re: Sorry Marcia
No major volcanic constructs have been identified in TM although source regions may remain buried.
It's that "source regions may remain buried" part.
If the material is volcanic air fall, the source(s) may be several thousand kilometers away [Hynek et al., 2002]."
It wouldn't surprise me, Olympus Mons may have spewed it's last breath 'millions' of years ago.
What's interesting is how flat and clean that middle section of Terra Meridiani is when compared to it's surroundings. Was it due from a volcanic bed/volcanism alone? Both volcanism and underground ice/hydrothermal? The sedimentary 'look' of the surrounding area is puzzling.
Thank you, Nick.