Historical State of the Sierra Nevada 1

How the Sierra was Formed:

The historical state of the Sierra Nevada is fairly young as far as mountain ranges go. "The major geologic events that created the Sierra Nevada began a little over 200 million years ago (MYA) when the granitic rocks that make up most of the Sierra Nevada began to be emplaced into the crust from below." (Michaelson, UCSB) Granite is an intrusive igneous rock, meaning that it is composed of formerly molten material that cooled below the surface. "The Sierra granitic rocks were formed as a result of the subduction of a denser oceanic crustal plate beneath the western margin of the North American plate." (Michaelson, UCSB)

The reason the Sierra formed where it did was because the location of the subduction zone was on the western edge of the Sierra. "As the oceanic crust was pushed farther beneath the surface it began to melt some distance east of the subduction zone." (Michaelson, UCSB) This is why there is a valley where the central valley is, and a mountain range just east of it. "Plumes of this molten material rose toward the surface through the overlying North American plate, melting and mixing with some of the continental crust. The granitic rocks that are now exposed in the Sierra Nevada cooled around 10km-20km beneath the surface and were most likely the roots of large volcanoes that were significantly higher than the modern Sierra Nevada (probably similar to the Andes Range). The oldest plumes are about 210 MYA, but the bulk are between 120 MYA and 80 MYA, with age generally decreasing moving eastward through the main portion of the Sierra Nevada (because the oceanic plate was moving eastward, so were the plumes of melting material which then rose to create the Sierra)" (Michaelson, UCSB). 

(http://www.geog.ucsb.edu/~joel/g148_f09/readings/sierra_nevada/sierra_nevada.html)