Volcanoes and Geology

Mars is roughly half the size of Earth, despite the fact that it contains several volcanoes that are many times larger than the ones on Earth. On Earth volcanoes are created during the process of plate tectonics. Mars has no current plate tectonics, which suggests that Mars' volcanoes are ancient. When the volcanoes on Mars erupted they kept building upon the last eruption debris and then again and again so they grew to be enormous. Mars does not have the same weathering as Earth does to erode the volcanoes. There aren't that many volcanoes on Mars, but the ones it has are huge. Also they are usually concentrated in two places, the Tharsis or the Elysium regions. The Elysium region contains some of the oldest volcanoes, meaning they have been inactive for a very long time.The rocks on Mars all have very similar geology. They are mostly porous and basaltic. This being true, more than half of these rocks came from volcanoes. A large portion of rocks are meteorites and other debris from space. It is presumed that all volcanic activity stopped on the Mars about 1.5 million years ago.

The Tharsis Region

The cluster of four large spots found in the northern hemisphere of Mars are referred to as the Tharsis Ridge or Bulge. It's called that because it is an area that literally bulges out of the surrounding plains. Some scientists think this was created by some internal faults letting lava leak through to the surface and finally it did and created the numerous volcanoes in that region. It is an area of Mars that was once bursting with volcanic activity. The area consists of four massive volcanoes named (large volcano in upper left of image) Olympus Mons. Then the northern most volcano of the three aligned is Ascraeus Mons (11°N, 104°W). Then moving south you will find Pavonis Mons (0° N, 133°W), Arisa Mons (9°S, 120°W), and about 4 minor volcanoes: Ulysses Patera, Biblis Patera, Ceranius Tholus, and Alba Patera. Tremendous lava flow patterns can be seen at the base of the volcanoes, especially Olympus Mons

The most amazing of these giant volcanoes is Olympus Mons (18°N, 133°W) which rises about 24km (15 miles) above the surrounding plains. Its base is more than 500km (340 miles) in diameter and on the south side is rimmed by a cliff 6km (4 miles) high. On the north side of the base instead of a steep cliff you can see an almost smooth transition to the ground made possible by the lava flow. At the caldera (peak or top) you can see several impact craters that widened the mouth of the volcano. It seems that since those meteor strikes, the volcano hasn't been active at all.

The Elysium Plains

The Elysium area of Mars seems to contain some of the oldest volcanoes on Mars. We can tell this because the lava flows in these plains are heavily cratered, meaning that they have had more time on the surface or have been exposed longer. These two volcanoes are Elysium Mons and Hecates Tholus. They are the largest in the area about the size of our Earth's volcanoes. On the right is a computer generated view of Elysium Mons from the base of the volcano.