The largest known volcano in the solar system is on Mars, and it is so wide that if you stood on its summit, you might not realise you were on a volcano at all — Olympus Mons rises from a base roughly the size of New Mexico, with slopes so gentle they would barely feel like a mountain.

The largest known volcano in the solar system is on Mars, and it is so vast that if you stood on its summit you might not realise you were on a volcano, or even a mountain, at all. Olympus Mons rises from a base roughly the size of New Mexico, and its slopes are so gentle that walking up them would barely feel like a climb.

That combination, enormous width and a shallow grade, is exactly what makes it hard to grasp. It is less a peak to be scaled than a continent-sized swelling of the planet’s crust.

A mountain the size of a country

The numbers are difficult to picture. Olympus Mons is more than 600 kilometres across at its base, covering an area of roughly 300,000 square kilometres. That is comparable to a large American state such as New Mexico or Arizona, or to a country like Italy. It is not a mountain sitting in a landscape so much as a feature the size of a landscape.

It is also tall. Olympus Mons stands about 22 kilometres above the surrounding plains, which is roughly two and a half times the height of Mount Everest measured from sea level. For comparison, the largest volcano on Earth, Mauna Loa in Hawaii, is about 120 kilometres across and rises around 9 kilometres from the ocean floor. Olympus Mons is several times wider and more than twice as high.

Why you would not feel the mountain

Height and width like that might suggest a dizzying, dramatic peak. The opposite is true, and the reason is the shape. Olympus Mons is a shield volcano, the same broad, low kind that built the Hawaiian islands, and its flanks slope at an average of only about five degrees.

Stand near the summit and the ground would fall away so gradually that it would read as a gently tilted plain rather than a mountainside. The base would be hundreds of kilometres off, far beyond the horizon, so you could not see the volcano’s edges or take in its overall shape from anywhere on it. Mars is also a smaller world than Earth, which brings its horizon closer and hides the distant flanks even more effectively. The sense of standing on a great mountain comes from seeing the land drop steeply away, and here it simply would not.

The cliff at the edge

There is one place where Olympus Mons is genuinely dramatic, and it is not the top. The whole edifice is ringed by an outer escarpment, a cliff that in places stands up to about 8 kilometres tall, a feature not seen around shield volcanoes on Earth. The volcano does not taper gently to nothing at its margins. It ends, in many places, in a towering wall.

The summit has its own drama on a smaller scale. It is capped by a cluster of nested calderas, an irregular pit roughly 60 by 80 kilometres across and up to a few kilometres deep, formed where the ground collapsed as magma chambers beneath it drained and emptied over time.

Why Mars could build something this big

The obvious question is why Mars has a volcano so much larger than anything on Earth, and the answer comes down to a difference in how the two planets are built. Earth’s surface is broken into tectonic plates that slowly move. When a plume of hot rock, a hotspot, sits beneath the crust, the moving plate slides over it, so instead of one giant volcano you get a chain of smaller ones. Hawaii is exactly this: an island chain, each island built in turn as the plate carried it off the hotspot.

Mars, as far as we can tell, does not have moving plates. Its crust stayed put over the hotspot, so lava piled up in the same place for an immensely long time, flow upon flow, letting a single shield grow to a size no Earth volcano can reach. Mars’s lower gravity helps too, allowing a taller structure to stand without collapsing under its own weight. Olympus Mons is, in effect, what a hotspot produces when nothing ever moves it along.

Old, but perhaps not finished

Olympus Mons is ancient, built up over a large part of the planet’s history. But some of the lava flows on its flanks appear to be surprisingly young in geological terms, with parts dated to within the last few tens of millions of years. That is recent enough that scientists are cautious about declaring the volcano entirely dead. Whether Mars is still capable of erupting anywhere, and whether Olympus Mons is merely dormant rather than extinct, is not settled, and it remains an open question worth watching.

Why it matters

Beyond the sheer spectacle, Olympus Mons is a record of how a planet without shifting plates evolves, and a natural experiment in what volcanism can build when it is left undisturbed for billions of years. It also quietly overturns an assumption. We tend to think a bigger mountain must be a more obvious one. On Mars, the largest of them all is something you could stand on top of and mistake for level ground.