Yosemite National Park spans a deep, glacially carved valley in the Sierra Nevada Mountains of California. The Sierra Nevadas are underlain by a vast batholith of granite and diorite. This batholith comprises part of a chain of batholiths that runs along the west coast of North America from southern Mexico to northwestern Canada. Rocks from these batholiths formed beneath a continental volcanic arc that erupted between 145 and 80 million years ago. Effectively, a batholith represents the roots of a volcanic chain—it consists of the magma that froze deep underground before it had a chance to erupt at the surface.

The volcanic mountains that once lay above the Sierra Nevada Batholith probably looked much like the volcanoes that are now found in the Andes Mountains of South America. Rivers and glaciers have stripped away the several kilometers of rocks that once covered the granite batholith, so that granite now crops out at the surface. During the last ice age, around 14,000 years ago, huge glaciers cut deep valleys in the Sierra Nevada; one of these is the valley that forms the heart of Yosemite National Park. Rock climbers delight in the durability of the rock that makes up the park’s cliffs. Granite, with its interlocking fabric, is a very hard rock that doesn’t crumble or break easily, so it serves well as a firm anchor for the exotic equipment that climbers use to anchor their safety ropes.

Abundant geologic evidence suggests that this continental volcanic arc formed when an oceanic plate, called the Farallon Plate, subducted beneath North America. Most of the Farallon Plate is now gone (it was consumed by subduction), but a small remnant still lies off the coast of Washington and Oregon. This fragment is known as the Juan de Fuca Plate, and its continued subduction yields the Cascade volcanic chain, including scenic wonders like Mt. Rainier and Mt. Baker. The infamous Mt. St. Helens, which exploded catastrophically during an eruption in 1980, also lies in this chain.

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