Geology Of The Sierra Nevada
by Mary Ann Resendes
It may be hard to imagine, but from 400 million to about 130 million years ago an ocean covered the area we know as the Sierra Nevada. Beneath the seafloor, geological processes were at work that would lead to the formation of the Sierra Nevada mountain range. Plate tectonics, the movement of the plates that form the earth’s crust played an important role in the formation of the Sierra Nevada, and is also responsible for the volcanic and seismic activity we experience today. Picture the earth’s crust as being made up of puzzle pieces that glide over a layer of molten magma. As two pieces of the puzzle, the North American and Pacific plates, converge, the Pacific Plate drops under the North American Plate in a process called subduction. During the late Paleozoic Era, approximately 250 million years ago, the pressure and friction that resulted from the grinding of the plates as they moved past each other caused the crust of the Pacific plate to melt, forming plumes of liquid plutonic rock that eventually floated up toward the surface.
These plutons came together to form the single, massive batholith, or deeply imbedded rock, that is the Sierra Nevada. As the batholith began to rise, about 80 million years ago, the layer of marine sedimentary rock that lay over the mountain was gradually eroded away and deposited in the valley. However, remnants of the marine rock, called roof pendants still cling to mountaintops. Because the uplift was greatest on the eastern side of the batholith, the mountain range tilts toward the west, creating a gradual western slope and a precipitous incline on the eastern side.
Table Mountains are an interesting feature of the Sierra Nevada foothills; these flat-topped mountains can be seen from Highway 168 as you drive toward Shaver Lake. Table Mountains are ribbons of solidified lava that once flowed from a volcanic eruption. The lava flowed easily down riverbeds as it became more diluted with water. Today, sections of these winding lava rivers continue to rise up from the foothills as the surrounding landscape gradually erodes away.
Once the mountains were formed, glaciers shaped the landscape. During the Pleistocene, there were many glacial periods in the Sierra Nevada. Glaciers are massive bodies of snow and ice that occur when snow accumulates more than it melts. As the snow begins to accumulate over the years, the crystals become compacted into ice. Rising temperatures in the day followed by freezing temperatures at night cause the crystals to refreeze and bond, forming a single mass of ice. The weight of the glacier causes it to flow downhill, often through canyons and stream channels, picking up sediment, rocks and boulders as it moves. The rock and boulders scrape along the bedrock, carving V-shaped stream valleys and canyon into U-shaped glacial valleys. Yosemite Valley was sculpted by several glacial events, one of which covered most of Half Dome. Glaciers form lake basins called cirques. These basins are formed when the glacier pulls rocks from an area below the mountaintop, near the beginning of the glacier, as it flows downward, forming a circular basin. Two glaciers moving down opposites sides of a mountaintop can erode away a sharp ridge called an arête, and three or more glaciers will form a horn. Glaciers can also transport large boulders, called erratics, and leave them stranded, sometimes precariously, at locations far from their origin. At the other end of the glacier, called the terminus, rocks and sediment that have been transported by the glacier are deposited in ridges called end moraines. As the glacier retreats it can leave a series of ridges, recessional moraines, or a shallow distribution of rocks and sediments called ground moraines. The moraines are a good means of identifying glacial activity because the rocks, which have been moved a great distance, are usually of different origin than the surrounding bedrock.
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