FRACTURES FAULTS FOLDS

Essential QuestionEssential Question How does Elastic Potential Energy cause the Earth’s crust to fracture, fault, and fold?

What is a fault? A fault is a fracture or series of fractures in Earth’s crust that occurs when stress is applied too quickly or stress is too great. Many faults occur along plate boundaries. Faults can also occur anywhere in the crust where rocks are bent (folded) until they break. Faults occur as a result of various types of stress.

Most Faults Have a Hanging Wall & Foot Wall The Hanging Wall is the one with the slope on which you could hang. The Foot Wall is the one with the slope on which you could walk. The type of fault is determined by the motion of the Hanging Wall. The fault line would also be called the fault plane. The area around it would be called the fault zone.

Is this the Hanging Wall or Foot Wall? The FOOT WALL is the one with the slope you would put your feet on. The HANGING WALL is the one with the slope you would hang from.

Types of Faults There are several different kinds of faults. These faults are named according to the type of stress that acts on the rock and by the nature of the movement of the rock blocks either side of the fault plane. Stresses can be compressional (push), tensional (pull) or shear (slide past each other). The type of stress determines how the Hanging Wall moves and therefore the type of fault.

3 Types of Faults Caused by 3 Different Types of Stress Normal Fault (Caused by Tension) Reverse Fault (Caused by Compression) Strike-Slip Fault (Caused by Shear Stress)

Normal Fault Occurs when tension pulls the fault apart and the Hanging Wall drops. The fault is an example of tension

Reverse Fault Occurs when compression pushes the two parts of the fault together and the Hanging Wall moves up. The fault is an example of compression

Strike-Slip Fault Shear causes the parts of the fault to slide past one another. This fault is an example of shear.

Examples of Faults

Folds Occur when stress is applied to both ends of a section of rock or rock layers. Some rocks would break, but a fold occurs when rocks bend. An anticline looks like a “rainbow”; a syncline looks like a “smile”.

Examples of Folds

Energy Transfer in Earthquakes F OCUS Q UESTION : What happens when energy stored in rock is released?

Procedure 1. Attach one end of the bungee cord to the wooden block and the other end to a pencil held at right angles to the bungee cord. 2. Place the block at one end of the piece of wood. The wood should have the sandpaper surface down. 3. Place a meter stick parallel to the piece of wood as shown in the picture with the 0 end of the meter stick at the end of the wood block. This will allow you to discuss how far the rubber band (simulating rock) is stretched before the larger block (simulating the larger rock structure) moves. 4. Pull the bungee cord out to a position that is straight but has almost no tension with the pencil. Continue to move the bungee 1cm at a time and record the movement. 5. Repeat 2 more times.

Discussion: 1.When the block moves, does it move in a predictable pattern? What evidence supports your conclusion? 2.The block model system is designed to simulate rock movement in the Earth’s crust. How does this model simulate rock movement in the Earth’s crust? 3.Using the information learned in this activity, why do you think it is difficult for scientists to predict earthquakes? ANSWER: No. The block will move when the bungee cord has enough elastic potential energy (EPE) to surpass its elastic limit. ANSWER: Once the bungee stretched out towards its elastic limit, it did not move at every cm but when it did move, it moved a significant distance. Similarly rocks in the Earth’s crust behave in this manner. Small amounts of kinetic energy around a fault will not cause significant movement of rock. Large amounts of kinetic energy around a fault will cause significant movement of rock. ANSWER: There is no observable pattern to when elastic potential energy will be transformed into kinetic energy to create an earthquake.

Essential Question: How does Elastic Potential Energy cause the Earth’s crust to fracture, fault, and fold? When compressive forces act on a material (rocks) but the elastic limit is not reached, folding occurs. If the elastic limit is exceeded, the rock will break causing a fracture in the rock to form. If the elastic limit is exceeded and movement occurs along the fracture, the fracture becomes a fault.