B-tags Earth Science Class: Faults/Stress by Sunjae Lee 2/3/2013

For the past few weeks we have been learning about faults and stress, but what exactly are faults and stress? Well to start off a fault is a break in the crust where slabs of crust slip past each other. There are three main types of faults which are the strike-slip faults, normal faults, and reverse faults. You can find these faults along plate boundaries. Along with faults you will also find something called stress. Stress is a force which is used to change the shape and volume of rock. Like faults there are three different types of stress which are shearing, tension, and compression. Each type of stress goes with a specific type of fault.
First let's talk about a strike-slip fault. Strike-slip faults are the result of when the rocks on either side of the fault slip past each other in a sideways motion. The type of stress which fits the strike-slip fault is shearing. Shearing pushes the mass of rocks into two opposite direction and can cause the rock to break and slip apart to change its shape. Shearing caused strike-slip faults to occur in areas which had a transform boundary. You can see strike-slip faults across the world, an example would be the Sand Andreas fault in California.

San Andrea's Fault

Source:http://www.sanandreasfault.org/4020_A.jpg

Next let's talk about what a normal fault is. Normal faults are faults which are at an angle and one block of rock is on top of the other rock. The half of the fault which is angled like a slide is called the footwall and the half of the fault which is shaped kind of like a cliff is called the hanging wall. In a normal fault the footwall is above and the hanging wall is below the footwall. Normal faults are paired up with tension which means that tension is causing the rock to be pulled at both sides of it and is causing it to become thinner in the middle. The tension's force causes the normal faults to appear where plates are diverging which means pull apart. An example of a normal fault can be seen in the Sandia Mountains in New Mexico which had created the mountain.

A picture of the Sandia Mountains in New Mexico

Source:http://www.filmapia.com/sites/default/files/imagecache/imgpreset_Scene_OnNode/filmapia/pub/place/sandia_mountains.jpg

Lastly let's talk about reverse faults. Basically the reverse fault is the exact same as a normal fault...except for one difference. Instead of the footwall being above the hanging wall this time it is the opposite with the footwall being below the hanging wall. Reverse faults are produced by the force of compression. Compression is a type of stress which squeezes the rock until it folds or break, and a plate pushing against another can cause the the rock to compress like a giant trash compactor. You can see reverse faults in areas with convergent boundaries, an example would be Mt. Gould in Glacier National Park.

For one of the homework's we were able to learn more about these concepts, and in class were able to visually see the motions with wooden blocks and review what we had learned the previous night.
For example, if we think about the reverse faults motion the footwall would move below the hanging wall. But for some people it would be hard to visualize, so to help understand the concept and the motion we used wooden blocks to demonstrate it.

Visual of the motion of a Reverse Fault
Source: http://www.tulane.edu/~sanelson/images/reverse.gif

Then we would also be able to tell that it would have compression as the stress because the rocks are squeezing against each other. (We could also tell where it occurs which is at a converging plate.)
We could also visualize with the wooden blocks what a normal fault is since we know it is the opposite of a reverse fault.

Visual of the motion of a Normal Fault
Source: http://hays.outcrop.org/images/lutge8e/Chapter_17/Text_Images/FG17_12A.JPG

Since a normal fault is the opposite of a normal fault we could figure out that it has tension as the stress and is found at areas where it is diverging.)
The only different one would be the strike-slip fault, though it easy because the motion is just two "wooden blocks" sliding past each other in opposite directions.

Visual of the motion of a Strike-Slip Fault
Source:http://geomaps.wr.usgs.gov/parks/deform/strikeslip.gif

You can tell from this information that it would have shearing as the stress and it would be in an area which has transform plates.

Now based on what you learned can you match these stresses and faults to each other?: