Pyroclastic Eruptions

O w in g to a g re a te r a tm o s p h e ric p re s s u re on V e n u s, n u c lé a tio n and d isru p tio n o f m agm a is c o n sid ere d to o ccu r at g reater depths on V enus than E arth {Garvin et aL, 1982; W i l s o n a n d H e a d , 1983; H e a d a n d W ilson, 1986; T h o r n h i l l , 1993). On E arth, m agm a fragm entation m ust occur at or below the surface if the to ta l v o la tile content in a b a salt ex ceed s ap p ro x im ately 0.1 wt% . On V enus how ever, the volatile w eight fraction m ust exceed a p p ro x im a te ly 2.5 w t% fo r H2O in b a s a ltic m ag m as and a p p ro x im ate ly 4 wt% fo r C O2 in any m agm a b e fo re e x p lo siv e e ru p tio n s occur: the results are specific for a surface atm ospheric p ressu re o f 10 M Pa, corresponding to the venusian low lands. This falls to 1.1 wt% for w ater in B asalt, 1.6 wt% for w ater in rhyolite and 2.0 wt% for C O2 in any m elt at an atm ospheric pressure o f 4 M Pa {Wilson an d Head, 1983; H ead an d Wilson, 1986). G iven the e n v iro n m e n ta l and v o la tile c o n d itio n s, the q u e stio n a rises can ex p lo siv e eruptions occur on V enus?

T he m inim um am ount o f w ater n eed ed fo r an eru p tio n at hig h a ltitu d e s on V enus (4 M Pa) is co n sid ered to be near the u p p e r end o f the range com m on in terre stria l m agm as. C O2 is co n sid ered m ore im portant in exsolution on V enus, than on E arth

{ H e a d a n d W ilson, 1982, 1986) as w ater is ex trem ely rare on V enus (see C hapter 1, 1.2.4).

H ow ever, it is generally accepted th at to trig g er su b stan tial m agm a d isru p tio n under the p resen t v en u sian co n d itio n s, v o latile c o n c e n tra tio n s m uch g re a te r th an th o se o b se rv e d in ty p ic a l terre stria l m agm as are needed (Garvin et aL, 1982). T his im plies that explosive volcanism is rare, unless a m ore soluble volatile can be su b stitu ted .

W hile m agm a volatile content m ay be less on V enus than on E arth , p a rtic u la r sty les o f e ru p tio n m ay re s u lt in p y ro c la stic a c tiv ity , fo r in stan c e S tro m b o lian e ru p tio n s. On E arth , b u b b le co alescen ce can co n cen trate gas su fficien tly to cause in te rm itten t ex p lo sio n s in lo w -v isco sity m agm as w hich ascen d slow ly to the su rface (B lackburn et aL, 1976; H e a d a n d W ilson, 1986). Such e ru p tio n s can p ro d u ce co n v ectio n co lu m n s 5-10 km in h eig h t.

G arvin et aL (19 8 2 ) show ed th a t on V en u s c o a le sc e n c e m ay produce bubbles 1 m etre in size for m agm a ascending at 0.03 m /s w ith a total wt% C O2 g reater than 0.5. H o w ev er, m ore ty p ical bubble sizes are predicted to lie betw een 0.1 and 100 mm.

V u lc a n ia n an d P e le a n e ru p tio n s ty le s m ay a lso b e resp o n sib le for localised pyroclastic deposits on V enus (H ead a n d Wi l s o n , 1986; F a g e n t s , 1994). The form ation o f debris in the vent and/or a chilled carapace results in a b uild-up o f gas p ressure in the u p p er p a rt o f the m agm atic system . T e rre stria l m o d ellin g o f V u lc an ian e ru p tio n s by S e lf et aL (1979) su g g e st p ressu re s o f 200-300 bars are needed before gas can escape. O n V enus, ow ing to the high am bient atm ospheric p ressu re, the en erg y from such an eruption is calculated to be only one fifth o f that expected from a sim ilar explosion on Earth.

A fu rth e r co n seq u en ce o f the g rea ter am b ien t tem p era tu re is the p roduction o f denser eruption clouds. The predicted resu lt is less c la st cooling, resu ltin g in the p ro b ab le fo rm atio n o f sp atter m aterial and lava flow s, rather than a py ro clastic cone (H ead a n d W i l s o n , 1986). H ence ignim brites are p red icted to w eld to g eth er m o re e a sily and e x te n d fo r g re a te r d ista n c e s u n d e r g re a te r v e n u sian tem p eratu res ( Ki ef f er , 1995).

T h e p re d ic tio n s ab o v e m ay be te s te d b y lo o k in g fo r c h aracteristic landform s (size o f calderas, fissu res, dykes, m aterial

w ith rad a r ch aracteristics and m o rp h o lo g ies sim ilar to te rre stria l p y ro clastic deposits) indicative o f specific volcanic activity, at S if and G ula M ontes and other large shield v olcanoes. F u rth erm o re, ob serv atio n s from w ell know n terre stria l v o lcan o es m ay be used to give possible insights into the evolution o f S if and G ula M ontes, as discussed in C hapter 5.

