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There was a "first stage" of geologic evolution that began the process of forming the Sierra Nevada mountains about 100 million years ago. But it took a lot more time for them to really develop and take the shape we see them in today. Some deep formations evolved in the beginning. Note that the Pacific plate and the North American plate form a boundary near this location, and as the Pacific plate subducts (goes beneath) the North American plate, there is tremendous heating of Pacific plate material below the western edge of the North American plate. The expansion of this material pushed up on the region, and set a "base" for the formation of this range. About 20 million years ago, there was a lot of vulcanism that set the stage for mountain forming. Then some 5 to 4 million years ago, there was more uplift, and also the "cutting away" of the sides of the string of mountains. There is a long history associated with the formation of these magnificent mountains, and by using the links below, you can begin to discover the story of their formation.

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Q: How old are the Sierra Nevada mountains?
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