Olympus Mons — The Tallest Volcano in the Solar System
If you stacked three Mount Everests on top of each other, you'd still be shorter than Olympus Mons.
Olympus Mons is the biggest volcano in the entire solar system. There's nothing else like it on any planet we've explored — not on Earth, not on Venus, not anywhere. It rises 22 km (almost 14 miles) above the surrounding plains, which makes it nearly three times taller than Mount Everest.
And it's not just tall. Its base is so wide it would cover the U.S. state of Arizona. Olympus Mons is the size of a country.
How Big Is Olympus Mons, Really?
Numbers can be hard to picture, so try these.
- It's 21.9 km tall — about 72,000 feet, or roughly 2.5 times taller than Mount Everest.
- Its base is around 600 km (370 miles) across. You could fit the entire state of Arizona inside it.
- It would cover most of Italy if you dropped it on Earth.
- The peak crater (called a caldera) at the very top is 80 km wide and 3 km deep. You could fit a whole large city inside the crater.
- The summit pokes up so high that it sticks out above most of Mars's atmosphere.
It's hard to overstate how massive this thing is. Astronauts standing at the base wouldn't be able to see the summit, because the slope curves away over the horizon long before you'd reach the peak.
Why Is Olympus Mons So Huge?
The short answer: Mars doesn't have plate tectonics, and Earth does.
On Earth, the crust is broken into pieces called tectonic plates that very slowly slide around on top of the hot rock below. When a volcano forms over a hot spot in the mantle, the plate eventually moves the volcano off the hot spot, and a new volcano starts somewhere else. That's how Hawaii's chain of islands formed — each one is an old volcano that drifted away from the hot spot, while a new one grew behind it.
On Mars, the crust doesn't move. So when a hot spot starts pushing magma toward the surface, the lava just keeps piling up in the same place. For billions of years. The result is one absolutely enormous volcano instead of a chain of smaller ones.
The lower gravity on Mars helped too. With only 38% of Earth's gravity, the rock and lava could pile higher before its own weight made it collapse.
What Type of Volcano Is It?
Olympus Mons is what scientists call a shield volcano. That means it has very gentle slopes — only about 5 degrees on average. It looks more like a giant warrior's shield laid flat on the ground than a sharp pointy mountain.
Earth has shield volcanoes too. Mauna Loa in Hawaii is the closest comparison. From the sea floor to its peak, Mauna Loa is about 10 km tall, which sounds impressive until you remember Olympus Mons is more than twice that.
Is Olympus Mons Still Active?
Probably not, but it's not totally dead either.
Studies of lava flows on Olympus Mons show that some are only about 2 million years old. That sounds like a long time, but in geology it's basically yesterday. So Olympus Mons was definitely erupting in fairly recent geological history. Scientists can't completely rule out that it could erupt again, though no eruption has ever been observed.
Some Mars researchers think there might still be hot magma deep under the surface. If they're right, Olympus Mons could be more dormant than dead.
The Other Tharsis Volcanoes
Olympus Mons doesn't sit alone. It's part of a huge volcanic region called the Tharsis bulge, a raised plateau covering about a quarter of Mars's surface. Three other massive volcanoes line up along the Tharsis ridge:
- Arsia Mons — about 19 km tall
- Pavonis Mons — about 14 km tall
- Ascraeus Mons — about 18 km tall
All three are taller than any mountain on Earth. Together with Olympus Mons, they make Mars the most volcanically dramatic planet in the inner solar system.
How Did We Find Olympus Mons?
People have been seeing Olympus Mons for a long time without realizing what it was. Back in the 1800s, astronomers using telescopes could spot a bright spot on Mars near the equator and called it Nix Olympica, "the snows of Olympus." They thought it was a snow-covered mountain.
The full size only became clear in 1971, when NASA's Mariner 9 orbiter became the first spacecraft to orbit another planet. Mariner 9 mapped Mars's surface in detail and revealed that the bright spot was actually a colossal volcano — by far the biggest ever seen anywhere.
Mariner 9 also discovered Valles Marineris on the same trip. Not a bad day's work.
Wow stat: If Mauna Loa from Earth were sitting next to Olympus Mons, you could fit Mauna Loa inside Olympus Mons more than 100 times.
Could a Person Climb Olympus Mons?
In theory, yes. The slopes are very gentle — most of them are no steeper than a wheelchair ramp. The hard part isn't the climb itself.
The hard part is the size. The volcano is so wide that a hike to the summit would take many days, possibly weeks. And the higher you went, the thinner the air would get, until you reached the peak — which is high enough that there's barely any atmosphere at all. Astronauts trying it in the future would need spacesuits the whole way up. There would be no easy turn-around.
It would be the most spectacular hike anywhere. From the summit caldera, you'd be standing higher than almost anywhere else in the entire solar system, looking down at clouds and dust storms far below.
Frequently Asked Questions
How tall is Olympus Mons?
Olympus Mons is about 22 km tall (around 14 miles, or 72,000 feet). That's nearly three times the height of Mount Everest, making it the tallest volcano anywhere in the solar system.
How wide is Olympus Mons?
The base of Olympus Mons is roughly 600 km (370 miles) across. That's about the size of the U.S. state of Arizona, or large enough to cover most of Italy.
Why is Olympus Mons so much bigger than any volcano on Earth?
Because Mars doesn't have moving tectonic plates like Earth does. On Earth, plates carry old volcanoes away from their hot spots, so volcanoes never get a chance to grow truly massive. On Mars, the crust stays still, so lava keeps piling up in the same place for billions of years.
Is Olympus Mons still active?
Probably not erupting today, but not completely dead either. Some lava flows on the volcano are only about 2 million years old, which is recent in geological terms. There may still be hot magma deep underneath, so scientists call Olympus Mons dormant rather than extinct.
Could you climb Olympus Mons?
In theory, yes. The slopes are very gentle — only about 5 degrees on average. But the volcano is so vast that a climb would take many days, the air gets thinner the higher you go, and the summit is so high it pokes up above most of Mars's atmosphere. Any future astronaut climber would need a spacesuit the entire way up.