Olympus Mons: The Solar System's Tallest Volcano, Three Times Higher Than Everest

A Mountain Beyond Scale

Mount Everest, the tallest mountain on Earth, stands approximately 8.8 kilometers above sea level. Olympus Mons on Mars stands 22 kilometers above the surrounding Martian plains — nearly two and a half times as tall. More striking than its height is its base diameter: approximately 600 kilometers across, or roughly the size of France. The volume of Olympus Mons is estimated at about 300 times the volume of Mauna Loa in Hawaii, which is by volume the largest volcano on Earth.

This scale has bizarre perceptual consequences. The slopes of Olympus Mons are remarkably gentle — an average gradient of about 5 degrees, similar to a moderate highway ramp — because the volcano's enormous width distributes its height across such an immense area. The summit caldera, a complex of nested collapse craters left by the withdrawal of magma from beneath the volcano, is itself about 80 kilometers wide and 3 kilometers deep. The outer edge of the volcano is defined by an escarpment — a cliff face up to 8 kilometers tall — that essentially marks where the volcanic structure ends and the surrounding plains begin. Standing at the base of this escarpment and looking up would give you a view of an 8-kilometer vertical face — roughly the height of Everest, just as a cliff.

Why Mars Builds Such Enormous Volcanoes

The reason Olympus Mons can grow so tall lies in one of the fundamental differences between Mars and Earth: the absence of plate tectonics on Mars. On Earth, the lithosphere — the rigid outer shell — is divided into tectonic plates that move slowly, driven by convection in the mantle below. As a tectonic plate drifts over a mantle "hot spot" — a fixed source of magma rising from deep within the Earth — it carries the surface over the heat source, building a chain of volcanoes as the plate moves. Hawaii is the most recent in a chain of islands formed this way, each island older and more eroded as it moved further from the hot spot over millions of years.

Mars has no such plate movement. When a hot spot forms beneath the Martian surface, the crust stays fixed above it. Magma erupts in the same location for billions of years, stacking lava flow upon lava flow, building a volcano that grows almost without limit. The weak Martian gravity — about 38% of Earth's — further reduces the tendency for the structure to collapse under its own weight. The result is a volcano that is almost three times as tall as any mountain Earth has produced in its entire history.

Mars as a Volcanic World

Olympus Mons is part of the Tharsis volcanic plateau — a vast elevated region on Mars that contains several other enormous shield volcanoes, including Ascraeus Mons, Pavonis Mons, and Arsia Mons, all three of which would individually be among the largest volcanoes in the solar system. The Tharsis bulge is so massive — containing enough material to measurably affect Mars's rotational axis — that it is thought to have altered the orientation of the entire planet relative to its rotation axis over deep geological time.

Whether Olympus Mons and its companion volcanoes are truly extinct or merely dormant is not definitively settled. Some models suggest that volcanic activity may have occurred as recently as 25 million years ago in geological terms, which is relatively recent. Evidence of lava flows that appear fresher and less crater-pocked than the surrounding terrain has been identified in some regions of the Tharsis plateau. The possibility that Mars is not yet entirely geologically dead — that beneath those vast shield volcanoes, heat still concentrates and magma still moves — remains an open question with significant implications for the search for life on Mars, since volcanic heat could sustain liquid water in subsurface environments long after the surface froze.