Exfoliating granite is a granite undergoing exfoliation, or onion skin weathering (desquamation). The external delaminated layers of granite are gradually produced by the cyclic variations of temperature at the surface of the rock in a process also called spalling. Frost and ice expansion in the joints during the winter accelerate the alteration process while the most unstable loosen external layers are removed by gravity assisted by runoff water.
Homogeneous granitic plutons are created in high-pressure environments and slowly solidify beneath the Earth's crust. Vertical compression of overburden releases through erosion, or removal of overlying rocks resulting in unloading. [1] [2] Other contributors of unloading are tectonic uplift, glacier retreat, and mass wasting. [3] The pressure is relieved when the granite is exposed at the surface, allowing it to expand towards the atmosphere. [1]
On the surface, if the granite is not jointed, or if it has few joints, then the exposed surface usually expands faster than the underlying granite. The surface layer, often a couple hundred feet thick, separates from the underlying granite along an expansion joint to form a shell. As this continues, several concentric shells may form to depths of 100 feet (30 m) feet or more. [4] Concentric slabs/shells of rock begin to break loose, onion-like layers subparallel to the exterior called exfoliating, sheet jointing, or fractures. [1] [2] As the granite expands the outer, most shells become susceptible to weathering by water pressure, freeze/thaw cycles, and functioning vegetation is a process called physical weathering. [5] The sheets of granite are large enough to shave off sharp edges on the granite's surface creating a dome shape. The overall activity creates exfoliation domes. [3]
Exfoliating rock can trigger rockfall. [6] Rock Falls in Yosemite National Park are common and pose threats to visitors. United States Geological Survey (USGS) conducted a study over a three-year period monitoring granite cracks within the park's Valley. Data collected by Park Geologist Greg Stock, and USGS civil engineer Brian Collins using deformation and temperature gauges. They concluded that there is an outward expansion with the change in thermal temperature up to an inch. With prolonged movement, the cracks expand over time and create exfoliation. [7]
Twain Harte Lake Dam is in Tuolumne County located within the Sierra Nevada Mountain Range. Engineers began construction on the dam in 1928 and placed the structure between two granite domes. In August 2014, the granite developed exfoliating joints and began to leak. The process was captured on video and is one of a couple ever to be caught on film. The lake was completely drained and the cost of reconstruction nine-hundred thousand dollars. In June 2016, the granite rock had to be closed off for a second time because of the continued exfoliation. [8]
Grove Karl Gilbert is considered one of the most influential geologists of his generation. He was the only president of the Geographic Society to receive the highest honor for geographic accomplishment; Hubbard Medal. [9] Working his way down the Great Basin, Gilbert knew the general formation of mountains through a tectonic process, and are composed of different rock types. He observed that the mountain ranges in this area contained similarities in strata, resembling bubbles and blisters on the earth's surface. He concluded that the mountains were once intrusive magma that had and became visible with erosion. Gilbert presented his findings in a report and contributed significantly in what future generations would know as Geomorphology. [9]
Yosemite National Park is a national park in California. It is bordered on the southeast by Sierra National Forest and on the northwest by Stanislaus National Forest. The park is managed by the National Park Service and covers 759,620 acres in four counties – centered in Tuolumne and Mariposa, extending north and east to Mono and south to Madera. Designated a World Heritage Site in 1984, Yosemite is internationally recognized for its granite cliffs, waterfalls, clear streams, giant sequoia groves, lakes, mountains, meadows, glaciers, and biological diversity. Almost 95 percent of the park is designated wilderness. Yosemite is one of the largest and least fragmented habitat blocks in the Sierra Nevada.
A batholith is a large mass of intrusive igneous rock, larger than 100 km2 (40 sq mi) in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, such as granite, quartz monzonite, or diorite.
An inselberg or monadnock is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. In Southern Africa a similar formation of granite is known as a koppie, an Afrikaans word from the Dutch diminutive word kopje. If the inselberg is dome-shaped and formed from granite or gneiss, it can also be called a bornhardt, though not all bornhardts are inselbergs. An inselberg results when a body of rock resistant to erosion, such as granite, occurring within a body of softer rocks, is exposed by differential erosion and lowering of the surrounding landscape.
A monolith is a geological feature consisting of a single massive stone or rock, such as some mountains. Erosion usually exposes the geological formations, which are often made of very hard and solid igneous or metamorphic rock. Some monoliths are volcanic plugs, solidified lava filling the vent of an extinct volcano.
Spheroidal weathering is a form of chemical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock that is known as saprolite. When saprolite is exposed by physical erosion, these concentric layers peel (spall) off as concentric shells much like the layers of a peeled onion. Within saprolite, spheroidal weathering often creates rounded boulders, known as corestones or woolsack, of relatively unweathered rock. Spheroidal weathering is also called onion skin weathering,concentric weathering,spherical weathering, or woolsack weathering.
