23 May 2009
Olympus Mons is a big volcano. It is almost unimaginably huge. It is 550 kilometers (342 miles) across at its base, and the volcanic crater (the technical term is ‘caldera’) at the peak is 80 kilometers (53 miles) long. If you were standing at the edge of the caldera, the volcano is so broad and the slopes are so gradual that the base of the volcano would be beyond the horizon. That’s right, it is a volcano so big that it curves with the surface of the planet.
And it is tall. 27 kilometers tall. That’s 16.7 miles from base to summit. 88,600 feet. That’s about three times as tall as Mt. Everest. Even Mauna Kea, Earth’s own giant shield volcano doesn’t come close. Measured from the sea floor to its summit, Mauna Kea is 33,476 feet (10.2 km) tall: taller than Everest, but only about 40% the height of Olympus Mons.
Ok, so throwing those numbers around is fun if you like stats, but it still doesn’t convey quite how tall Olympus Mons is. So here’s an eye opener. Olympus Mons is so tall that it essentially sticks up out of Mars’s atmosphere. The atmosphere on Mars is thin to begin with, but at the summit of Olympus Mons, it is only 8% of the normal martian atmospheric pressure. That is equivalent to 0.047% of Earth’s pressure at sea level. It’s not quite sticking up into space, but it’s pretty darn close. In fact, it was first confirmed to be a huge mountain when Mariner 9 saw it towering above the top of a global dust storm like an island in a rust-colored sea.
Finally, since it is fun to compare Olympus Mons to Mauna Kea, what would the pressure be like at the summit if we placed Olympus Mons next to Mauna Kea in the Pacific? In that case, the summit of Olympus would be 21 km (68,897 ft) above sea level: still higher than Everest, and about twice as high as normal jets fly. The atmospheric pressure at the summit would be about 4.6% that at sea level. For comparison, at the top of Everest it is about one third the pressure at sea level, and most people still need to use oxygen canisters.
Olympus Mons is huge. Hopefully these numbers give you a little better idea of just how huge. If you’re wondering how it got to be so big, I already wrote about that in this post about shield volcanoes, so go check it out!