t a s tab le , convec t ing e rup t i on c loud can form. Instead, a much lower, f o u n t a i n -

l i k e c loud forms over the vent , and from t h i s s t r u c t u r e a m ix tu re o f hot p a r t i c l e s , v o l -

can ic gas and some en t ra ined atmospher ic gas descends to the ground in the form of a dense

but f l u i d , ground-hugging cloud c a l l e d a p y r o c l a s t i c f low. A cons ide rab le amount o f segre-

ga t i on o f gas and p a r t i c l e s occurs in the ou te r par ts o f such a f oun ta i n (Sparks e t a l . ,

1978; F isher , 1979; Sher idan, 1979), l ead ing to a lower, r e l a t i v e l y dense, p a r t i c l e -

dominated l a y e r and an upper, more d i l u t e , gas-dominated l a y e r in the p y r o c l a s t i c f low

produced. Small and i n t e r m e d i a t e s ized c l as t s are t r anspo r ted f o r a t l e a s t tens of km

in many types of p y r o c l a s t i c f low, whereas very la rge rock c l as t s commonly s e t t l e very

q u i c k l y through the body o f the f low and are emplaced c lose to the vent as lag depos i t s

(Wright and Walker, 1977). A wide v a r i e t y o f p y r o c l a s t i c depos i ts can be produced in

events o f t h i s k ind (Wright e t a l . , 1981), from r e l a t i v e l y t h i n , f i n e - g r a i n e d p y r o c l a s t i c

surge depos i t s (F i she r , 1979) to the much more massive and ex tens i ve rock bodies c a l l e d

i g n i m b r i t e s (Wilson and Walker, 1982). As a la rge p y r o c l a s t i c f low t r a v e l s away from

the vent ( a t a speed which is l i k e l y to l i e in the range 30 to 300 m/s - Sparks et a l . ,

1978), gas and small ash p a r t i c l e s escape from i t s upper sur face and take advantage of

a second chance to i n t e r a c t c o n v e c t i v e l y w i th the atmosphere, the p a r t i c l e s u l t i m a t e l y

land ing over a wide area to form a c o - i g n i m b r i t e a i r - f a l l depos i t (Walker, 1972). Ca l cu la -

t i o n s show t h a t the co l l apsed -c l oud c o n d i t i o n is the consequence of an e x c e s s i v e l y low

gas con ten t in the erupted g a s - p y r o c l a s t m ix tu re , or o f an e x c e s s i v e l y wide vent and,

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346 L. Wilson

hence, high magma d ischarge ra te (Sparks and Wilson, 1976; Sparks e t a l . , 1978; Wilson

et a l . , 1980). Whenever the a p p r o p r i a t e c o n d i t i o n s are met dur ing an e rup t i on - commonly

towards the end - the changeover from a high, convect ing cloud to a low, co l lapsed cloud

can occur. I t is a lso c l ea r t h a t , dur ing some t e r r e s t r i a l e rup t i ons , c o n d i t i o n s may

o s c i l l a t e back and f o r t h between the two s ta tes (Freundt and Schmincke, 1984); a l t e r n a -

t i v e l y , f i e l d ev idence (Walker and Croasdale, 1971; Wilson and Walker, 1982; Nairn and

Se l f , 1978) suggests t ha t par ts o f the same e r u p t i o n c loud may be in one s ta te and par ts

in the o the r s t a te a t the same t ime.

MODELLING ERUPTION PROCESSES

The fo rego ing d e s c r i p t i o n o f vo l can i c e rup t i on s t y l es ( f o r the most pa r t in the

form t h a t they take on Ear th) is o f l i t t l e va lue in i t s e l f f o r unders tanding e rup t i ons

on o the r p lane ts : a t best a q u a l i t a t i v e idea o f the i n f l uence of the env i ronmenta l cond i -

t i ons can be ob ta ined, I t is on ly when e x p l i c i t phys ica l - and, subsequent ly , mathemat ica l

- d e s c r i p t i o n s o f the processes are made (such d e s c r i p t i o n s being termed models) t h a t

q u a n t i t a t i v e statements can be made. In vo l cano logy , the re is a s t rong t r a d i t i o n of

deve lop ing such models on a gene ra l i sed , m u l t i - p l a n e t basis ( e . g . , McGetchin and U l l r i c h ,

1973; Wilson and Head, 1981; K i e f f e r , 1982), In the subsequent sec t ions , the major models

o f t h i s k ind t h a t are c u r r e n t l y a v a i l a b l e w i l l be o u t l i n e d ; i l l u s t r a t i o n s w i l l be g iven

of the ways in which these models p r e d i c t , or a l l ow us to i n t e r p r e t , e rup t i on s t y l e v a r i a -

t i ons between the var ious p lane ts .

Masma mot ion in the l i t h o s p h e r e

A l l magmas are a complex m ix tu re o f l i q u i d s i l i c a t e s , en t ra ined c r y s t a l s (o f high

me l t i ng p o i n t m ine ra l s ) and bubbles o f exsolved gas. At s u f f i c i e n t l y g rea t depths in

the l i t h o s p h e r e the gas bubble con ten t may be n e g l i g i b l e , and in a ve ry hot magma the

c r y s t a l con ten t may be smal l , but in general a l l t h ree components are present . The rheo-

l o g i c a l p r o p e r t i e s of magmas ( i . e . , the ways in which they respond to shear ing fo rces )

are s i m i l a r l y complex, but are commonly model led as being those o f a Bingham p l a s t i c .

No deformat ion or f low takes p lace in such a ma te r i a l u n t i l the shear s t ress exceeds a

c h a r a c t e r i s t i c va lue ca l l ed the y i e l d s t reng th , Y. When f low does occur, the r a t i o o f

the shear s t ress in excess of the y i e l d s t reng th to the ra te o f s t r a i n is a cons tant ,

ca l l ed the p l a s t i c v i s c o s i t y , E.

