Yogi is a large basalt-type rock.
From there we decided we should pretty much go straight toward Yogi, which
is the large, big rounded rock which is almost perched in some of the
rover images. It looks like it's sitting on a point and it's a big round
rock. When we got close to it, it turned out to be more difficult to get
the APX up against it and we had our first fenderbender on Mars where the
wheel of the rover actually went up against Yogi. But we did back off and
get the APX up against it and found that that turned out to be a less
silca-rich rock, more like a basalt primitive mantle melt that we kind of
expected on Mars. Except that part of the surface looks like it was
covered with dust and that if you subtract that dust away, then Yogi could
be a higher-silca rock as well. Both of which are truly outstanding
discoveries.
Yogi is a really large rock and it does have some texture in it that has
some clues to what it is. It has a radiating bundle of fractures in the
upper left-hand side. These cracks could be result of cooling from a melt.
I've seen cracks like this in lava flows; they've also been found in
welded pyroclastic flows, which are volcanic rocks which have been
exploded and deposited by eruption through the air. But there are other
possibilities: it's possible this could be some kind of sedimentary
structure as well.
Yogi has a two-toned appearance which makes it an extremely interesting
rock in terms of surface processes. One side of the rock is very reddish,
crusted appearance; the other side appears to be scoured by the wind and
free of that dust and weathered material. This tells us it has probably
been sandblasted to remove that outer crust.