QUOTE (Stu @ Jul 14 2007, 02:01 AM)
Interesting - very thick - crust on the big feller
Many of those vesicular basalt float pieces display a thin layer that is not filled with bubbles. There are several ways such a textural contact can form in basalts, but most likely both layers have very similar chemical compositions. Mainly, it depends on how quickly the volatiles can expand, float upwards, and escape from the flow, and how quickly the flowing molten rock solidifies. As you might imagine, there are a lot of variables that can influence those processes.
Since we are only seeing pieces of a basalt flow that once existed, it is difficult to put it into a context where we might speculate further about the nature of that contact. I continue to hope that Spirit manages to hobble over to an outcrop where some of this basalt is still in place, if such a place still exists nearby.