The exposed geology of the Yosemite area includes primarily granitic rocks with some older metamorphic rock. 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. The sediment that formed the area first settled in the waters of a shallow sea, and compressive forces from a subduction zone in the mid-Paleozoic fused the seabed rocks and sediments, appending them to the continent. Heat generated from the subduction created island arcs of volcanoes that were also thrust into the area of the park. In time, the igneous and sedimentary rocks of the area were later heavily metamorphosed.
The Wichita Mountains are located in the southwestern portion of the U.S. state of Oklahoma. It is the principal relief system in the Southern Oklahoma Aulacogen, being the result of a failed continental rift. The mountains are a northwest-southeast trending series of rocky promontories, many capped by 500 million-year old granite. These were exposed and rounded by weathering during the Pennsylvanian and Permian Periods. The eastern end of the mountains offers 1,000 feet (305 m) of topographic relief in a region otherwise dominated by gently rolling grasslands.
Moro Rock is a granite dome rock formation in Sequoia National Park, California, United States. It is located in the center of the park, at the head of Moro Creek, between Giant Forest and Crescent Meadow. A stairway, designed by the National Park Service and built in 1931, is cut into and poured onto the rock, so that visitors can hike to the top. The view from the rock encompasses much of the Park, including the Great Western Divide. Use of this trail is discouraged during thunderstorms and when it is snowing.
The Stawamus Chief, officially Stawamus Chief Mountain, is a granitic dome located adjacent to the town of Squamish, British Columbia, Canada. It towers over 700 m (2,297 ft) above the waters of nearby Howe Sound. It is one of the largest granite monoliths in the world.
The Regular Northwest Face of Half Dome was the first Grade VI big wall climbing route in the United States. It was first climbed in 1957 by a team consisting of Royal Robbins, Mike Sherrick, and Jerry Gallwas. Its current aid climbing rating is VI 5.9 A1 or 5.12 for the free climbing variation. It is recognized in the historic climbing text Fifty Classic Climbs of North America and considered a classic around the world.
Exfoliation joints or sheet joints are surface-parallel fracture systems in rock, often leading to the erosion of concentric slabs.
A joint is a break (fracture) of natural origin in a layer or body of rock that lacks visible or measurable movement parallel to the surface (plane) of the fracture. Although joints can occur singly, they most frequently appear as joint sets and systems. A joint set is a family of parallel, evenly spaced joints that can be identified through mapping and analysis of their orientations, spacing, and physical properties. A joint system consists of two or more intersecting joint sets.
Granite domes are domical hills composed of granite with bare rock exposed over most of the surface. Generally, domical features such as these are known as bornhardts. Bornhardts can form in any type of plutonic rock but are typically composed of granite and granitic gneiss. As granitic plutons cool kilometers below the Earth's surface, minerals in the rock crystallize under uniform confining pressure. Erosion brings the rock closer to Earth's surface and the pressure from above the rock decreases; as a result the rock fractures. These fractures are known as exfoliation joints, or sheet fractures, and form in onionlike patterns that are parallel to the land surface. These sheets of rock peel off the exposed surface and in certain conditions develop domical structures. Additional theories on the origin of granite domes involve scarp-retreat and tectonic uplift.
The Sierra Nevada Batholith is a large batholith that is approximately 400 miles long and 60-80 miles wide which forms the core of the Sierra Nevada mountain range in California, exposed at the surface as granite.
Little Yosemite Valley is a smaller glacial valley upstream in the Merced River drainage from the Yosemite Valley in Yosemite National Park. The Merced River meanders through the 3.5 mi (5.6 km) long flat valley, draining out over Nevada Fall and Vernal Fall before emptying into the main Yosemite Valley. It can be reached by a day hike from the main valley, and is the most popular area in the Yosemite Wilderness. The Valley provides access to nearby destinations such as the back side of Half Dome, Clouds Rest and the High Sierra Camp at Merced Lake.
Sentinel Rock is a granitic peak in Yosemite National Park, California, United States. It towers over Yosemite Valley, opposite Yosemite Falls. Sentinel Rock lies 0.7 miles (1.1 km) northwest of Sentinel Dome.
Yosemite National Park is located in the central Sierra Nevada of California. Three wilderness areas are adjacent to Yosemite: the Ansel Adams Wilderness to the southeast, the Hoover Wilderness to the northeast, and the Emigrant Wilderness to the north.
The Royal Arches is a cliff containing natural occurring granite exfoliation arches, located below North Dome and rising above Yosemite Valley, in Yosemite National Park, California.. These are not literal arches, but rather arch shaped indentations in the cliff face.
A panhole is a depression or basin eroded into flat or gently sloping cohesive rock. Similar terms for this feature are gnamma or rock holes (Australia), armchair hollows, weathering pans and solution pans.
Sentinel granodiorite is a type of granodiorite found in Yosemite National Park. It is a poorly understood western "outlier" of the ~93-85-Ma Tuolumne Intrusive Suite of the Sierra Nevada batholith. It is only slightly older than the undated Yosemite Creek Granodiorite and the Kuna Crest Granodiorite.