Isostasy

Mountain ranges don't simply rest on top of the Earth's crust the way a brick sits on a table; instead, they float on the denser, plastic-like rock of the mantle beneath them, in much the same way an iceberg floats on water. This principle, called isostasy, explains why the crust beneath the tallest mountain ranges is disproportionately thick, extending far deeper into the mantle than the crust beneath flatter regions, rather than mountains simply being tall piles of extra rock sitting on an otherwise uniform layer of crust. Because continental crust is less dense than the mantle rock underneath it, it floats in a state of buoyant equilibrium, and just as a larger iceberg needs a correspondingly larger submerged portion to stay balanced, a taller mountain range needs a deeper crustal root extending downward into the mantle to support its visible height above the surrounding terrain. This buoyant balance isn't static -- it actively responds to changes in weight on the surface. When a massive ice sheet melts, for instance, the crust that had been pressed down under the ice's weight gradually rises back upward over thousands of years as the mantle beneath it slowly flows to reestablish equilibrium, a process called post-glacial rebound that's still measurably occurring today in regions like Scandinavia and parts of Canada that were heavily glaciated during the last ice age. Similarly, when erosion strips away rock from a mountain range's surface over millions of years, the reduced weight allows the remaining crust to rise slightly in response, partially offsetting the height lost to erosion, which is one reason ancient, heavily eroded mountain ranges can still stand taller than a simple erosion-rate calculation alone would predict.