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VOLCANISM ON OTHER PLANETS 197

Fig. 13.6 The ∼120 km diameter lunar crater Alphonsus.
The small, dark, elongate craters on the floor surrounded by
dark haloes are interpreted to be sites of transient Vulcanian
explosions. (Apollo 16 metric frame #AS16-M-2478; NASA
Fig. 13.7 The region around the lunar sinuous rille Rima
image.)
Bode. The low reflectivity of the surface in the ∼150 km
diameter region occupying the middle one-third of the
that was propagating toward the surface or within image is due to the mixing into the fragmental surface
about 3 km of the surface in a dike that had already regolith of a pyroclastic deposit of volcanic glass beads.
opened and was erupting magma. The largest (Part of Lunar Orbiter IV image 109H2; NASA image.)
amounts of gas that could be formed in an ongoing
eruption were a few hundred parts per million, i.e., up into an enormous number of tiny droplets. If
at least ten times less than in typical basaltic erup- these droplets were thrown to great distances from
tions on Earth. Even so, the great expansion of the vent they were shaped into spheres by surface
this gas into the almost perfect vacuum above the tension and then rapidly chilled to become glassy
lunar surface more than compensated for its small solids before landing. At the other extreme, if they
amount and threw out pyroclasts to form cinder were not thrown far, then the large numbers and
cones and ash blankets at least as large as those on small sizes meant that they formed fire-fountains in
Earth, in some cases up to 3 km in diameter. These which droplets in the outer parts of the fountains

features are called dark halo deposits (Fig. 13.6), shielded those in the inner parts and prevented
because the reflectivity of the dark basaltic pyro- them from radiating away their heat into space.
clasts is less than that of the rocks on which they are Most of the droplets thus landed without having
deposited. In many cases these pyroclastic features cooled at all, and formed hot ponds feeding lava
must have been buried by the very large volumes flows instead of forming the spatter or cinder cones
of lava being erupted, but in short-lived eruptions so common on Earth. The Moon has given us a great
they were sometimes preserved. The much larger deal of insight into this pattern of behavior in explo-
amounts of gas generated in the low-pressure re- sive eruptions on planets with little or no atmo-
gions behind the tips of new dikes were able to throw sphere, and we shall come back to it when the
pyroclasts out to even greater distances, up to sev- eruptions on Mars and Io are considered.
eral tens of km, to form dark mantle deposits
(Fig. 13.7). Pyroclasts from a few of these deposits
were collected by the Apollo astronauts, and con- 13.5 Mars
sist of submillimeter-sized glass beads.
The small size is due to the fact that almost every A succession of spacecraft missions to Mars has
gas bubble that formed in a magma on the Moon shown that about 60% of the surface area of Mars,
eventually expanded and burst when exposed to mainly in the southern hemisphere, preserves a
the vacuum at the surface, thus tearing the magma very ancient highland terrain covered with impact