The high plateau of Tharsis lies at the western end of the rift of Valles Marineris in Mars' northern hemisphere. In ancient times this highland plain was the site of immense volcanic upswelling, and indeed the central regions have been thrust up some 10km above the mean elevation of the planet. Across this great volcanic bulge lie a series of long-extinct shield volcanoes. The largest and best known of these is Olympus Mons, but across the plateau runs of line of three similar volcanoes that form the Tharsis Montes.
The Tharsis Montes form a line running for some 1,800km from southwest to northeast. At the southwestern end is Arsia Mons, the broadest of the three volcanoes with a base measuring some 475km in diameter. The central mountain is Pavonis Mons, the smallest of the three, though it still dwarfs any similar feature on Earth. The third, northeastern volcano is Ascraeus Mons, not quite so broad as Arsia Mons, but the highest of the Tharsis Montes with a peak rising 18km above its base on the already high Tharsis plateau.
The fact that the three volcanoes form an almost exact line, and that other less prominent volcanic features also fall along this line, implies that they formed as part of a common geological structure or event in the turbulent past of the bulging volcanic plain of Tharsis.