Olympus Mons, the shield volcano on Mars, is the largest known volcano in the solar system. Due to the thick, rigid Martian crust, which does not drift as Earth crust plates do, the Martian surface can support the massive weight of a volcano that rises 16 miles in the air and has a base the size of the state of Montana.
Gargantuan Olympus Mons is similar to the shield volcano Mauna Loa in Hawaii, though Olympus Mons has more lava than the whole Hawaiian volcanic chain. Earthly and Martian shield volcanoes alike are formed when large quantities of free-flowing lava erupt from vents and spread widely, gradually building up a low, domed mountain. Thousands of separate lava flows construct shield volcanoes leaving the upper slopes lava-terraced.
All the volcanoes on Mars are found along three parallel lines on the planet which run from southwest to northeast (Earth volcanoes are also found in a general beltway) and since impact craters are rare on Martian volcanoes, whereas they are abundant elsewhere on the planet, there is a strong indication that the volcanoes, like Earth's, have been active in recent geologic time. Recent lava flows would cover over impact craters on the volcanoes' surface leaving them, unlike the rest of the planet which has no method of erasing the impacts, relatively crater free.
Earth and Mars are obviously radically different planets, yet the presence of volcanic activity on both planets, and throughout the universe, remind us of each planet's place in a solar system.
