Tharsis Montes is a region on Mars from which four giant shield volcanoes rise. Three of these volcanoes can be seen in the photo above. They are Arsia Mons in the lower left, Pavonis Mons in the middle, and Ascreus Mons in the upper right. The largest volcano of all on Mars (and in the solar system), Olympus Mons, is also in the Tharsis Region and lies off the image to the upper left.
Arsia Mons, Pavonis Mons, and Ascreus Mons dominate the center of the Tharsis region, forming a line nearly 1500 km long. They are about equally spaced, nearly 700 km apart, and reach a similar altitude to Olympus Mons, that is, 25–26 km. However, the volcanoes of the Tharsis region sit on a large mound of lava flows nearly 10 km high, so their true height is "only" about 15 km. All three seem to have formed at about the same time and were active for a long period. Arsia Mons appears to be slightly older than Pavonis Mons, which, in turn, slightly predates Ascreus Mons, indicating that volcanism in the Tharsis region gradually shifted north over time. A similar progression of volcanism is found in the Hawaiian Islands. Almost certainly, the giant shield volcanoes on Mars formed over a hotspot in the mantle as did their Hawaiian counterparts.