Sierra Nevada (U.S.) - Geologic History

Geologic History

For central Sierra Nevada geology, see Geology of the Yosemite area.

The earliest granite of the Sierra started to form in the Triassic period. This granite is mostly found east of the crest and north of 37.2°N. In the Triassic and into the Jurassic, an island arc collided with the west coast of North America and raised a set of mountains, in an event called the Nevadan orogeny.

In the Cretaceous, a subduction zone formed at the edge of the continent. This means that an oceanic plate started to dive beneath the North American plate. Magma formed through the subduction of the ancient Farallon Plate rose in plumes (plutons) deep underground, their combined mass forming what is called the Sierra Nevada batholith. These plutons formed at various times, from 115 Ma to 87 Ma. The earlier plutons formed in the western half of the Sierra, while the later plutons formed in the eastern half of the Sierra. By 65 Ma, the proto-Sierra Nevada had been worn down to a range of rolling low mountains, a few thousand feet high.

Twenty million years ago, crustal extension associated with the Basin and Range Province caused extensive volcanism in the Sierra. About 10 Ma, the Sierra Nevada started to form: a block of crust between the Coast Range and the Basin and Range Province started to tilt to the west. Rivers started cutting deep canyons on both sides of the range. The Earth's climate cooled, and ice ages started about 2.5 Ma. Glaciers carved out characteristic U-shaped canyons throughout the Sierra. The combination of river and glacier erosion exposed the uppermost portions of the plutons emplaced millions of years before, leaving only a remnant of metamorphic rock on top of some Sierra peaks.

Uplift of the Sierra Nevada continues today, especially along its eastern side. This uplift causes large earthquakes, such as the Lone Pine earthquake of 1872.