This is an image of some Martian volcanoes.
Click on image for full size
Image from: NASA
The History of Martian Volcanism
During its earliest history, Mars was bombarded with *planetismals*. The impacts of these asteroid-like boulders caused the surface regions of Mars to become warm enough for continents to drift across the surface just as they do on Earth to this day. When the lithosphere became immovable, what may once have been a supercontinent froze in place, in the southern hemisphere, becoming what is now the highlands of Mars.
Evidence for this theory can be found with an examination of the global geography of Mars, together with the cratering pattern.
The cratering evidence suggests that Mars may have warmed from the inside late in its forming history, causing volcanism *after* the lithosphere became immovable. This period of volcanism is what created the Tharsis Bulge, Olympus Mons, and the other volcanoes. The volcanoes poured out a new surface over the lowlands of Mars, which received a lighter load of bombardment than did the older highlands. The lowlands, where the volcanoes are found, are cratered at a rate which suggests an age of 3.7-3.8 Billion Years. The highlands are much older.
The cratering record suggests that after this period however, all volcanic activity on Mars ceased. There seem to be no features younger than 3.5 Billion Years.
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