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Name: Toby D.
Status: Student
Age: 30s
Location: N/A
Country: N/A
Date: December 12, 2002


Question:
I would like to know specifically what "general shear" is when mentioned with respect to strain in rocks?



Replies:
This is a difficult concept to explain in this type of forum. I suggest that you review available textbooks in structural geology to get a better explanation. However, here is a shot at it.

General shear is a combination of two theoretical types of strain, pure shear and simple shear. In the case of pure shear, the material does not rotate, but is sort of "squished", to use a non-technical term in response to an applied stress. That is, the material uniformly elongates in one direction and uniformly shortens in the perpendicular direction.

Simple shear, on the other hand, involves a rotational strain in which in the material moves parallel to a given axis. (Think of a deck of cards being fanned.) In simple shear, the axis perpendicular to the shear plane does not shorten. (The deck of cards does not get thinner when it is fanned.)

In actual materials, neither of the two theoretical shear types are encountered in isolation. Instead, most shear deformation is some combination of the two. Consequently, geologists use the term general shear to describe the combined case.

Andy Johnson



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