Thursday 28 September 2017

British tourist killed by rockfall on El Capitan, Yosemite National Park.

A British tourist has been killed and his female companion, also British, has been seriously injured, following a rockfall on El Capitan, a mountain in Yosemite National Park, California, on Wednesday 27 September 2017. The couple were initially thought to have been climbing the peak when the incident occurred, but are now known to have been walking beneath a cliff popular with climbers when a chunk of rock about 40 m high, 20 m wide and 3 m thick, and estimated to weigh about 1170 tonnes, fell from the rockface. The couple will not be named until relatives in the UK can be contacted. About 30 climbers were on the cliff-face when the incident occurred, though none of them are reported to have suffered any injuries.

Dust from a rockfall on El Capitan, Yosemite National Park, on 27 September 2017. Tom Evans/US National Park Service.

El Capitan is a granite outcrop reaching 2308 m above sea-level, and about 900 m above the valley bellow, in the Sierra Nevada Mountains. Mountains appear to be permanent structures to humans, but are actually dynamic active systems, being constantly uplifted (in the case of the Sierra Nevada by the subduction of the last chunks of the ancient Farallon Plate along the Pacific Coast of North America), and constantly worn down by erosion. The granite outcrops of the Sierra Nevada ate the remains of an ancient batholith, formed in the Cretaceous Period by a mantle plume that rose up beneath the area, but did not reach the surface to any great extent. This allowed the liquid magma of the plume to cool slowly, allowing the large mineral crystals that make up granite to form. This granite has been exposed at the surface by uplift and erosion over the last four million years. Because granite is made up of large crystals of different minerals, with different physical properties, it is particularly prone to erosion, as heating and cooling of the rock on a seasonal, or even daily, cycle, causes the minerals to expand at different rates, leading to the formation of cracks. These cracks can be further widened by water entering them and freezing, pushing them further open, and allowing more water to enter on the next cycle.

Granite from the Sierra Nevada, showing the large crystals that give it both its distinctive appearance and make it vulnerable to erosion. Roc Doc Travel.

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