Computer-generated perspective view of the Solar System's largest volcano, Olympus Mons.
THE VOLCANOES OF MARS
The largest and most well known volcanoes occur in the Tharsis region of Mars. The Tharsis volcanoes are the three edifices of the Syria rise, Arsia, Pavonis, and Ascraeus Mons. Alba Patera to the north and Olympus Mons to the east are also included in the Tharsis group.
Olympus Mons
Olympus Mons (once called Nix Olympia or "Snows of Olympus") is the biggest and probably the most famous Solar System volcano. Olympus rises 24 kilometres above the surrounding plains and has a diameter of 550 kilometres. It is a shield volcano built up from many, perhaps thousands, of smaller flows from the summit. The roughly circular shape shows that the amount of lava flowing in each direction was symmetrical.
Olympus Mons
The Solar System's largest volcano, Olympus Mons.	Comparison of Olympus Mons, Mount Everest, and Mauna Loa.	Olympus Mons, as seen by the Mars Global Surveyor, with late afternoon clouds.
The main crater, or "caldera", at the top of the volcano is 80 kilometres across. The caldera was once occupied by molten lava. After a time the lava lake drained back into the volcano. Smaller caldera can be seen in the surface of the main crater. These nested caldera show that lava returned to the summit six times.
At the edge of the main shield, which has quite a gentle slope, is a steep scarp or cliff. In places it is 6000 metres high. Large landslides can be seen where sections of the scarp have collapsed.
Viking Orbiter 1 view of the caldera at the top of Olympus Mons.
A series of gigantic lobes on the plains encircle the main shield. They extend some 700 kilometres from the edge of the shield. The terrain is fairly flat and is covered with a complicated system of grooves. No one is sure how it formed. Some think that the lobes were made by ash, while others have suggest that they are the result of huge landslides or material squeezed out from beneath the volcano.
Arsia, Pavonis and Ascraeus
The shields of Arsia, Pavonis and Ascraeus Mons are also very distinctive. They are dark and stand out well against the lighter coloured plains. They are aligned, occurring on a large fissure running south-southwest to north-northeast. Each is about 350 kilometres to 400 kilometres across and roughly 16 kilometres high. Each have large summit craters, with smaller nested craters superimposed on the main caldera. Their gentle slopes (about 5o) have a have a fine radial texture.
Ascraeus Mons
Pavonis Mons
Arsia Mons
Size
So why have Martian volcanoes have grown so big? The answer lies in the stability and thickness of the planet's crust.
On Earth, because the plates can move, the volcano doesn't have a chance to grow as big as it might. The Hawaiian volcanoes occur in a line because the plate moves over the volcanic source or "hotspot". If there was no plate movement there might be just one giant Hawaiian volcano, instead of a chain. On Mars, because the crust stays in one place and the volcano remains above the same hotspot, it can grow. It will keep growing as long as there is a supply of molten rock.
Other volcanoes
Though the landscape of the Tharsis region is dominated by the large shields there are a number of smaller shields. These are nonetheless, large volcanoes and range from 50 kilometres to 200 kilometres in diameter and include Uranius, Ceraunius, Tharsis and Hecates. They have steeper slopes and tend to be more conical than the larger shields. They have radial channels on their surface, but it is not certain how these were formed.
Uranius Patera, Ceraunius Tholus, and Uranius Tholus.
Uranius
Volcanism is not confined to the Tharsis area. There are some volcanoes in the more heavily cratered southern hemisphere. These, while not as impressive as the Tharsis volcanoes, have some interesting characteristics and tell us something of volcanic activity in Mars's early history. Tyrrhena Patera and Hadriaca Patera are both low shield volcanoes covering a large area, badly eroded and not very well defined. However, they're cut by a very distinctive gully system. Radiating in all directions from the low summit are very broad flat bottomed channels. These channels appear to have formed very easily as they are quite deep and there are so many of them. This may be because the volcano shield is coated with a deep layer of ash. Their complicated flower shape, when viewed from above, earned these volcanoes the nickname "dandelions".
As well as shields of all sizes and plains volcanism, there are also many small cone volcanoes occurring in groups, similar to those on Venus. One of the densest clusters of small volcanoes occurs in the Cydonia region.
The heavily eroded shield volcano Tyrrhena Patera.
Volcanic history
Many Martian volcanoes are thought to be young because they look fresh. The detail on their surfaces is preserved and they have sharp crater rims. The very low crater counts on these volcanoes suggests that they indeed very young and that if volcanic activity isn't continuing today it must only have stopped very recently. In the case of Olympus Mons, maybe as recently as 30 million years ago, which is only yesterday in geological time.
Surface ages of volcanoes are related to the number of large craters found on their flanks. There is a wide range of crater densities showing that volcanism operated through much of Mars's history. But there is a large margin of error and the possibility that the rate of impact cratering on Mars was not the same as elsewhere in the Solar System could lead to bias. Knowledge of the chemistry of Martian rocks will lead to a better idea of how the volcanoes on Mars have evolved in the way they have. The role of water, for instance, is still uncertain.
In recent years analysis of meteorites found on Earth has shown that some contain very small bubbles of gas, sealed in volcanic glass. The gas is similar to that measured by Viking lander so these meteorites are thought to have come from Mars. One of the basaltic meteorites contains some carbonate, an indication that water was involved in its formation.
More about Martian meteorites.
Another clue to the type of volcanic activity comes from the images taken by the Viking and Pathfinder landers. These boulder fields don't look much like a lava flow, but collapse and erosion over millions have broken it up, creating the rock strewn plains we see today. The images show that many of the rocks have a very rough or pitted surface. These pits, known as "vesicles", are caused by bubbles of gas escaping from the molten rock. These pitted and crinkly rocks contrast with the smooth slabby rocks of the Venusian plains, which appear to have formed in a regime where gases (volatiles) were absent and volcanism was not explosive as on Mars or Earth.
Viking Lander 2 pan of the boulder fields of Utopia Planitia.		The boulder fields surrounding the Mars Pathfinder landing site.
The most active and explosive volcanism operated early in Mars's history while there were still volatiles in the mantle. The escape of these gases (through volcanism), cooling, and crustal thickening led to progressively less widespread and less explosive volcanism. The large shields of more recent times were built up by more steady outpourings of lava before they stopped altogether.