Olympus Mons spans a France-sized area but has slopes gentler than a ramp

On Earth, the biggest volcanoes are built by plate tectonics: as a plate drifts over a hot spot, eruptions punch through at shifting points, producing chains like the Hawaiian Islands. Mars has no moving plates, so a single hot spot can keep erupting in the same place for a very long time — and Olympus Mons is the result. ^[1][2]

A giant with surprisingly shallow slopes

Olympus Mons is a shield volcano, a broad dome built by runny, low-viscosity lava that spreads far before solidifying. Its flanks typically rise at only about 5 degrees, and even the steepest parts near the base stay close to 10 degrees. By comparison, a typical wheelchair ramp climbs at roughly 5 to 8 degrees, so the largest volcano in the solar system is, in a literal sense, gentler than a ramp. ^[1][3]

The volcano's size is usually given as roughly 600 km across, a footprint comparable to the area of France. Its summit caldera complex contains multiple overlapping collapse craters, and the entire edifice is ringed by a distinctive scarp, or cliff, that in places rises several kilometres above the surrounding plains. ^[2][3]

Why Mars could build something so big

• A stationary crust: without plate motion, the magma source stays under the same vent for millions of years. ^[2]
• Lower surface gravity, about 38% of Earth's, lets erupted material pile higher before collapsing under its own weight. ^[1]
• Repeated, long-lived eruptions of fluid basalt built the broad, low-angle shield rather than a steep cone. ^[2][3]

1. 1971

   Mariner 9 becomes the first spacecraft to orbit another planet and reveals the giant Martian volcanoes.

2. 1970s

   Viking orbiters map Olympus Mons in detail, confirming its shield shape and enormous extent.

3. 2000s–present

   High-resolution cameras and laser altimeters refine its dimensions, slopes and volcanic history.

The volcano is thought to be very old but not necessarily extinct. Crater-counting suggests its main bulk formed billions of years ago, with some lava flows perhaps as young as a few million years — recent enough that some scientists consider it potentially dormant rather than dead. ^[1][3]

That combination — immense scale, shallow slopes, and a possible pulse of recent activity — is why Olympus Mons remains a prime target for future Mars missions and a landmark for anyone trying to picture the planet's true proportions. ^[2]