Mars Geology

Volcanoes

One of Mars' most notable types of features is its volcanoes. Volcanoes are built by the accumulation of lavas that erupt from deep reservoirs of molten rock, from successive deposits of exploded ash and cinder from gas-rich explosive eruptions, or mixtures of the two. On Earth, plate tectonics shifts the Earth's lithosphere horizontally so that a volcano eventually moves off its deep magma source. That limits the volume of lava that can erupt over time, and therefore how high a volcano can grow. But the Martian lithosphere is fixed in place, so that a volcano's height is limited by the longevity and volume of the underlying magma source. As a result, the heights of many of Mars' volcanoes are extreme by Earth standards. For example, the largest Martian volcano, Olympus Mons, is three times taller base-to-summit than Earth's largest volcano, the island of Hawaii. The summit of Olympus Mons protrudes above more than 85% of the planet's atmosphere. Three other large volcanoes, called the Tharsis Montes, crown the Tharsis volcanic plateau. Many dozens of smaller volcanoes of different types and sizes are distributed across the rest of the planet.

Olympus Mons is larger than the U.S. state of Arizona and three times taller than Earth's largest volcano (the island of Hawaii). (Image credit NASA/USGS.)