Page 222 - Fundamentals of Physical Volcanology

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VOLCANISM ON OTHER PLANETS 199

convection is not the same on Mars as on Earth and, Earth. At the top of a martian volcano 20 km tall,
coupled with the smaller size of Mars, this appears the atmospheric pressure is only ∼13% of its value
to have significant consequences. Computer simu- at the foot of the volcano, where it is already 200
lations of patterns of mantle convection predict times less than on Earth. This 1500-fold difference
that the Earth should have 20 to 30 major areas of has a profound effect on the eruption of any magma
upwelling in the mantle, and this is consistent with containing enough volatiles to cause it to fragment
the number of volcanic hot spots that we see. These as it nears the surface (and martian basalts are
same computer models predict that Mars should expected to have similar amounts of gases to basalts
have only a very small number of such regions of on the Earth). The greater expansion of the released
upwelling, perhaps only two or three, and that they gas leads to greater fragmentation of the magma,
should be more extensive than those on Earth. It is making smaller pyroclast sizes, and much more
very tempting to identify Mars’ two major volcanic acceleration of the erupting materials so that they
provinces, Tharsis and Elysium, with these man- reach much greater speeds. Finally the lower accel-
tle hot spots. If this is the case then it would be eration due to gravity means that pyroclasts thrown
expected that they might have been active for all of out to form cinder or spatter deposits will travel to
Mars’ history, and this is supported by the finding even greater ranges. Table 13.2 compares the erup-
that the range of ages estimated by counting impact tion of a basalt containing 0.25 wt% water on Earth
craters for the various parts of Tharsis does span a at sea level and on Mars at the top of a Tharsis shield
very large fraction of martian geological time. volcano. Eruption speeds on Mars are twice as large
The martian shield volcanoes have morpholo- as on Earth and pyroclast ranges are more than 11
gies very similar, apart from their size, to those of times greater. This means that the eruption of sim-
basaltic shield volcanoes on Earth. Their surfaces ilar volumes of material on the two planets would
appear to be dominated by lava flows and they all lead to a cinder cone more than 130 times less high
have one or more collapse calderas at their sum- on Mars than on Earth, and thus very much harder
mits (Fig. 13.8). All of the remotely sensed spectro- to identify in a spacecraft image.
scopic evidence from orbiting spacecraft suggests Similar issues relate to dispersal of clasts from
that the compositions of the volcanoes are basaltic martian eruption plumes. Computer models of the
to andesitic. Although no rock samples have been rise of Plinian eruption clouds predict that on Mars
collected from Mars we are certain that some mete- the clouds produced by a given eruption rate will
orites come from Mars, because the gases trapped rise about five times higher than on Earth, and so

in them are identical to the martian atmosphere the martian winds (which are typically twice as
sampled by the two Viking spacecraft that landed strong as ours) should disperse materials over a
on the surface. These meteorites are volcanic rocks much greater area. The only place where a fall
with essentially basaltic chemistry. As with the
overall sizes of the volcanoes, the scales of indi-
vidual features are large: whereas lava flows on Table 13.2 Comparison of conditions in eruptions of basalt
the Hawaiian volcanoes tend to be 5 to 20 km long, containing 0.25 wt% water on Mars and Earth.
those on Mars range from 30 to 300 km in length.
Earth Mars Units
Terrestrial basaltic calderas are rarely more than
3 km in diameter and 200 m deep, but martian
Depth at which gas 133 348 m
examples are typically 20 to 40 km wide and up to starts to exsolve
2000 m deep. Depth at which 34 90 m
Much of this difference can be understood in magma fragments
terms of the differing environmental conditions, Final amount of gas 0.2285 0.2499 wt%
especially the atmospheric pressure and the grav- exsolved
Eruption speed of gas 66.4 139.6 m s −1
ity. For example, there are few pyroclastic deposits,
Maximum range of 450 5210 m
and one possible reason for this is that we expect
ejected pyroclasts
them to have a much greater dispersal than on