I f a magma w i th these p r o p e r t i e s f lows upwards in a laminar fash ion through a f i s s u r e

w i th w id th W and length L (both measured in a h o r i z o n t a l p lane) , the speed, U, w i l l be

g iven by (Ske l land , 1967)

U : (W 2 G D - 2 Y W)/(12 E) ( I )

where D is the e f f e c t i v e d e n s i t y d i f f e r e n c e (between the d e n s i t i e s o f the sur rounding

c rus t and the magma) which is d r i v i n g the magma upwards ( t y p i c a l l y o f o rder 200 kg/m 3)

and G is the a c c e l e r a t i o n due to g r a v i t y . D may be j u s t equal to the d e n s i t y d i f f e r e n c e

when f r a c t u r e s are opened by t e c t o n i c forces ex te rna l to the magma r e s e r v o i r i t s e l f ; how-

ever , i f an excess pressure P in the magma chamber is respons ib le f o r i n i t i a t i n g the erup-

t i o n , D w i l l be equal to the t r ue d e n s i t y d i f f e r e n c e between c rus ta l rocks and magma plus

an ex t ra c o n t r i b u t i o n equal t o P/(G L) where L is the depth of the magma chamber roo f

below the sur face. Values of P up to about I0 MPa ( I00 bars) are l i k e l y on any of the

s i l i c a t e p lane ts , g iven t h a t the s t reng ths o f coherent rocks are of t h i s o rder or less,

and values of L are commonly a t l eas t 1 km. To g ive a p l a u s i b l e example, i f P is taken

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Planetary Environments and Volcanic Eruption Styles 347

as 6 MPa and L is equal to 3 km, then on Earth, w i th G = 9.8 m/s 2, the e x t r a c o n t r i b u t i o n

to D is about 200 kg/m 3, which happens to be the same s ize as a t y p i c a l t r ue d e n s i t y

d i f f e r e n c e . On the Moon, w i th i t s s i x - f o l d sma l l e r g r a v i t y than the Earth, the c o n t r i -

bu t ion from t h i s excess pressure w i l l be s ix t imes l a r g e r .

Equat ion ( I ) shows t h a t i f (½ W G D) is not g r e a t e r than Y, i . e . i f the s t resses

app l i ed to a magma do not exceed i t s y i e l d s t reng th , then i t w i l l not e rup t , even though

a pathway may be a v a i l a b l e to i t . C l e a r l y , f o r a g iven va lue o f W and a g iven magma

rheo logy , a small va lue of G w i l l lead to a reduced chance of e rup t ions t ak i ng p lace.

Conversely , t h i s means t h a t l a r g e r s t resses must be b u i l t up on l o w - g r a v i t y p lane ts be fo re

pathways a l l o w i n g magma to e rup t can be opened. I f these s t resses are due s p e c i f i c a l l y

to magma accumulat ion in l o c a l i s e d reg ions beneath a r i g i d l i t h o s p h e r e ( r a t h e r than p l a t e

t e c t o n i c processes which, as we have seen, have not occurred on the sma l le r p l ane ts ) t h i s

may in tu rn imply t h a t l a rge volumes o f magma must accumulate before an e r u p t i o n can s t a r t .

When f i s su res do then open, they are l i k e l y to be on average w ider than f i s su res on h i g h e r -

g r a v i t y p lane ts , and the r e s u l t i n g e rup t i ons w i l l i n v o l v e both l a r g e r erupted volumes

and h igher e r u p t i o n ra tes . I t is p robab ly s i g n i f i c a n t , t h e r e f o r e , t h a t the Moon, Mars

and Mercury a l l appear to have had episodes in t h e i r h i s t o r i e s in which ex tens i ve areas

of t h e i r sur faces were f looded by b a s a l t i c lava f lows, the g rea t lengths of which imply,

as we sha l l see l a t e r , t h a t they were emplaced from high e f f us i on ra te e rup t i ons .

E f f us i ve e rup t i ons

Once a lava f low forms on a p l a n e t a r y sur face ( e i t h e r by d i r e c t o v e r f l o w from the

vent or accumulat ion of hot magma c l o t s from a f i r e f o u n t a i n ) i t s form is determined by

the l oca l topography ( i f i t f lows i n t o a p r e - e x i s t i n g v a l l e y or depress ion) or , i f i t

moves o f f onto a r e l a t i v e l y smooth p l a i n , by i t s i n t e r n a l r h e o l o g i c a l p r o p e r t i e s . I f

p r e - e x i s t i n g topography c o n t r o l s the w id th o f a f low, d e f i n i n g a na tu ra l channel w i t h i n

which i t moves, the on ly independent f l ow parameter which can be p red i c ted is the mean

f low v e l o c i t y , which w i l l be determined in a complex way by the c r o s s - s e c t i o n a l shape

o f the channel , the s lope o f the f l o o r o f the channel and the r h e o l o g i c a l p r o p e r t i e s Y

and E: Johnson (1970) g ives many examples of these r e l a t i o n s h i p s . Because the mot ion

of a f low is so comp le te l y d i c t a t e d , in t h i s case, by the topography over which i t moves,

i t is not usefu l to a t tempt to compare the p r o p e r t i e s of such f lows having s i m i l a r e f f u s i o n

ra tes and rheo log ies but erupted on d i f f e r e n t p lane ts . Such a comparison can be p r o f i t -

ab ly made, however, in the a l t e r n a t i v e s i t u a t i o n of an e rup t i on onto a r e l a t i v e l y smooth

s lope ( i . e . , any e r u p t i o n f o r which the depth o f the f low which forms is much g r e a t e r

than the he igh ts o f the sur face i r r e g u l a r i t i e s over which i t moves). In these c i rcum-

stances, the lava forms i t s own s t a t i o n a r y l a t e r a l banks ( c a l l e d levees) in such a way

t h a t the levee he igh t , H, and w id th , B, are g iven by (Hulme, 1974):

H = Y/(G R A) (2)

B = H/A (3)

where A is the s lope of the ground beneath the f low and R is the lava d e n s i t y , I t must

be s t ressed t h a t the lava forming the levees is not n e c e s s a r i l y a p p r e c i a b l y coo le r than

t h a t in the cen t re o f the f low; the levees remain in p lace s imply because the s t ress l e v e l s

w i t h i n them are too small to overcome the y i e l d s t reng th .

The w id th , C, o f the cen t ra l channel w i t h i n which the lava moves is g iven by

C (24 F E / Y A 2) ½ (4)

where F is the volume of lava per second l eav ing the vent , and the equat ion app l i es as

long as the levees are w ider than the c e n t r a l channel (Wilson and Head, 1983), which is

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348 L. Wilson

commonly the case. The c e n t r e - l i n e th ickness of the f low, T, is g iven by

T = ( 2 - ~ R + H2)½ (5)

(Hulme, 1974) and the mean v e l o c i t y , V, o f the lava advancing down the cen t ra l channel

is g iven by the r a t i o o f the volume f l u x to the c r o s s - s e c t i o n a l area of the channel, which

leads to

V = F/ [ ~ y R (T3 - H3)] (6)

The maximum d is tance , X, from the ven t which can be reached by a lava f low is l i m i t e d

by coo l i ng , ma in ly from the f l o w ' s upper sur face. Theo re t i ca l arguments (Hulme and F ie l de r ,

1977) and f i e l d obse rva t i ons (Walker, 1973b; P inker ton and Sparks, 1976) suggest t h a t

the r e l a t i o n s h i p is

V T 2 X - 300 K (7)

where K is the thermal d i f f u s i v i t y o f the lava and the f a c t o r 300 has been determined

e m p i r i c a l l y .

