Major Sites of Volcanism
Mars has the largest shield volcanoes in the solar system. It also has a wide range of other volcanic features. These include large volcanic cones, unusual patera structures, mare-like volcanic plains, and a number of other smaller features. However, volcanic features are not common. There are less than 20 named volcanoes on Mars, and only 5 of these are giant shields. Also, volcanism occurs mostly within three regions. Even the mare-like plains cluster near these regions. The main cluster of volcanoes and lavas is in Tharsis. A much smaller cluster of three volcanoes lies in Elysium. Lastly, a few paterae are near the Hellas impact basin.
Differences from Moon
Like the Moon, volcanism on Mars is very old. The mare-like plains on Mars are the same age as the lunar mare, roughly 3 to 3.5 billion years old. However, volcanism lasted much longer on Mars than on the Moon. It also seems to have changed over time. Volcanism in the highland paterae and mare-like plains on Mars stopped 3 billion years ago, but some of the smaller shields and cones erupted only 2 billion years ago. The giant shield volcanoes are even younger. These volcanoes formed between 1 and 2 billion years ago. The youngest lava flows on Olympus Mons are only 20 to 200 million years old. These flows are very small, however, and they probably represent the last gasp of martian volcanism. Thus, the odds of finding an active volcano on Mars today are very small.
Like the Moon, Mars shows no sign of plate tectonics. It has no long mountain chains, and there is no clear global pattern to the volcanism. Over half of Mars is heavily cratered like the lunar farside. Unlike the Moon, however, most martian volcanism lies outside large impact basins. Instead, the mare-like plains are mostly near the largest volcanoes. These plains also are not limited to the lowest elevations. Indeed, some lava plains are much higher than the cratered uplands. Lava plains may lie at lower elevations as well. However, thick layers of dust and sediment cover both the Northern Lowlands and the large basin floors. These layers reflect a long history of winds, glaciers and flood events. They also hide any volcanism that may have occurred in the low areas on Mars.
The concentration and duration of volcanism into these two regions are attributed to the evolution of a long-lived mantle hotspot.
More Mars Volcano Information can be found at the "Geology of Mars" website curated by Albert T. Hsui, University of Illinois at Urbana-Champaign.
As well as:
NASA’s Mars Exploration Program
NASA Human Spaceflight