The Sierra Nevada

Another Lens on Life blog from Chuck Kopczak, PhD

11/2/2020
Mountains of rock, bare of almost anything but patches of snow, jut up into a blue sky
Chuck Kopczak, PhD

Early morning light on the eastern face of the Sierra Nevada range in California

Today’s photo was taken in the Alabama Hills near the town of Lone Pine in California’s Owens Valley. It was taken with a Canon EF28-135 mm f/3.5-5.6 IS USM lens set to a focal length of 85 mm on a Canon EOS 5D Mk. III. The exposure was set at 1/8 sec. at f/11 and ISO 100.

Growing up in Los Angeles, California, it wasn’t unusual to encounter people who had moved to California from another part of the United States, or other places in the world. Just as California’s population is notable for being made up a people from many different places, California’s geological past is notable for very similar reasons.

Prior to about 400 million years ago, the western edge of the continent that was to become the North America that we know today lay somewhere along a line roughly tracing the western borders of Montana and Wyoming, cutting down the middle of Utah, and almost following the western border of Arizona. California as a landmass simply didn’t exist. But between then and 65 million years ago, at least three large island arcs, created by tectonic activity, welded themselves to the western edge of North America. While this didn’t complete the assembly of California, it is illustrative of the processes that were involved to create the place we know today.

It was during this period that the heart of what was to become the present-day Sierra Nevada range began to form. Almost 200 million years ago, as the ancient oceanic Farallon plate began to dive under the edge of the North American plate, the friction created enough heat to melt the rock, and turn it into magma that began to rise up through the overlying North American plate like hot air balloons rise up through the atmosphere. As the great balloons of magma cooled, they became enormous globs of granite below the surface. Such aggregations of subsurface granite produced by intrusion of magma from below is called a batholith.

This process happened at least twice more, once 140 million years ago, and again 80 million years ago, as subduction along the western edge of the North America plate continued. The mass of magma created by these three events covers an area of about 25,000 square miles (65,000 square kilometers; the size of West Virginia), and is six miles (9.6 kilometers) thick, for a total volume of something like 150,000 cubic miles (625,230 cubic kilometers). It is known as the Sierra batholith.

Between erosion of the older, overlying material, and less than 20 million years ago, the stretching and breaking of the Earth’s crust to the east, many parts of the surface of the Sierra batholith were exposed. Then, about five million years ago, the eastern edge of the batholith began to rise, while the western edge remained anchored beneath the middle of what is now the San Joaquin Valley. The exact cause of this uplift is still not understood, but the rather abrupt elevation of the eastern headwall of the Sierra Nevada range is clear evidence of its occurrence. The Sierra Nevada lie like a huge tilted block of granite embedded in the Earth’s crust.

Today’s photo shows some of that eastern headwall, rising abruptly from the rather flat expanse of the Owens Valley. The tall peak on the left side of the frame is Lone Pine Peak, rising some 12,949 feet (3,947 m) above sea level. While it appears to be the tallest peak in the image, it is actually some 1500 feet (457 m) shorter than Mount Whitney, the highest prominence on the right side of the frame at 14,505 feet (4,421 m), the tallest peak in the continental United States. In the foreground are the eroded Alabama Hills. Basically, all of the rock that you can see in this photo is part of the massive Sierra batholith, which geologists estimate must weigh about a quadrillion tons, which is a thousand trillion tons. That’s a lot of granite.

Sources

McPhee, J. (1993). Assembling California. New York: Farrar, Straus and Giroux.

http://www.ucmp.berkeley.edu/science/profiles/erwin_0609geology.php

https://pubs.geoscienceworld.org/gsa/geosphere/article/8/3/592/132515/magmatic-growth-and-batholithic-root-development