Olympus Mons is the tallest volcano in the solar system, standing roughly two and a half times the height of Mount Everest measured from sea level, and it is still there for one stubborn reason: the crust of Mars does not move. The volcano has been sitting on the same magma source for billions of years, piling lava on lava, because there are no plates beneath it to slide the mountain off the heat that feeds it.
On Earth, that same trick is impossible. Hawaii tried. The Big Island is drifting northwest, riding the Pacific plate away from the hotspot that built it. Every few million years a new island rises in its place and the old one starves, cools, and sinks. Mars skipped that whole conveyor belt.
A volcano the size of Arizona
Olympus Mons covers an enormous area of the Martian surface, comparable to the state of Arizona. The summit caldera alone is wide enough to swallow the greater metropolitan area of London and leave room for the suburbs. The whole edifice is a shield volcano, built from countless slow, runny basalt flows rather than the explosive stratovolcano eruptions that produce Fuji or St. Helens.
The scale is staggering: according to research on Martian volcanic activity, Mars hosts some of the largest volcanoes in our solar system. Olympus Mons, the biggest mountain in our solar system, is a giant volcano.
The slopes are so shallow that a hiker standing on the flank would not necessarily register the incline. The mountain is wider than it is tall by a factor of roughly fifteen. If you set it down in the middle of France, the outer scarp would reach the German border before the summit came into view.
Why Earth’s volcanoes shrink and Mars’s don’t
Earth has tectonic plates, all in constant, slow motion. Hotspots — the deep mantle plumes that punch magma toward the surface — stay put. The crust above them slides. That is how you get the Hawaiian-Emperor seamount chain, a trail of extinct volcanoes stretching from the Big Island northwest toward the Kamchatka Peninsula. Each seamount is a former Hawaii, dragged off its heat source and left to erode.
The same mechanism produced the Ninetyeast Ridge in the Indian Ocean, one of the longest linear volcanic features on the planet, traced back to the Kerguelen hotspot beneath the southern Indian Ocean. Plate motion turns single volcanoes into chains.
Mars has no chain. It has one enormous volcano, and three slightly smaller ones lined up nearby on the Tharsis bulge — Arsia Mons, Pavonis Mons, and Ascraeus Mons — which together suggest the same plume region has been feeding roughly the same patch of crust for most of the planet’s history. The crust never left.
The gravity discount
Martian gravity is about 38 percent of Earth’s. That matters more than it sounds. On Earth, a shield volcano can only grow so tall before its own weight causes the underlying crust to sag or the flanks to collapse in massive landslides. Mauna Loa, measured from its base on the Pacific seafloor, rises nearly twice as tall as Mount Everest above sea level — and its flanks are riddled with ancient slump scars where whole sides of the mountain slid into the ocean.
Lower gravity means Mars can pile lava higher before the structure gives way. Combine that with a stationary crust and a magma source that kept delivering for billions of years, and the arithmetic runs to extraordinary elevation above the surrounding plains.
Still active, technically
Olympus Mons appears to have erupted within the last few tens of millions of years, according to the crater-counting technique planetary scientists use to date volcanic surfaces. The method works by comparing the density and size distribution of impact craters on a surface with calibrated rates derived from lunar samples. Fewer craters means a younger surface, because lava flows wiped the older impacts away.
By that metric, parts of Mars look startlingly recent. As a survey of Solar System volcanism in Discover notes, some Martian volcanic features appear geologically recent. The planet is not dead. It is dormant, resting between eruptions on a timescale that makes human civilization look like a camera flash.
Recent gravity and seismic data reinforce the picture. The heat is still there. The crust is still not moving.
Not the only kind of volcano on Mars
The Red Planet’s northern lowlands are dotted with tens of thousands of small conical hills, many with tiny craters at the summit. For years these looked like miniature volcanoes, but their composition puzzled geologists. New laboratory work suggests many of them are mud volcanoes, formed when water-rich sediments trapped underground were forced back to the surface under pressure.
