STRESS, FAULTS, AND FOLDSSTRESS, FAULTS, AND FOLDS

Deformation is the bending, tilting, and breaking of the Earth’s crust. Plate tectonics is the major cause of crustal deformation.

Thicker and heavier crust sink deeper into the mantle where thinner and lighter crust will rise higher on the mantle.

The up and down movement of crust is balanced by two pressures. One is the crust pressing down and the other is the mantle pushing up. The balancing of these two forces is called isostasy.

The up and down movement of the crust to reach isostasy is called isostatic adjustment. During this adjustment the rocks in the crust are bent causing deformation.

There are 3 basic kinds of stress that the isostatic adjustment causes, compression, tension, and shearing.

Compression occurs when crustal rocks are squeezed together.

Tension is the force that pulls rocks apart. Here rocks tend to become thinner.

Shearing pushes rocks in opposite directions. Sheared rocks bend, twist and break.

The results of stress are folding and faulting.

When a rock has stress put on it and does not break it is called folding. Folds appear as wave-like structures in rock layers. Some folds are small and can be seen in individual rocks and some folds are huge and can only be seen from the air.

The 3 general types of folds are anticlines, which are upcurved folds where the oldest rock layers are in the center,

synclines which are downcurved fold in which the youngest layers are in the center

and monoclines in which both ends stay horizontal but one side is lower than the other.

Rocks don’t always bend, sometimes they break. When the rock moves and breaks it is called a fault.

There are several different kinds of faults. There are two sides to a fault. The side that is above the fault plane is called the hanging wall.

Hanging Wall

When the hanging wall moves down it is called a normal fault. Normal faults occur in places where there is tension or the rocks are being pulled apart.

When the hanging wall moves up it is called a reverse fault. Reverse faults are caused by compressional forces.

A low angle reverse fault is called a thrust fault because one side is being thrust onto the other.

The last type of faults are called strike-slip faults. Strike-slip faults slide horizontally past one another.

If you are looking across to the other side of a strike-slip fault and that side moves to the left of you it is called a left lateral strike-slip fault. Strike-slip faults occur in and around transform plate boundaries like where we live near the San Andreas fault. This is also where shearing takes place.