Exfoliating granite is a

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

undergoing exfoliation, or onion skin weathering (

desquamation Desquamation, or peeling skin, is the shedding of dead cells from the outermost layer of skin. The term is . Physiologic desquamation Keratinocytes are the predominant cells of the epidermis, the outermost layer of the skin. Living keratin ...

). 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

spall Spall are fragments of a material that are broken off a larger solid body. It can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ba ...

ing.

Frost Frost is a thin layer of ice on a solid surface, which forms from water vapor that deposits onto a freezing surface. Frost forms when the air contains more water vapor than it can normally hold at a specific temperature. The process is simila ...

and

ice Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 ° C, 32 ° F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally oc ...

expansion in the joints during the winter accelerate the alteration process while the most unstable loose external layers are removed by

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

assisted by

surface runoff Surface runoff (also known as overland flow or terrestrial runoff) is the unconfined flow of water over the ground surface, in contrast to ''channel runoff'' (or ''stream flow''). It occurs when excess rainwater, stormwater, meltwater, or other ...

of water.

Geology

Homogeneous granitic

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

s are created in high-pressure environments and slowly solidify beneath the

Earth's crust Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...

. Vertical compression of

overburden In mining, overburden (also called waste or spoil) is the material that lies above an area that lends itself to economical exploitation, such as the rock, soil, and ecosystem that lies above a coal seam or ore body. Overburden is distinct from tai ...

releases through

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

, or removal of overlying rocks resulting in unloading. Other contributors of unloading are

tectonic uplift Tectonic uplift is the orogeny, geologic uplift of Earth#Surface, Earth's surface that is attributed to plate tectonics. While Isostasy, isostatic response is important, an increase in the mean elevation of a region can only occur in response to ...

, glacier retreat, and

mass wasting Mass wasting, also known as mass movement, is a general term for the movement of rock (geology), rock or soil down slopes under the force of gravity. It differs from other processes of erosion in that the debris transported by mass wasting is no ...

. The pressure is relieved when the granite is exposed at the surface, allowing it to expand towards the atmosphere. GeologicalExfoliationOfGraniteRock

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 (about or so) thick, separates from the underlying granite along an

expansion joint A expansion joint, or movement joint, is an assembly designed to hold parts together while safely absorbing temperature-induced expansion and contraction of building materials. They are commonly found between sections of buildings, bridges, s ...

to form a shell. As this continues, several concentric shells may form to depths of or more. Concentric shells or layered slabs of rock begin to break loose, onion-like layers subparallel to the exterior called exfoliation joints, sheet jointing, or

fractures Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...

. As the granite expands the outermost shells may be further widened by processes of physical

weathering Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...

: water pressure, freeze—thaw cycles, and the effects of vegetation. 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. Chemical

occurs in granite exfoliation by changing the mineral composition. Mechanical weathering is the breakdown of rocks into smaller fragments or pieces.

Dangers

Rock falls

Exfoliating rock can trigger rockfall. Rock Falls in

Yosemite National Park Yosemite National Park ( ) is a List of national parks of the United States, national park of the United States in California. It is bordered on the southeast by Sierra National Forest and on the northwest by Stanislaus National Forest. The p ...

are common and pose a threat to visitors.

United States Geological Survey The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on Mar ...

(USGS) conducted a study over a three-year period, monitoring granite cracks within the park's Valley. Data was 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 of up to an inch, with a change in thermal temperature. With prolonged movement, the cracks expand over time and create exfoliation.

Infrastructure failure

Twain Harte Dam is in

Tuolumne County, California Tuolumne County (), officially the County of Tuolumne, is a County (United States), county located in the U.S. state of California. As of the 2020 United States census, 2020 census, the population was 55,620. The county seat and only incorpora ...

, within the

Sierra Nevada The Sierra Nevada ( ) is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primari ...

mountain range. The structure, completed in 1928, is between two granite domes. In August 2014, the granite developed exfoliating joints and began to leak. The process was captured on video, one of only a few ever to be caught on film. The lake was completely drained; the cost of reconstruction works was . In June 2016, the granite dome known as "the Rock" was closed off for a second time as a safety precaution because of continued exfoliation.

References

External links

Exfoliation: Onion Skin Weathering – diagram and explanation

ideo IDEO () is a design firm, design and consulting firm with offices in the U.S., England, and China. It was founded in Palo Alto, California, in 1991. The company's 500 staff uses a design thinking approach to design products, services, environmen ...

b
Geographer Online
– via YouTube.
Granite dome exfoliates, damaging the dam at Twain Harte Lake

uploaded by CondorEarth – via YouTube.

Images

"Exfoliating granite, Mount Kinabalu, Sabah, Borneo. 1985"
Image by Dombrovskis, Peter. ransparency– via

Trove Trove is an Australian online library database owned by the National Library of Australia in which it holds partnerships with source providers National and State Libraries Australia, an aggregator and service which includes full text documen ...

.
"Exfoliating Granite: Sierras"
''DIVA@Academic Technology''. San Francisco State University
A large pile of exfoliating granite on Shuteye Ridge just south of Yosemite National Park in the Sierra national forest.
(Stock image) {{refend Weathering Geological processes Granite

Geology

Dangers

Rock falls

Infrastructure failure

See also

Examples

Processes

References

Further reading

External links

Images