Volcanic Plains

Flows on the flanks of the volcano and adjacent volcanic plains that were erupted from fissures east of the volcano are among the youngest flows on Mars. The extreme height of the Martian volcanoes can be attributed to the low surface gravity and the lack of relative motion between the lithosphere and the magma source. The presence of shield volcanoes on Mars and their absence on the Moon can be attributed to differences in the viscosities of the erupted lavas. A significant gravity anomaly is associated with the Tharsis uplift. This gravity anomaly can be explained if the volcanic construct is partially supported by the elastic lithosphere on Mars. Because Mars is smaller than the Earth, it would be expected to cool more efficiently – it has a larger surface area to volume ratio – and has a thicker lithosphere, other factors being the same. This additional thickness and the smaller radius give the elastic lithosphere on Mars a much greater rigidity. Another major tectonic feature on Mars is an enormous canyon system, Valles Marineris, extending eastward from Tharsis for about 4500 km. Individual canyons are up to 200 km wide and several kilometers deep. In the central section (Figure 1–68), the system is about 600 km wide and over 7 km deep. The Valles Marineris system might be a complex set of fractures in the Martian crust caused by the large topographic bulge containing the Tharsis volcanic region. The system is roughly radial to this bulge, as are other prominent fractures. Numerous channels are widely distributed over the Martian surface. They display a variety of morphologic forms, including braiding and stream-lined islands, strongly suggestive of formation by flowing water. If water did flow on the surface of Mars some time in the past, the water may have originated by the melting of subsurface ice. This is supported by the association of the apparent sources of many channels with so-called chaotic terrain: areas of large, irregular blocks probably formed by collapse following the removal of a subsurface material such as ground ice. Chaotic terrain is visible in the photomosaic in Figure 1–67 at the eastern end of the Vallis Marineris system; a broad collapsed area extends outward from Capri Chasma. Many of the north-trending channels in this area appear to originate in this chaotic terrain. Martian channels give the impression of having been formed by episodic flooding of large areas, as might be expected from the sudden release of large amounts of subsurface water. Possible terrestrial analogs to these channels are the scablands of the Columbia plateau in the United States and the Sandur plains in Iceland, both of which formed by the sudden release of large quantities of glacial meltwater.