Tharsis quadrangle Mount Everest and about as big in area as all of Europe. Tharsis contains a group of large volcanoes. Olympus Mons is the tallest. Tharsis is a land of great volcanoes. Olympus Mons is the tallest known volcano in the Solar System; it is 100 times larger than any volcano on Earth. Ascraeus Mons and Pavonis Mons are at least 200 miles across and are over six miles above the plateau that they sit on—and, the plateau is three to four miles above the zero altitude of Mars. Pavonis Mons, the middle in a line of three volcanoes, sits at
Tharsis Montes is about across and stands above the ocean floor. The Tharsis Montes volcanoes lie near the equator, along the crest of a vast volcanic plateau called the Tharsis region or Tharsis bulge. The Tharsis region is thousands of kilometers across and averages nearly above the mean elevation of the planet. Olympus Mons, the tallest known mountain in the Solar System, is located about northwest of the Tharsis Montes, at the edge of the Tharsis region. The Tharsis Montes were discovered by the Mariner 9 spacecraft in 1971. They were among the few surface features visible as the spacecraft entered orbit