The area of Yosemite National Park was astride a passive continental margin (similar to the east coast of present-day North America) during the Precambrian and early Paleozoic. Starting in the mid-Paleozoic, a convergent plate boundary developed off the coast and transported seabed sediments into the area of the park in the pre-late Devonian possibly during the Antler orogony. Subduction led to the creation of an island arc of volcanoes on the west coast of Laurentia (proto-North America) between the late Devonian to Permian. These rocks were incorporated into the proto-North America by the middle Triassic, some of which found their way to the area of the park. Most of these volcanic and sedimentary rocks have since been heavily metamorphosed, uplifted and eroded away. The most prominent outcrops of this material can be found in the eastern part of the park.
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. Mount Dana is made out of these meta-volcanic rocks.
The first phase of regional plutonism started in the late Triassic 210 million years ago and continued throughout the Jurassic to about 150 million years before present. 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 below the surface.
The second, and as far as the park area is concerned, the major pluton emplacement phase lasted from about 120 million to 80 million years ago during the Cretaceous. All told there have been more than 50 plutons found in the park.
During much of the early Cenozoic, warm wet climates caused a great deal of chemical weathering of the granite and other rocks in the area of the park. The water especially took advantage of the numerous joint planes so that relatively joint-free features.
In the late Cenozoic, extensive volcanism occurred east of the park area. Within the Yosemite region, andesitic lava flows and lahars flowed north of the Grand Canyon of the Tuolumne River and volcanic dikes and plugs developed from faults on the flanks of Mount Dana.
There is evidence for a great deal of rhyolitic ash covering the northern part of the Yosemite region 30 million years ago. This, and later ash deposits, have been almost completely eroded away (especially during ice ages). Also starting about 30 million years ago, was a series of glaciations that further modified the area by accelerating mass wasting through ice-wedging, glacial plucking, scouring/abrasion and the release of pressure after the retreat of each glaciation. Severe glaciations formed very large glaciers that tended to strip and transport top soil and talus piles far down glacial valleys while less severe glaciations deposited a great deal of glacial till further up in the valleys.
In the late Cenozoic, tilting of the Sierra block and the resulting uplift of the Sierra Nevada increased the gradient of western-flowing streams. The streams consequently ran faster and thus cut their valley's more quickly (most notably in Yosemite Valley). Tributary streams, however, did not have their gradients increased, so their rate of valley cutting was not significantly affected. The result were hanging valleys and cascading waterfalls where the tributaries met the main streams. Additional uplift followed after major faults developed to the east and the Owens Valley was formed due to extensional forces associated with the ongoing creation of the Basin and Range Province.
This exposed the granitic rock to surface pressures and it responded by exfoliation (responsible for the rounded shape of the many domes in the park) and mass wasting following the numerous fracture joint planes (cracks; especially vertical ones) in the now solidified plutons.
Since the end of the Tioga ice age 10,000 years ago (see Wisconsinan glaciation), the so-called Little Ice Age led to the development of the Lyell and Maclure glaciers along with several smaller glaciers in the park.
Some domes in the park were covered by glaciers and were modified into roche moutonnées, which are characterized by having a smooth, rounded side and a steep face. The rounded side was where the glacier flowed over the dome and the steep side is where the glacier flowed away from it. The steepness is caused by glacial plucking of rock along fracture joints. Good examples in the park are Liberty Cap, Lembert Dome, and Mount Broderick (Half Dome was created by a different process).
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The origin of the geological landscapes of the park have been under debate since 1865. 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. John Muir proposed that Yosemite Valley and Hetch Hetchy Valley were formed purely by glacial action. In 1930, Francois Matthes proposed an hybrid hypothesis, where most of the depth of the valley was gouged by water erosion, the rest by glacial action. The glacial action also widened the valley.
Recently, the debate has been reopened by Jeffrey Schaffer, who
suggests that the role that glaciers and other erosion processes has been dramatically overstated. Schaffer states that Yosemite Valley above 5600 feet, for example, has changed relatively little in the past 30 million years and other than being slightly larger the major features would be recognizable to the modern eye. Schaffer believes that the numerous joint planes have had the greatest impact on the geomorphology of the Park's major features. This is contradiction to the previous consensus view that huge highly abrasive glaciers combined with a great deal of uplift over just the past couple million years was the primary shaping force of the features (such rapid uplift would have greatly accelerated all types of erosion).
Controversy
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