The biggest volcano in the solar system was discovered in 1971 by the space probe Mariner 9, though it is actually visible from most backyard telescopes. This volcano is called Olympus Mons and it is located on Mars. Its peak lies 14 miles above its base, making it more than four times taller than Mt. Everest. Its slope is very gradual, but the entire volcano is about the size of Arizona.
A digital reconstruction of Olympus Mons, The Museum of Unnatural Mystery
Olympus Mons is one of the key indicators for scientists on Earth that there is tectonic activity on Mars. It is unlikely that Olympus Mons is or was ever a stratovolcano (think tall, cone-shaped, and very explosive, such as Mt. Vesuvius or Mt. St. Helens) since it is very large and seems to have gradual lava flows down its sides that have built up the base, rather than intermittent explosive eruptions. Volcanoes like this on Earth, called shield volcanoes, often form above hotspots, hotter locations in the Earth’s mantle where magma rises to the surface and is ejected to the crust in slow, steady eruption of basaltic lava. The chain of Hawaiian islands are an example of this: as the Pacific plate moves northwest, volcanoes form above the hotspot but then move with the plate, only building up until that location on the plate is no longer above the hotspot (Space.com).
Astronomers have yet to find evidence of multiple tectonic plates on Mars, indicating that there is no plate movement or that the surface of the crust is just one big plate that may move with turbulence in the mantle. Thus, Olympus Mons has been sitting above the same hotspot since it first started to form (likely, since the hotspot first formed beneath the surface). The volcano thus had the opportunity to continue to grow and grow, layers of lava building up on top of each other with successive eruptions and creating the biggest volcano in the world. Evidence of these eruptions is visible in the multiple calderas seen on top of the volcano. Want to know more about Olympus Mons? Check out this informational page.
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