This picture from one of NASA's Viking Orbiters shows the presence of large extinct volcanoes on the Martian surface. Mars has a number of volcanoes, which are generally larger than those on Earth. This is thought to be due to the lower gravity on Mars and more importantly, the fact that because Mars has never been tectonically active, magma welling up from the planet's interior continued to erupt from a volcano at the same location. On Earth the movement of tectonic plates means that this does not occur. There is currently no volcanic activity on Mars, but with the most recent volcanic features dated at between 10 and 100 million years ago, relatively recently in geological terms, scientists have not ruled out the posibility of volcanoes erupting again in the future.
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