Table Of Content56 Inlernational Colloquium on Venus
H20 content, Co)low-density overburden, or (c) lesser horizontal with diameters greater than 100 km, arethe sites of some of the most
length scale of flow. voluminous eruptions. Head et at. [1] have identified 158 of these
Relevant to these hygotheses is that differertdadon on Vertus structures. Their spatial distribution isneither random (see Fig. 1)
requires more than adiabatic upwelling and pressure-release melt- nor arranged in linear chains as on the Earth; large volcanos on
hag. In models of this process on Earth [e.g., 5,6] the availability of Venus are concentrated in two large, near-equatorial clusters that
mantle material to flow into the region istaken as given; the concern are also the site of many other forms of volcanic activity [1].
is about the rising of magma within asolid matrix, inparticular, the The set of conditions that must be met on Venus that controls the
mechanism(s) concemrating the melt in anarrow vertical slab. The change from widespread, distributed volcanism to focused, shield-
viscosity of the melt, and hence its ability to separate, isaffected by building volcanism isnot well understood. Future studies of la-ansi-
water content. Differentiation from a plume, without pull-apart, tional features will help to address this problem. It is likely,
under alayer of higher strength or lower density (as ismore likely however, that the formation and evolution of aneutral buoyancy
on Venus), requires asurplus of heat, and thus islikely to lead to a zone (NBZ) plays an important role in both determining the style of
lower rate of magmatism for agiven heat flow. Also the inhibition the volcanism and the developmem of the volcanic feature once it
of upward flow by alow-density overlying layer may lead to less has begun toerupt. Head and Wilson [2] have suggested that the high
differentiation of crust. Application of models of aplume under a surface pressure on Venus may inhibit volatile exsolution, which
lithosphere [7] to Venus features such asAria and Beta indicate that may influence the density distribution of the upper crust and hence
appreciably higher upper mantle viscosities may cause pressure control the nature and location of aNBZ. The extreme variations In
gradients to aec,ount for these great peaks in the geoid. pressure with elevation may result in significantly different charac-
We have applied finite element modeling to problems of the teristics ofsuch aNtiZ atdifferent locations on the planet. In order
interaction of mantle convection and crust on Venus [8]. The main to test these ideas regarding the imlxntance of N'BZ development in
emphasis has been on the tectonic evolution of Ishtar Terra, as the the evolution of a large shield and to determine the style of
consequence of convergent mantle flow. The early stage evolution volcanism, three large volcanos that occur at different basal eleva-
is primarily mechanical, with crust being piled up on the down- tions were examined and the distributlon of large volcanos as a
stream side. Then the downflow migrates away from the center. In function of altitude was determined.
the later stages, after more than I00 m.y., thermal effects develop The evolution of Sapas Mons. a 600-kin-diameter shield vol-
due to the insulating influence of the thickened crust. An important cano, was studied [3]. Six flow units were identified on the basis of
feature of this modeling isthe entrainment of some crustal material radar properties and spatial and temporal relations. The distinctive
in downflows. variation between units was attributed to the evolution of magma
An important general theme in both convergent and divergent in a large chamber at depth. The presence of summit collapse
flows isthat of mixing vs. stratification. Models of multicomponent stauctures and radial fractures, interpreted to be the surface expres-
solid-state flow obtain that lower-density crustal material can be sion of lateral dikes, s_ this suggestion. Theory predicts that
entrained and recycled, provided that the ratio of low-density to volcanos located at the altitude of Sapas Mons should have large
high-density material is small enough (as in subducted slabs on magma chambers located at zones of neutral buoyancy at relatively
Earth). The same considerations should apply in upflows; asmall shallow depths beneath the substrate [2]. Not only does the evidence
percent partial melt may be carried along with its matrix and never from the flow units suggest that such azone ispresent, but the size
escape to the surface. Models that assume melt automatically rising of the summit collapse and the near-surface nature of the radial dikes
to the crust and no entrainment or other mechanism of recycling indicates that the chamber isboth large (on the order of 100 km in
lower-density material [e.g., 9] obtain oscillatory behavior, because diameter) and shallow.
it takes a long time for heat to build up enough to overcome a In comparison to Sapas Mons, two other volcanos at different
Mg-rich low-density residuum. HoweveT, these models develop elevations were examined. Theory predicts that the volcano atthe
much thicker crust than consistent with estimates from crater
depth:diameter ratios [1].
