Geologic map of the Phoenicis Lacus Quadrangle of Mars

The Phoenicis Lacus quadrangle (named Lake of the Pheonix by Schiaparelli, 1877) includes some of the largest geologic features recognized on the terrestrial planets. Arsia and Pavonis Montes, (“South Spot” and “Middle Spot” of Mariner 9) rise 18 and 17 km, respectively, above the surrounding plateau to an elevation of about 27 km above the 6.1 millibar base level. These mountains and the two large volcanoes Ascraeus and Olympus Montes, located outside the quadrangle to the north, are also about 27 km in height, about three times as high as any volcano on Earth. The summit caldera of Arsia Mons. more than 100 km in diameter, is the largest known. Syria and Sinai Plana are part of the highest plateau on Mars, the Tharsis plateau; their relative elevation, 10 km, is twice that of the Tibetan Plateau, the highest plateau on Earth. Noctis Labyrinthus, the great system of fault valleys at the west end of Valles Marineris which is located outside the quadrangle to the east, displays a more clearly developed system of faults than the largest continental fault system on Earth, the East African rift valleys. The land slopes down to the northwest into Amazonis Planitia (fig. 1); the difference in elevation between the volcanic peaks and Amazonis low plains is greater than the relief form the top of the Andes to the bottom of the Peruvian trench. This great slope on Mars is similar to the continental shelf and slope that marks the transition between the continents and ocean basin floors of the Earth.