Olympus Mons is an extinct volcano on Mars. It is the largest known volcano in the solar system, rising about 25 km above the surrounding plain. It has a base diameter of 624 km and is surrounded by a steep cliff 6 km high. In addition to being high, Olympus Mons is also especially wide, namely about 550 kilometers.
Olympus Mons is believed to have formed in a similar way to Hawaii. It is a shield volcano, in which material comes very deep from the planet to the surface and thus forms land. The fact that the volcanoes have grown so large and have not sunk means that Mars had already cooled enough by then to have a lithosphere that could support such a large mass. The largest volcano on Earth is Mauna Kea in Hawaii, 10 kilometers high from the seabed and 250 kilometers wide. Compared to the rest of the Martian surface, Olympus Mons is very young, only about 165 million years old. This corresponds to the age of Shergottite meteorites from Mars.
The top of the volcano is formed by an 85 by 60 kilometer wide caldera. This consists of 6 overlapping pit craters, and is 3 kilometers deep at the lowest point. The volcano is surrounded by an area known as the Olympus Mons aureole (Latin for circle of light). It consists of gigantic ridges and blocks that extend up to 1000 kilometers from the foot. This landscape shows signs of a dynamic Martian crust and glacial activity. There are still many unanswered questions about this type of terrain.
The volcano was already observed from Earth through telescopes in the 19th century. The volcano was initially called Nix Olympica ("Olympic snow"). The US spacecraft Mariner 9, orbiting Mars, took pictures of the volcano in 1972.
Mars has even more very large volcanoes. Most are located in the Tharsis Ridge, such as Olympus Mons, but also in Elysium Planitia.
Olympus Mons on Google Mars
Location with NASA World Wind[dead link]