5.1 Earth’s Crust in Motion

Stress in the Crust

• What is an earthquake?• The movement of Earth’s plates creates

powerful forces that squeeze or pull the rock in the crust

• These forces are examples of stress• Because stress is a force, it adds energy

to the rock• The energy is stored until the rock either

breaks or changes shape – deformation!

Types of Stress

• Three different kinds of stress occur in the crust:

1. Shearing 2. Tension3. Compression

• Shearing, tension, and compression work over millions of years to change the shape and volume of rock

Shearing

• Stress that pushes a mass of rock in 2 opposite directions is called shearing

• Shearing can cause a rock to break and slip apart or change its shape

Tension• The stress force

tension pulls on the crust, stretching rock so that it becomes thinner in the middle

• Occurs when 2 plates are moving apart

Compression

• Compressionsqueezes rock until it folds or breaks

• One plate pushing against another can compress a rock like a giant trash compactor

Kinds of Faults

• When enough stress builds up in a rock, the rock breaks, creating a fault

• Faults usually occur along plate boundaries, where the forces of plate motion compress, pull, or shear the crust so much that the crust breaks

• There are 3 types of faults:1. Strike slip faults2. Normal faults3. Reverse faults

Strike-Slip Fault

• Similar to transform boundaries

• Rocks on either side of the fault surface are moving past each other

• Rocks are subject to shearing stress (tearing)

• Example – San Andreas Fault

San Andreas Fault

Normal Faults• Tension pulls rocks

apart• Similar to divergent

plate movement• Rock above the

fault surface moves downward in relation to the rock below the fault surface

• Example – Sierra Nevada Mountains

Sierra Nevada Mountains

Reverse Fault• Result from

compression forces generated at convergent plate boundaries

• Rocks above the fault surface are forced upward and over the rocks below the fault surface

• Example – Himalayan Mountains

Himalayan Mountains

Mountain Building

• Over millions of years, fault movement can change a flat plain into a towering mountain range

• Mountains can be formed by either folding or faulting

Mountains Formed By Faulting

• Fault-block mountains form when normal faults uplift a block of rock

• Examples are the Sierra Nevada and the Grand Teton mountains

Grand Tetons

Mountains Formed by Folding

• The collision of 2 plates can cause compression and folding of the crust

• Examples include the Appalachian Mountains

Satellite Image of the Appalachians

Anticlines and Synclines

• Describes the upward and downward folds in a rock

Anticline (makes an “A”) Syncline (makes an “U”)

Which is it? Anticline or Syncline?

Plateaus• The forces that raise

mountains can also raise plateaus

• Some plateaus form when vertical faults push up a large, flat block of rock

• Example – “Four Corners” region of AZ, CO, UT, and NM (the Colorado Plateau)

Kaibab Plateau