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Shear stress – this is when you get slippage or translation on rock. This is most likely to contribute to the twisting of rock in some tangible way.
Rocks deforming means they are under strain. We define strain as any change in the shape, size, or total volume on a rock. Now that you have stress and strain on rock, you can see that it will deform the rock. Rock doesn't deform in a random way. It goes through several stages of deformation as this happens. Let's look at these stages of deformation so you can see how a rock can change as force is applied to it: •
Stage 1 – this is called elastic deformation. Like any deformation, it is completely reversible if the pressure on the rock is removed.
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Stage 2 – this is called ductile deformation. The strain is irreversible by now but the rock is still intact.
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Stage 3 – this is also irreversible and happens as the rock is fractured completely.
DUCTILE VERSUS FRAGILE ROCK Of course, all rocks do not behave the same. You know intuitively that some rock is fragile and more brittle, while other rocks are stronger and perhaps more stretchy. In geology, we divide these rocks into different types based on how they behave under stress: •
Brittle rock will not be ductile enough to remain intact. It will fracture easily.
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Ductile rock will be more elastic so that it will stretch more or bend before fracturing.
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Temperature affects how ductile a rock will be. Like anything, high temperature means more ductile behavior.
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The presence of confining pressure will diminish the chances of a fracture. Low pressure around a rock will alternatively increase the chances of a fracture.
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Strain rate matters – if the rate of strain is too fast, there is a greater chance for a fracture compared to a slower strain rate.
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