Planetary Volcanism 831
FIGURE 2 The upper three layers of gray, dark, and bright
material are air-fall pyroclastic deposits from the 1875 Plinian
eruption of Askja volcano in Iceland. They clearly mantle earlier,
dark, more nearly horizontal pyroclastic deposits. (Photograph
by L. Wilson.)
to extensive melting of the continental crustal rocks—the
potential exists for the occurrence of very large scale explo-
sive eruptions in which finely fragmented magma is blasted
at high speed from the vent to form a convecting erup-
tion cloud, called a Plinian cloud, in the atmosphere. These
clouds may reach heights up to 50 km, from which pyroclas-
tic fragments fall to create a characteristic deposit spread-
ing downwind from the vent area (Fig. 2). Under certain
circumstances, the cloud cannot convect in a stable fashion
and collapses to form a fountain-like structure over the vent,
which feeds a series of pyroclastic flows—mixtures of incan-
descent pyroclastic fragments, volcanic gas, and entrained
air—that can travel for at least tens of kilometers from the
vent at speeds in excess of 100 m/s, eventually coming to
rest to form a rock body called ignimbrite. These fall and
flow deposits may be so widespread around the vent that no
appreciable volcanic edifice is recognizable; however, there
may be a caldera, or at least a depression, at the vent site
due to the collapse of the surface rocks to replace the large
volume of material erupted from depth.
It should be clear from the foregoing descriptions that
the distribution of the various types of volcano and charac-
teristic volcanic activity seen on Earth are intimately linked
with the processes of plate tectonics. A major finding to
emerge from the exploration of the solar system over the
last 30 years is that this type of large-scale tectonism is
currently confined to the Earth and may never have been
active on any of the other bodies. Virtually all of the ma-
jor volcanic features that we see elsewhere can be related
to the eruption of mantle melts similar to those associated
with the midocean ridges and oceanic hot spots on Earth.
However, differences between the physical environments
(acceleration due to gravity, atmospheric conditions) of the
other planets and Earth lead to significant differences in
the details of the eruption processes and the deposits and
volcanic edifices formed.1.2 The Moon
During the 1970s, analyses of the samples collected from
the Moon by theApollomissions showed that there were
two major rock types on the lunar surface. The relatively
bright rocks forming the old, heavily cratered highlands of
the Moon were recognized as being a primitive crust that
formed about 4.5 Ga (billion years) ago by the accumulation
of solid minerals at the cooling top of an at least 300 km
thick melted layer referred to as a magma ocean. This early
crust was extensively modified prior to about 3.9 Ga ago
by the impacts of meteoroids and asteroids with a wide
range of sizes to form impact craters and basins. Some of
the larger craters and basins (the mare basins) were later
flooded episodically by extensive lava flows, many more than
100 km long, to form the darker rocks visible on the lunar
surface. [SeeTheMoon;PlanetaryImpacts.]
Radiometric dating of samples from lava flow units
showed that these mare lavas were mostly erupted between
3 and 4 Ga ago, forming extensive, relatively flat deposits
inside large basins. Individual flow units, or at least groups
of flows, can commonly be distinguished using multispec-
tral remote sensing imagery on the basis of their differing
chemical compositions, which give them differing reflectiv-
ities in the visible and near-infrared parts of the spectrum.
In composition, these lavas are basaltic, and their detailed
mineralogy shows that they are the products of partial melt-
ing of the lunar mantle at depths between 150 and more
than 400 km, the depth of origin increasing with time as
the lunar interior cooled. Melting experiments on samples,
supported by theoretical calculations based on their miner-
alogies, show that these lavas were extremely fluid (i.e., had
very low viscosities, at least a factor of 3 to 10 less than those
of typical basalts on Earth) when they were erupted. This
allowed them to travel for great distances, often more than
100 km (Fig. 3) from their vents; it also meant that they had
a tendency to flow back into, and cover up, their vents at
the ends of the eruptions. Even so, it is clear from the flow
directions that the vents were mainly near the edges of the
interiors of the basins that the flows occupy. Many vents
were probably associated with the arcuate rilles found in
similar positions. These are curved grabens, trench-like de-
pressions parallel to the edges of the basins formed as parts
of the crust sink between pairs of parallel faults caused by
tension. This tension, due to the weight of the lava ponded
in the middle of the basin, makes it easier for cracks filled
with magma to reach the surface in these places.