Page 221 - Fundamentals of Physical Volcanology

9780632054435_4_013.qxd 12/10/2007 12:34PM Page 198

198 CHAPTER 13

craters. The rest of the surface, mainly the northern
hemisphere, is lower in average elevation and con-
sists of extensive plains made mainly of lava flows
and finer grained windblown materials. The current
martian atmosphere is thin, with a surface pressure
about 200 times smaller than that of Earth. This,
combined with the fact that Mars is further from the
Sun than Earth, causes the mean surface temper-
ature to be about −60°C; only in a few places for a
few hours on summer afternoons does the temper-
ature rise above the freezing point of water. The
low atmospheric pressure means that water boils
at between 1 and 2°C, and so the surface of Mars is
currently both very cold and very dry.
All the available evidence implies that Mars still
has plenty of water, present as ice in pore space and
fractures in the outer few kilometers of the crust
and as water trapped in pore space and fractures
beneath the icy layer. In the lowlands surrounding
the north polar cap there are deposits, currently
being eroded by the wind, that may be the remains
of the mud that collected on the floor of an anci-
ent ocean that surrounded the north pole. The
presence of such an ocean would imply that the
atmospheric pressure was higher in the early his-
tory of Mars. The ghosts of old impact craters show
through these sediments in places, and add to the
evidence from the craters in the southern highlands
that there has been little or no large-scale distur-
bance of the original crust of Mars, in other words Fig. 13.8 The summit region of Arsia Mons, one of the

that plate tectonics has never occurred on Mars. giant shield volcanoes in the Tharsis region of Mars. The
summit caldera dominating the image is ∼110 km wide, and
The bulk of the volcanically erupted material on
the entire volcano is more than 500 km in diameter and
Mars is concentrated into two major provinces, ∼19 km high. The complex depressions on the SW and NE
Tharsis and Elysium, which contain a series of sides mark rift zones that extend in these directions. (Mars
giant shield volcanoes (Fig. 13.8). The four largest Odyssey THEMIS image courtesy of NASA/JPL/Arizona
of these, Olympus Mons, Ascraeus Mons, Pavonis State University.)
Mons, and Arsia Mons, are in Tharsis and are typ-
ically 20 km high and 600 km wide. The three vol- giant neighbors but are partly buried by their
canoes forming the Elysium group, Hecates Tholus, lava flows so that it is hard to judge true sizes. There
Elysium Mons, and Albor Tholus, are much smaller, are also a few much older and heavily eroded volca-
with the largest, Elysium Mons, being about half as noes clustered around the Hellas impact basin in
tall and wide as the Tharsis shields. To the north of the cratered southern highlands.
Tharsis lies the volcano Alba Patera, much lower The large sizes of the martian volcanoes and their
(about 4 km tall) but wider (about 1000 km) than grouping into two main provinces raise issues
the four Tharsis shields, and scattered around about the state of the martian mantle. If plate tec-
Tharsis is a group of volcanoes (Uranius Paters, tonics never developed on Mars, the crust has
Ceraunius Tholus, Biblis Patera, Tharsis Tholus, and always been stationary relative to the mantle. This
Jovis Tholus) that are apparently smaller than their means that the upper boundary control on mantle