References: [1] Sharpton V. L. and Edmunds M.S. (1991) Eos, _5 Ij i I I t t t t I I t t t I _ I I I
72, 289. [2] Kaula W. M. (1992a) Proc. IUGG Syrup. Chem. Evol. III expected#
Planets, in press. [3] Kaula W. M. (1980) JGR, 85, 7031. [4] Kaula 20 [] actual #
W. M. (1992b) Proc. IAG Syrup. Gray. Field Det. Space Air Meas.,
inpress. [5] McKenzie D. P. (I 985)EPSL, 74, 81. [6] Scott D. R.and
Stevenson D. J. (1989) JGR, 94, 2973. [7] Sleep N. H. (1990) JGR,
JBI
95, 6715. [8] l_e-aardic A. etai. (1991) GRL, 18, 2209. [9] Parmentier E.=
E. M. and Hess P. C. (1992) LPSC XXIII, 1037
N93-14333 q fog
LARGE SHELD VOLCANOS ON VENUS: THE EFFECT OF
NEUTRAL BUOYANCY ZONE DEVELOPMENT ON EVO- 0 _ll I l I I _ I l i l I I l I l _I I
LUTION AND ALTITUDE DISTRIBUTION, S. Keddie and B_N 65N 45N 25N 5N 15S 35S SSS 755
latitude
J. Head, Department of Geological Sciences. Brown University,
Providence g102912, USA.
Fig. 1. Location oflarge volcanos ss afunction of latitude. Thedark-shaded
columns indicate where volcanos would be located if they were randomly
The Magellan mission to Venus has emphasized the importance distributed ontbe surface as afunction of the peree_tage of area atagiven
of volcanism in shaping the surface of the planet. Volcanic plains latitude. The striped columns show where thevolcanos actually occur. Note
make up 80% of the terrain and hundreds of regions of localized the paucity of volcanos at higher latitudes and the concentration m the
eruptions have been identified. Large volcanos, defined as edifices equatorial region of the planeL
LPI Contribution No. 789 57
highest basal elevation, Mut Mons, should have awell-developed,
large, and relatively deeper NBZ and that the volemm at the lowest 60S5 'JI, , ,,1 ,, ,, I .... t , , , , I , , , • 1, t ,,
altitude, an unnanmd volcano located southwest of Beta Regio at
10", 273*, should have either apoorly developed magma chamber 6054 ] o _b 0
ornone at all [2]. Preliminary mapping of Mast Mona [3] identified o
at least six flow units that exhibit greater variations in morphology
6053 ._ Oo oofito OooooOoo
and radar proposes than the flows of Sapas Mons. These units are
also spatially mad temporally distinct and suggest the eruption of a
6052 o o
continuously evolving magma. Although smaller in diameter, the
summit caldera ismuch better def'med than the depression atSepas. _%_ o o _ o
The inferred young age of Mut (Klose et el. [4] suggest that it may 6051 o oo_[_,
even be "active") may mean that the chamber has not yet grown to 1_oo oo
"full size," explaining the relatively smaller ealderL There is no 6050"1 .... , .... , .... , , • • t '
0 1 2 3 4 5 6
evidence of radial fr_tme_ at Mast Mons, suggesting that if lateral
Height (kin)
dike pro_gation occurred, itwas sufficiently deep that there was no
surface expression. In contrast, the urm_tmed volcano has no summit
features, no radial dikes, and only three flow units that exhibit Fig. 3. Graph showing the heights of 110 large volcanos as afimction of
considerable morphologie variations within units [3]. These obser- basal altitude. The majority of volcanos cluster between 6051 and6053 krn
vations suggest that either the NBZ is very poorly developed or it andthereisaweak positive correlation between height andbasal altitude, The
does not exist and the magma erupts directly atthe surface. Thus the majority of volcanos thatoccur above 6052_8krnandaretaller than 2.6kmare
character of three large volcanos on Venus supports the suggestion located inzones of mantle upwellin 8and/or rifting.
