Olympus Mons: The Tallest Volcano in the Solar System

What you would actually see

Approach Olympus Mons and the first thing that defeats your intuition is that you cannot take it in as a mountain at all. A terrestrial peak like Everest or Denali presents a recognizable triangular silhouette. Olympus Mons does not. Its ~600 km base is so broad and its ~5° slopes so shallow that from the ground the volcano curves away below the visible horizon — you would be standing on a gigantic dome without ever perceiving a summit. Only from orbit does its true form resolve: a near-circular shield of stacked lava flows crowned by a pockmarked central pit.

The volcano's edge is its most dramatic feature. Instead of tapering gently into the plains, Olympus Mons is ringed by a basal escarpment — an outward-facing cliff up to about 8 km tall in places. Imagine a wall six or seven times the height of the White Cliffs of Dover, running for hundreds of kilometers around the entire mountain. Why the volcano has such a sharp scarp is still debated; explanations range from ancient landslides and the collapse of oversteepened flanks to erosion by long-vanished glaciers or seas.

At the top sits a complex caldera roughly 60 by 80 km across and up to 3.2 km deep — a nested set of at least six overlapping collapse pits. Each pit records a time when an underground magma chamber emptied and the roof above it sank. Beyond the mountain proper, a vast rumpled apron of grooved terrain called the Olympus Mons aureole extends outward as far as ~750 km, most likely the debris of enormous flank collapses that slid off the growing volcano.

Why Mars could build a mountain this tall

The single most important fact about Olympus Mons is not its size but the reason for it: Mars has no plate tectonics. On Earth, the crust is fractured into plates that drift a few centimeters a year. Hawaii's volcanoes sit over a stationary plume of hot mantle — a hotspot — but the Pacific plate slides steadily over that plume. Each volcano is carried away before it can grow too large, and the hotspot punches out a fresh one behind it. The result is the Hawaiian island chain: many modest volcanoes instead of one giant.

Mars froze that conveyor belt. Its lithosphere is a single rigid shell that does not move over the mantle beneath. So when a hotspot in the Tharsis bulge began feeding magma to the surface, every eruption for hundreds of millions of years piled onto the same spot. Lava sheet after lava sheet stacked up in place, and the volcano simply kept growing.

Two other factors let it climb so high before it stopped:

• Low gravity. Mars's surface gravity is about 3.72 m/s² — roughly 38% of Earth's. A mountain fails when its own weight exceeds the strength of the rock at its base; weaker gravity means a pile of rock can grow far taller before it crushes itself.
• A thick, strong lithosphere. Mars's cold, rigid outer shell — estimated at many tens of kilometers (often >70 km) thick beneath Tharsis — could bear the immense load without the crust buckling and sinking, as a thinner, hotter lithosphere would.

Combine a stationary crust, a long-lived hotspot, weak gravity, and a strong lithosphere, and you get a volcano that had no natural reason to stop until it ran out of magma.

The numbers, and how to read them honestly

Quoting the height of Olympus Mons requires care, because Mars has no sea level. Elevations are measured against a mathematical reference surface called the areoid (the Martian datum). Relative to that datum the summit reaches about 21.3 km. But the volcano sits on a slightly raised platform, so measured against the immediately surrounding plains it stands about 21.9 km tall. And because the plains to its northwest — Amazonis Planitia — are themselves a deep basin, the drop from summit to that lowland floor can approach 26 km. All three figures are correct; they simply answer different questions.

For comparison, Mount Everest rises 8,849 m above sea level, and Mauna Loa — Earth's most massive volcano — stands about 9 km above the Pacific seafloor from which it grew. Olympus Mons is roughly two and a half times either of those, and in sheer bulk it dwarfs them: its volume is on the order of 100 times that of Mauna Loa, spread over an area of about 300,000 km².

A common misstatement is that Olympus Mons is undisputedly "the tallest mountain in the Solar System." It is the tallest volcano, but its height crown is contested. The central peak of Rheasilvia, an impact basin at the south pole of the asteroid Vesta, rises an estimated 20–25 km from its floor — comparable to, and possibly greater than, Olympus Mons. So the honest statement is that Olympus Mons is the tallest known volcano, and among the tallest known mountains, but not the uncontested single highest peak.

