To understand the evolution of the Sierra Nevada, one must have a grasp of the theory of plate tectonics, which states that the surface of the earth is composed of rigid plates between 5 and 100 kilometers thick. The plates, either oceanic or continental, float on a partially molten layer that allows them to move relative to one another at velocities of a few centimeters per year. Oceanic plates are much younger than continental plates and, in fact, are created continuously along the ridges that line the world's ocean basins. Magma (molten rock) from deep in the earth erupts at the mid-oceanic ridge and cools, forming new sea floor. To make room for the new material, old sea floor is removed from the ocean basins at trenches. Trenches are …show more content…
Ridges and subduction zones, therefore, define two types of plate boundaries: divergent (where plates move apart) and convergent (where plates come together), respectively.
The geology and morphology of the Sierra Nevada tell a story that spans 0.5 billion years—of which subduction dominated the last 200 million years. For the first few hundred million years, however, the area of the Sierra Nevada was quiet. Sediments were deposited in shallow water on the continental shelf of Nevada to the east and in deep water on the Pacific Ocean floor to the west. (The edge of the North American continent during this time was several hundred kilometers farther east.) Approximately 225 million years ago, the scene changed. The Pacific Ocean began to …show more content…
For the next 50 million years, erosion dominated. Streams stripped the volcanoes from the surface, exposing the batholith and metamorphic rocks that lay below. The removed material was deposited as soil and gravel in the rolling hills between the Sierra Nevada and the Pacific Ocean. Eventually, the landscape was eroded to a uniform level. Subduction below California stopped completely approximately 25 million years ago. The rise of the modern Sierra Nevada began 20 million years ago. The continental crust between the Sierra Nevada and Utah stretched and broke apart, producing normal faults oriented in an almost northerly direction. This stretched province of narrow mountains and wide valleys is the Basin and Range Province. The westernmost of the normal faults in the Basin and Range Province defines the eastern edge of the Sierra Nevada. Rocks west of the Sierra fault moved up to create the modern mountains, while those to the east moved down to form a valley. Eruption of lava and ash accompanied the stretching of the continental crust as spaces formed to allow the passage of molten rock from deep in the earth. Lavas frequently flowed down the old stream channels. New streams circumvented the lavas, leaving them as high grounds in the landscape. Finally, glaciers of the Wisconsin glacial epoch (60,000 to 90,000 years ago) carved