Olympus Mons is a shield volcano so broad that a person standing on its lower slope would not see the summit above the horizon. Its summit rises roughly 16 miles above the surrounding Martian plains, close to three times Mount Everest’s height above sea level. The comparison is striking, but the volcano’s immense width is as important as its elevation.
Height on Mars needs a defined reference
Earth mountains are commonly measured above mean sea level, a reference Mars does not have. Scientists instead use a Martian datum based on gravity and atmospheric pressure, or compare a feature with its surrounding plains.
NASA’s Olympus Mons image note explains the height comparison. Different reference choices produce slightly different Olympus Mons figures. The familiar near-three-times-Everest comparison uses an elevation around 25 kilometers, while relief from the local base can be somewhat smaller.
Lava built a shield hundreds of miles wide
Olympus Mons formed through repeated flows of relatively fluid lava. Each flow spread across a large area before cooling, producing a shield profile rather than the steep cone associated with some explosive volcanoes.
The volcano spans roughly 600 kilometers, comparable to a large U.S. state. Its average slopes are only a few degrees, which is why the enormous height would feel gradual on the ground.
Mars lacked moving plates over the hotspot
On Earth, a tectonic plate moves over a relatively fixed mantle plume, creating chains such as the Hawaiian Islands. A volcano is carried away from its magma supply while a new one forms.
Mars does not have active plate tectonics of the same kind. A volcanic source could therefore feed one location for a very long time, allowing flows to pile up into a single colossal structure.
Lower gravity helped the mountain grow
Surface gravity on Mars is about 38 percent of Earth’s. Rock weighs less there, reducing the stress that limits how high a mountain can stand before its base deforms or collapses.
JPL’s Mars volcano explainer describes why the shield grew so large. Lower gravity was not the only ingredient. Long-lived volcanism, a thick stationary crust and the absence of strong erosion by rivers and oceans all helped preserve the shield.
A giant cliff marks much of the base
An escarpment several kilometers high surrounds large portions of Olympus Mons. Beyond it lies an extensive apron of ridged terrain, evidence that parts of the volcano spread or collapsed under their own weight.
The summit contains a complex caldera formed when magma chambers emptied and the overlying ground subsided. Olympus Mons is therefore not one simple peak but a landscape of lava plains, cliffs and nested collapse pits on a scale difficult to picture from Earth.
A high summit crowns an immense volcanic province
Olympus Mons rises above much of the Martian atmosphere, so pressure near its summit is even lower than on surrounding plains. Future aircraft and human systems would face extreme elevation within an already thin atmosphere.
The youngest mapped lava surfaces may be only millions of years old, geologically recent but far older than civilization. No eruption has been observed, and present activity remains unconfirmed.
Overlapping pits in the summit caldera show that separate magma reservoirs emptied and collapsed at different times. Crater counts on nearby flows provide a relative chronology.
Lava channels and tubes cross the shield, recording flows that traveled great distances on gentle slopes. Low viscosity and sustained eruption built area as effectively as height.
Everest rises through colliding plates and is cut by glaciers and rivers. Olympus Mons grew on stationary crust with little liquid-water erosion, making the comparison one between different planetary systems.
The volcano’s shallow grade would make a climb long rather than continuously steep. The basal escarpment presents the sharpest obstacle in many places, after which the shield rises gently across hundreds of kilometers. From low on the flank, curvature would hide both the distant summit and much of the base.
Mars’s lower gravity allowed a taller edifice, but the planet’s stationary crust supplied the time. Repeated eruptions could return to the same vent system instead of forming an island chain as a moving plate does over Hawaii. The result combines material strength, gravity, erosion and tectonic history rather than reflecting one exceptional eruption.
Elevation figures also depend on where the base is drawn. Measuring from the Martian datum gives a value near 25 kilometers, while measuring relief above nearby terrain yields a smaller number. Both exceed Everest by a wide margin, but stating the reference explains why reputable descriptions do not always print the same height.
The broad summit contains calderas rather than one open crater. Each collapse marks withdrawal of magma from below, and later eruptions overprinted older surfaces. This layered structure shows that Olympus Mons grew through many episodes, not one eruption capable of building a 16-mile mountain at once.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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