How it erupts: a shield built by runny lava

Olympus Mons is a basaltic shield volcano, the same broad, low-profile family as Hawaii's — and emphatically not the steep, explosive cone we picture when we hear "volcano." Steep stratovolcanoes like Mount Fuji or Vesuvius are built by sticky, gas-rich, silica-heavy magma that erupts violently and piles up close to the vent. Shield volcanoes are built by fluid, low-viscosity basalt that runs for great distances before freezing, spreading into thin, wide sheets. Thousands of such flows stacked over eons produce a shape that genuinely resembles a warrior's shield lying on the ground.

That fluid-lava mechanism is exactly why the flanks slope at only about 5°. It is also why Olympus Mons grew outward as fast as it grew upward, reaching that ~600 km footprint. On its slopes, spacecraft cameras have imaged lava channels, collapsed lava tubes, and long ridges of overlapping flow fronts — a frozen record of rivers of molten rock.

The volcano's activity was not steady but episodic. Crater-counting — dating a surface by how many impact craters have accumulated on it — tells us the caldera floors collapsed in stages roughly 350 to 150 million years ago, and some lava flows on the flanks are astonishingly young, as recent as about 2 million years. In geological terms that is the day before yesterday, which means Olympus Mons is probably best described as dormant rather than extinct. Whether it could ever erupt again is unknown, but nothing rules it out.

Discovery: from a smudge of 'snow' to a mapped giant

Olympus Mons was, in a sense, seen long before anyone knew what it was. In 1879 the Italian astronomer Giovanni Schiaparelli — the same observer whose sketches of Martian canali sparked a century of speculation — mapped a persistent bright patch on Mars and named it Nix Olympica, Latin for "Snows of Olympus." Through a telescope it appeared as a recurring light-colored spot, probably clouds and hazes clinging to high ground, and no one could tell it was a mountain at all.

The truth arrived with NASA's Mariner 9, which in 1971 became the first spacecraft to orbit another planet. It reached Mars during a planet-wide dust storm that hid nearly the entire surface — except for a handful of dark spots poking up through the dust. As the storm cleared, those spots resolved into the summits of enormous volcanoes rising above the haze. The largest, sitting right where Schiaparelli's Nix Olympica had been, was confirmed to be a colossal shield volcano and renamed Olympus Mons, "Mount Olympus."

Every mission since has sharpened the picture. Laser altimetry from Mars Global Surveyor (whose MOLA instrument, active from 1997, produced the topographic map that pinned down the volcano's true height), together with high-resolution imaging from Mars Express, Mars Reconnaissance Orbiter, and others, has let geologists count craters, trace individual lava flows, and reconstruct the mountain's growth over billions of years — turning a telescopic smudge into one of the best-studied landforms off Earth.

Standing on it, and what it teaches us

Popular accounts often say that a person standing on Olympus Mons "wouldn't even know they were on a mountain," and for once the cliché is literally true. The slope is so slight and the volcano so vast that the summit is always hidden below the horizon; there is no vista, no peak, no sense of altitude. In this sense Olympus Mons is less a mountain to be climbed than a small continent to be crossed.

It is also a warning against Earth-based intuition. We tend to assume that bigger planets or more violent processes make bigger features. In reality, Olympus Mons is a monument to stillness: it is enormous precisely because Mars is geologically quiet, its crust locked in place, its gravity gentle, its lithosphere strong and cold. Earth, more geologically active, can never build such a thing — plate tectonics keeps recycling the crust and dispersing our volcanoes into chains.

Studying Olympus Mons therefore does more than break records. It lets planetary scientists read Mars's deep history — the thickness and strength of its lithosphere, the longevity of its mantle hotspots, and the timing of its last volcanic gasps. Those clues feed directly into the biggest questions about the planet: how long Mars stayed warm and volcanically active, whether that activity could have sustained liquid water and habitable environments, and why a world that once resembled the early Earth ended up frozen, dry, and still. A single mountain, in other words, is also a clock and a thermometer for an entire planet.

Olympus Mons versus Earth's largest shield volcano, Mauna Loa

Property	Olympus Mons (Mars)	Mauna Loa (Earth)
Height above local base	~21.9 km	~9 km (from Pacific seafloor)
Base diameter	~600 km	~120 km
Approximate volume	~100× Mauna Loa	baseline (~75,000 km³)
Average flank slope	~5°	~12°
Surface gravity	3.72 m/s² (0.38 g)	9.81 m/s² (1 g)
Built over	A fixed crust (no plate motion)	A moving plate over a hotspot