Deforming the Earth’s Crust Faults & Earthquakes Deforming the Earth’s Crust

Deformation The process by which the shape of a rock changes because of stress is called deformation. There are two basic types of deformation: Plastic deformation Elastic deformation When rock deforms in a plastic manner, it folds like a piece of molded clay. With elastic deformation, the rock stretches like a rubber band until it breaks. Elastic deformation can lead to earthquakes.

Stress Stress is a force that acts on rock to change its shape or volume. Because stress if a force, it adds energy to the rock. This energy is stored in the rock until the rock either breaks, or changes shape.

There are three types of stress that occur in the Earth’s crust: Compression Tension Shearing

Compression The stress force called compression squeezes rock until it folds, or breaks. Compression makes a mass of rock occupy a smaller space. When compression occurs at a convergent boundary, large mountain ranges can form.

Tension The stress force called tension pulls on the crust, stretching rock so that it becomes thinner in the middle. Tension occurs where two plates are moving apart, such as mid-ocean ridges, or rift valleys.

Shearing Stress that pushes a mass of rock in two opposite, horizontal directions is called shearing. Shearing can cause rock to break and slip apart. Shearing occurs at transform boundaries.

Folding The bending of rock layers because of stress in the Earth’s crust is called folding. Undisturbed rock layers are horizontal, so when we see a fold we know that deformation has taken place.

Faults When the stress on rocks causes them to break and slip past each other, a fault is formed. The blocks of crust on each side of the fault are called fault blocks. When faults are not vertical one side of the fault block will be called a hanging wall and the other the footwall. The position of the fault block will determine which it is.

Faults There are 3 main types of faults: Normal fault Reverse Fault Reverse, or thrust fault Strike-slip

Normal Faults Tension forces cause normal faults. The hanging wall lies above the fault and the footwall lies below the fault. When movement occurs along the fault line, the hanging wall slips downward. Normal faults are found at divergent plate boundaries, where plates pull apart.

Reverse Faults Compression forces produce reverse faults. Reverse faults have the same basic structure as a normal fault, but the blocks move in the opposite direction. When movement occurs along the fault line, the hanging wall slides up and over the footwall. Reverse faults are found at convergent plate boundaries, where plates are pushed together.

Strike Slip Faults Shearing creates strike-slip faults. The rocks on either side of the fault slip past each other sideways with little up or down motion. A strike-slip fault that forms the boundary between two plates is called a transform boundary.

Fault Block Mountain When the tension in a normal fault uplifts a block of rock, a fault-block mountain forms. The Grand Tetons in Wyoming are an example of a fault-block mountain range.

Folded Mountains Folded mountains form at convergent boundaries where continents have collided. The Appalachian Mountains, the Alps, and the Himalayas are examples of folded mountains.