Olympus Mons doesn’t just dwarf Earth’s mountains—it makes them look like geological hiccups.
We’re talking about a volcano so preposterously large that if you stood at its base, you couldn’t see the summit because it would curve beyond the horizon. The thing is 374 miles across—roughly the distance from Los Angeles to San Francisco—and rises 16 miles above the Martian datum. Mount Everest, by comparison, barely scratches 5.5 miles. It’s not even close. The entire chain of the Hawaiian Islands could fit inside Olympus Mons’s caldera, that collapsed crater at the summit, which itself spans 53 miles wide. This isn’t a mountain. It’s a planetary-scale geological monument to what happens when plate tectonics take a permanent vacation.
Here’s the thing about Mars: it doesn’t have moving plates.
On Earth, volcanoes like Kilauea or Mount Etna sit atop hotspots—plumes of scorching mantle rock rising from deep below. But our crust keeps drifting, carried by convection currents, so the volcanoes migrate away from their magma source. That’s why we get volcanic island chains instead of single supersized peaks. Hawaii’s islands, for instance, formed over millions of years as the Pacific Plate crept northwest at about 3.2 inches per year. Each island represents a different chapter in the hotspot’s history. Kilauea is the current active volcano, while Kauai, 450 miles away, went extinct roughly 5 million years ago after drifting off the plume.
When a Hotspot Refuses to Let Go of Its Favorite Spot
Mars said “nah” to all that shuffling around. Its crust locked into place billions of years ago, possibly because the planet cooled faster than Earth or lacked the internal heat engine to keep convection going. So when a hotspot formed beneath what would become Olympus Mons, it just… stayed there. For hundreds of millions of years. Maybe longer. The volcano kept building on itself, layer after layer of basaltic lava flows, like some obsessive geological hoarder who never throws anything away. Estimates suggest Olympus Mons could be around 200 million years old, though some of its youngest lava flows might be as recent as 2 million years ago—which in geological terms is practically yesterday.
The result? A shield volcano so gently sloped—averaging just 5 degrees—that you could theoretically drive up it without realizing you’re climbing the tallest volcano in the solar system. Until, you know, you run out of atmosphere. Mars’s thin air (about 1% the density of Earth’s) means Olympus Mons pokes well above most of the planet’s weather systems. Its summit sits in near-vacuum conditions, where atmospheric presure drops to barely 0.03% of Earth’s sea level pressure.
Turns out this is exactly what you’d expect from a shield volcano on steroids. Shield volcanoes form from runny, low-viscosity basaltic lava that spreads out in thin sheets rather than piling up steeply. Hawaii’s Mauna Loa, Earth’s largest active volcano, works the same way—it’s just that Mauna Loa had to share its hotspot real estate as the Pacific Plate kept moving. Olympus Mons never had to share. It’s the geological equivalent of an only child with unlimited resources and no supervision.
Wait—maybe the weirdest part isn’t the size but the cliffs.
The Escarpment That Makes Geologists Question Their Career Choices
Olympus Mons sits ringed by a escarpment—a cliff face that drops up to 5 miles in places. Five. Miles. That’s taller than any cliff on Earth, and nobody’s entirely sure how it formed. Some researchers think it’s from the volcano’s sheer weight causing the surrounding terrain to collapse, like setting a bowling ball on a mattress. Others propose that glacial or debris flow erosion carved away the base over time, though Mars’s thin atmosphere makes sustained erosion tricky to explain. A 2004 study published in the Journal of Geophysical Research suggested that massive landslides, triggered by the volcano’s gravitational instability, might have created the scarp. The debri from these collapses could have spread hundreds of kilometers across the surrounding plains.
The volcano’s flanks tell another story entirely. High-resolution imaging from Mars Reconnaissance Orbiter, operational since 2006, revealed countless lava flow channels, some stretching over 60 miles. These channels show evidence of lava tubes—underground tunnels where molten rock once flowed—that later collapsed, leaving sinuous depressions across the surface. Some tubes remain intact, and scientists have speculated these could serve as natural shelters for future Mars missions, offering protection from radiation and temperature extremes.
Olympus Mons also features something called an “aureole”—a weird, lobed terrain of ridges and valleys extending up to 435 miles from the volcano’s base. The leading hypothesis? Massive sheets of ice that once surrounded the volcano melted and lubricated gigantic landslides, creating this crumpled, chaotic landscape. We’re talking about rockslides so enormous they’d make terrestrial disasters look like sandbox spills.
And yet despite all this drama—the size, the cliffs, the apocalyptic landslides—Olympus Mons probably grew almost unbearably slowly. Shield volcanoes don’t explode like stratovolcanoes; they seep. Each eruption might have added just a few meters of new rock, spread across decades or centuries of quiet activity. The Hawaiian comparison helps here: Mauna Loa has been erupting on and off for at least 700,000 years, building itself grain by grain. Olympus Mons had millions of years to work with, zero plate drift to interrupt it, and possibly lower Martian gravity (38% of Earth’s) that allowed lava to flow farther before solidifying.
What we’re left with is less a mountain and more a testament to geological patience—a reminder that the biggest transformations don’t always come from violent explosions but from relentless, incremental accumulation. Though honestly, when you’re talking about cliffs five miles tall, “patient” might not be the first word that comes to mind.
