BellringerDescribe how Plate Tectonic forces and

earthquakes might be related.

Forces in Earth’s Crust

Notes

Types of Stress

•When Earth’s plates move, rocks are pushed and pulled. The pushes and pulls are called stress.

•Stress adds energy to rocks. Rocks keep storing the energy until they cannot stand any more more stress. Then the rocks break or change shape.

•If a rock only changes shape, this is known as a fold. If a rock breaks, this is a fault.

Fold

Fault

Forces in Earth’s Crust

•Tension is stress that pulls and stretches rocks. Tension makes rocks thinner in the middle. Tension happens when two plates move apart, acting like two suction cups.

Forces in Earth’s Crust

•Compression is stress that squeezes rocks. Compression makes rocks fold or fault. Compression happens when two plates push together.

Forces in Earth’s Crust

•Shearing is stress that pushes rocks in opposite directions. Shearing makes rocks break, slip apart, or change shape. Shearing happens when two plates slip past each other in opposite directions.1

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Kinds of Faults•A fault is a break in Earth’s crust

where rocks are under stress.

•In many faults, the fault line is slanted. So the block of rock on one side of the fault is above the bock of rock on the other side of the fault. The overhead wall is called the hanging wall. The downward block is called the footwall.

Kinds of Faults•There are three different types of

faults (one for each type of stress): normal, reverse, and strike-slip.

•A normal fault happens when tension pulls rocks apart (divergence). In a normal fault, the hanging wall appears to move down relative to the footwall.

•Normal faults are found all along the Rio Grande rift valley in New Mexico.

Kinds of Faults•A reverse fault happens when compression pushes rocks together (convergence). In a reverse fault, the hanging wall slides up and becomes higher then the footwall.

•Movement along reverse faults produced part of the northern Rocky Mountains in the western United States and Canada.

Kinds of Faults

•A strike-slip fault happens when shearing pushes rocks in opposite directions (transform). In a strike-slip fault, two blocks of rock move past each other, but neither block moves up or down.

•The San Andreas fault in California is an example of a strike-slip fault.

Changing Earth’s Surface

•Stresses in Earth’s crust cause the surface to change. Different stresses cause different changes.

•Compression causes folding. Folding is the bending of rock without breaking. Folding is like a rug getting wrinkled up when it is pushed across the floor.

Changing Earth’s Surface

•Folds that bend upward into ridges are called anticlines. Folds that bend downward into valleys are called synclines.

•Anticlines = think how “A” points upward.

•Synclines = “sync” or “sink” down.

•The central Appalachian mountains, the Himalayas, and the Alps are all folded rocks.

Changing Earth’s Surface

•Tension causes stretching. When crust stretches, many normal faults form.

•Sometimes a block of rock moves upward between two normal faults. The block forms a mountain called a fault-block mountain.

•When the block between two normal faults moves downward, this is called a rift valley.

Changing Earth’s Surface

•Stresses in the crust can also form plateaus. A plateau is a large area of flat land that has been lifted up above sea level.

•Forces deforming the crust uplifted the Colorado Plateau in the “Four Corners” region of Arizona, Utah, Colorado, and New Mexico. Much of the Colorado Plateau lies more than 1,500 meters above sea level.

The flat land on the horizon is the Kaibab Plateau, with forms the North Rim of the Grand Canyon in Arizona. The Kaibab Plateau is part of the Colorado Plateau.

This uplift of the crust is what allowed the Colorado River to erode its way down over hundreds of millions of

years to its current position.