Scientists investigating the Rocky Mountains have found two clues about their early history. Ammonites, on the side 8000 feet high, are evidence that the area was once under the sea. Traces of iron in granite is evidence that rock, pushed up from 15 miles below the surface, tilting the Flatirons. And it didn't just happen here, but along approximately a thousand miles of the American Rockies. Geologists now needed to find out what monumental forces were responsible for this massive upheaval.

One hundred million years ago, most of North America was covered by a vast inland sea. 70 million years ago, the sea retreated. And the Rocky Mountains began to rise, forming a great mountain range. Scientists trying to piece together their geological past, needed to solve the mystery of what lifted them up.

A force capable of that amount of heavy lifting would have to have been on a global scale. Geologists believe this force was caused by plate tectonics. The earth's crust is broken up into a series of interlocking plates. These plates are continuously on the move. Over millions of years, they collide and break apart, forming new continents and geological features around the world. When the Rockies formed, two of these plates smashed into each other at the American west coast.

What we know is that at the time of this grand uplift, on the western margin of North America, ocean crust, and Oceanic Plate, was subducting beneath the North American Plate. And it was doing so at a high rate of speed. As such, it was transferring stress into the interior of the continent.

As the two plates move towards each other, they squeeze the crust. Over millions of years, it folded and buckled, forming tall mountains. This was the birth of the American Rockies. But a mystery …

小编有约："Geologists believe this force was caused by plate tectonics. " Daisy想知道，地质学家给的这个假设，你觉得怎样？所以Daisy今天的问题就是要问问大家，你了解板块构造论吗？

来源:可可英语 http://www.kekenet.com/Article/201110/158662.shtml