Rock Deformation: Types and Causes
Rocks can deform in two main ways: brittlely or ductilely. Brittle deformation occurs when rocks break or fracture under stress. Ductile deformation occurs when rocks bend or flow under stress. The type of deformation that occurs depends on a number of factors, including the type of rock, the temperature and pressure conditions, and the rate at which the stress is applied.
Brittle Deformation
Brittle deformation is most common in the upper crust of the Earth, where the temperature and pressure conditions are relatively low. Brittle deformation can produce a variety of features, including:
- Faults: Faults are fractures in rocks along which there has been displacement. Faults can be caused by a variety of stresses, including compression, tension, and shear.
- Joints: Joints are fractures in rocks that do not show any displacement. Joints are often caused by cooling or drying of rocks.
- Breccia: Breccia is a type of rock that is made up of angular fragments of other rocks. Breccia is often formed by brittle deformation of rocks.
Ductile Deformation
Ductile deformation is most common in the lower crust and mantle of the Earth, where the temperature and pressure conditions are relatively high. Ductile deformation can produce a variety of features, including:
- Folds: Folds are bends in rocks. Folds can be caused by a variety of stresses, including compression and tension.
- Foliation: Foliation is a layering that can be seen in some metamorphic rocks. Foliation is caused by the alignment of minerals in the rock during ductile deformation.
- Lineation: Lineation is a linear feature that can be seen in some metamorphic rocks. Lineation is caused by the alignment of minerals or elongated grains in the rock during ductile deformation.
Rocks Are Stressed
Did you know that rocks experience stress? While not the same kind of stress that you might experience on a bad day, the stress rocks are subjected to still has quite an impact. Stress causes rocks to deform, meaning the rocks change size or shape.
There are different kinds of stress that rocks experience, and these determine how the rocks deform.Tensional stress is when rock is stretched apart. This is similar to pulling on a string from both ends after the string has already been fully extended. Compressional stress is when rock is pressed together. Here, rocks are squeezed together, like a car caught in the middle of a long pile-up on the highway.
Shear stress is when rock slips in a horizontal direction. With shear stress, the rock is being pulled in opposite directions but on different ends. To understand this, try putting your palms together and then rubbing them back and forth. Now imagine that there is a rock in the middle and you can see how one end goes forward while the other end gets pulled backward.
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| Weird Rock Formations at Agia Pavlos. by G. Shuttleworth |
Stages of Rock Deformation
As rocks are stressed, they go through stages of deformation. At first, the rock is strained enough that its shape or size may change, but the change is reversible. This is the first stage, called elastic deformation. Think of this 'elastic' change like the elastic in your waistband. If you pull it and then let it go, the stretch is reversible because it can go back to its original shape.
Rocks may also become so deformed that the change is not reversible, which we call ductile deformation. Ductile means that something can be changed into a new shape, but once this happens, it stays that way. This is like the copper wiring in your house. Copper is ductile, meaning you can stretch it into long, thin wires. However, once you've made this change, you can't 'unstretch' it, and the same is true for rocks in this stage of deformation.
Finally, if rocks are stressed enough, they fracture, which is when the change is irreversible and the rock breaks. If you fall down hard enough, you may fracture a bone in your body, and rocks experience the same thing when the stress is great enough.
How Rocks Handle Stress
Some people handle stress better than others, and rocks are the same way. So, just like there are various types of rock stress, there are also different responses to the stress for different types of rock material. The ability of a rock to handle stress depends on its elasticity, or the flexibility of the rock.
You might be surprised to learn that rocks are flexible. Under the right conditions, they can 'flow' very slowly. This movement is similar to silly putty. Push on silly putty very hard and it feels like a solid, but pull it apart and it moves like a liquid. Rocks are not quite as fluid as silly putty, but sometimes they can act similarly.
Do you remember from before that ductile materials like copper can be reshaped? The same is true for rocks. Ductile rock is rock that flows in response to stress. These rocks are more flexible than not. Things like clay and mica minerals are ductile materials, which you've experienced if you've ever played with clay to mold it into different shapes.
Alternately, there is also brittle rock, which breaks or fractures in response to stress. This type of rock is less flexible and therefore cracks under pressure more easily than ductile rock. Examples of this type of material are quartz and feldspar minerals, which would break very easily if you dropped them on the ground.
The material of the rock is just one factor in how it deforms. Temperature also plays a role because at high temperatures, material can 'flow' more easily, so it's more ductile. The opposite is true for low temperatures, so materials are more brittle and break more easily.