Olympus Mons formed the same way shield volcanoes form on Earth: through repeated eruptions of fluid lava that spread outward and piled up layer after layer over an extraordinarily long time. What made it grow into the largest volcano in the solar system, roughly 600 km across and 22 km tall, is a combination of factors unique to Mars: a stationary crust that never dragged the volcano away from its magma source, lower gravity that let the structure pile higher, and an eruptive history spanning well over a billion years. The result is a feature so large it would cover most of France, with a summit caldera wide enough to swallow a mid-sized city.
Lava Flows, Not Explosions
Olympus Mons is a shield volcano, a type built almost entirely from successive outpourings of basaltic lava rather than explosive blasts of ash and rock. On Earth, the Hawaiian Islands are the closest comparison. Mauna Loa and Kilauea grow the same way: thin, runny lava spreads out over wide areas, cools, and hardens into gently sloping layers. Over thousands of eruptions, the result is a broad, dome-shaped mountain with relatively shallow flanks. Olympus Mons has average flank slopes of only about five degrees, which is gentle enough that standing on its lower slopes you would have trouble telling you were on a volcano at all. The immense width relative to height is the hallmark of this eruptive style.
The lava that built Olympus Mons was almost certainly basaltic, similar in composition to the dark volcanic rock common across Earth’s ocean floors and Hawaiian volcanoes. Basaltic lava flows easily and travels long distances before cooling, which is why shield volcanoes end up so wide. On Mars, the combination of lower gravity and potentially higher eruption rates may have allowed individual lava flows to travel even farther than their terrestrial counterparts, contributing to the volcano’s staggering footprint.
Why Mars Builds Bigger Volcanoes Than Earth
The single most important reason Olympus Mons grew so large is that Mars lacks plate tectonics. On Earth, the tectonic plates that make up the crust are constantly in motion, drifting over the deeper mantle. When a volcanic hotspot punches through the crust, the plate above it slowly slides sideways, carrying the active volcano away from its magma supply. The hotspot then burns through fresh crust and starts building a new volcano. This is exactly how the Hawaiian island chain formed: a string of volcanoes, each one active only while it sat above the hotspot, then going extinct as the Pacific Plate carried it northwest.
Mars never developed this conveyor-belt system. Its crust sits still. So when a deep magma source began feeding eruptions at the location that would become Olympus Mons, the volcano just kept growing in the same spot, eruption after eruption, for billions of years.1Mars Education | Arizona State University. Tectonics If Hawaii’s hotspot had been feeding a single stationary volcano for that long, the result would be dramatically larger than any individual Hawaiian island. On Mars, that scenario played out, and the outcome is a volcano roughly a hundred times the volume of Mauna Loa.
Lower surface gravity also helped. Mars has about 38 percent of Earth’s gravitational pull. A rock structure on Mars can pile higher before its own weight causes it to collapse or spread apart. This does not mean gravity is irrelevant to Olympus Mons, as its sheer mass has caused significant deformation of the surrounding crust, but the threshold at which gravity becomes a limiting factor is much higher on Mars than it would be on Earth.
How Long It Took to Build
Olympus Mons did not appear overnight, even by geological standards. Crater-counting studies and topographic modeling suggest that the volcano’s main growth phase stretched across roughly a billion years, beginning around 3.7 billion years ago and continuing until at least 2.5 billion years ago.2Elsevier. The volcanic history of Olympus Mons from paleo-topography and flexural modeling That time frame places most of the volcano’s construction during the Hesperian period of Martian history, a transitional era when Mars was losing its thicker atmosphere and surface water but still had enough internal heat to drive massive volcanism.
Some of the youngest lava flows on Olympus Mons appear to be much more recent, perhaps only a couple hundred million years old, which by planetary science standards is practically yesterday. Whether the volcano is truly extinct or merely dormant is still debated. Mars almost certainly retains some residual internal heat, and there is no definitive evidence that the deep magma source beneath the Tharsis region has completely shut off. If eruptions did resume, they would add to a structure that has already been building intermittently for most of the planet’s history.
The Tharsis Rise and Its Role
Olympus Mons does not sit in isolation. It perches on the northwestern edge of the Tharsis rise, an enormous volcanic plateau near the Martian equator that hosts several other giant shield volcanoes, including Arsia Mons, Pavonis Mons, and Ascraeus Mons.3Elsevier. Olympus Mons volcano, Mars: A photogeologic view and new insights The Tharsis region represents the largest concentration of volcanic activity on Mars, and its formation involved a colossal upwelling of hot material from the planet’s interior that bulged the crust upward and cracked it with enormous fracture systems, including the Valles Marineris canyon network thousands of kilometers to the east.
The deep thermal anomaly responsible for Tharsis was likely active very early in Martian history, possibly within the first billion years of the planet’s formation. As mantle material rose toward the surface, it generated enormous volumes of magma that fed the Tharsis volcanoes over billions of years. Olympus Mons, sitting slightly off the main plateau, tapped into this system and became the most productive single vent. The fact that all four giant Tharsis shields grew to enormous sizes reinforces the point about plate tectonics: without crustal motion to distribute volcanic output across a chain, each vent simply keeps accumulating material in one place.
The Summit Caldera
At the top of Olympus Mons sits a complex of nested craters collectively forming the summit caldera, a depression roughly 80 km across and up to 3 km deep. This caldera did not form from a single event. High-resolution imaging has revealed a sequence of at least six overlapping collapse craters, each produced when a subsurface magma chamber partially drained and the roof above it dropped.4Lunar and Planetary Institute. EVOLUTION OF THE OLYMPUS MONS CALDERA, MARS
The process worked something like this: during active eruptions, magma filled large chambers beneath the summit. When eruption rates slowed or magma migrated sideways into flank vents, the chamber lost pressure and could no longer support the rock above it. The roof then collapsed downward in a roughly circular pattern, forming a pit crater. Later episodes of magma filling and draining produced additional collapses, each slightly offset from the last, creating the overlapping “nested” pattern visible today. The final collapse event involved a vertical drop of about 350 m. Benches and terraces along the caldera walls record intermediate stages where lava lakes partially drained, leaving shelves of solidified lava at different levels, much like bathtub rings marking former water levels.
