Olympus Mons stretches 21 kilometers into the Martian sky—roughly two and a half times the height of Everest—and yet if you were standing on its slopes, you’d probably miss the fact that you’re on a volcano entirely. The gradient is so gentle, about 5 degrees on average, that it would feel more like walking up a highway onramp than scaling a mountain.
Which is weird, right? We think of volcanoes as these explosive, cone-shaped mountains that announce themselves with drama and smoke. But Olympus Mons is more like the geological equivalent of a slowly spreading puddle—except the puddle is made of basaltic lava and covers an area roughly the size of Arizona. The caldera at its summit measures 80 kilometers across and plunges 3 kilometers deep, a collapsed volcanic depression that could swallow entire cities. NASA’s Viking 1 orbiter first captured detailed images of this behemoth in 1978, and scientists have been scratching their heads about it ever since.
Here’s the thing about Mars: it doesn’t have plate tectonics.
On Earth, volcanic hotspots move—or rather, the crust moves over them. Hawaii’s volcanic chain formed this way, with each island representing a different moment in geological time as the Pacific Plate drifted northwest over a stationary mantle plume. The Big Island sits over the hotspot now, but Kauai formed roughly 5 million years ago when that same spot was punching through crust hundreds of kilometers away. But Mars? The crust just sits there. So when a hotspot opens up beneath the surface, lava keeps piling onto the same location for millions—possibly billions—of years. Olympus Mons likely formed over the past 2 billion years, with some flows dating back 200 million years and others as recent as 2 million years ago based on crater counting analysis published in the Journal of Geophysical Research in 2004.
The result is less volcano and more monument to geological persistence. Shield volcanoes on Earth—like Mauna Loa in Hawaii, which measures about 120 meters at its summit—seem almost quaint by comparison. Mauna Loa rises roughly 9 kilometers from its seafloor base, impressive until you realize Olympus Mons could fit three of them stacked vertically and still have room left over. The Martian giant’s base spans 600 kilometers, ringed by cliffs up to 8 kilometers tall in places, scarps that formed either through landslides or the spreading of the volcanic edifice under its own colossal wieght.
Wait—maybe the strangest part isn’t the size but the loneliness. Olympus Mons sits in the Tharsis region along with three other massive shield volcanoes: Arsia Mons, Pavonis Mons, and Ascraeus Mons. But even those neighbors pale next to Olympus, which dominates the landscape like someone left the geological faucet running and forgot to turn it off. The Tharsis bulge itself—the elevated region containing these volcanoes—rises 10 kilometers above the mean planetary radius and may have literally tilted Mars on its axis, a possibility suggested by research in Nature in 2010 analyzing the planet’s rotational dynamics.
The Volcano That Punched Through an Atmosphere Too Thin to Notice
Turns out, being the tallest mountain in the solar system comes with perks. Olympus Mons extends so high that its summit pokes above much of Mars’s atmosphere, which is already about 100 times thinner than Earth’s. Atmospheric pressure at the peak measures roughly 2% of the already-sparse Martian surface pressure. Any hypothetical visitor standing up there would be experiencing conditions closer to interplanetary space than planetary surface—exposed to intense solar radiation with almost no atmospheric protection whatsoever.
This creates bizarre weather dynamics. Clouds occasionally form around the volcano’s flanks when moisture in the thin atmosphere condenses, creating comma-shaped formations visible from orbit that the Mars Express orbiter photographed in 2018. But the summit? Dry as ancient bone. The volcano essentially creates its own microclimate zones, with conditions varying wildly from base to peak across those 21 vertical kilometers.
When Geological Time Moves Slower Than Human Comprehension Allows
The last eruption on Olympus Mons probably happened somewhere between 2 and 20 million years ago—geologically speaking, practically yesterday. Some planetary scientists think it might not be extinct at all, just dormant, waiting for the right conditions to reactivate. Mars Global Surveyor data suggested possible ongoing volcanic activity elsewhere on Mars as recently as 2 million years ago based on pristine lava flows analyzed in 2004.
But even if Olympus Mons erupted tomorrow, nobody would be around to film it for the evening news. The eruptions wouldn’t be explosive anyway—shield volcanoes ooze rather than explode, their low-viscosity basaltic lava spreading out in thin sheets rather than shooting skyward. Imagine watching paint dry, except the paint is molten rock and the canvas is the size of Poland. That’s shield volcanism.
What’s genuinely fascinating is what Olympus Mons tells us about planetary evolution. Earth’s plate tectonics constantly recycle the crust, erasing ancient volcanic features through subduction and mountain-building. Mars preserved its geological history like a museum exhibit, freezing volcanic structures in place once the planet’s interior cooled and volcanic activity declined. Olympus Mons isn’t just a volcano—it’s a monument to a planet that stopped changing, a reminder that geological vitality requires internal heat, and heat eventually runs out.
Sometimes the most impressive things in the universe are also the loneliest.
