{{Short description|Granite spalling in superficial layers due to temperature cycles}} {{use dmy dates|date=April 2025}}
[[File:Half Dome Trek 15 (4245933444).jpg|thumb|350px|right|Exfoliating slabs of granite, on [[Half Dome]] in [[Yosemite National Park]], [[United States|USA]]]] '''Exfoliating granite''' is a [[granite]] undergoing [[Exfoliation joint|exfoliation]], or onion skin weathering ([[desquamation]]). The external [[delamination|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]]ing. [[Frost]] and [[ice]] expansion in the joints during the winter accelerate the alteration process while the most unstable loose external layers are removed by [[gravity]] assisted by [[surface runoff]] of water.
== Geology == Homogeneous granitic [[pluton]]s are created in high-pressure environments and slowly solidify beneath the [[Earth's crust]]. Vertical compression of [[Overburden pressure|overburden]] releases through [[erosion]], or removal of overlying rocks resulting in [[Weathering|unloading]].{{r|petersen}}<ref name="Lutgens">{{cite book |last1=Lutgens |first1=Frederick K. |last2=Tarbuck |first2=Edward J. |title=Essentials of Geology |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, New Jersey, US |isbn=0131497499 |pages=118–119 |edition=9th}}</ref> Other contributors of unloading are [[tectonic uplift]], [[Glacial motion|glacier retreat]], and [[mass wasting]].{{r|mcgivney}} The pressure is relieved when the granite is exposed at the surface, allowing it to expand towards the atmosphere.<ref name="petersen">{{cite book |last1=Petersen |first1=James F. |last2=Sack |first2=Dorthy |last3=Gabler |first3=Robert E. |title=Fundamentals of Physical Geography |date=2011 |publisher=Brooks – Cole |location=Belmont, California, US |isbn=978-0-538-73463-9 |pages=292–293}}</ref>
[[File:GeologicalExfoliationOfGraniteRock.jpg|thumb|upright=1.35|Exfoliating granite, at [[Enchanted Rock]] State Natural Area, Texas]]
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 {{convert|200|ft|m|round=5|disp=out}} or so) 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 {{convert|100|ft|m}} or more.<ref name="konigsmark">{{cite book |last1=Konigsmark |first1=Ted |title=Geologic Trips, Sierra Nevada |date=2003 |publisher=Bored Feet Press; Geopress |isbn=0-9661316-5-7 |page=[https://www.geologictrips.com/sn/snttyv.pdf 232] |id=[Excerpt]}}</ref> Concentric shells or layered slabs of rock begin to break loose, onion-like layers subparallel to the exterior called [[exfoliation joint]]s, sheet jointing, or [[Fracture (geology)|fractures]].{{r|petersen|Lutgens}} As the granite expands the outermost shells may be further widened by processes of physical [[weathering]]: water pressure, freeze—thaw cycles, and the effects of vegetation.<ref>{{cite web |last1=Schutter |first1= Paul de |title=Exfoliation Domes |url=http://www.ougseurope.org/index.php?id=49 |website=OUGS Mainland Europe |publisher=The Open University Geological Society |archive-url=https://web.archive.org/web/20220521224137/http://www.ougseurope.org/index.php?id=49 |archive-date=21 May 2022 |date=2003}}</ref> 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.<ref name="mcgivney">{{cite web |last1=McGivney |first1=Annette |title=Forming Yosemite's Granite Domes |url=https://www.myyosemitepark.com/things-to-do/natural-wonders/mountains-landscapes/granite-domes/?scope=anon |website=Yosemite National Park Trips |date=4 July 2019}}</ref>
Chemical [[weathering]] occurs in granite exfoliation by changing the mineral composition.{{r|blatt|pp=245–246}} Mechanical weathering is the breakdown of rocks into smaller fragments or pieces.<ref name="blatt">{{cite book |last1=Blatt |first1=Harvey |last2=Middleton |first2=Gerard V. |last3=Murray |first3=Raymond C. |title=Origin of Sedimentary Rocks |date=1980 |publisher=Prentice-Hall |location=Englewood Cliffs, New Jersey, US |isbn=0136427103 |edition=2nd}}</ref>{{rp|247–250}}
