QUESTION:
Mars has no plates and the center of the planet is very cold.  How 
can there be volcanoes?

ANSWER from Jeff Plescia on May 9, 1997:
While plate tectonics and the boundaries between plates does have a
major role in the formation of volcanoes on Earth, all volcanoes are
not associated with plate boundaries. Typically the volcanoes around
the edge of the Pacific Ocean (the ring of fire) are associated with
the subduction of one plate under another; Iceland, for example, is
associated with a spreading ridge (the mid Atlantic ridge). However,
many other volcanoes occur in the middle of plates away from the
boundaries and are believed to be caused by very hot places in the
mantle, called mantle plumes or hot spots, where hot rot is rising
toward the surface. Hawaii, the Snake River Plains - Yellowstone in 
the western US, and many other Pacific ocean islands are volcanoes 
over such hot spots.

In the case of Hawaii, the hot spot is stationary while the Pacific
plate moves across the Earth. One volcano forms and is then carried
away from the hot spot by the plate motion. Another one then forms 
over the hot spot and is carried away, etc. The result is a line of 
islands which get progressively older. For Hawaii, the chain gets 
older to the northwest and the island of Hawaii is the youngest of 
the set and still has active eruptions. A new island is forming to 
the south but has not yet reached the surface, it is called Loihi.

Mars has only 1 plate, rather than no plates. But the process of hot
spots still works there. There are probably two major hot spots, one
under the Tharsis region and one under the Elysium region. So the hot
material in the mantle rises up and produces volcanoes at the surface
of Mars. But since there is no plate motion, the volcanics just keep
piling up. If the total volume of volcanic material from the entire
Hawaiian island chain were combined, it would be the same as the 
volume of material in Olympus Mons - the largest martian volcano. 
Whether the other large volcanoes in the Tharsis area represent a 
small amount of movement of the hot spot or plumes of different age 
is unknown. Volcanism on Mars has occurred over billions of year and 
there may still be active volcanism today.

ANSWER from Jeff Plescia on May 6, 1997:
Yes, there are volcanoes on Mars; there are quite a few.
And some of them may be active today. The interior of
Mars may still be hot enough in some places to generate
volcanism. Some of the volcanic surfaces appear very fresh
and young and they have very few craters.

The SNC meteorites, which are volcanic in origin and believed to
have come from Mars, have ages as young as 130 million
years. From a geologic perspective that's fairly young and
there may be places where volcanism still occurs.

An eruption on Mars has never been observed as it was
on Io, one of Jupiter's moons.