Mars, volcanoesMuch of the northern hemisphere of Mars shows signs of volcanism with extensive lava plains sparsely pock-marked with impact craters: an indication of an age considerably less than the 3500 million years assigned to the heavily cratered southern hemisphere. This disparity may arise from the fact that the mean Martian surface lies at a lower level in the northern hemisphere than in the southern.
The two main areas of volcanic activity, both situated on bulges in the Martian crust, are the Tharsis region – with three huge volcanoes on the Tharsis Ridge and the immense Olympus Mons to the NW of the Ridge – and the Elysium Planitia. The volcanoes are of the shield type, having long gently sloping flanks. Counts of the numbers of impact craters on the slopes suggest ages between 200 million and 800 million years for the Tharsis volcanoes, and about 1500 million years for those of Elysium Planitia. The extraordinary size of the volcanoes may be related to the absence of plate tectonics on Mars, so that the crust remains locked in relation to volcano-inducing ‘hot spots' in the mantle below. See also Mars, surface features.