QUESTION:
How do you explain the presence of rocks of andesitic composition (from the
preliminary APXS data) on Mars given that all the tell-tale signs of active
or ancient plate tectonics (eg. mountain ranges etc.) are absent?

Rocks of this composition usually imply significant fractionation of a
primitive magma, commonly only seen in convergent plate-tectonic settings on
Earth (eg. the Andes, north island of New Zealand etc.).

ANSWER from Bruce Jakosky on September 1, 1997:
That's a good question, and one of the puzzles from the Pathfinder results.
If we rule out as improbable the idea that the andesite is actually an Earth
rock transported to Mars by impact, then we are left with several possible
alternatives.  First, the apparent andesites could be impact breccias,
formed by the squeezing together of grains from many different kinds of
rocks.  Second, there could be a substantial contamination of the rock due
to the presence on its surface of airborne dust that was high in silicon
content.  Third, it could be a volcanic rock erupted from a differentiated
magma chamber.  This last may not be too implausible.  For example, we find
andesites and, in fact, pure olivine, in some volcanic eruptions in Hawaii;
these generally occur as late-stage volcanism, but this is a similar
environment to Mars in that we would expect something more basaltic. 

Unfortunately, without having more information about what sort of geologic
environment was the source for these rocks, it's awfully difficult to know
how to interpret them.  This is the difficulty of dealing with a "grab bag"
landing site that contains rocks that came from other regions.  We might be
able to make a better interpretation after a large number of rocks have been
examined and we can get statistics on the types of rocks that are present.


ANSWER from Phil Christensen on August 11, 1997:
The best response to the question of andesites on Mars is that we have 
very limited experience with volcanoes and "plumbing systems" the size 
of the volcanoes on Mars.  What type of differentiation might occur 
beneath a volcano 600 km in size and 26 km high?  How does differentiation 
proceed in magma chambers of this scale?  What are the recharge time 
scales and volumes from the source below the volcano?  Perhaps magmas 
reside in chambers for a very long time. Also, we don't have a good 
idea of the water content of the martian mantle, nor its effect on 
differentiation.  I would suggest that if andesites are confirmed then 
it will force us to rethink how more differentiated magmas can form 
in the absence of plate tectonics. I suspect we have a lot to learn.