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.

Islands in the SkyMountains also have a gentler side. Because they are so rugged and hard to traverse, they are home to some of the rarest and least seen species of animals and plants, such as giant pandas, snow leopards and big horn sheep, just to name a few high-profile mountain creatures.

What makes mountains prone to rare species is elevation and isolation. Where mountains soar alone above lowlands, the highland species are literally trapped. A panda, for instance, that lives off only one kind of bamboo, which grows only at one elevation, can't spread to a faraway mountain of the same height because it would starve in the intervening lowlands. These pandas and many other mountain species are hemmed in. They might as well be surrounded by water.

In Nevada, range after range of mountains rise out of the desert — each with small, rare verdant zones up high. Most host species that live only on one or a few ranges and nowhere else on the planet. The oldest living things on the planet are among these isolated species — the 5,000-year-old Bristlecone pine trees, for example. These mountains are true islands in the sky.
Riches & War ZonesHumans can bring new sorts of violence to mountains. Because we tend to prefer lowlands for agriculture and building cities and towns, mountain ranges have often served as borderlands between nations. As a result, many wars tend to take place in mountainous regions. It's been true from Kashmir to Cambodia. Struggles between governments and ethnic minorities often play out in rugged places, which give smaller forces better odds.

Mountains are also a major part of the world's illegal drug market. Almost all of the world's heroin and cocaine comes from mountainous regions of the Myanmar-Thailand-Laos border trisection, the border of Pakistan and Afghanistan, and the Bolivia-Columbia border.

There's also the mineral wealth of the Earth that's far more accessible in mountainous regions. Whole mountaintops are being torn down to extract coal in Appalachia. Copper and gold mines in Nevada and Utah remove nearly entire mountains. It's a new mountain-destroying force never before seen on Earth.

Good News: Because soils are meager in mountains and the land is rugged, humans still prefer lowlands. This is the mountains' greatest protection. People like to visit mountains to hike, ski or just escape the heat of lowlands in summertime. Tourism has spurred more protection efforts of these places. After all, no one wants to visit the mountains just to sit in traffic and peer through smoggy air.

Credits: http://dsc.discovery.com/convergence/planet-earth/guide/mountains.html

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