QUESTION:
I read on this website that Mars has only one plate, unlike Earth which has
13.  This raises a question about the vocanic activity and mountains on
Mars.  Since on Earth, volcanoes and mountain ridges are associated with the
boundary's of these plates, two plates would be nessary for such things as
mountains and vocanoes to exists.  Since Mars boasts the largest known
vocano in the solar system, how could it have only one plate?

ANSWER from Jeff Plescia on September 5, 1997:
Mars is suggested to have only one plate.  That is, there are no areas on
Mars were plates are moving apart (as at the Mid Atlantic Ridge on the
Earth) or coming together (as in the Pacific Ocean Trenches).  Most of the
Earth's volcanism is centered at plate margins (basaltic volcanism at
spreading centers; andesitic volcanism at convergent margins), but not all. 

There are many examples of significant volcanism within a plate.  These best
examples are the Hawaiian islands (which occur in the middle of the Pacific
plate) and Yellowstone (within the North American plate).  Each of these
major centers of volcanism within a plate occur over a hot spot - a zone in
mantle of the Earth where a plume of hot material is rising from the
interior and causing volcanism on the surface.  On Mars we believe there are
similar hot spots. In the case of Hawaii, the plate moves over the hot spot
and a trail of islands is formed.  On Mars there is not movement of a plate
so the volcanics just pile up in one place.

Olympus Mons, the largest of the Martian volanoes, has a volume about equal
to that of the entire Hawaiian island chain.  In Hawaii there volcanic
material is spread out, at Olympus Mons it's one big pile.

It is the thermal conditions of the mantle which govern the location and
amount of volcanism on Mars.