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How and Why Martian Geology is So Different to that of
Earth Vic Gostin Visiting Fellow, |
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Abstract: With only half the earth’s diameter and much less internal radiogenic heat, the crustal evolution of Mars has been vastly different to our own. Since its formation and primary melting, the Martian lithosphere (brittle crust) has thickened to such a degree that it has remained rigid over most of its history. Thus about two-thirds of its surface is extremely old and heavily cratered, like that of our Moon. Initial planetary differentiation created a small core, and later, an overheated mantle. These hidden processes probably led to mantle overturn, forming the major depression of the northern hemisphere, and then to the Tharsis bulge with its accompanying shield volcanoes. Evidence of crustal extension or stretching is displayed in numerous closely spaced parallel faults. The long-term stability of such a stress field is indicated by the limited occurrence of cross-cutting younger fault sets resulting from changes in the stress directions. Crustal extension has also played a role in the formation of the enormous interconnected canyon system of Valles Marineris, but its full history is complex and yet to be unravelled. Unlike our Earth, Mars has no plate tectonics. There has been no compression, thrusting, or overturning of sequences; no strike-slip faulting, or major deformation and elevation of mountain ranges. There are no regional metamorphic rocks on the Martian surface (eg. slate, shist, gneiss, quartzite, marble). Where layering is visible, it is usually horizontal and not deformed. There are no deep "ocean" trenches, volcanic island arcs, or oceanic greenstone belts. Without recycling of the "continental" crust, there are no granites or acid volcanics. Clearly, Mars is a small planet deprived of much of Earth’s great geological variety! Ancient flood basalts cover vast areas, while younger shield volcanoes grew to much greater heights than those on Earth. Water ice is not only present at the poles but forms substantial permafrost areas. Glaciers played an important role in the past, while wind erosion and resulting sedimentation continue to be active. Liquid water may be present subsurface, but only intermittently has it flowed over the Martian landscape. Thus while water-carved channels exist, and deltas grew into large lakes, the presence of unweathered olivines and pyroxenes indicates that there was little surface moisture required for much chemical weathering. The discovery of sulphates and haematite shows some aqueous chemical activity, but where are the bugs? It is possible that Mars is now a "dead planet", and only continued exploration will reveal its ancient secrets. PROFILE: Dr Victor Gostin is a
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