The Martian volcano
Olympus Mons is one of the largest in the solar system. The volcano is more
than 360 miles (600 km) across and 16 miles (27 km) high. (Courtesy of NASA)
Olympus Mons volcano, Mars The largest-known shield volcano
in the solar system, Mars's Olympus Mons stands some 75,000 feet (22,860 m)
high. Olympus Mons is one of several large volcanoes in the Tharsis region of
the northern hemisphere. The Martian volcanoes appear to have played a major
role in shaping the topography of that hemisphere in relatively recent times,
although the existence of older volcanoes elsewhere on Mars indicates that
volcanism has been active on Mars for much of the planet history. The major
Martian volcanoes are larger by far than any on Earth. The evident absence of
plate tectonics on Mars may account for the impressive size of its volcanoes.
Unlike Earth, where horizontal motion of crustal plates can prevent a volcano
from growing past a certain height because the volcano can move out from its
source, Mars appears to have a fixed, comparatively immobile crust. This lack
of movement gives a volcano time to reach tremendous size by terrestrial
standards. Because the processes that create volcanoes along crustal plate
margins on Earth are apparently lacking on Mars, it is believed that
radionuclides underground may supply the heat required for volcanism there.
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Although little is known about the geology of Mars compared
to that of Earth, volcanism is known to have played a large part in shaping the
surface of the red planet. Unmanned probes to Mars have revealed the existence
of large volcanoes, such as Olympus Mons and the volcanoes along nearby Tharsis
Ridge in the northern hemisphere. Olympus Mons is the largest volcano in the
solar system to the best of our knowledge. Although volcanic mountains are
evident on its surface, Mars appears to lack the pattern of shifting and
colliding crustal plates that characterizes tectonic and volcanic activity on
Earth. It has been suggested that material expelled through the volcanoes of
Tharsis Ridge came from magma underlying Valles Marinensis, a great valley
thought to have been formed by subsidence as molten material underground was
removed through eruptions. Volcanism evidently has given the northern
hemisphere a geomorphology differing from that of the southern hemisphere in
ways besides the mere existence of volcanic mountains. Impact cratering is much
more visible in the southern hemisphere, whereas in the northern hemisphere,
tephra from eruptions apparently has covered many impact craters and other such
features. The Martian volcanoes affect the planet's meteorology on a small but
interesting scale; clouds form in the lee of Olympus Mons and are thought to
have been observed through telescopes on Earth at various times before probes
reached Mars. Mars, like the moon, appears to be much quieter than Earth from a
seismic viewpoint. Evidence of erosive activity on the Martian surface
indicates that Mars once possessed substantial amounts of liquid water and a
denser atmosphere than at present. Although the Martian atmosphere is only a
fraction as dense as that of Earth, occasional dust storms on Mars are capable
of transporting large quantities of sediment aerially. These storms are capable
of hiding even the giant Martian volcanoes from view.
shield volcano
A volcano that is shaped like an ancient shield. It is a
relatively flat mound with gradual slopes, formed by eruptions of basaltic
lava. The basalt has such low viscosity and lacking in ejecta that it cannot
stack up a steep slope. Olympus Mons on Mars and the Hawaiian Islands are
examples of shield volcanoes.
