Shear in Rocks
Name: Toby D.
Date: December 12, 2002
I would like to know specifically what "general
shear" is when mentioned with respect to strain in rocks?
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
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.
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Update: June 2012