Mars Academy

Volcanoes

There are two types of eruptions that occur on Mars: those that happen repeatedly out of the same conduit and build mountains about them, and eruptions from fissures that create extensive lava plains. Because of the lack of plate tectonism on Mars that usually carries a volcano away from its source, volcanoes remain in the same place and can continue to grow as long as magma is available.

Volcanoes are mostly common on the Tharsis Bulge and Elysium near the equator. Just off the northwest edge of the Tharsis Bulge is Olympus Mons, the tallest volcano on the planet. It measures 550 to 600 km across, rises 24 km and, in some places, is rimmed by a 6 km cliff. Around the volcano lies ridged terrain extending almost 1000 km away from the mountain. It is believed to be the ash flow of a pre-Olympus volcano. Olympus Mons resembles the large terrestrial shield volcanoes such as those in Hawaii, the main difference being size. The size probably results from a combination of the deep source of magma that pushes through and the lack of plate tectonism on Mars. The average height of Martian volcanoes implies that their magma came from very deep down and was under great pressure. To reach the top of Olympus Mons, magma comes up from depths of 150 to 200 km (as versus 60 km in Hawaii). The large volcanoes have gentle slopes of 6 degrees or less. Tholi domes that appear on Mars are convex in shape and generally have a slope of about 8 degrees. The difference in formation of these Tholi and the larger volcanoes may be that the material of the tholi was more viscous during the eruptions, or that the eruptions had a smaller volume and consequently piled up closer to their source. There are also some collapsed shield volcanoes on Mars. They generally have shallow slopes and scalloped, complex edges. They have been modified by collapse and erosion. Because of their susceptibility to collapse they are believed to be made of loose materials such as ash flows. They are also thought to be the oldest volcanoes as they are so eroded.