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At 8,848 meters above sea level, Mt. Everest reaches farther into the sky than any other peak on our planet.  It's often called "the roof of the world" but it might also be pretty close to the ceiling, the upper limit of mountain height. 

See, a mountain can't just rise indefinitely.  There are limits.  Much of this has to do with how mountains form.  Most major mountain ranges come about where two segments of Earth's crust are crashing slowly into each other.  Though the Earth's crust is solid rock, it still changes shape under enough pressure, and when rock pushes up against rock, you end up with what's called crustal shortening as the plates squish together and crustal thickening as the crust bunches and bulges like a carpet pushed against a wall.

This can gradually push parts of a plate kilometers into the air, forming mountains through what geologists call uplift.  Mt. Everest and the surrounding Himalayas are a result of a particularly forceful collision between what is now India and the rest of Asia, which created a lot of uplift, but like with everything on this planet, gravity ultimately gets the final say.

Mountains are really heavy and that mass causes the underlying crust to bend and sag like a person sitting on a trampoline, which lowers the peak somewhat, and if those peaks grow tall enough, they can become so heavy that their weight overpowers the tectonic forces pushing them upward and uplift stops.  As the pieces of crust keep pushing together, the pressure leads to shortening and thickening under lower peaks instead, causing the slopes on either side of the central peaks to rise, so when the mountains can't grow upward anymore, they expand outward into plateaus. 

This has been observed in mountains all over the world, from the Rockies to the Andes to the Himalayas, and though we don't have an exact number for max height, we can assume Everest is pretty close to it, since it and other mountains in its range started growing outwards instead of up, but the mountain might be a bit shorter now than it used to be, and that's because there's another possible limit around mountain size: ice.

The idea is that once a peak reaches high enough into the atmosphere, glaciers begin to form which then slowly carve the summit.  You see, mountains all around the world seem to top out at about 1500 meters above the local snowline, an altitude which varies depending on the climate of the region, and this trend suggests that glacial erosion carves away mountains faster than they can grow their tallest peaks, a hypothesis known as the glacial buzzsaw.  

But there are some mountains that don't seem to suffer from the glacial buzzsaw: highly active volcanoes.  It's not because their lava melts the ice or anything, but because they form somewhat differently from other mountains.  Mauna Kea on the island of Hawaii measures more than 10,000 meters from its base at the sea floor to its peak, making it technically taller than Everest, though the majority of it is underwater.  It's so big because it's a shield volcano that formed through successive eruptions depositing layer upon layer of lava.

Like other mountains, Mauna Kea causes the crust beneath it to sag downward.  It and its neighbor Mauna Loa depress the seafloor by as much as 6,000 meters, but that weight doesn't keep it from growing since its growth doesn't depend on tectonic uplift and it's thought that the speed of volcanic build-up can outpace glacial erosion.  In fact, Mauna Kea probably would be taller except it lost its fuel source.

The mountain formed because a mantle plume beneath the crust produced magma, which powered the volcano, but now the plate has moved it away from the plume, slowing the pace of eruptions.  There are bigger mountains in our solar system, though.  On Mars, the crust doesn't move around like it does on Earth, and there's less gravity pulling things down, which is how Olympus Mons is able to reach a staggering 25,000 meters around the surrounding planes.  

Sadly, a mountain that large is probably not possible on Earth.  Physics just aren't on our side.  How big can mountains get is the kind of question that forces you to apply everything you learned in science, math, and engineering courses, and if you like that kind of thing, you'll probably really enjoy a subscription to Brilliant.org.

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