A team led by Petr Broz of the Czech Academy of Sciences, working with the German Aerospace Center, poured water-rich mud into a low-pressure chamber that simulated Martian surface conditions. The result, published in Nature Geoscience and reported by the Free Press Journal, was striking. According to Broz’s research reported in the Free Press Journal, the mud flows under Mars’s low atmospheric pressure behave similarly to ‘pahoehoe’ or ‘ropy’ lava found on volcanoes in Hawaii and Iceland.
The mud, in other words, freezes and boils simultaneously in the thin Martian atmosphere, forming a chilled crust while the interior stays fluid — mimicking the flow behavior of 1,100-degree lava on Earth. Two very different processes, one visual signature.
What the missing plates cost Mars
Plate tectonics does more than move volcanoes around. On Earth, subduction — where one plate slides beneath another — is what produces granite, recycles carbon into the mantle, and keeps the long-term climate roughly stable by scrubbing CO2 from the atmosphere over geologic time. Mars has basalt in abundance but essentially no granite, because there is no subduction zone to make it.
The absence also means volatiles released by ancient Martian volcanism — water vapor, carbon dioxide, sulfur compounds — had no way to be cycled back into the interior. They leaked to space through the thin, unmagnetized atmosphere. Volcanism serves as a mechanism for taking volatiles from deep within the planetary interior and launching them into the atmosphere.
On Earth, that gas-delivery service helped build a breathable atmosphere. On Venus it created a runaway greenhouse that pushed surface temperatures to extreme levels. On Mars, the outgassing arrived and then, over billions of years, mostly slipped away.
The Io comparison
For a sense of what a truly active volcanic world looks like, the closest analog is not Mars but Jupiter’s moon Io. NASA’s Juno mission has spotted some of the most powerful volcanic activity on Io ever recorded from spacecraft, with plumes rising hundreds of kilometers into space and lava lakes larger than the Great Lakes.
Io’s heat comes from tidal flexing, not a stationary hotspot, but it demonstrates what continuous, high-energy volcanism does to a surface: it resurfaces the entire moon rapidly, erasing craters entirely. Mars kept its craters. Io does not have any. Olympus Mons sits between the extremes — a mountain built by patient repetition rather than continuous fury.
How you build a 22-kilometer mountain
The mechanics of a shield volcano are almost boring compared to the drama of a stratovolcano. Basalt lava, low in silica and gas, flows out of a central vent and travels for tens of kilometers before cooling. Each eruption adds a thin veneer — sometimes only a few meters thick — over the previous flow. Repeat for three billion years, and the layers stack up.
On Earth, the process is described in general geological references like Live Science’s overview of volcano types: shield volcanoes grow broad and gentle because their lava travels far before it stops. The Hawaiian Islands are the textbook case. Olympus Mons is the same recipe with the timer left running for an extra couple of billion years and the conveyor belt switched off.
The escarpment around the base of Olympus Mons — a cliff up to 8 kilometers high where the volcano’s edge drops abruptly to the surrounding plain — is itself a puzzle. It may be the scar of ancient flank collapses, or it may mark the edge where the volcano’s own weight has pushed the crust down. Either way, it is the kind of feature you only get when a single volcano sits in one place long enough to become a geological fixture.
The mountain that will still be there
Mars runs on a much older energy budget — the slow leak of primordial heat from a small planet’s core. That heat drove the volcanism. When it finally dies, so does Olympus Mons’s slow growth, if it has not already ended.
Even then, the mountain will still be there. Erosion on Mars is glacial in the literal sense: no rain, no rivers to speak of, just wind and dust and the occasional dust devil scratching at the flanks. A feature the size of Arizona does not blow away.
Earth will not preserve Mauna Loa the same way. Ten million years from now, the Big Island will be a low seamount, drowned and silent, somewhere northwest of its current position, and a new Hawaiian volcano — the currently submarine Kama’ehuakanaloa, once known as Loihi — will be taking its turn above the hotspot. Fifty million years from now that one will be sinking too.
Olympus Mons will not have moved a meter.
