Yosemite in Depth: Geology

Rocks are made of minerals. Minerals can break down and form new minerals in different settings. There are three types of rocks: Igneous, sedimentary, and metamorphic. Igneous rocks are made when molten magma solidifies. Sedimentary rocks form when rock debris becomes cemented. Metamorphic rocks form when rocks begin to melt again and change composition.

Overview: Formation of the Sierra Nevada Range

Granitic rocks are a type of igneous rock. Igneous rock is formed by the cooling and solidification of molten material (magma). There are two types of igneous rock: volcanic and plutonic. Yosemite was formed from plutonic rock that cooled deep beneath the surface of the Earth. The bedrock of much of the Sierra Nevada range is part of a vast field of rock that extends downward many miles.

This field of rock is called a batholith (from the Greek word bathos, meaning deep, and lithos, meaning rock). The batholith is not one uniform mass of granite. It is made up of a group of individual masses of rock called plutons (named for Pluto, god of the underworld). Plutons can vary in size from 1.6 kilometers (1 mile) to many kilometers in extent. Some plutons are next to each other. Other plutons are separated by areas of metamorphic rock. The Sierra Nevada batholith is a collection of granite plutons.

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The plutons formed during separate episodes of magma intrusion. Molten rock, lighter than the older host rock surrounding it, rose slowly and intruded into these host rocks. As the magma came within a few kilometers of the surface, it began to cool. After a long time the magma cooled completely. The minerals in the magma crystallized and solidified to form the igneous granitic rock of the Sierra Nevada.