The Tharsis Montes are three large shield volcanoes in the Tharsis region of the planet Mars. From north to south, the volcanoes are Ascraeus Mons, Pavonis Mons and Arsia Mons. Mons is the Latin word for mountain; it is a descriptor term used in astrogeology for mountainous features in the Solar System. The three Tharsis Montes volcanoes are enormous by terrestrial standards, ranging in diameter from to . Ascraeus Mons is the tallest with a summit elevation of over, or base-to-peak. For comparison, the largest volcano on Earth, Mauna Kea in Hawaii, 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 9spacecraft in 1971. They were among the few surface features visible as the spacecraft entered orbit during a global dust storm. Appearing as faint spots through the dusty haze, they were informally christened North Spot, Middle Spot, and South Spot. A fourth spot corresponding to the albedo featureNix Olympica was also visible and later named Olympus Mons. As the dust cleared, it became obvious that the spots were the tops of enormous shield volcanoes with complex central calderas. The three Tharsis Montes volcanoes are evenly spaced about apart from peak to peak, in a line oriented from southwest to northeast. This alignment is unlikely to be coincidental. Several smaller volcanic centers northeast of the Tharsis Montes are on an extension of the line. The three volcanoes, most notably Arsia Mons, also all have collapse features and rifts, from which flank eruptions issued, that transect them along the same northeast-southwest trend. The line clearly represents a major structural feature of the planet, but its origin is uncertain.
