Page 225 - Fundamentals of Physical Volcanology

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202 CHAPTER 13

episodic. First, swarms of graben can be seen radi-
ating from many of the Tharsis volcanoes. Some
scientists argue that these are mainly created by
tension due to the way the developing volcanic
edifices stress the underlying lithosphere. However,
many of the graben have collapse craters or small
volcanic features associated with them, and some
show low mounds aligned along their floors
(Fig. 13.10). It is tempting to assume that the mounds
are the tops of dikes which formed the graben, the
dikes being fed from the central magma reservoir
under the volcano at the center of the graben
swarm. Graben creation would then be most logi-
cally associated with the episodes of upsurge in
magma production in the mantle which, as seen
above, may occur at 100 million year intervals.
Many of the enormous valleys formed by water
floods that are seen in various places on Mars have
sources located either within graben of this kind or
within nearby complex areas of collapsed terrain
that appear to be underlain by sills fed by the dikes.
Indeed, in some of these places deposits occur that
appear to be the products of hydrovolcanic explo-
sions. Thus the volcanic episodes may be the direct
cause of the water-release events. However, they do
this not by melting cryosphere ice but by frac-
turing the cryosphere and releasing some of the
vast supply of water trapped in aquifers beneath it.
And if future volcanic eruptions are possible, so are
large catastrophic water flood events. The sites of
these volcanically triggered water floods are the
Fig. 13.10 Part of the Memnonia Fossa graben on Mars.
most likely places to look for evidence of living
The linear hills aligned roughly east–west on the floor of the
graben may be outcrops of the top of the dike that caused organisms that might be surviving in the under-
the subsidence of the graben floor. The gap in the north ground water reservoirs.
wall of the graben is ∼5.5 km wide; it was eroded by the
overflow to the north of water from deep underground that
filled the valley. (Part of THEMIS visible image #V04762003,
13.6 Venus
courtesy NASA/JPL/ASU.)
Of all the terrestrial planets, Venus is closest in size
cyclicity that is emerging, plus the fact that being in to the Earth, and should be at a similar stage in its
an active phase does not have to mean being active thermal and volcanic history. We cannot see the
all the time, it is just as likely that all of the volca- surface directly in visible light from orbiting space-
noes are currently in dormant phases or between craft because the complete water droplet cloud
eruptions. Humans may yet see eruptions on Mars, cover in the upper part of the dense carbon dioxide
although the long time scales imply that an eruption atmosphere blocks light transmission. However,
during any one person’s lifetime is not very likely. three missions have taken radar scanners to
There are two consequences of the idea that Venus and produced radar images of its surface
the magma supply from martian mantle plumes is (Fig. 13.11) with high enough resolution (∼100 m)