Geology of the Yosemite area
The first rocks were laid down in Precambrian times, when the area around Yosemite National Park was on the edge of a very young North American continent.
Most of the rock now exposed in the park is granitic, having been formed 210 to 80 million years ago as igneous diapirs 6 miles (10 km) below the surface.
The area of the park was astride a passive continental margin (similar to the east coast of present-day United States) during the Precambrian and early Paleozoic.
Starting in the mid-Paleozoic and lasting into the early Mesozoic, a convergent plate boundary transported many of those seabed sediments into the area of the park (possibly during the Antler orogeny).
Heat generated from the subduction led to the creation of an island arc of volcanoes on the west coast of Laurentia (proto-North America) between the late Devonian and Permian periods.
Later volcanism in the Jurassic intruded and covered these rocks in what may have been magmatic activity associated with the early stages of the creation of the Sierra Nevada Batholith.
The resulting Nevadan mountain range (also called the Ancestral Sierra Nevada) was 15,000 feet (4500 m) high and was made of sections of seafloor and mélange.
This was directly part of the creation of the Sierra Nevada Batholith, and the resulting rocks were mostly granitic in composition and emplaced about 6 miles (10 km) below the surface.
A few miles (several km) of material was eroded away, leaving the Nevadan mountains as a long series of hills a few hundred feet (tens of meters) high by 25 million years ago.
There is also evidence for a great deal of rhyolitic ash covering the northern part of the Yosemite region 30 million years ago.
Volcanic activity persisted past 5 million years BP east of the current park borders in the Mono Lake and Long Valley areas.
Additional uplift occurred when major faults developed to the east, especially the creation of Owens Valley from Basin and Range-associated extensional forces.
(Example of a graben valley is Death Valley in California) The uplifting and increased erosion exposed granitic rocks in the area to surface pressures, resulting in exfoliation (responsible for the rounded shape of the many granite domes in the park) and mass wasting following the numerous fracture joint planes (cracks; especially vertical ones) in the now solidified plutons.
Most of these long, linear and very deep cracks trend northeast or northwest and form parallel, often regularly spaced sets.
Neoglaciation in the range culminated during the "Little Ice Age," a term originally coined by François E. Matthes in the Sierra Nevada, but now widely accepted as referring to a period of global glacial expansion between about AD 1250 to 1900.
The longest glacier in the Yosemite area ran down the Grand Canyon of the Tuolumne River for 60 miles (95 km), passing well beyond Hetch Hetchy Valley.
Some domes in the park were covered by glaciers and modified into roche moutonnées, which are characterized by having a smooth, rounded side and a steep face.
At that time, Josiah Whitney, then chief geologist of California, proposed that Yosemite Valley is a graben: a downdropped block of land surrounded by faults.
In 1930, François E. Matthes proposed a hybrid hypothesis, where most of the depth of the valley was gouged by water erosion, the rest by glacial action.
More recently, the debate has been reopened by Jeffrey Schaffer, who suggests that the role of glaciers and other erosion processes has been dramatically overstated.
