QUESTION:
If molten rock comes from the earth's mantle for its volcanoes, where does
the molten rock for Mars' volcanoes come from?  What is the composition of
the Martian basalt, and does it flow farther because of lack of air pressure
and lower gravity?
 
ANSWER from Jeff Plescia on June 23, 1997:
Molten rock (magma) for martian volcanoes almost certainly comes from the
martian mantle (as the Earth's molten rock comes from the Earth's mantle). 
In many cases on the Earth, the volcanoes (for example Hawaii) lie above a
"mantle plume". A mantle plume is a zone in the mantle where hot rocks are 
rising from depth (possibly the core / mantle boundary).  When it reaches
near the surface, it breaks through the crust and builds a volcano.  This
may also be occurring on Mars as most of the volcanism is restricted to a
few areas and within those areas, only a few volcanoes occur.

Based on the shape and size of the lava flows, it is assumed that they are
either basalt or komatiite.  They flow long distances because the lava is
very hot, has very low viscosity and is erupted at a high rate.  The rate of
cooling is similar to that on the Earth.