Tharsis is the vast volcanic region on Mars that contains several of the planet's enormous shield volcanoes. This elevated province dominates much of Mars's western hemisphere and stretches for thousands of kilometers near the equator.
The three volcanoes known as the Tharsis Montes—Arsia Mons, Pavonis Mons, and Ascraeus Mons—form a prominent northeast-to-southwest alignment across the region. Tharsis also includes Alba Mons, while Olympus Mons is usually associated with the broader Tharsis volcanic province even though it lies just off the main bulge's western edge.
Martian shield volcanoes are built by repeated eruptions of relatively fluid basaltic lava. They can grow much larger than most terrestrial volcanoes because Mars lacks Earth-like plate tectonics: a volcanic source can remain beneath one area for extremely long periods instead of being carried away by a moving plate.
Tharsis is sometimes confused with Elysium, another major Martian volcanic province. Elysium also contains large volcanoes, but the best-known concentration of gigantic shield volcanoes—including the Tharsis Montes—is in Tharsis.