Examinat ion of these equat ions shows t h a t the loca l g r a v i t y a c c e l e r a t i o n exe r t s

some con t ro l on H, B, T, V and X but not on C. The r e l a t i o n s h i p s are complex and are

best i l l u s t r a t e d by an example. Table 2 g ives the va lues of a l l the above v a r i a b l e s f o r

t y p i c a l small lava f lows on the Earth and Moon (which have the h ighes t and lowest g r a v i t y

a c c e l e r a t i o n s o f the s i l i c a t e p lane ts , r e s p e c t i v e l y ) . In both cases the f lows are formed

from a t y p i c a l b a s a l t i c lava having E = I00 Pa s, Y = 300 Pa and d e n s i t y 2800 kg/m 3 which

is being erupted a t a ra te o f I00 m3/s onto a sur face w i th s lope 0.01 rad ians (about 0.57

of a degree) . I t w i l l be seen t h a t , because o f the lower g r a v i t y , the lunar f low has

levees which are 6 t imes w ider and t h i c k e r than the t e r r e s t r i a l f low. The cen t ra l channel

o f the lunar f low has the same w id th as the t e r r e s t r i a l f low but is 4.7 t imes deeper.

The lava in the t e r r e s t r i a l f low moves down i t s channel r a t h e r more than 5 t imes f a s t e r

than i t s l unar e q u i v a l e n t , but the g r e a t e r th ickness and, hence, heat r e t e n t i o n of the

lunar f low a l lows i t to advance more than 4 t imes f u r t h e r than i t s t e r r e s t r i a l coun te rpa r t .

While these r a t i o s w i l l va ry w i th the exac t va lues taken f o r E, Y, F and A, the r e l a t i v e

t rends are preserved f o r a l l e rup t i on c o n d i t i o n s . Other f ac to r s being equal , Ma r t i an

and Mercur ian f lows would be expected to have p r o p e r t i e s i n t e r m e d i a t e between those of

t e r r e s t r i a l and lunar f lows, and f lows on Venus would be expected to be s i m i l a r to those

on Earth. There are some comp l i ca t i ons to t h i s p i c t u r e , due to the f ac t t h a t the atmos-

pheres o f the Earth, Mars and Venus a l l have d i f f e r i n g a b i l i t i e s to convect heat away

from the sur face of a f low whereas f lows on the Moon and Mercury could on ly lose heat

by d i r e c t r a d i a t i o n i n t o space; however, these d i f f e r e n c e s are not enough to change the

pa t t e rn o u t l i n e d above to a s i g n i f i c a n t e x t e n t (Wilson et a l . , 1984a).

Table 2. Comparison of lava f lows on the Earth and Moon produced by the e rup t i on of a lava w i th E = I00 Pa s and Y = 300 Pa a t a ra te o f I00 m3/s onto a s lope of 0.01 rad ians . Symbols are as de f ined in the t e x t .

Levee Levee Channel Channel Lava Flow Tota l Flow Planet Width Thickness Width Depth Speed Length Width

B/m H/m C/m T/m V/(m/s) X/km (C+2B)/km

Earth 109 1.09 200 1.51 0.380 4.15 0.42

Moon 657 6.57 200 7.06 0.073 17.39 1.52

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Planetary Environments and Volcanic Eruption Styles 349

The above p a t t e r n o f r e l a t i v e lava f low p r o p e r t i e s w i l l be changed d r a m a t i c a l l y ,

o f course, i f t he re are la rge sys temat ic d i f f e r e n c e s between the common va lues o f F ( the

e rup t i on r a t e ) , A ( the topograph ic s lope) or Y and E ( the r h e o l o g i c a l parameters) between

p lane ts . The on l y p l ane t from which we have rock samples is the Moon, and enough of these

samples are de r i ved from lava f lows to show t h a t , towards the end of the major era of

lunar volcanism (between 4 and 3 Ga ago), the vas t m a j o r i t y o f the lunar lavas were

b a s a l t i c (Tay lo r , 1975), w i th composi t ions and e rup t i on temperatures such as to g ive them

low values o f E and Y r e l a t i v e to present day basa l t s on Ear th. Most o f the geometr ic

p r o p e r t i e s of these f lows ( l eng th , w id th , th i ckness , e t c . ) can be measured from the photo-

graphs ob ta ined by the Apo l l o missions or the e a r l i e r O r b i t e r probes, and many o f the

f lows have g rea t lengths, in excess of 200 km. When the equat ions g iven above are used

to analyse these f lows (Hulme and F i e l d e r , 1977), i t is found t h a t , even w i th the appro-

p r i a t e a l lowances f o r lunar g r a v i t y and the low va lues o f E and ¥, the lengths and shapes

o f these f lows imply t h a t the e r u p t i o n ra tes which formed them were o f ten I00 t imes g r e a t e r

than p resen t -day b a s a l t i c e rup t i on ra tes on Ear th. This f i n d i n g adds we ight to the ideas,

mentioned in the prev ious sec t ion , t h a t these lunar e rup t i ons invo lved the accumulat ion

of unusua l l y la rge volumes of r e l a t i v e l y hot magma beneath a t h i c k , r i g i d c rus t .

The c u r r e n t s t a t e o f e x p l o r a t i o n o f Mars shows t ha t , l i k e the Moon, i t has many

long lava f lows exposed on i t s sur face. We do not have samples of these f lows f o r chemical

i d e n t i f i c a t i o n and age de te rm ina t i ons , but i n d i r e c t evidence suggests t h a t many of them

are o f b a s a l t i c composi t ion and were formed in r e l a t i v e l y high e rup t i on ra te events .

When we have a more complete p i c t u r e o f the chemis t ry o f the mar t ian lavas, perhaps from

the planned Mars Geochemical O r b i t e r miss ion which w i l l de tec t the r a d i a t i o n products

from n a t u r a l l y r a d i o a c t i v e elements in the mar t ian c rus t , we should be ab le to bu i l d up

a p i c t u r e o f the e v o l u t i o n of the mar t ian l i t h o s p h e r e . A s i m i l a r pa t t e rn of i n v e s t i g a -

t i o n is poss ib le f o r Mercury i f and when a s u i t a b l e o r b i t i n g probe is sent the re ; however,

a t the moment, we do not even have images at high enough r e s o l u t i o n to i d e n t i f y i n d i v i d u a l

lava f low boundar ies.

Perhaps the most i n t e r e s t i n g l e v e l o f amb igu i t y about the i d e n t i f i c a t i o n of lava

f lows is t h a t c u r r e n t l y e x i s t i n g f o r Venus. Radar measurements show e longa te patches

of ma te r i a l w i th d i f f e r i n g roughness and r e f l e c t i v i t y c h a r a c t e r i s t i c s which, a t the best

a v a i l a b l e r e s o l u t i o n of a few km, are c o n s i s t e n t w i th t h e i r being lava f lows (Campbell

e t a l . , 1984). However, i t is poss ib l e t h a t a t l eas t some of these fea tu res are debr is

f lows ( i . e . , l a n d s l i d e s ) or vo l can i c ash f lows. I f they are lava f lows, then the longer

ones must be products o f high e f f u s i o n ra te e rup t i ons l i k e the longes t f lows on the Moon

and Mars (Head and Wilson, 1984). C u r r e n t l y planned radar missions to Venus, t oge the r

w i th f u r t h e r Earth-based measurements, should enable these fea tu res to be i d e n t i f i e d

unambiguously dur ing the next few years .

