Olympus Mons stands 21 kilometers tall. That’s roughly two and a half Mount Everests stacked on top of each other, which sounds like the kind of thing a planetary scientist would say after too much coffee, except it’s true.
Here’s the thing about Mars: it doesn’t have plate tectonics. Earth’s crust shuffles around like a restless sleeper, constantly repositioning itself over the mantle’s hot spots. A volcano forms, the plate drifts, and boom—you get a new volcano somewhere else. The Hawaiian Islands work exactly this way, with each island representing a different episode of the same volcanic hot spot punching through the Pacific Plate as it crawled northwest at about 7 centimeters per year. The Big Island sits over the hot spot right now, which is why Kilauea kept erupting until 2018, when it finally took a break after destroying 700 homes in a single eruption sequence.
Mars just sits there.
No tectonic shuffling means a Martian hot spot stays put beneath the same patch of crust for hundreds of millions of years, possibly billions. Imagine holding a blowtorch to the same spot on a metal plate—eventually, you’re going to build quite a pile of melted metal. Olympus Mons represents roughly 3 billion years of volcanic activity in the same location, layer upon layer of basaltic lava flows stacking up like the world’s most patient geological construction project. The volcano’s base sprawls across an area roughly the size of Arizona, which seems excessive until you realize Mars had nothing better to do.
Turns out gravity matters more than anyone expected. Mars has only 38% of Earth’s surface gravity, which means lava flows can pile higher before collapsing under their own weight. On Earth, mountains start crumbling when they hit around 10 kilometers—the rock literally can’t support itself beyond that point. But on Mars? The structural limits stretch much higher. It’s like building with lighter bricks that somehow don’t compress as much.
When Planets Forget How To Recycle Their Own Crust Properly
Earth’s volcanism works on a recycling program. Oceanic crust slides into subduction zones, melts, and resurfaces as volcanic arcs—think Japan, Indonesia, the Andes. The system resets itself every 200 million years or so. Volcanic regions don’t accumulate indefinitely because the conveyor belt keeps moving. Mars abandoned this system entirely, or maybe never developed it in the first place, which planetary geologists still argue about at conferences.
Wait—maybe size matters too. Mars is only about half Earth’s diameter, which means it cooled faster. A smaller planet has more surface area relative to its volume, so heat escapes more efficiently, like how small coffee cups cool faster than large ones. This matters because plate tectonics requires a hot, convecting mantle to drive the whole system. Mars likely had tectonics early in its history, maybe for the first billion years, but then the engine cooled down and everything froze in place. The volcanos kept erupting through the now-stationary crust, building upward instead of spreading sideways across multiple vents.
The Atmospheric Pressure Problem That Nobody Talks About Enough
Mars has virtually no atmosphere—surface pressure sits around 600 pascals, less than 1% of Earth’s. This changes everything about how lava behaves. On Earth, atmospheric pressure keeps volatiles dissolved in magma until they’re close to the surface. On Mars, gases escape more easily, which affects eruption dynamics in ways that scientists are still modeling. Lower atmospheric pressure also means less resistance to vertical growth, though this effect is probably minor compared to the gravity and tectonics issues.
The Tharsis region, where Olympus Mons and several other massive shield volcanos sit, represents one of the solar system’s most dramatic volcanic provinces. The entire region bulges upward by about 10 kilometers, probably because magma pooled beneath the crust for billions of years without anywhere else to go. It’s like planetary indigestion that never resolved itself. Alba Mons, another Tharsis volcano, covers more surface area than any volcano in the solar system—nobody even knows how to classify it properly because Earth has no equivalents.
Olympus Mons last erupted maybe 25 million years ago, based on crater counting studies published in 2004. That sounds ancient until you realize it’s practically yesterday in geological terms. Some researchers think the volcano might still be active in some meaningful sense, just taking a very long nap between eruptions. Mars doesn’t have seismographs measuring deep magma movement, so nobody really knows what’s happening beneath the surface right now.
The caldera at Olympus Mons’s summit contains six overlapping collapse pits, each representing a different episode where the magma chamber drained and the surface collapsed inward. The youngest pit formed around 150 million years ago, which suggests volcanic activity continued episodically for billions of years. Each collapse event probably released enormous volumes of lava—maybe hundreds of cubic kilometers per eruption, dwarfing anything Earth produces.
Mauna Loa in Hawaii, Earth’s largest volcano by volume, rises about 9 kilometers from the ocean floor and contains roughly 75,000 cubic kilometers of rock. Olympus Mons holds maybe 10 times that volume, though estimates vary because nobody’s drilled core samples from Mars yet. The sheer scale breaks most intuitive comparisons—it’s a planetary feature, not just a mountain.
