The canyons of Valles Marineris terminate to the west near the crest of the Tharsis rise, a vast bulge on the Martian surface more than 8,000 km (5,000 miles) across and 8 km (5 miles) high at its centre. Near the top of the rise are three of the planet's largest volcanoes—Ascraeus Mons, Arsia Mons, and Pavonis Mons— which tower 18, 17, and 14 km (11.2, 10.5, and 8.7 miles), respectively, above the mean radius. Just off the rise to the northwest is the planet's tallest volcano, Olympus Mons. To the north is the largest volcano in areal extent, Alba Patera. It is 2,000 km (1,250 miles) across but only 7 km (4.3 miles) in height. Between these giant landforms are several smaller volcanoes and lava plains. Tharsis itself is a vast pile of volcanic rock, and although it had largely formed by 3.7 billion years ago, it has been a centre of volcanic activity ever since.
The presence of the Tharsis rise has caused stresses within, and deformation of, the crust. A vast system of fractures radiating from Tharsis and compres-sional ridges arrayed around the rise are evidence of this process. The radial faulting around Tharsis appears to have contributed to the formation of the Valles Marineris system.
Another volcanic rise is located in the northern region of Elysium at about 215° W longitude. The Elysium rise is much smaller than Tharsis, being only 2,000 km across and 6 km (3.7 miles) high, and is also the site of several volcanoes.
The largest volcano in the solar system and the highest point on Mars is the imposing mountain Olympus Mons. Centred at 19° N, 133° W, Olympus Mons consists of a central edifice almost 22 km (14 miles) high and 700 km (400 miles) across. Around its perimeter an outward-facing cliff ascends as high as 10 km (6 miles) above the surrounding area. At the summit is an 85-km- (53-mile-) diameter crater, or caldera, comprising several mutually intersecting craters. For comparison, the largest volcano on Earth, Mauna Loa, Hawaii, measures 120 km (75 miles) across at its widest extent and rises 9 km (5.6 miles) above the ocean floor. Broad, gradually sloping flanks and the presence of numerous long flows and lava channels identify Olympus Mons as a shield volcano and suggest that it was built up from eruptions largely of fairly fluid basaltic lava. Its tremendous size has been attributed to the stability of the Martian crust and to a long accumulation time, possibly more than a billion years.
Olympus Mons, the highest point on Mars, in a computer-generated oblique view made by combining photos obtained by the Viking mission in the 1970s with topographic data gathered by Mars Global Surveyor a quarter century later. NASA/JPL/MOLA Science Team
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