Sinuous r i l l e f o rma t ion

The i n i t i a l morphology o f a lava f low w i l l be de f ined by a combinat ion of i t s rheo-

l o g i c a l c h a r a c t e r i s t i c s and the p r e - e x i s t i n g topography, as o u t l i n e d in the p rev ious

sec t i on . I f the f low mot ion is t u r b u l e n t , i t is r e l a t i v e l y easy to show t ha t the main

source of heat loss is i n i t i a l l y r a d i a t i o n from the sur face (Hulme, 1973). This is a

very good approx imat ion i f no atmosphere is p resent (as is the case on the Moon, where

sinuous r i f l e s were f i r s t recogn ised) , an adequate approx imat ion i f the atmospher ic d e n s i t y

i f not too high (on Mars and the Ear th ) , and a poor approx imat ion i f convec t i ve heat loss

from the f low sur face is s i g n i f i c a n t (as is the case f o r e rup t i ons on Venus - Head and

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350 L. Wilson

Wilson, 1984 - and f o r underwater e rup t i ons on Earth - Wilson e t a l . , 1984b). To the

ex ten t t ha t the approx imat ion holds, the mean lava temperature can be c a l c u l a t e d as a

f unc t i on o f d i s tance from the vent and the heat t r a n s f e r ra te i n t o the ground can be

obta ined a t any po i n t . This in tu rn determines the e ros ion ra te and, hence, the depth

o f the r e s u l t i n g channel : the g r e a t e s t depth, D M, occurs at the vent . Erosion can on l y

take p lace out to a f i n i t e d is tance , X M, from the vent so t h a t beyond t h i s d is tance the

f low remains above the p r e - e x i s t i n g sur face l e v e l .

Hulme (1973) has shown t ha t X M is d i r e c t l y r e l a t e d to the volume e f f us ion ra te ,

F, from the vent and the r a t i o o f the e r u p t i o n temperature of the lava , @E' to the me l t i ng

temperature o f the ground, @S:

3u ~ X M 8~ C F = (8)

R Z ( ] -o~ /e~)

where ~ is the Stefan-Bol tzmann constant , ~ is the e m i s s i v i t y o f the lava (c lose to u n i t y )

and Z is i t s s p e c i f i c heat . Since @S can be est imated f o r lunar m a t e r i a l s from measure-

ments on the Apo l l o samples (Tay lo r , 1975), F can be obta ined from X M i f (@S/@E) can be

determined. Cons idera t ion of the eroson ra te as a f unc t i on o f temperature shows t ha t

the depth o f the r i l l e channel , D L, a t a d i s tance X L from the vent can be expressed in

terms o f the depth at the vent , D M, and the t o t a l r i l l e length X M by:

E =

where

Thus (@E/@S) can be der ived by f i t t i n g measurements of r i l l e dep th as a f u n c t i o n of

d i s t a n c e from t he ven t to t h e s e e q u a t i o n s and t hen used in e q u a t i o n (8) to f i n d the mean

volume e r u p t i o n r a t e , F, d u r i n g the e v e n t which formed t he r i l l e . Measurements of t h i s

kind have been made f o r s e v e r a l l u n a r r i l l e s hav ing ×M in t he range 40 to 100 km (Head

and Wilson, 1981) and show t h a t l ava e r u p t i o n r a t e s of 104 to 105 m3/s must have been

i nvo lved , wi th t he e r u p t i o n s c o n t i n u i n g f o r t y p i c a l l y s e v e r a l months to produce t he

observed r i l l e d e p t h s of 100 to 300 m. The t o t a l l ava volumes produced in t h e s e e v e n t s

ranged up to a few thousand cub ic k i l o m e t e r s and each one r e p r e s e n t e d a s i g n i f i c a n t c o n t r i -

b u t i o n to t h e f l o o d i n g of t he l u n a r impact b a s i n s .

Var ious types of s inuous channe l have been i d e n t i f i e d on Mars (Baker , 1982), some

of which may be t h e r m a l l y eroded lava c h a n n e l s of the same kind as t he l u n a r s inuous r i l l e s

(Mouginis-Mark e t a l . , ]984) . I f t h i s i s t he c o r r e c t i n t e r p r e t a t i o n , t hen t he l e n g t h s

of some o f these r i f l e s imply e rup t i on ra tes up to 106 m3/s, even l a r g e r than the lunar

va lues.

For comparison, i t may be noted t ha t g e o l o g i c a l l y recent t e r r e s t r i a l b a s a l t i c erup-

t i ons have r a r e l y taken p lace at ra tes in excess of 104 m3/s; e rup t i on ra tes in the 104

to 108 m3/s range are c u r r e n t l y assoc ia ted w i th a n d e s i t i c to r h y o l i t i c magmas on Earth.

However, we have seen t h a t the lengths o f long, m o r p h o l o g i c a l l y normal, lava f lows on

the Moon and Mars imply t ha t very high e r u p t i o n r a t e b a s a l t i c e rup t i ons have taken p lace

on both o f these bodies at va r ious t imes in t h e i r h i s t o r i e s . The sinuous r i l l e channels

serve to emphasise t h i s conc lus ion . Perhaps the most e x c i t i n g recent development in

connect ion w i th the sinuous r i l l e f o rma t ion mechanism is the suggest ion (Huppert e t a l . ,

1984) t ha t thermal e ros ion of the subs t ra te was a common consequence of the emplacement

Page 19: The influences of planetary environments on the eruption styles of volcanoes

Planetary Environments and Volcanic Eruption Styles 351

of koma t i i t e lavas dur in 8 the ea r l y h i s t o r y of the Earth. These hot, low v i s c o s i t y

l i q u i d s form lava depos i ts which have unusual coo l ing tex tu res (probab ly re la ted to the

fac t t ha t t h e i r motion was t u r b u l e n t ) and which are o f ten associated w i th n ickel su lphide

m i n e r a l i s a t i o n . The i m p l i c a t i o n is t ha t the ore bodies were formed by reac t ion between

n ickel from the lava and sulphur which was incorporated as the f low melted and then ass imi -

la ted under ly ing su lphu r - r i ch sediments.

