Rock solid pre-Lesson Assessment (3 points)

1. 
   Of what material(s) is the earth's crust made? _______________________________________________________________
   
2. 
   How do rocks break in nature? ________________________________________________________________
   
3. 
   How would the breaking of a large rock affect people? ________________________________________________________________________________________________________________________________
   

We see rocks outside every day, in both landscaping and nature. In fact, the entire earth is basically one gigantic rock! The earth's crust is entirely made of solid rocks, so there are huge rocks in the ground covering the entire planet! We cannot see many of these rocks. Sometimes dirt covers them and we must dig very deep to find them, but huge rocks cover the entire earth's crust, even under the oceans. That means that all of our buildings, all of our bridges, all of our roads, and even your home, are sitting on rock.

The following are questions that geotechnical engineers think about when determining locations to place structures. Geotechnical engineers understand what causes rocks to break. They know how to identify different types of rocks, and determine if a certain rock is likely to break. They work with structural engineers to plan the best way to build structures in different rock conditions.

1. 
   What might happen if some of these huge rocks broke?
   

2. 
   What types of natural disasters might be caused?
   

3. 
   What would happen to the bridge or skyscraper resting upon one of those massive rocks?
   

Furthermore, a less serious reason why we care about the strength of rocks is for development, which means building and expanding the use of land (such as new shopping centers, schools or homes being built in a town). Many building plans require deep foundations, making it necessary to excavate or dig out rock. An engineer provides information about the best way to excavate the rock, so as to build an adequate foundation.

When pressure is applied to an area, such as a rock, it is called stress. If you press your hands together, you can feel the forces of stress. In nature, stress can cause rocks to break, and one way that stress occurs is by the natural movements of the earth's crust (remember that the earth's crust is basically floating on liquid magma, and so it moves often). There are three types of stress

1. 
   Compressional stress is when a rock is pressed together into itself, like when crust movements cause two rocks to squeeze another one between them. Another example is when mountains are formed at a convergent boundary, like the Rocky Mountains. Press your hands together again. You can feel that the inner parts of your hands are being smashed by compressional stress from the muscles in your hands pushing inward.
   
2. 
   Tensional stress is when a rock is pulled apart. For example, if a rock wedged itself into the crack of another rock, and movement of the earth's crust caused it to wedge even further until the rock broke apart. Another example is a divergent boundary, like the Mid-Atlantic Ridge, which is formed by two tectonic plates pulling apart from each other to allow lava to flow upward. Use one of your hands to pull a finger on your other hand. You can feel the tensional stress because your hand is pulling your finger one way, and your other hand is attached to your finger, pulling it the other way by holding it in place.
   

3. 
   Shear stress is when a rock is pulled on one side but pushed on the other side. This can happen if the crust movements on one side of a rock are opposite of those on the other side of the rock. An example of this is the San Andreas Fault, which is on a transform boundary, with the California plate moving southward and the Pacific Ocean plate moving northward. Put your hands together again, but this time press upward with your right hand and downward with your left hand. If you press hard, you should notice that the skin on your right hand is being pulled down because of the forces from your left hand pulling down, and the skin on your left hand is being pulled up because of your right hand. (It may be easier to see the skin being pulled if you use an area on your body where the skin is looser, such as your hand pressing upward against your arm or cheek.)
   

• 
  Physical weathering is when a physical action breaks the rock, such as the forces of wind or water. A common example is the freeze/thaw action of water in rock cracks. As the water freezes, it expands, causing stress (pressure) that breaks the rock. (Note: If students ask what kind of stress this is, tell them that the process is complicated and includes both tensional and compressional stress.)
  

• 
  Chemical weathering is when the rock is chemically broken down. Some common examples of this are rust forming on granite or acid rain breaking down limestone. This type of weathering is not considered a type of stress because there is no pressure on the rock (remember that stress is pressure applied to an area).
  

