Folding and Faulting

Folds A fold is when the earth’s crust is pushed up from its sides. There are six types of folds that may occur: Anticline Syncline Tight Fold Overfold Recumbent Fold Nappe Fold

Anticline An anticline occurs when a tectonic plate is compressed by movement of other plates. This causes the center of the compressed plate to bend in an upwards motion. Fold mountains are formed when the crust is pushed up as tectonic plates collide. When formed, these mountains are usually enormous like the newly formed Rocky Mountains in Western Canada and the United States To the top right is a picture of an anticline. Beneath is a picture of the Rocky Mountains.

Syncline A syncline is similar to an anticline, in that it is formed by the compression of a tectonic plate. However, a syncline occurs when the plate bends in a downward motion. The lowest part of the syncline is known as the trough. To the top right is a diagram of a syncline fold (The bottom of the fold center is the trough). Beneath, is an example of a syncline in California. Can you distinguish the trough in this picture?

Tight Fold A tight fold is a sharp peaked anticline or syncline. It is just a regular anticline or syncline, but was compressed with a greater force causing the angle to be much smaller. Folds such as these occur to form steep mountain slopes like those in Whistler, British Columbia. To the left is a photo of a tight fold formed by extreme pressure on these rocks.

Overfold An overfold takes place when folding rock becomes bent or warped. Sometimes the folds can become so disfigured that they may even overlap each other. An example of overfolding is shown in the diagram below.

Recumbent Fold This type of fold is compressed so much that it is no longer vertical. There is a large extent of overlapping and it can take the form of an “s”. To the right is a diagram that shows the process of recumbent folding.

Nappe Folding This fold is similar to a recumbent fold because of the extent of folding and overlapping. However, nappe folding becomes so overturned that rock layers become fractured. To the right is a picture of someone standing under a fractured fold.

Faults A fault is when tension and compression associated with plate movement is so great that blocks of rock fracture or break apart. This process can occur very rapidly, in the form of earthquakes. The damage caused by this event can be very destructive and cause severe changes to the earths surface. There are five types of faults that can occur: Normal Fault Reverse Fault Tear Fault Rift Valley Horst Fault

Normal Fault This occurs when rocks move away from each other due to the land moving apart. When the rocks move apart, the side with the less stable tectonic plate drops below the side with the more stable plate. On the top right is the movement of a normal fault. A picture is also shown below. Notice the displacement of the different types of rock on each side of the fault.

Reverse Fault Reverse faults are the opposite of normal faults. Rocks are compressed such that one plate moves up while the other descends below it. When plates compress and crack, usually the more dense one is forced under the less dense one. This is similar to the action of the continental crust colliding with the oceanic crust. Here the more dense crust, being the oceanic crust is forced under the continental crust. To the right is an animation of a reverse fault. Below that is a real picture of what a reverse fault looks like.

Tear Fault A tear fault, also known as a transform fault, occurs when two tectonic plates slide in a lateral motion past each other. This type of fault causes the most severe earthquakes because they grind against each other. These earthquakes can either be shallow or deep and cause tremors over a short or long period of time. Tear faults can occur frequently, especially along the coast of California.

Rift Valley A rift valley is when two normal faults occur parallel to each other and the land sinks between the faults. There are two major examples of this. One being the Great Rift Valley in North Africa and the other, the San Andreas Fault in California. The top right picture is San Andreas Fault and on the bottom right is a diagram of what a rift valley looks like.

Horst Fault A Horst is the opposite of a rift valley. The land between the parallel faults is forced upward because the two faults are being pushed together. This process can take a long time to occur because the average plate movement is one inch per year. There are examples of horst faults on the left.

Summary Folding and faulting has a major influence on the way the earth looks. Mountains form and disappear over time, as well as large rift valleys and other features. This has an impact on where and how we live.

The End