Planetary Volcanism 833
FIGURE 4 The Olympus Mons shield volcano
on Mars with the Hawaiian Islands
superimposed for scale. (NASA image with
overlay by P. J. Mouginis-Mark. Reproduced by
permission of the Lunar and Planetary
Institute.)suggests that most of the magmas erupted on Mars are
basalts or basaltic andesites. [SeeMars: Surface and In-
terior; Mars: Landing Site Geology, Mineralogy,
and Geochemistry.]
The most obvious volcanic features on Mars are four ex-
tremely large (∼600 km diameter, heights up to>20 km)
shield volcanoes (Olympus Mons, Ascraeus Mons, Pavonis
Mons, and Arsia Mons) with the same general morphology
as basaltic shield volcanoes found on Earth (Fig. 4). There
are also about 20 smaller shields on Mars in various stages of
preservation. Counts of small impact craters seen in high-
resolution (∼10 m/pixel) spacecraft images show that the
ages of the lava flow units on the volcanoes range from more
than 3 Ga to less than∼50 Ma. Complex systems of nested
and intersecting calderas are found on the larger shields,
implying protracted evolution of the internal plumbing of
each volcano, typified by cycles of activity in which a volcano
is sporadically active for∼1 Ma and then dormant for∼ 100
Ma. Individual caldera depressions are up to at least 30 km
in diameter, much larger in absolute size than any found on
Earth, and imply the presence of very large shallow magma
reservoirs during the active parts of the volcanic cycles. The
large size of these reservoirs, like that of the volcanoes them-
selves, is partly a consequence of the low acceleration due
to gravity on Mars and partly due to the absence of plate
tectonics, which means that a mantle hot spot builds a single
large volcano, rather than a chain of small volcanoes as on
Earth. The availability of large volumes of melt in the man-
tle beneath some of the largest shield volcanoes has led to
the production of giant swarms of dikes, propagating radi-
ally away from the volcanic centers for more than 2000 km
in some cases.
Most shields appear to have flanks dominated by lava
flows, many more than 100 km long. The flanks of Elysium
Mons contain some sinuous channels like the sinuous rilles
on the Moon that we think are caused by hot, turbulent,
high-speed lavas melting the ground over which they flow.
Some of the older and more eroded edifices, like Tyrrhena
Patera and Hadriaca Patera, appear to contain high pro-
portions of relatively weak, presumably pyroclastic, rocks.
There is a hint, from the relative ages of the volcanoes and
the stratigraphic positions of the mechanically weaker lay-
ers within them, that pyroclastic eruptions were commoner
in the early part of Mars’ history. More contentious is the
suggestion that some of the plains-forming units, generally
interpreted as weathered lava flows, in fact consist of pyro-
clastic fall or flow deposits.1.4 Venus
Because of its dense, optically opaque atmosphere, the only
detailed synoptic imaging of the Venus surface comes from
orbiting satellite-based radar systems. Despite the differ-
ences between optical and radar images (radar is sensitive
to both the dielectric constant and the roughness of the
surface on a scale similar to the radar wavelength), numer-
ous kinds of volcanic features have been unambiguously
detected on Venus. Large parts of the planet are covered
with plains-forming units consisting of lava flows, having
well-defined lobate edges and showing the clear control of
topography on their direction of movement (Fig. 5). The
lengths (which can be up to several hundred kilometers)
and thicknesses (generally significantly less than 30 m, since
they are not resolvable in the radar altimetry data) of these
flows suggest that they are basaltic in composition. This in-
terpretation is supported by the (admittedly small) amounts
of major-element chemical data obtained from six of the
Soviet probes that soft-landed on the Venus surface. Some
areas show concentrations of particularly long flows called
fluctus (Latin for floods). Most of the lava plains, judging