Also spelled Ascraeus Mons, this is one of the Tharsis Montes, a range of gigantic, extinct volcanoes on Mars which includes Olympus Mons, Arsia Mons and Pavonis Mons. At over 25km above the surface of Mars, Ascreus Mons is almost as tall as Olympus Mons, and almost as wide (440km in diameter), but much of its height is made up by the fact that it, together with Arsia Mons and Pavonis Mons, stands on the Tharsis Ridge, a huge geological feature that makes up approximately 25% of the Martian surface.
Like the other giant volcanoes on Mars, Ascreus Mons rises at a very slow angle from the land around. It is a shield volcano encircled by a fracture or ridge, and contains a very large volcanic caldera at its centre, in which many smaller craters can be seen. Its past eruptions, as can be seen from its very small angle of ascent, have been slow, basalt flows which are technically known as 'effusive', slowly seeping vast lava flows which have built up the land around its centre over hundreds of millions of years.
Until recently it was thought that Ascreus Mons, along with the other Tharsis Montes, had not erupted for 1 or 2 billion years, but new evidence gathered by the Mars Global Surveyor suggests that it may have been active more recently. See Arsia Mons for more detail.