Tharsis Ridge(thar -siss) The principal area of volcanic activity on Mars, measuring 5000 km in diameter and elevated by up to 7 km from the average level of the planet's surface. It lies on the Martian equator and includes three of the largest Martian volcanoes: Ascraeus Mons, Pavonis Mons, and Arsia Mons. The highest Martian volcano, Olympus Mons, lies several hundred km to the NW. On the E flank of the Tharsis Ridge is a vast maze of interconnecting canyons, Noctis Labyrinthus, which in turn lie at the W end of the Valles Marineris. See also Mars, volcanoes.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006