that basal altitude can play acritical role in the development of a
NBZ. Examination of other volcanos at agreater range of altitudes
will help to further test this hypothesis. tively shallow depths beneath the surface. A survey of 110 large
In addition to studying the detailed evolution of three large volcanos found that this altitude distribution appears to be observed
volcanos, the altitude and height distribution of all volcanos was on Venus.
determined. Although in general there is a broad distribution of The height of volcanos is also related to basal altitude and the
large volcanos as a function of altitude, there is somewhat of a development of zones of neutral buoyancy. There isaweak corre-
paucity of large volcanos at elevations below 6051 k:m(Fig. 2). lation of volcano height with basal elevation (Fig. 3). Although
Between 6051 and 6053 km the number of volcanos is slightly many factors need to be considered to explain this correlation, in a
greater than expected and above this an absence of volcanos isagain general sense NBZ development is responsible. As magma cham-
observed. This absence at the highest elevations isprobably due in bers become larger, and thus the "life" of the volcano islengthened,
large part to the predominance of tessera terrain atthese elev ations. there isan opportunity for a greater number of repeated, relatively
Those volcanos that do occur inthis area areassociated with regions small volume eruptions. This type of eruption enhances edifice
of uplift and rifting probably caused by mantle upweUing. Head and growth. Head and Wilson [2] suggested that NBT_.s will grow to
Wilson [2] suggested that below an elevation of 6051 km it is relatively larger sizes atgreater altitudes. Therefore this correlation
unlikely that aNBZ would develop, due largely to the high atmo- of height with altitude isexpected.
spheric pressure, and thus edifice growth would beinhibited. They Although agood deal more work ne__ds to be done to test the idea
also found that the Inst few kilometers above 6051 km would be that basal altitude plays asignificant role in the development and
most favorable for edifice growth as NBZs develop early at rela- evolution of neu_ buoyancy zones on Venus, studies of the
altitude distribution and heights of many large volcanos, as wall as
the evolution of individual volcanos, indicates that NBZ develop-
ment is occurring, that it is varying, apparently as afunction of
30 ,I,l,l,l,l,i,l,lll,l,l,l,,,I,Iit,ltlll,I, altitude, and that the morphology and history of large edifices is
being strongly influenced.
25
References: [1] Head J. W. et al. (1992) JGR, submitted.
[2] Head J. W. and Wilson L. (1992) JGR, 97, 3877-3903. [3]
20
Keddie S. T. and Head J. W. (1992) LPSCXXIlI, 669--670. [4] Klose
K. B. etal. (1992)JGR, submitted. - F_
N93-14334
10
MANTLE PLUMES ON VENUS REVISITED. Walter S. Kiefer,
5 Code 921, Goddard Space Flight Center, Greenbelt MD 20771,
USA.
The Equatorial Highlands of Venus consist of a series of
quasicircular regions of high topography, rising up to about 5krn
above the mean planetary radius [l ]. These highlands are strongly
Fig. 2. Location of largevolcanos asafunction of basal altitude. The dark-
correlated with positive geoid anomalies, with apeak amplitude of
shaded columns indicate where volcanos would be located if they were
120 m at Atla Regio [2.3]. Shield volcanism is observed at Beta,
randomly distributed onthe surface as afunction of thepercentage of areaat
agiven altitude. The slriped columns show where thevolcanos actually occur. Eistla, Bell, and Asia Regiones and in the Hathor Mons-lnnini
See textfor adiscussion of the implications of this distribution Mons-Ushas Mons region of the southern hemisphere [4-10].