Steady exp los ive erupt ions

In a l l cases where a magma conta ins enough v o l a t i l e s to cause i t s continuous d i s -

rup t ion i n to p y r o c l a s t i c ma te r ia l and released gas by the t ime i t reaches the surface,

the e rup t ion products emerge from the vent at r e l a t i v e l y high speeds. The erup t ion w i l l

be c l a s s i f i e d as P l i n i an i f most of the p y r o c l a s t i c fragments are small enough to be

ent ra ined i n to the gas stream and Hawaiian i f a la rge f r a c t i o n of the pyroc las ts decouple

from the e rup t ion cloud to f o l l ow near ly b a l l i s t i c paths back to the surface. The gas

content requ i red to cause continuous d i s r u p t i o n is a func t ion of the atmospheric pressure

at the surface, and de ta i l ed c a l c u l a t i o n s ( e . g . , Wilson et a l . , 1981) show tha t the minimum

values necessary are roughly 0.01 weight % on Mars, 0.07 wt % on Earth and 3 wt % on Venus.

The values f o r Mars and Earth are wel l w i t h i n the range of magma water and CO 2 contents

commonly found in t e r r e s t r i a l b a s a l t i c melts but the value fo r Venus is not, being much

more c h a r a c t e r i s t i c of r h y o l i t i c magmas on Earth. Unless such evolved magmas e x i s t on

Venus (which is of course poss ib le , but not proven), or unless o ther v o l a t i l e s are common

in Venusian magmas (again poss ib le but hard to i n v e s t i g a t e ) , i t is poss ib le tha t exp los ive

erupt ions are rare on Venus.

Examination of many de ta i l ed model l ing c a l c u l a t i o n s on exp los ive erupt ions (McGetchin

and U l l r i c h , 1973; Pai e t a l . , 1978; Wilson e t a l . , 1980; Wilson and Head, 1981; Ke i f f e r ,

1982) shows tha t the exact d e t a i l s of the gas exso lu t i on and expansion process are not

too impor tant (Wilson, L., 1980) and tha t the e rup t ion speed, S, can be wel l approximated

by

½ S 2 = ~ N Q @ M l°ge(PD/PA) ( l l )

where N is the weight f r a c t i o n of gas exsolved from the magma, PA is the atmospheric

pressure, PD is the pressure at the leve l below the surface where the magma d i s rup ts ,

@ is the magma temperature, M is the molecular weight of the magmatic gas, Q is the

un iversa l gas constant and X is a f a c t o r which takes account of var ious f r i c t i o n a l and

p o t e n t i a l energy losses and is c lose to un i t y f o r most sets of e rup t ion cond i t i ons . PD

is i t s e l f determined by the amount of gas exsolved from the magma (Wilson et a l . , 1980)

and is given w i th s u f f i c i e n t accuracy by

PD N Q S R = 3 M ( 1 2 )

where the f a c t o r 3 is re la ted to the amount of gas bubble generat ion needed to cause the

magma to d i s r u p t (Sparks, 1978). The main importance of the e rup t ion v e l o c i t y , S, is

tha t , toge ther w i th the dens i t y of the released magmatic gas (which is in turn propor -

t i o n a l to the surface atmospheric pressure) , i t determines the maximum size of the pyro-

c l a s t i c fragments tha t can be t ranspor ted out of the vent. Equating the weight of such

fragments to the drag force exerted on them by the gas shows tha t the product of maximum

diameter ~ and dens i t y o f o r such c las ts is given by

o 3 ~ N PA = _ _ lOge(PD/PA) (13)

2G

Page 20: The influences of planetary environments on the eruption styles of volcanoes

352 L. Wilson

Maximum c l a s t s izes can be ca l cu l a ted from t h i s equat ion using the obse rva t i on t h a t pumice

d e n s i t y on Earth is r a r e l y less than 800 kg/m 3 and the reasonable assumption t ha t the

same is t rue on the o the r p lane ts .

These equat ions revea l a number o f i n t e r e s t i n g p r e d i c t i o n s . F i r s t , p l a n e t a r y g r a v i t y

has a n e g l i g i b l e i n f l uence on the e rup t i on speeds in gas - r i ch e rup t i ons . Second, the

composi t ion of the magma gas phase is very impor tan t in de te rmin ing the e rup t i on speed: a

la rge value o f M, the mo lecu la r we igh t , causes a small va lue of PD in equat ion (12) and

t h i s enhances the e f f e c t o f a la rge va lue of M in reducing S in equat ion ( l l ) , We thus

a n t i c i p a t e t h a t e rup t i on speeds w i l l be s y s t e m a t i c a l l y smal le r , f o r a g iven magma gas

content , wherever high mo lecu la r we ight species dominate the magma v o l a t i l e s . Th i rd ,

the amount o f the v o l a t i l e phase re leased from the magma and the magma temperature both

have the same kind of i n f l uence as the mo lecu la r weight but in the oppos i te sense: la rge

va lues of N or @ lead to la rge va lues of S. Fourth, the lower the atmospher ic pressure

at the p l a n e t a r y sur face, the g r e a t e r the amount o f gas expansion and the h igher the

e rup t i on v e l o c i t y . And, f i n a l l y , f o r a f i x e d magma v o l a t i l e content , the maximum s ize

of e jec ted py roc l as t s w i l l be p r o p o r t i o n a l to the atmospher ic pressure and i n v e r s e l y propor-

t i o n a l to the p l a n e t a r y g r a v i t y .

The above r e l a t i o n s h i p s are i l l u s t r a t e d in Table 3 which shows e rup t i on v e l o c i t i e s

and maximum p y r o c l a s t s izes in exp l os i ve e rup t i ons on Venus, Earth and Mars. Values are

g iven f o r H20 and CO 2 as magma v o l a t i l e s . Both are common magmatic gases on Earth and

are l i k e l y contenders f o r t h i s r o l e on Mars, being present in the atmosphere as gases

and at the poles and in sediments as s o l i d ice p a r t i c l e s . CO 2 is the main c o n s t i t u e n t

o f the Venus atmosphere, but water may be h e a v i l y dep le ted on t ha t p l ane t (Ahrens, 1981);

none the less, the va lues are g iven f o r comparison. Two magma gas contents are used:

0. I wt %, a common va lue in basa l t s on Earth, and 3 wt %, more t y p i c a l o f t e r r e s t r i a l

r h y o l i t e s . The major t rends in the t a b l e are seen to be those of i nc reas ing e rup t i on

v e l o c i t y and decreasing p y r o c l a s t s ize as the atmospher ic pressure decreases from Venus

to Earth to Mars. P y r o c l a s t i c b locks may be at l eas t ten t imes l a r g e r in e rup t i ons on

Venus than on Earth f o r comparable magma gas contents , ranging up to tens o f metres in

s ize . However, on l y mm to tens of mm sized p y r o c l a s t s are l i k e l y to have been produced

in t y p i c a l Mar t ian e rup t i ons .

