The Tharsis Montes includes Olympus Mons, the largest known volcano in the solar system, which is northwest of the Tharsis Montes chain. Taller than three Mount Everest’s and about as wide as the entire Hawaiian Island chain, this giant volcano appears nearly as flat as a pancake from space. Olympus Mons is the largest of the major Tharsis volcanoes, rising 25 kilometers (15.5 miles) and stretching over nearly 550 kilometers (340 miles) east-west. The summit caldera, a composite of as many as seven roughly circular collapse depressions, is 66 by 83 kilometers (41 by 52 miles) across.

Elysium Mons is one of three large volcanoes that occur on the Elysium Rise, the others are Hecates Tholus (northeast of Elysium Mons) and Albor Tholus (southeast of Elysium Mons). The volcano rises about 12.5 kilometers (7.8 miles) above the surrounding plain. Elysium Mons has many craters on its surface. Some of these probably formed by meteor impact, but many show no ejecta pattern characteristic of meteor impact. Some of the craters are aligned in linear patterns that are radial to the summit caldera.  These most likely formed by collapse as lava was withdrawn from beneath the surface, rather than by meteor impact. Other craters may have formed by explosive volcanism. Evidence for explosive volcanism on Mars has been very difficult to identify from previous Mars spacecraft images.[1]