Mars shows abundant signs of past volcanic activity spanning a wide range of geologic ages and volcanic rocks cover most of the surface. The most obvious features are the large volcanoes in the Tharsis province, such as Olympus Mons, and the extensive plains formed from volcanic flows. Volcanoes are also important to climate science: in the past they would have injected large quantities of gases into the Martian atmosphere, e.g. water vapor, carbon dioxide (CO2), and sulfur dioxide (SO2), which may have enhanced greenhouse warming on early Mars. From the viewpoint of comparison with the Earth, the interior of Mars has cooled more rapidly over geological time because Mars is smaller, only half the diameter of Earth, and only 1/10th the mass. Consequently, the heat flow from the interior of the planet, which causes volcanic and geothermal activity at the surface, is now so small that there are no known active volcanoes.

Arsia Mons (MOC2-179)

Mars shows abundant signs of past volcanic activity spanning a wide range of geologic ages and volcanic rocks cover most of the surface. The most obvious features are the large volcanoes in the Tharsis province, such as Olympus Mons, and the extensive plains formed from volcanic flows. 

Pavonis Mons (MOC2-609)

Estimates of ages of Martian volcanic features can be made from the number of impact craters and the layering relationships of various geologic units. Published estimates of the formation age of the surfaces of the Tharsis volcanic shields range from 2500 million years ago to only 100 million years ago - so there is a great deal of uncertainty. Recent imaging data from NASA's Mars Global Surveyor suggests that the great shield caldera of Arsia Mons is no older than 40-100 million years old, based on the lack of impact craters. Globally, volcanic activity is thought to have peaked about 3000-3500 million years ago (when the Martian ridged plains were formed) and the rate has since dropped off exponentially so that today there are no signs of active volcanoes. Nevertheless, extrapolation of the volcanic activity curve to present day suggests that geothermal activity may not have completely vanished; it is estimated that a lava flow has been extruded about once every 10,000 years in the past few hundred million years. If there are any "hot spots" on Mars they would provide a source of energy. Thus they would be of great interest scientifically since a hot spot is where liquid water and possible vestiges of life are most likely to be found near the surface.