At the moment we are not ab le to v e r i f y these p r e d i c t i o n s f o r e x p l o s i v e e rup t i ons

on e i t h e r Venus or Mars. In the case of Venus we do not ye t have radar observa t ions

which are capable o f un ique ly i d e n t i f y i n g the s p a t i a l d i s t r i b u t i o n pa t te rns of the products

o f exp l os i ve e rup t i ons (Head and Wilson, 1984) and none o f the Venera probes has landed

c lose to a suspected vent area. On Mars, we do have i n d i c a t i o n s t h a t the f l anks of some

of the volcanoes are b lanketed w i th p y r o c l a s t i c m a t e r i a l s (Reimers and Komar, 1979);

however the o r b i t a l images o f the r e l e v a n t areas have s p a t i a l r e s o l u t i o n s which are at

Table 3. Values o f e rup t i on v e l o c i t y , S, and maximum p y r o c l a s t s ize , ~, in s teady exp lo - s ive e rup t i ons on Venus, Earth and Mars (us ing sur face pressures of 6 x I0 ~, 9 x I0 and 400 Pa, r e s p e c t i v e l y ) f o r fou r combinat ions of magma gas con ten t and gas compos i t ion . A nu l l en t r y imp l ies t ha t exp l os i ve a c t i v i t y cannot take p lace.

H20 (M = 18) CO 2 (M = 44)

N = O.l wt % N = 3 wt % N = O.l wt % N = 3 wt % Planet S/(m/s) q~ S/(m/s) d# S/(m/s) (b S/(m/s) d#

Venus

Earth

Mars

- - 177 36 m

44 30 mm 414 2.7 m

89 1.5 mm 593 64 mm

- 26 1,9 m

20 15 mm 240 2.2 m

53 1.3 mm 360 59 mm

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Planetary Environments and Volcanic Erup[ion Stylus 353

best some tens of metres, and n e i t h e r o f the V ik ing Lander probes was loca ted c lose to

a vent . For both p lane ts , we are in the p o s i t i o n of having to regard the p r e d i c t i o n s

in Table 3 as a guide to p lann ing f u t u r e miss ions. The kinds of measurements needed are

v i sua l images from s o f t - l a n d e r probes t a rge ted near suspected vents and o r b i t a l radar

measurements of sur face roughness (which w i l l be c o n t r o l l e d by p a r t i c l e g r a i n - s i z e ) using

radars work ing in the mm to metre ranges. A d d i t i o n a l l y , the Mar t ian atmosphere is

s u f f i c i e n t l y t h i n to a l l ow thermal i n f r a - r e d de te rm ina t i ons of sur face hea t in 8 and coo l i ng

ra tes to be made (Zimbelman and K i e f f e r , 1979) and these can p rov ide added i n f o r m a t i o n

on the g r a i n - s i z e d i s t r i b u t i o n s of sur face p a r t i c l e s .

Atmospheric e r u p t i o n clouds and p y r o c l a s t i c f lows

Convect ing e r u p t i o n clouds can form over the vent in a wide range o f e x p l o s i v e

e rup t i ons on Venus, Earth and Mars as lon 8 as most o f the p y r o c l a s t i c f ragments produced

are much sma l l e r than the s izes g iven in Table 3 f o r the a p p r o p r i a t e magma gas con ten t

and type. Models o f the c loud r i s e process (Wilson et a l . , 1978; S e t t l e , 1978) show t ha t

the he igh t reached by the top of an e r u p t i o n c loud is p r o p o r t i o n a l to the f ou r t h roo t

o f the heat (and, hence, mass) re lease ra te from the vent . The cons tan t o f p r o p o r t i o n -

a l i t y is a complex f unc t i on o f the atmospher ic s t r u c t u r e (Morton e t a l . , 1956) and takes

va lues such t h a t , f o r a g iven mass f l u x , e r u p t i o n clouds should r i s e about 5 t imes h igher

on Ma~s than on Earth (Wilson e t a l . , 1982) but should be 1.7 t imes lower on Venus than

on Earth (Head and Wilson, 1984; Espos i to , 1984). The widths of the depos i t s from such

clouds should be app rox ima te l y equal to the he igh ts o f the clouds producing them (Wilson,

1978), whereas the downwind ex ten ts o f the depos i t s depend s t r o n g l y on the average wind-

speed a t the t ime o f the e r u p t i o n . Cons idera t ions of the e rup t i on c o n d i t i o n s which p revent

s tab le e r u p t i o n clouds formin 8 over vents (Sparks et a l . , 1978) show t h a t the maximum

he igh ts which can ever be reached by e rup t i on clouds are about 35 km on Venus, 60 km on

Earth and about 250 km on Mars. There should be a genera l t rend, t h e r e f o r e , o f t h i c k e r ,

more l o c a l i s e d p y r o c l a s t i c f a l l depos i t s in P l i n i a n e rup t i ons on Venus than on Earth,

whereas Mar t ian P l i n i a n depos i t s should be more widespread and t h i n n e r than t h e i r t e r r e s -

t r i a l e q u i v a l e n t s . The i d e n t i f i c a t i o n o f such depos i t s on Venus su f fe rs from the f a c t

t h a t , w i th present radar data, they can e a s i l y be confused w i th lava f lows. I t seems

l i k e l y , however, t h a t a t l eas t one la rge a i r - f a l l depos i t has been unambiguously i d e n t i -

f i ed on Mars, near the summit o f the vo lcano Hecates Tholus (Mougin is-Mark et a l . , 1982),

and the e r u p t i o n c o n d i t i o n s forming t h i s depos i t have been analysed in d e t a i l .

When convec t in8 e r u p t i o n clouds are uns tab le under Venusian or Mar t ian cond i t i ons ,

p y r o c l a s t i c f lows w i l l be formed as on Earth (Wilson and Head, 1983). The mode l l i ng of

c h a r a c t e r i s t i c s such as the d i s tance t r a v e l l e d by a p y r o c l a s t i c f low from i t s vent , or

the th ickness o f the f i n a l f low depos i t , as a f unc t i on o f the e rup t i on c o n d i t i o n s is s t i l l

in a p r i m i t i v e s t a t e , even f o r the Ear th, desp i t e va r i ous recent advances (Sparks et a l . ,

1978; Wilson and Walker, 1982; Wilson, C.J .N. , 1980) in unders tanding the mechanisms

i nvo l ved . We are not c u r r e n t l y ab le to make usefu l p r e d i c t i o n s about the d e t a i l e d

behav iour o f p y r o c l a s t i c f lows on Venus or Mars.

Some o f the images of the sur face of Venus re tu rned by the Venera s o f t - l a n d i n 8

probes show what appear to be w e l l - d e f i n e d layered s t r uc tu res in the sur face rocks.

