4.1 Forces in Earth’s Crust Chapter 4: Earthquakes 4.1 Forces in Earth’s Crust

Stress Stress is a force that acts on rock to change its shape or volume. Energy caused by stress is stored in rocks until it changes shape or breaks.

Stress Tension: stress force that pulls on the crust and thins rocks in the middle; occurs when plates pull apart. Compression: stress force that squeezes rock until it folds or bends; occurs when plate comes together. Shearing: stress that pushes a mass of rock in opposite directions; occurs when plates slide past each other.

Normal Faults formed from rocks being pulled apart at divergent boundaries one block, called the hanging wall, sits over the fault and the other block, called the footwall, sits beneath the fault. The hanging wall slides down and the footwall moves up.

Normal Faults

Reverse Faults formed from rocks being pushed together at convergent boundaries The hanging wall moves up and the footwall moves down.

Reverse Faults

Strike-slip Faults The rocks on either side of the fault slip past each other at a transform boundary. There is little up and down movement.

Folding Earth’s Crust Folds are bends in rock that form from compressions shortening and thickening the crust. Upward fold into an arch – anticline Downward fold into a V shape – syncline

Forming Mountains Compression from convergent plates colliding can build mountain ranges. Fault block mountains form from tension in Earth’s crust and faulting. If there are two normal faults near each other, the land between could eventually drop down creating a valley.

Forming Mountains

Plateaus Plateau: large area of flat land elevated high above sea level. Forces push up large, flat blocks of rock.

4.2 Earthquakes and Seismic Waves Chapter 4: Earthquakes 4.2 Earthquakes and Seismic Waves

Seismic Waves Earthquake: shaking and trembling that results from movement of rock beneath Earth’s surface. Plate movements and stress in Earth’s crust can lead to earthquakes, releasing great amounts of energy.

Seismic Waves Seismic waves: vibrations that are similar to sound waves; travel through Earth carrying energy from earthquakes. The focus of an earthquake is the area beneath Earth’s surface where the rocks are shifted (action that causes Earthquake). The epicenter is the point on Earth’s surface above the focus.

Seismic Waves