• 
  Biological weathering is when living organisms break the rock. A typical example is a tree root breaking a rock due to the stress caused by its pressure. (Note: If students ask what kind of stress this is, tell them that the process is complicated and includes both tensional and compressional stress.)
  

Rocks cover the entire earth, and we need to know how they break. Who can tell me why? If we know how rocks break, then we can predict many types of natural hazards, which can, in turn, save structures or even our lives. Also, because we live on the earth and build structures on and in the earth, we need to know about how rocks break. Geotechnical engineers know all about rocks in the earth's crust. They know how to identify different rocks, and determine if a certain rock is likely to break. They do tests and simulations to predict volcanoes, earthquakes or rockslides. Geotechnical engineers also need to know how rocks break so that other engineers can excavate or dig into them to build the deep foundations or basements of buildings such as parking garages, bridge piers or skyscrapers.

Do you know how rocks break? Well, rocks break from stress. Stress is when pressure is applied to an area, such as a rock. There are three types of stress. Compressional stress is when something is pressed together, like when mountains form. Tensional stress is when something is pulled apart, like when a ridge such as the Mid-Atlantic Ridge is formed. Shear stress is when something is pulled one way on one side and the other way on the other side, like the San Andreas Fault.

Now that we've reviewed how rocks break, let's demonstrate these types of stresses ourselves! Using our own hands, do you think we could create enough stress to break a rock? Probably not! What are some things that we can break with our hands? Soap is a good model because with it, we can show the three different types of stress. Since we are not strong enough to break rocks, we can imagine that the bars of soap are rocks, and break them in different ways.

1. 
   Choose one team member to place TENSIONAL STRESS on the bar of soap. Watch what happens carefully. Draw a picture of the soap and how it was broken. Draw arrows showing the direction of the force placed on the soap. (2 points)
   

2. 
   Describe the area where the soap broke. (Smooth, rough, jagged, stretched, etc.) (2 Points)
   

3. 
   What is a real life example of tensional stress? (5 Points) ______________________________________________________________________________________________________________________________________________________
   

1. 
   Choose one team member to place COMPRESIONAL STRESS on one of the pieces left from the bar of soap. Watch what happens carefully. Draw a picture of the soap and how it was broken. Draw arrows showing the direction of the force placed on the soap. (2 points)
   

2. 
   Describe the area where the soap broke. (Smooth, rough, jagged, stretched, etc.) (2 points) ________________________________________________________________________
   

3. 
   What is a real life example of compressional stress? (5 Points) ______________________________________________________________________________________________________________________________________________________
   

Figure 2. Using compression (pressed together pressure) to break soap.

1. 
   Choose one team member to place SHEAR STRESS on one of the pieces left from the bar of soap. Watch what happens carefully. Draw a picture of the soap and how it was broken. Draw arrows showing the direction of the force placed on the soap. (2 Points)
   

2. 
   Describe the area where the soap broke. (Smooth, rough, jagged, stretched, etc.) (2 Points) ________________________________________________________________________
   

3. 
   What is a real life example of compressional stress? (5 Points) ______________________________________________________________________________________________________________________________________________________
   

Figure 3. Using shear stress to break soap. One hand pulls up, the other hand pulls down.

How might you demonstrate combination stresses using a piece of soap?

1. 
   Draw a picture of the soap and how it was broken. Draw arrows showing the direction of the force placed on the soap. (2 Points)
   

2. 
   Describe the area where the soap broke. (Smooth, rough, jagged, stretched, etc.) (2 Points) ________________________________________________________________________
   

3. 
   What is a real life example of combination stress? (5 Points) ______________________________________________________________________________________________________________________________________________________
   

1. 
   Today we demonstrated the three different types of stress. How would some of these stresses occur on a bigger scale in the natural world? BE SPECIFIC! (10 Points)
   

_____________________________________________________________________

2. 
   Why do geotechnical engineers need to understand stress in rocks? BE SPECIFIC! (Hint: think natural disasters, human made objects, etc.) (10 Points)
   

___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________