I n d i v i d u a l l aye rs have th icknesses in the tens to hundreds of mm range and appear to

cons i s t o f f i n e g ra ined (a t l e a s t sub-mm) m a t e r i a l s (Garv in e t a l . , 1984). I t has been

suggested t h a t these layers cons i s t o f s l i g h t l y l i t h i f i e d p y r o c l a s t i c s (F lo rensky et a l . ,

1983), e i t h e r de r i ved from p y r o c l a s t i c f lows or emplaced as a i r - f a l l depos i t s . I f b e t t e r

Page 22: The influences of planetary environments on the eruption styles of volcanoes

354 L. Wilson

i n fo rmat ion about the d e t a i l e d s i z e - d i s t r i b u t i o n of these ma te r i a l s were ava i l ab l e , i t

would be poss ib le to g ive some idea" of the d is tance t ha t they might have t r a v e l l e d from

a source vent (Head and Wilson, 1984) i f they were emplaced from an atmospheric e rup t ion

cloud. However, so poor is our understanding of the p y r o c l a s t i c f low process at present

tha t having the gra in s ize data would not help us to deduce the e rup t ion cond i t i ons i f

t h i s were the emplacement mode.

Steady erupt ions on Io and the Moon

On p lanets w i thou t a s i g n i f i c a n t atmosphere, equation ( I I ) breaks down, since we

cannot a l low PA to approach zero. Phys ica l l y , the impor tant f ac t is t ha t the escaping

vo lcan ic gas phase expands to a very la rge volume and low pressure. The now w ide ly

separated pyroc las ts cannot supply enough heat to the gas to mainta in i t s temperature

and i t cools a d i a b a t i c a l l y . The gas v e l o c i t y approaches a l i m i t i n g value ( K i e f f e r , 1982)

as the dens i t y decreases and even the smal les t py roc las ts decouple complete ly from the

gas f low to f o l l o w b a l l i s t i c paths back to the surface (Wilson and Head, 1981; Strom et

a l . , 1982). The l i m i t i n g e rup t ion v e l o c i t y , SL, is given w i th s u f f i c i e n t accuracy

(Wilson, L., 1980 )by

½S 2 = ¥ Q O N (14)

where ¥ is the r a t i o of the s p e c i f i c heats of the gas phase. Table 4 conta ins some values

of S L over a wide range of gas weight f r a c t i o n s , N, in the e rup t ion products fo r four

values of the molecular weight , M, of the gas, represent ing some common so la r system

v o l a t i l e s .

For the anc ient lunar b a s a l t i c e rup t ions , in which the released gas was probably

carbon monoxide (Housley, 1978), N may have ranged up to O.l wt % imply ing values of S L up

to about 50 m/s. At these e rup t ion speeds, the t y p i c a l l y sub-mm sized p y r o c l a s t i c s

(produced by the thorough d i s r u p t i o n of the magma as the gas expanded) would have formed

loca l i sed depos i ts extending to about 1500 m from the vent. Low ramparts of t h i s order

of s ize are indeed v i s i b l e around some of the c ra te rs and f i ssu res suspected of being

vents f o r the lavas f l ood ing the lunar basins.

Present day erupt ions on Io appear to invo lve sulphur and SO 2 as the major v o l a t i l e s

(Smith e t a l . , 1979; K ie f f e r , 1982). Some of the p y r o c l a s t i c fragments are observed to

reach heights up to 300 km, imply ing values of S L up to about lO00 m/s. Such high

v e l o c i t i e s in high molecular weight gases imply tha t N must be r e l a t i v e l y large, so tha t

the released gases dominate the erupted ma te r i a l s . However, the mean dens i t y of Io,

toge ther w i th the de tec t ion of h i l l s and va l l eys w i th r e l i e f up to at l eas t 2 km, imply

tha t the c rus t and mantle of Io must be dominated by s i l i c a t e s (Sagan, 1979; Cart et a l . ,

1979). The observed sulphur volcanism is t he re fo re almost c e r t a i n l y dr iven by under ly ing

Table 4. Values of maximum e jec t i on v e l o c i t y , S L (see equat ion (14) ) , of gas and small py roc las ts in steady exp los ive erup t ions tak ing place in a vacuum as a func t ion of the molecular weight , M, of the gas and the weight f r a c t i o n , N, of the e rup t ion products which i t represents,

N= 0. I w t% M

18 61

28 49

44 39

64 32

l wt % lO wt % lO0 wt % v o l a t i l e

192 608 1920 H20

154 488 1540 CO

123 389 1230 CO 2

I02 323 I020 S 2 o r SO 2

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Planetary Environments and Volcanic Eruption Styles 355

s i l i c a t e volcanoes. The d e t a i l s of the ways in which heat is t r ans fe r red from the magma

to the sulphur layers w i thou t producing much p y r o c l a s t i c mate r ia l are s t i l l obscure,

however. I t is a n t i c i p a t e d t ha t the combinat ion of v i sua l , i n f r a - r e d and u l t r a v i o l e t

measurements to be made by the planned G a l i l e o J u p i t e r O r b i t e r probe w i l l help to c l a r i f y

the s i t u a t i o n by he lp ing to def ine the thermal s t ruc tu res of the e rup t ion plumes and the

s izes of the so l i d p a r t i c l e s w i t h i n them.

Unsteady exp los ive erupt ions

The t reatments of e rup t ion processes in which gas is released e p i s o d i c a l l y , as in

Strombol ian or Vulcanian exp los ions, ra the r than in a more near ly uniform manner, as in

Hawaiian or P l i n ian erup t ions , presents many problems, due mainly to the need to keep

t rack of the vary ing acce le ra t i ons of the components of the system. A common fea tu re

of t r a n s i e n t exp los ions is the concent ra t ion of gas i n to l oca l i sed pockets, in such a

way tha t on ly a par t o f the t o t a l a v a i l a b l e magma is e jec ted in the exp los ion . As a

r e s u l t , i t is of l i t t l e consequence whether the gas is der ived so le l y from the magma or

is produced p a r t l y or complete ly from the vapor i sa t ion of near-sur face v o l a t i l e layers .

Attempts to produce q u a n t i t a t i v e models of t r a n s i e n t explos ions ( e .g . , Wilson, L., 1980)

show tha t equat ions s i m i l a r to ( I I ) and (14) can be der ived, but tha t v e l o c i t y co r rec t i on

fac to rs , t ak ing values cons iderab ly less than un i ty , are needed to a l low fo r the reduced

e f f i c i e n c y of usage of the energy released as the gas phase expands.

Ca lcu la t ions of the gas and py roc las t speeds expected in lunar Strombol ian erupt ions

have been made (Wilson and Head, 1981) and these i nd i ca te tha t accumulat ion of gas i n to

la rge bubbles emerging through the surface of a lava lake can cause e f f e c t i v e values of

N up to I0 wt % in the exp los ion products, so tha t equat ion (14) p red i c t s e j ec t i on

v e l o c i t i e s up to 500 m/s and r e s u l t i n g p a r t i c l e ranges of up to I00 km. Some of dark,

r eg iona l -man t l i ng depos i ts seen on the Moon (Head, 1974) were probably formed in prolonged

Strombol ian erup t ions of t h i s kind. Other, more l oca l i sed (2 to 3 km d iameter) dark

mantles on the Moon, such as those surrounding some of the small c ra te rs ins ide the large

c ra te r Alphonsus, are probably the resu l t s of a lunar vers ion of Vulcanian a c t i v i t y (Head

and Wilson, 1979): b a s a l t i c magma invaded the fragmental lunar surface layer ( the r e g o l i t h ,

produced by the f requent , sub-metre scale impact c ra te r i ng of the sur face) and c h i l l e d

from the top downwards; gas exsolved from the magma at depth and accumulated under the

coo l ing plug in the vent, even tua l l y reaching a pressure high enough to overcome the

s t rength of the plug and cause an exp los ion . Erupt ions of t h i s type were not common on

the Moon, and seem to have requ i red the spec ia l circumstances of magma j u s t reaching the

surface w i thou t over f low ing the vent to form a lava f low.

