Faults & Earthquakes

Deforming the Earth’s Crust

Deformation• The process by which the shape of a rock

changes because of stress is called 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.

• There are two basic types of deformation:– Plastic deformation– Elastic deformation

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.

Folds• The two most common

types of folds are:– Anticlines– Synclines

• Anticlines are upward-arching folds.• Synclines are downward, trough-like folds.

• Another type of fold is a monocline. • In a monocline, both ends of the fold

are horizontal.

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, or thrust fault

– Strike-slip

Reverse Fault

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.