In this video lesson, you will learn about how rock can be deformed from stress and the effect this has on the rock. You will identify different types of deformation based on the composition of the rock as well as external stress factors.
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.
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.
Water is also important, and the less water present in a rock, the more brittle it is. Let's use clay as an example again. Wet clay is easy to mold and shape because it still has a lot of water in it. But when you bake it in the kiln, this removes the water and you can no longer change its shape without breaking it apart. Cement is the same way. When you add water, you can pour and shape the material almost any way you like. But as soon as it dries, you have to break it apart with a jackhammer to get it to change at all!
If you think you're having a bad day, try being a rock. Rocks experience so much stress that sometimes they deform, or change size and shape. There are a few different types of stress that rocks may experience. Tensional stress is when rock is stretched apart, similar to stretching a string as far as possible from both ends. Compressional stress is when rock is pushed together, similar to a car being smashed from both ends in a long pile-up. Shear stress is when rock slips horizontally, like how it would if you put it between your hands and then moved them in opposite directions.
As rocks are stressed, they go through three phases of deformation. First is elastic deformation, which is when the deformation is reversible. If the change is not reversible, then this is ductile deformation. If the stress is so much that the change is not reversible and the rock breaks, we call this fracture.
Many factors determine how a rock reacts to stress. Ductile rock will flow in response to stress because ductile materials are flexible enough to be reshaped without breaking. On the other hand, brittle rock will simply break in response to stress because it's not very flexible material. In addition to the material, things like temperature and water also play a role. Heat and water tend to increase the rock's elasticity, or flexibility. But in cool temperatures and dry conditions, rock becomes more brittle, breaking more easily.
After this lesson, you should have the ability to:
- Describe the types of stress that rocks experience
- Explain three phases of deformation that rocks go through in response to stress
- Differentiate between ductile and brittle rock
- Summarize the effects that heat and water have on a rock's elasticity