The high atmospheric pressure on Venus causes s i g n i f i c a n t d i f f e rences between

Strombol ian exp los ions on tha t p lane t and the Earth. Magma c lo t s w i l l be expel led at

speeds up to a few tens of m/s (Garvin et a l . , 1982) which is a f ac to r of several smal le r

than the speeds encountered in t e r r e s t r i a l Strombol ian explos ions (Blackburn et a l . , 1976).

The high drag fo rce exer ted by the dense Venusian atmosphere should l i m i t the ranges of

the e jec ted fragments to a few tens of metres from the vent. Even i f the reso lu t i on of

o r b i t i n g radar de tec to rs is improved by two orders of magnitude over what is c u r r e n t l y

ava i l ab l e , the near-vent depos i ts b u i l t up from such fragments w i l l be hard to de tec t .

L i t t l e has been done to s imula te the consequences of Strombol ian (or o ther t r a n s i e n t )

e rup t ions on Mars. However, i t may be expected tha t , as is the case w i th steady erupt ions

(Wilson et a l . , 1982), e j ec t i on v e l o c i t i e s of py roc las ts w i l l be g rea te r on Mars than

Page 24: The influences of planetary environments on the eruption styles of volcanoes

356 L. Wilson

on Earth ( though sma l le r than on the Moon). The main candidates f o r being the products

o f t r a n s i e n t exp l os i ve e rup t i ons on Mars are the dome- and c o n e - l i k e f ea tu res some tens

to hundreds o f metres in d iameter which are found in c l us te r s in some lowland areas (Frey

and Jarosewich, 1982). A f a c t o r o f ten improvement in r e s o l u t i o n over t h a t a v a i l a b l e

from the V ik ing O r b i t e r cameras w i l l be needed on f u t u r e Mars missions to enable progress

to be made in the ana l ys i s o f these fea tu res using o r b i t a l photography.

SUMMARY

Our unders tanding o f vo l can i c processes is a t an i n t e r e s t i n g stage. The dynamics

of most o f the r e l a t i v e l y steady types o f a c t i v i t y ( i . e . , those in which the t ime ra te

of change of the v a r i a b l e s is smal l ) are reasonab ly we l l understood, whether they i n v o l v e

exp los i ve a c t i v i t y or less v i o l e n t e f f u s i v e a c t i v i t y . The major excep t ion to t h i s

g e n e r a l i s a t i o n concerns the i n t e r n a l mot ion of p y r o c l a s t i c f lows. In the o the r cases,

we are able to make some e x p l i c i t p r e d i c t i o n s about the in f luences of the c o n t r o l l i n g

v a r i a b l e s on the e rup t i on s t y l e and many of these p r e d i c t i o n s have been v e r i f i e d by observa-

t i ons of e rup t i ons on Earth. I t is then r e l a t i v e l y s t r a i g h t f o r w a r d to extend the p r e d i c -

t i ons to take account o f the env i ronmenta l c o n d i t i o n s on o the r p l a n e t a r y bodies. In some

instances we a l ready have the necessary i n f o rma t i on , c o l l e c t e d by o r b i t i n g or f l y - b y space-

c r a f t , to t e s t these p r e d i c t i o n s and to i d e n t i f y the r e l e v a n t e rup t i on s t y l es on o ther

p lane ts . In o the r cases, however, i t is c l e a r t h a t we do not ye t have adequate data to

make unique i d e n t i f i c a t i o n s ; but we can use the mode l l i ng c a l c u l a t i o n s to de f i ne the kinds of

measurements which should be made dur ing f u t u re missions in o rder to reso lve the ambigu i t ies .

The s i t u a t i o n is much less s t r a i g h t f o r w a r d when we cons ider unsteady vo l can i c

processes, e s p e c i a l l y t r a n s i e n t exp l os i ve e rup t i ons in which la rge a c c e l e r a t i o n s are

i nvo l ved . Qui te apar t from the ma themat i ca l -phys i ca l problems assoc ia ted w i th f o r m u l a t i n g

t rea tments o f these kinds of a c t i v i t y , they tend to i n v o l v e the i n t e r a c t i o n of magma w i th

near -sur face v o l a t i l e l aye rs , and commonly lead to the excava t ion of some of the p re-

e x i s t i n g coun t ry rock surrounding the vent . This produces g rea t u n c e r t a i n t i e s in the

r a t i o s of gas to rock mass invo lved in the ensuing exp los ions , and makes the g e n e r a l i s a t i o n

of models o f these processes even more d i f f i c u l t than might o therw ise be the case.

Attempts to produce comple te ly general models o f vo l can i c e rup t i on mechanisms, which

are v a l i d on any p lane t , have been a major s t imu lus to modern vo lcano logy , and promise

to con t inue to serve t h i s purpose: the d i scove ry of the cu r ren t exp l os i ve a c t i v i t y on

Io, i n v o l v i n g as i t almost c e r t a i n l y does the r a t h e r e x o t i c i n t e r a c t i o n of s i l i c a t e magma

from the i n t e r i o r w i th su lphur compounds on the sur face, serves to draw a t t e n t i o n to the

g rea t amount o f work which remains to be done.

ACKNOWLEDGEMENTS

I am g r a t e f u l to many co l leagues f o r d iscuss ions on va r ious aspects o f t e r r e s t r i a l

and p l a n e t a r y vo l cano logy . I p a r t i c u l a r l y thank G.P.L. Walker, J.W. Head and R.S.J. Sparks

in t h i s respect .

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Muon radiography of volcanoes and the challenge at Mt ...people.na.infn.it/~strolin/MU-RAY.pdf · eruption. Mt. Vesuvius is among the most studied volcanoes in the world. The volcano

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Intro Objective: 7 November 2011 Compare and contrast the three types of volcanoes. Why do you think different eruption types make differently shaped volcanoes?

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Volcanoes and their eruption patterns

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The Powerful SupervolcanoSupervolcanoes are exploding volcanoes. They are not the volcanoes where lava flows out more slowly. Rocking the world A supervolcano eruption would change

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Volcanoes of Canada - University of Albertaunsworth/UA-classes/EAS105/CanadianVolcanoes-CH2005.pdf• Anatomy of an Eruption ... Although none of Canada’s volcanoes are erupting

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