== Dangers == === Rock falls === Exfoliating [[Rock (geology)|rock]] can trigger [[rockfall]].<ref>{{cite journal |last1=Collins |first1=Brian D. |last2=Stock |first2=Greg M. |title=Rockfall triggering by cyclic thermal stressing of exfoliation fractures |journal=Nature Geoscience |date=May 2016 |volume=9 |issue=5 |pages=395–400 |doi=10.1038/ngeo2686 |url=https://www.nature.com/articles/ngeo2686 |url-access=subscription |language=en |issn=1752-0908 |quote=From a thermodynamic analysis, we find that daily, seasonal and annual temperature variations are sufficient to drive cyclic and cumulative opening of fractures. }}</ref> Rock Falls in [[Yosemite National Park]] are common and pose a threat to visitors. [[United States Geological Survey]] (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.<ref>{{cite web |last1=Gordon |first1=Leslie C. |title=Hot Days Can Trigger Yosemite Rockfalls |url=https://www.usgs.gov/news/national-news-release/hot-days-can-trigger-yosemite-rockfalls |website=United States Geological Survey |language=en |date=March 28, 2016}}</ref>
=== Infrastructure failure === [[Twain Harte Dam]] is in [[Tuolumne County, California]], within the [[Sierra Nevada (U.S.)|Sierra Nevada]] 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 {{Currency|900 thousand|USD}}. 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.<ref>{{cite web |last=McCaffrey |first=Tim |title=History of the Twain Harte Lake |url=https://www.thlake.org/history |website=Twain Harte Lake |publisher=Twain Harte Lake Association |language=en |date=2016}}</ref>
== See also == === Examples === * [[Cannon Mountain (New Hampshire)]] in the [[White Mountains (New Hampshire)|White Mountains]] * [[Enchanted Rock]], Texas * [[Geography of the Yosemite area]] ** [[Half Dome]], Yosemite ** [[Royal Arches]] in [[Yosemite National Park]]
=== Processes === * {{annotated link|Discontinuity (geotechnical engineering)}} * {{annotated link|Erosion surface}} * {{annotated link|Foliation (geology)}} * {{annotated link|Spheroidal weathering}}
==References== {{Reflist|25em}}
== Further reading == {{refbegin}} * {{cite AV media |last1=Adelson |first1=Mitch |last2= Seelig |first2=Willliam|title=Rockfalls in California's Sierra Nevada |url=https://www.usgs.gov/media/videos/pubtalk-102016-rockfalls-californias-sierra-nevada |publisher=U.S. Geological Survey |language=en |date=27 October 2016|id=[PubTalk 10/2016] |type=Lecture |ref=none}} * {{cite journal |last1=Collins |first1=Brian D. |last2=Stock |first2=Greg M. |last3=Eppes |first3=Martha-Cary |last4=Lewis |first4=Scott W. |last5=Corbett |first5=Skye C. |last6=Smith |first6=Joel B. |title=Thermal influences on spontaneous rock dome exfoliation |journal=Nature Communications |date=22 February 2018 |volume=9 |issue=1 |doi=10.1038/s41467-017-02728-1 |ref=none|pmc=5823905 }}
== External links == * [https://www.youtube.com/watch?v=nL8ZwPOPMcc Exfoliation: Onion Skin Weathering – diagram and explanation] [video] by [https://www.thegeographeronline.net/ Geographer Online] – via YouTube. * [https://www.youtube.com/watch?v=wJUHq6nX1iE Granite dome exfoliates, damaging the dam at Twain Harte Lake] [video] uploaded by CondorEarth – via YouTube.
=== Images === * [https://nla.gov.au/nla.obj-149049410/view "Exfoliating granite, Mount Kinabalu, Sabah, Borneo. 1985"]. Image by Dombrovskis, Peter. [Transparency] – via [[Trove]]. * [https://diva.sfsu.edu/collections/pestrong/bundles/218786 "Exfoliating Granite: Sierras"]. ''DIVA@Academic Technology''. San Francisco State University * [http://www.freeimages.com/premium/exfoliating-granite-1624937 A large pile of exfoliating granite on Shuteye Ridge just south of Yosemite National Park in the Sierra national forest.] (Stock image) {{refend}}
[[Category:Weathering]] [[Category:Geological processes]] [[Category:Granite]]