Mars has Olympus Mons—a volcano so absurdly large it makes Everest look like a speed bump. At 72,000 feet tall and roughly the size of Arizona, it’s the solar system’s heavyweight champion of volcanic structures. Earth’s tallest volcano, Mauna Kea in Hawaii, measures about 33,500 feet from its seafloor base. Olympus Mons is more than twice that height.
When Planetary Gravity Decides to Play Favorites With Molten Rock
Here’s the thing about Mars: its gravity is only 38% of Earth’s. That seemingly boring fact changes everything about how volcanoes behave. On Earth, gravity pulls magma columns down with enough force that they collapse under their own weight once they reach a certain height. The pressure becomes unsustainable. But on Mars? That gravitational leash is so much looser that lava can pile up, and up, and up, building structures that would immediately slump into pancakes on our planet. It’s like comparing sandcastles built in normal gravity versus sandcastles built on the Moon—except with molten rock traveling at temperatures exceeding 2,000 degrees Fahrenheit.
Turns out planetary mass matters more than anyone initially thought.
The Bizarre Physics of Why Io Makes Earth Look Boring
Jupiter’s moon Io is what happens when tidal forces and volcanic activity have an extremely dysfunctional relationship. With over 400 active volcanoes, it’s the most volcanically active body in the solar system. Its gravitational environment is chaos—Jupiter’s immense gravity, combined with gravitational tugs from nearby moons Europa and Ganymede, literally kneads Io’s interior like dough. This generates enough heat to keep the moon’s subsurface molten. Io’s gravity is only 18% of Earth’s, so its volcanic plumes shoot sulfur dioxide gas up to 300 miles into space. On Earth, the 1980 eruption of Mount St. Helens sent ash about 15 miles high. That’s the difference gravity makes—Io’s eruptions reach altitudes twenty times higher simply because there’s less gravitational force yanking everything back down.
Venus Plays by Different Rules Despite Similar Gravity
Venus has roughly 91% of Earth’s gravity, so you’d expect its volcanoes to behave similarly. They don’t. The planet hosts volcanic structures like Maat Mons, which rises 5 miles above the surrounding plains. Soviet Venera missions in the 1980s captured images of what appeared to be recent lava flows. Scientists using data from the European Space Agency’s Venus Express mission in 2015 detected temperature spikes consistent with fresh volcanic activity. But here’s where it gets weird: Venus’s crushing atmospheric pressure—92 times Earth’s at the surface—changes how eruptions work. Lava flows spread out in broader, flatter patterns because the dense atmosphere resists vertical plume formation. Gravity might be similar, but the environmental context transforms the whole volcanic experience into something alien.
Wait—maybe gravity is only half the story.
The Moon’s Ancient Volcanic Scars Tell Uncomfortable Truths
The Moon’s “maria”—those dark patches visible from Earth—are ancient basaltic lava flows from eruptions that occured billions of years ago. With only 17% of Earth’s gravity, you’d think the Moon would have towering volcanic peaks. It doesn’t. Most lunar volcanoes are low, sprawling shield structures because the lava was so fluid it spread across vast distances before solidifying. The Moon’s volcanic activity largely ceased around 3 billion years ago when its interior cooled enough that tidal heating from Earth couldn’t sustain magma generation. The gravity was low, sure, but without sufficient internal heat and tectonic activity, that gravitational advantage became irrelevant. Low gravity enables tall structures, but it can’t manufacture the molten rock needed to build them in the first place.
Gravity shapes volcanoes the same way wind shapes sand dunes—the force itself is consistent, but the materials and environment create infinite variations. Olympus Mons proves low gravity allows monumental construction. Io demonstrates how gravitational chaos drives hyperactive volcanism. Venus shows that atmospheric pressure can override gravitational expectations.
And Earth? Our volcanoes are gravitationally constrained compromises—tall enough to be impressive, compact enough to stay structurally sound, and just active enough to remind us that the planet beneath our feet is still very much alive.
