QUESTION:
Which three basic rock types (igneous, metamorphic, or sedimentary) would be
the most and least likely to be found on Mars?

ANSWER from Jeff Plescia on November 18, 1997:
The types of rocks on the martian surface and their lithology are still very
poorly constrained topics.  We have not yet made definitive petrologic
examinations on the surface of Mars and the only direct samples are the SNC
meteorites.

Based on orbital images, one might expect igneous and sedimentary rocks.
Much of the Tharsis and Elysium is covered with volcanic rocks and the
morphology of the flows and the volcanoes suggests they are basaltic or
ultramafic in nature.  Most of the SNC meteorites have compositions which
are similar to basalt, although the rocks appear to be shallow intrusive
rather than surface flows.  But they might be very thick flows which cooled
slowly to produce larger crystals.  The X ray data from the Pathfinder site
is consistent with basalt or basaltic andesite and much of the texture of
the rocks at both Pathfinder and the Viking landing sites suggests the rocks
are largely volcanic.

The northern plains appear to be sedimentary, both in terms of windblown
material and material deposited by the floods.  Whether the sediments are
lithified or not and their composition is unknown.  Several people on
Pathfinder have suggested that conglomerates and sandstones occur at the
landing site.  But this is based on the morphology and seen in the images.

The real question is the nature of the cratered highlands.  The one SNC
meteorite (ALH 84001) which has an age consistent with it coming from the
highlands has a composition of pyroxenite.  Whether this is a typical sample
of the highlands or just one that was strong enough to make it to the Earth
is unknown.  In addition to what ever the crust is made of, there are
clearly sedimentary, and younger volcanic deposits.

Probably, overall, igneous rocks - volcanic and plutonic - would be the most
voluminous, with sedimentary second.  It is doubtful that there are much
metamorphic rocks (gneisses, schists, etc.) exposed on the surface of Mars. 
Although such rocks may occur at depth, they might only be exposed as
fragments in ejecta from large craters and in some of the faulted rings
around large impact basins.

There does not appear to be active plate tectonics on Mars.  There are no
spreading ridges or subduction zones observed.  The large size and volume of
the Tharsis shields suggest they are stationary points over a hot spot. It
has been estimated that the volume of the entire Hawaiian chain is
equivalent to Olympus Mons.  Plate tectonics may not have developed on Mars
either because the mantle never had sufficient convection or because there
was not a lot of water in the system.