This type of caldera formation is well understood on Earth. Kilauea’s summit caldera in Hawaii has gone through similar cycles of filling, draining, and collapse, though on a much smaller scale. The Olympus Mons caldera is simply the same process scaled up to match a volcano that dwarfs anything on our planet.
The Towering Basal Scarp
One of the most striking features of Olympus Mons is the cliff that surrounds much of its base. This escarpment rises up to 10 km above the surrounding plains in places, a sheer wall taller than the entire height of Mount Everest above sea level.5ScienceDirect. What can Olympus Mons tell us about the Martian lithosphere? The origin of this scarp has been a long-running puzzle, and the answer involves the volcano’s own weight working against it.
As Olympus Mons grew taller and heavier, the outer edges of the edifice began to spread outward under gravity, a process geologists call gravitational spreading. Imagine a thick stack of pancakes slowly slumping outward at its edges under its own weight. On Olympus Mons, this spreading created radial fractures called leaf grabens on the lower flanks and caused the outermost portions of the volcano to push outward, leaving a steep scarp where the main edifice meets the displaced apron of material around its base. The spreading also tells researchers something about the strength of the Martian crust beneath the volcano: the lithosphere had to be strong enough to support the bulk of the load without the entire structure collapsing, but not so rigid that spreading could not occur at the margins.
The basal scarp gives Olympus Mons its distinctive appearance in orbital images, making it look almost like a mesa or a giant pedestal rather than a smoothly tapering dome. From a distance, the volcano resembles a vast raised plateau with gently sloping upper flanks, abruptly ending at a cliff edge that drops precipitously to the plains below.
Ice at the Base of a Volcano
Mars has water ice, and some of it has interacted directly with Olympus Mons. Orbital observations have identified deposits around the base of the volcano that resemble debris-covered glaciers, thick ice flows buried under rocky rubble. These glacial features banked up against the basal escarpment, and in some cases lava flows descending from the upper slopes crossed the cliff edge and encountered the ice below.6Lunar and Planetary Institute. Olympus Mons Debris-Covered Glaciers: Formation, Evolution and Volcano-Ice Interactions
When hot lava meets glacial ice, the results are dramatic. On Earth, such interactions produce violent steam explosions, unusual rock textures, and chaotic terrain. On Mars, the evidence for these encounters includes blocky deposits around the volcano’s base that do not closely match any single terrestrial analog, suggesting the low atmospheric pressure and different ice compositions on Mars produced interaction styles somewhat different from what we see on Earth. These deposits are scientifically valuable because they record periods when ice was present at equatorial latitudes on Mars, which constrains models of the planet’s past climate. If ice accumulated against the flanks of Olympus Mons, atmospheric conditions at the time must have supported ice stability at those latitudes, something that is not the case under today’s Martian climate.
Could Olympus Mons Erupt Again
The youngest dated lava flows on Olympus Mons are geologically recent enough to raise the question of whether the volcano is extinct or sleeping. A few hundred million years sounds ancient, but for a volcano that has been intermittently active for close to four billion years, a pause of that length is not necessarily terminal. Earth has volcanoes that went quiet for hundreds of thousands of years before reactivating, and Mars operates on even longer timescales because its interior cools more slowly relative to its smaller size.
There is currently no seismic or thermal evidence of active magma movement beneath Olympus Mons. NASA’s InSight lander, which operated on Mars from 2018 to 2022, detected marsquakes originating from various regions but did not specifically identify volcanic tremor beneath Tharsis. The absence of evidence is not definitive proof of extinction, especially since InSight was stationed far from Tharsis and had limited sensitivity to distant, deep events. Future seismic networks on Mars would provide much clearer answers.
Even if Olympus Mons never erupts again, its long eruptive history has already shaped Mars in ways that go far beyond one mountain. The Tharsis volcanic province, of which Olympus Mons is the crown jewel, released enough gas over its history to substantially alter Mars’s atmosphere and may have contributed to the planet’s early greenhouse warming. Sulfur dioxide and carbon dioxide vented during Tharsis eruptions would have thickened the atmosphere and potentially kept Mars warm enough for liquid water on its surface during the planet’s first couple of billion years. As volcanism waned and the atmosphere thinned, Mars transitioned into the cold, dry world we see today.
Why Earth Will Never Build One
Earth has plenty of volcanic activity, far more than Mars does today, but our planet’s geology prevents anything like Olympus Mons from forming. Plate tectonics is the main constraint. Every hotspot volcano on Earth is riding a moving plate, so no single edifice gets fed for more than a few million years before being carried away. Even if a plate somehow stalled over a hotspot, Earth’s stronger gravity would limit how high the structure could rise before deforming under its own weight, and erosion from wind, rain, and ocean waves would carve it down as fast as it grew.
There is also the issue of crustal thickness and ocean floors. Many of Earth’s most productive hotspots, including Hawaii, punch through thin oceanic crust. The volcanoes they build start on the seafloor several kilometers below sea level, and by the time they breach the ocean surface their visible height is only a fraction of their true stature. Mauna Loa, measured from its base on the ocean floor, is about 9 km tall, which is genuinely impressive but still less than half the height of Olympus Mons. And Mauna Loa has been building for less than a million years. Give it another three billion years of uninterrupted eruption over a stationary crust and you would get something in the same league, but Earth’s geology will never allow that experiment to run.