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Early telescopic observations of Mars revealed a series of dark spots in the Tharsis region, which were originally thought to represent a string of lakes on the Martian surface. The northernmost of these was given the name Ascraeus Lacus, 'Lake Ascraeus', probably named for the birthplace of the Greek poet Hesiod. Once Mars could be mapped by orbiting spacecraft, it became clear that Ascraeus and its companions were not lakes at all - there are in fact no lakes anywhere on Mars - but a group of immense volcanoes. Thus the northernmost 'lake' was renamed as Ascraeus Mons, or Mount Ascraeus.
The entire highland plain of Tharsis in Mars' northern hemisphere is volcanic in origin, but it boasts a series of extinct volcanic peaks rising even higher above its terrain. Prominent among these are the Tharsis Montes, an almost straight line of three huge mountains, with Ascraeus Mons lying at the northeastern end of the line. Ascraeus is not the largest of the three, but it is the tallest, reaching more than 18km above the mean planetary radius at its highest point.
Like the other volcanoes of Tharsis, Ascraeus Mons is not a single narrow peak. Rather it is a shield volcano, built up over time by cumulative lava flows to form a wide, dome-like structure spreading out across the surface of Mars. In the case of Ascraeus Mons, these lava flows have created mountain that measures some 480 km in diameter, with vents in the sides of the mountain spreading lava even further to the north and south. At the highest point of the mountain, in its centre, lies its complex of calderas, craters formed by the collapse of the volcano around its main vent. The central caldera is some 24 km across, and is surrounded by the remains of no less than four lesser structures of the same kind.
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