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Exfoliating slabs of granite, on Half Dome in Yosemite National Park, USA. 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.
In effect, chemical weathering changes the original set of minerals in the rock into a new set of minerals that is in closer equilibrium with surface conditions. True equilibrium is rarely reached, because weathering is a slow process, and leaching carries away solutes produced by weathering reactions before they can accumulate to equilibrium ...
Decomposed granite is a kind of granite rock that is weathered to the point that the parent material readily fractures into smaller pieces of weaker rock. Further weathering yields material that easily crumbles into mixtures of gravel-sized particles known as grus that further may break down to produce a mixture of clay and silica sand or silt ...
Spheroidal or woolsack weathering in granite on Haytor, Dartmoor, England Spheroidal weathering in granite, Estaca de Bares, A Coruña, Galicia, Spain Woolsack weathering in sandstone at the Externsteine rocks, Teutoburg Forest, Germany Corestones near Musina, South Africa that were created by spherodial weathering and exposed by the removal of surrounding saprolite by erosion.
One hypothesis is that granite domes are uplifted blocks. This is the case with some granite domes but the fracture related exfoliation joints are what controls the steep slopes. Another theory that regards isolated bornhardts is that they remain after long-distance scarp recession. Moisture related weathering is what causes scarp recession.
Case hardening is a weathering phenomenon of rock surface induration.It is observed commonly in: felsic alkaline rocks, such as nepheline syenite, phonolite and trachyte; pyroclastic rocks, as pyroclastic flow deposit, fine air-fall deposits and vent-filling pyroclastic deposits; sedimentary rocks, as sandstone and mudstone.
Chemical weathering of granite occurs when dilute carbonic acid, and other acids present in rain and soil waters, alter feldspar in a process called hydrolysis. [50] [51] As demonstrated in the following reaction, this causes potassium feldspar to form kaolinite, with potassium ions, bicarbonate, and silica in solution as byproducts.
Joints have a profound control on weathering and erosion of bedrock. As a result, they exert a strong control on how topography and morphology of landscapes develop. Understanding the local and regional distribution, physical character, and origin of joints is a significant part of understanding the geology and geomorphology of an area.