Volcanism on Mars
Mars has one single Plate (no `tectonics'), but has two distinct terrains: The Northern plains and the Southern Highlands. An enormous escarpment several km high separates them. The Southern Highlands are rough, highly cratered and old. The North is much younger, with extensive resurfacing. (Movie)
Why are the volcanoes so large?
1) Lack of erosion (water or air), 2) No motion of
plates combined with a long lived mantle hotspot and 3) Low GRAVITY.. about 1/3 of
Earth's gravity.
The lack of erosion helps in sorting out the geological history. It shows that volcanism has been extensive through out Mars' history. Some volcanoes are several b.y.o. Yet, it has also been active fairly recently. Olympus Mons is thought to be at most 200 m.y.o. It's not known if volcanism is presently still active or not.