Olympus Mons doesn’t just dwarf Earth’s volcanoes—it makes them look like geological hiccups.
The thing is roughly 374 miles wide at its base, which means you could cram the entire state of Arizona inside its footprint with room to spare for a few national parks. Its peak towers 16 miles above the Martian plains, making it nearly three times the height of Mount Everest. But here’s the thing: despite being the largest volcano in the solar system, Olympus Mons probably didn’t blow its top in some catastrophic explosion. It just sort of… oozed into existence over millions of years, one lava flow at a time.
Turns out Mars is absurdly good at building volcanoes.
On Earth, tectonic plates shuffle around like restless dinner guests, moving volcanic hotspots away from their magma sources. Hawaii’s volcanic chain formed this way—each island represents a moment when the Pacific Plate drifted over a stationary plume of molten rock. The Big Island sits over the hotspot now, but in a few million years, it’ll drift away too, and some new island will start forming. Mars, though? Mars said “no thanks” to plate tectonics entirely. Its crust just sits there, frozen in place like a geological couch potato. So when a volcano starts forming over a hotspot, it stays put. For eons. Building up, layer after layer, flow after flow, until you get these absurd shield volcanos that make terrestrial mountains look quaint.
When a Planet Forgets How to Move Its Own Skin
The Tharsis region—home to Olympus Mons and three other massive shield volcanoes—covers an area roughly the size of North America. These four giants (Olympus Mons, Ascraeus Mons, Pavonis Mons, and Arsia Mons) formed over a mind-bending timeline. We’re talking activity spanning potentially 3.5 billion years, with some lava flows dated to as recently as 2 million years ago. That’s practically yesterday in geological terms.
Wait—maybe that’s why they got so stupidly large?
Mars also has lower gravity than Earth—about 38% of what we experience. That means lava can pile higher before collapsing under its own weight. The slopes of Olympus Mons average only about 5 degrees, which sounds gentle until you realize you’re climbing a mountain that’s 16 miles tall. You wouldn’t even notice you were ascending if you were driving across its flank. It’d feel more like crossing Kansas than climbing Kilimanjaro.
The volcano’s summit hosts a caldera complex—a nested series of collapse craters—that spans 53 miles across and plunges nearly 2 miles deep. That’s where the magma chamber repeatedly drained and refilled, causing the overlying rock to collapse inward like a geological sinkhole. Six overlapping calderas sit there now, each one recording a different episode of emptying and collapse. The youngest is probably around 150 million years old, which means Olympus Mons was still erupting when dinosaurs roamed Earth.
The Escarpment That Makes No Geological Sence
One of the weirdest features of Olympus Mons is the massive cliff that rings most of its base—a scarp that drops up to 6 miles in places. Nobody’s entirely sure how it formed. Maybe the volcano grew so heavy it sagged into the Martian crust, creating a moat of collapsed terrain around its edges. Or maybe glaciers once surrounded the volcano, and when they melted, they left behind this absurd cliff. Or perhaps—and this is the really wild part—the volcano literally slumped outward under its own weight, like a geological avalanche frozen in time.
Compare this to Earth’s largest volcano, Mauna Loa in Hawaii, which rises about 6.3 miles from its base on the ocean floor. Impressive, sure. But Olympus Mons is more than twice as tall and covers an area about 100 times larger. If you placed Olympus Mons on Earth, its peak would breach the stratosphere, and commercial airliners would fly below its summit.
The other Tharsis volcanoes aren’t exactly modest either. Ascraeus Mons reaches 11 miles high. Pavonis Mons sits nearly dead-center on Mars’s equator, which is probably just cosmic coincidence but feels weirdly deliberate. And Arsia Mons hosts cave entrances—skylights into ancient lava tubes—that could potentially shelter future Mars explorers from radiation. In 2007, the Mars Odyssey orbiter spotted seven of these cave openings, some plunging hundreds of feet into darkness.
What really gets me is how quiet these monsters are now.
Mars lost most of its magnetic field billions of years ago, which meant solar wind gradually stripped away its atmosphere. Without atmospheric pressure, water couldn’t stay liquid on the surface. Without water, weathering slowed to a crawl. So these volcanoes just sit there, frozen in time, their flanks still scarred with lava channels from eruptions that happened when Earth was a very different place. The youngest flows on Olympus Mons date to about 2 million years ago—recent enough that some scientists won’t rule out future activity entirely. Mars might not be geologically dead. Just sleeping.
And if Olympus Mons ever wakes up? Well, with Mars’s thin atmosphere and low gravity, lava fountains could shoot miles into the sky, creating eruption plumes visible from Earth with a decent telescope. That’s about as dramatic as planetary geology gets—a volcano so large it defies comprehension, erupting on a world where plate tectonics never existed and geological time moves at its own stubborn pace.
