From:
"John Vancamp" <jvcstnwrks@zzzzzzzzzzzzz>
Date:
Fri, 16 Dec 2005 15:18:19 -0600
Subject:
planes in granite
John, I don't think this is exfoliation, because these layers are very
thick. In the photo in front of me I can pick out planes that are 2 to
20 ft thick and extend at least 60 ft down from the original surface of
the dome. And I know there are more below the bottom edge of the
picture. I have a human on top for scale.
Norman, a short google search on exfoliation brought up this---from a usgs
open file report 99-385.
The sheet or exfoliation joint set J1 closely parallels the cliff face.
Sheet structures typically form in environments of high differential
stress, particularly upon vertical unloading of a rock mass that formed at
depth under high triaxial compression and is now exposed at the surface due
to uplift and erosion. Individual exfoliation sheets thicken perpendicular
to the topographic surface of the rock mass and sheets tend to be thinner
in fine-grained rocks than in coarse-grained rocks (Holzhausen, 1989).
Gilbert (1904) noted that sheet structure in the domes of the Sierra Nevada
tends to parallel all topographic surfaces and that the separation of
sheets penetrated to depths between 50 and 100 ft perpendicular to the
surface. At this rock-fall release the thickness of the most exfoliation
sheets visible on the cliff surface, probably ranges only from 3 to 6 ft
thick.
The same search also brought up a link to a great picture of Enchanted
Rock-- the exfoliation dome I am familiar with.
JVC
- Follow-ups
- message 00510: planes in granite - Norman Watts (19 Dec 2005)
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