Monday, August 9, 2010

Geology field notes: granite exfoliation

The "Geology field notes" is intended to be be a series of short posts on observations, measurements or geologic features I was intrigued by, awed or excited enough to take a picture of while in the field.

I returned recently from a trip to the Sierra Nevada and I was lucky to be able to spend some time in the Yosemite area.  What a paradise for geologists and outdoor enthusiasts alike!

Exfoliation features, or exfoliation joints, are commonly seen in Yosemite.  There is still debate about their genesis, but is it believed that they occur when concentric shells or plates of a rock break from its surface.  Exfoliation is common in intrusive igneous rocks such as granite, formed under great pressure and temperatures. When granitic rocks are exposed to the surface, the pressure is released and the granite expands slightly, resulting in exfoliation.

There are many examples of exfoliation feature in Yosemite; I will show here only a couple, the Royal Arches and the North Dome, both visible from the Glacier Point.
© RomaniaRocks

The Royal Arches (labeled RA in the figure above) formed during the Pleistocene glaciation, when glaciers peeled away the outer exfoliation shells along the side of an exfoliation dome, the North Dome (labeled ND in the picture above).  The smooth and broad arches were possible to form primarily because of the strong and homogenous fabric of the rock.  The more geologically inclined folks would be excited to know that the rock that made the arches possible is a coarse-grained granodiorite of Cretaceous age (87 my old),  with well-formed plates of biotite and long rods of black hornblende, an especially good rock to provide the geologist with exfoliation features.  Also in the picture above (and below) is the Washington Column (WaC), separated from the Royal Arches by a steep gully, which is a major joint in the granodiorite.
© RomaniaRocks

The outer arch of the Royal Arches spans a distance of ~550m (1800 feet), is about 60m (200 feet) thick and 300m (1000 feet) high.
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1 comment:

  1. Exfoliation is common in intrusive igneous rocks such as granite, formed under great pressure and temperatures.Marble Countertop


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