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One-fifth of the Earth's land surface is mountains. But only one in 10 people live in these rugged and beautiful places.
Until just a few decades ago, the secrets of making mountains were largely a mystery. Geologists could make sense of how volcanoes build themselves higher with their own lava, but how do ocean sediments end up on top of the Andes of South America?
It was the theory of plate tectonics — accepted by geologists in the 1960s — that finally provided some sensible answers. Simply put, where the rafts, or plates, of Earth's brittle crust collide, that's where most mountains tend to be. It's also where most of the earthquakes, volcanoes and other geological violence tend to gang up and make a ruckus.
The Himalayan Range, for instance, is growing every day as the Indian Plate continues to smash north into the much larger Asian Plate. The rumpled, contorted rocks of the collision form the highest mountain range on the planet. Among those rocks are old ocean sediments — right up at the very summit of Mount Everest. Just like in the Andes.
What Goes Up ...
But as sure as they rise, rain, ice and snow are tearing mountains down. That's the irony of being a mountain on planet Earth: the higher a mountain reaches, the more clouds gather round its heights, dumping rain and snow on it to erode it away. The Appalachians were once a towering mountain range. But because the tectonic collision that pushed them up ended long ago, the old range is slowly melting away.
The Himalayas are putting up a better fight. The Indian monsoons wear them down, but the ongoing collision of plates keeps pushing the mountains up as well — and so a balance has been struck.
The flip side to all this mountain weather is that it feeds glaciers and fills rivers with just about all the freshwater on the planet. By forcing air to rise into chillier altitudes, mountains force water vapor to condense and deposit rain or snow on the land. All that water eventually feeds the rivers, lakes and streams that sustain our crops and provide water to cities and industries.
Mountains also come down more violently. Avalanches are an extreme danger in some mountainous regions. The worst are in places where old layers of rock are tilted downslope and then lubricated with water.
Volcanoes can also blow themselves apart in an instant — as was seen when Mount St. Helens literally lost its top in 1980.
