We're going to talk for a minute about
stress and strain. Now, we use those words very casually in our everyday speech, but
they have very specific meanings when it comes to science and the mechanics of the behavior of
materials. So, tectonic forces cause deformation, and stress is a force. Remember from your physics
class that a force has magnitude and direction. So, when you talk about stress, you need to talk
about the kind of stress, the direction of stress, and its magnitude. Geologically speaking, there
really are three that we're concerned with: there is compression, which is a
squeezing stress; there's tension, which is a stressing stretching or pulling apart,
and there's shearing, which is a sliding stress. Now, you know this already from plate
tectonics, right? Because you know that there are three predominant kinds of
plate boundaries. You can either have plates coming together or pulling apart or
sliding past each other. Same thing here; you're talking about tectonic forces deforming
rocks in those same three motions: compression, tension, and shear. Now, strain is the response
to stress. It's what happens to the rock when you put it under stress. This is a sequence
of four experiments from the rock deformation lab at Oklahoma University. You can see in the
first cylinder on the left, that's the undeformed cylinder. And then we have one that broke by
cracking, right? So that's brittle behavior; it's breaking. In the middle, we have maybe
a little bit of cracking but more flowing, more ductile behavior, and on the right,
a fully ductile deformation of that rock. Now, it's maybe a little bit hard
to think about rocks as flowing, as deforming ductilely, but they do.
Under high pressure and temperature, rocks can behave very much like silly putty.
So, two predominant responses to stress: ductile and brittle behavior. So you can
see then that there are six potential end members for the kinds of deformation that you
might have. You can have compression, tension, and shearing stresses behaving in either ductile
or brittle ways. So, see if you can pause this recording and make a little chart for yourself and
draw a picture, make one or two layers, let's say, of rock, and then think about squeezing them.
What would happen if the rock layers behaved ductilely? What would happen if they behaved
brittlely when you squeeze them? Then, do the same thing for stretching them or for sliding them
past each other. Take a minute or two to do that. Now, maybe you came up with something that
looks like this: compressive features, tensional features, and shearing features
that you might see in rocks. When you compress something and it behaves ductilely, it
will fold. When it behaves brittlely, it will break and form a fault where the rocks
are squeezed together, and one side's pushed up. When you pull something apart, then the ductile
behavior will be stretching and thinning, and the faulting will be stretching apart and dropping
down. And when you provide a shearing stress, then you can bend something in a horizontal plane
or you can make a strike-slip kind of fault.