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VOLCANISM ON OTHER PLANETS 211

ferentiated asteroid: pieces of vesicular surface lava might expect to have volcanic activity now, or to
flow and slowly cooled magma that intruded into have been volcanically active in the past, appear
the crust; pieces of olivine-rich rock representing to do so, although the evidence is still poor for
the mantle after basaltic melts have been removed Mercury due to lack of data.
from it; and fragments of the surface regolith, • None of these other volcanically active planets
the outermost layer of rocks broken up and mixed appear to have, or to have had in the past, a plate
by large numbers of small impact craters. The tectonic system like that of the Earth, and we still
Hubble image of 4 Vesta shows an irregular sur- have no complete understanding of why this is
face with one giant crater punching through the so. The inference is that the volcanism is driven
crust into the mantle (Fig. 13.21). This is exactly by mantle plumes forming hot spots, and the
the configuration to be expected after analyzing presence of giant shield volcanoes on Mars and
the meteorites. Venus is consistent with this.
There are other groups of meteorites where • The commonest volcanic rock type erupted on
all the members of the group have the same trace- all of the bodies for which there are good data is
element chemistry and isotope ratios, and have some variety of basalt, as we would expect for
clearly all come from the same differentiated body. primary melts from planetary mantles.
The Aubrite meteorites are the residual mantle of • Theory suggests that the lower the acceleration
an asteroid that was rich in volatiles, but there are due to gravity on a body the wider the dikes that
no meteorites found that would represent the crust can form in its lithosphere, the larger the magma
of this body. We infer that explosive eruptions reservoirs that can form, and the larger the length
occurred when basalts from the mantle of this body and volume of typical lava flows. This prediction
reached the surface, and that these were so vigor- seems to be borne out by the great sizes of lava
ous that all of the pyroclastic droplets were erupted flows on Mars, Venus, the Moon, and Io.
with speeds greater than the escape velocity from • The lack of atmospheres on the Moon and Io,
the asteroid. Thus no significant basaltic crust ever and the low atmospheric pressure on Mars, mean
formed on this body. The Ureilites are another group that vigorous explosive eruptions are more likely
of rocks from an asteroid mantle. The detailed on these bodies. Clear evidence for this is seen on
chemistry of these meteorites is throwing light on the Moon and Io but is less clear for Mars, where
the mechanisms whereby basaltic partial melts complex surface processes due to the presence
separate from their source rocks, either quickly by of the thin atmosphere – wind and weathering –

forming dikes or slowly by percolating along min- may help to hide the resulting features.
eral grain boundaries. In some meteorites, e.g., the • Some of the most interesting evidence about
Acapulcoite, Lodranite, and Angrite groups, veins what happens deep within the Earth, which we
are present that represent cracks containing melt cannot reach directly, comes from examining
and sometimes gas bubbles; these seem to be small the fragments of the interiors of differentiated
dikes caught in the act of migrating to the surface to asteroids that have been broken up by impacts
feed eruptions. Needless to say volcanic rocks from and delivered to Earth as meteorites.
asteroid mantles are invaluable in trying to under-
stand process in planetary interiors: it is only very
rarely that eruptions on Earth bring up mantle rocks 13.12 Further reading
from depths corresponding to those of many me-
teorite samples from asteroids.
Cattermole, P. (1996) Planetary Volcanism. Wiley,
Chichester.
Davies, A. (2007) Volcanism on Io. Cambridge
13.11 Summary University Press, 376 pp. ISBN: 9780521850032.
Lopes, R.M.C. & Gregg, T.K.P. (Eds) (2004) Volcanic
• All of the other planetary bodies with composi- Worlds: Exploring the Solar System Volcanoes.
tions roughly similar to that of the Earth that we Springer-Praxis, Chichester.