Duration Of Mountains
Mountains, like every other thing in the natural world, go through a life cycle. They rise, from a variety of reasons, and wear down over time, at various rates. Although humans have always used mountains to represent eternity, individual mountains do not last very long in the powerfully erosive atmosphere of the earth. Mountains on the waterless worlds of Mars and the moon are billions of years old, but Earth's peaks begin to fracture and dissolve as soon as their rocks are exposed to air. The permanent part of a mountain range is not the shape taken by the rocks at the surface, but the huge folded shapes that the rocks were deformed into by the original orogenic event. (Orogeny is the process of mountain formation.) Throughout their almost four-billion year history, the continents have been criss-crossed by many immense ranges of mountains. Most of the mountain ranges in the planet's history rose and wore away at different times, a long time ago. Where did these mountains go?
A range of mountains may persist for hundreds of millions of years, like the Appalachians. At several different times, the warped, folded rocks of the Appalachians were brought up out of the continent's basement and raised thousands of feet by tectonic forces. In order to stand for any considerable length of geologic time, a mountain range must experience continuous uplift. A tectonically quiet mountain range will wear down from erosion in a few million years. In North America's geologic past, for example, eroded particles from its mountains were carried by streams and dumped into the continent's inland seas, some of which were as large as the present-day Mediterranean. Those rivers and seas are gone from the continent, but the sediments that filled them remain, like dirt in a bathtub when the water is drained. The roots of all the mountain ranges that have ever stood in North America still exist, and much of the sand and clay into which the mountains were transformed still exists also, as rock or soil formations. This is true of all the continents of Earth.
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