Page 218 - Fundamentals of Physical Volcanology

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VOLCANISM ON OTHER PLANETS 195

Fig. 13.3 The central part of the lunar linear rille Hyginus.
The numerous collapse craters aligned and elongated along
its length imply a strong volcanic association. (Part of Lunar
Orbiter III frame 73M; NASA image.)

Fig. 13.2 Some of the lava flow units that flooded the
interior of the Mare Imbrium impact basin on the Moon.
produces heat in proportion to the number of
The image is ∼32 km wide. (Part of Apollo 17 Hasselblad
radioactive atoms it contains, which is proportional
frame #AS17-155-23714. NASA image.)
to its mass and hence to its volume, in turn propor-
tional to its radius cubed. When that heat reaches
which are examples of graben, depressions where the surface (whether it gets there volcanically or by
the crust has subsided between two parallel, nearly conduction), the rate at which it is lost by radiation
linear faults. These features form where tensional into the surrounding space is proportional to the
forces stretch the crust until the rocks break. In surface area which is in turn proportional to the
some cases there are minor volcanic features – radius squared. Thus the ratio (heat produced/heat
domes, cones, and small flows – associated with lost) is proportional to (radius cubed/radius squared),
these rilles (Fig. 13.3), and where these occur the i.e., is proportional to the radius. So large planets

rilles tend to be deeper, implying a greater horizon- have more trouble getting rid of their heat than
tal extension of the crust than usual. This is easy to small planets. Conversely, small planets lose heat
understand as the result of the need to make space efficiently and, as a result, are not volcanically active
for a dike nearing the surface. The implication is for very long. This idea acts as a guide to what we
that much of the crust of the Moon was invaded should expect on other bodies.
by dikes that stalled not far below the surface as Although the most common volcanic features on
intrusions because they could not quite reach the the Moon are the long lava flows flooding the
surface to produce major eruptions. They were, impact basins, there are other features interpreted
however, able to force a small amount of magma to to be evidence for large-volume eruptions: the
the surface to form the minor features that we see. sinuous rilles (Fig. 13.4). The inference is that in
The duration of the Moon’s volcanically active some places the eruption rate of lava was so great
phase, deduced by using the proportions of radio- that the flows were turbulent rather than laminar,
active elements in rock samples returned by the and the constant stirring of the lava increased the
Apollo missions to measure the time since the lavas efficiency with which heat was transferred to the
froze on the surface, was only about 1000 million underlying ground. Some of these eruptions con-
years, less than one-quarter of lunar history. The tinued for long enough – several weeks – that the
reason it was so short is the small size of the ground, often consisting of older lava flows, was
Moon compared with the Earth. A planetary body heated to its solidus and began to melt. The molten