2 . 7 S u m m a r y

V arious m odels are proposed to explain the stratigraphy and volcanic landform s seen on the surface o f V enus. B a s ile v s k y a n d H e a d (1994, 1995a, 1995b, 1996), H e a d a n d B a sile v s k y (1996) and B a silevsky et al. (1996, 1997) have d eveloped a p relim in ary stratig rap h ie m odel for V enus. T he m odel im plies a sequence of events in w hich different styles o f volcanic and tectonic activ ity acted during separate periods o f geological tim e, each represents a discrete stage in the p lan et’s form ation. The volcanic and tectonic ev en ts o ccu rred at the sam e tim e on a g lo b al scale b u t have d e c lin e d fro m re g io n a l to lo c a l p r o c e s s e s c o n s id e r e d a consequence of crustal thickening (Phillips and Hansen, 1997).

From the low crater population on V enus (< 1000) and the o b serv atio n th at m ost are unm odified by tecto n ism or volcanism , an a v e r a g e a b s o l u t e age o f the v e n u sia n su rfa c e h as b een calcu lated betw een 800 and 190 M a, ty p ically considered betw een 500 and 300 M a (Strom et al., 1992; S c h a h e r et al., 1992; P h i l l i p s et al., 1992; Strom et al., 1994; Price and Suppe, 1994; Price et al.,

1996). W hile som e w orkers co n sid er the sp atial and hy p so m etric distribution o f craters to be random (Schaber et al., 1992; Strom et a l., 1992), others suggest that the paucity corresponds to areas of m ost rec en t geological activity (Phillips et al., 1992). H ence, two opposing m odels for the resurfacing o f V enus are considered: the global resu rfacin g m odel (Schaber et al., 1992; Strom et al., 1994, 1995), im p ly in g a re la tiv e ly rap id re s u rfa c in g o f V en u s and su b se q u e n t d eclin e in g e o lo g ica l a c tiv ity , and the e q u ilib riu m resu rfacin g m odel (ERM) in w hich localised patches o f the surface are ren e w e d d estro y in g craters at the sam e rate o f p ro d u ctio n

{Phillips et al., 1992; H e rric k an d P hillips, 1994; H errick et al.,

1 9 9 5 ).

D e ta ile d m ap p in g p re se n te d in C h ap ter 3 (to g e th e r w ith o th e r s tra tig ra p h ie o b s e rv a tio n s ) w ill be u se d to te s t the d ire c tio n a l n a tu re o f the g lo b al stra tig ra p h ie m o d el. T he new re su lts also test w hich o f the resu rfac in g m odels (if any) are applicable to the V31 and V19 m apping quadrangles, and for the contiguous regions o f G uinevere and Sedna Planitiae.

Tw o volcanic landform s p rev io u sly id en tified on V enus are coronae and large shield volcanoes {Barsukov et al., 1984, 1986). T h ree stag es o f c o ro n a e v o lu tio n are in fe rre d fro m p rev io u s o b se rv a tio n s, in itia l u p lift and v o lcan ism , the fo rm atio n o f an an n u lu s o f e ith e r rid g es or g rab en , and la stly by to p o g rap h ic red u ctio n and late stage volcanism {Stofan an d Head, 1990; S t o f an et al., 1992; Squyres et al., 1992). W ork presented in C hapter 4 is u se d to v a lid a te the th ree stage e v o lu tio n ary m o d el, and test w h eth er the stratigraphie m odel proposed by B asilevsky and H ead apply to coronae observed in V31.

L arge shield volcanoes on V enus are considerably low er and b ro a d e r th an th eir terre stria l co u n terp arts { S c h a b e r a n d Kozak,

1989). H e a d a n d W ilso n (1992) su g g est the g rea ter w idth to h e ig h t ra tio is a c o n seq u en ce o f e n v iro n m en ta l c o n d itio n s on V enus w hich favour lateral over vertical m ovem ent o f the m agm a re se rv o ir, w ith n eu tral buoyancy zones g en erally b ein g lo w er or a b se n t w ith in the v o lcan ic p iles in co m p ariso n w ith te rre stria l v o lcan o es. F urtherm ore, it is suggested that m agm a reserv o irs are la rg e r on V enus {H ead a n d W ilson, 1992). As a co n seq u en ce, calderas are predicted to be larger. Ow ing to the lack o f w ater, and g re a t a tm o sp h eric p ressu re , p y ro cla stic eru p tio n s are co n sid ered to be rare, unless a build-up o f exsolved volatiles occurs.

C h ap ter 5 p resen ts a d etailed d escrip tio n o f S if and G ula M ontes and com pares their edifice and flow m orphology w ith w ell d o c u m e n te d te rre s tria l v o lca n o es. As p a rt o f th is stu d y , the m odels o f large shield volcanism outlined above w ill be tested by lo o k in g fo r c h a ra c te ristic lan d fo rm s in d ic ativ e o f the p ro cesses describ ed above acting at the two volcanoes.

Chapter 3

Guinevere and Sedna Planitiae: Mapping,