6.3 Deforming Earth’s Crust

Deformation The process by which the shape of a rock changes in response to stress. Stress The amount of force per unit area on a given material.

Rock layers can deform by bending and by breaking. Describe two ways in which rock layers can deform when stress is placed on them. Rock layers can deform by bending and by breaking.

Folding The bending of rock layers in response to stress in Earths crust.

Two types of folds Anticline A fold in which the oldest rock layers are in the center of the fold. Rock limbs slope down from the center to form an arch. Syncline A fold in which the youngest rock layers are in the center of the fold. The limbs slope up from the center to form a trough.

Shapes of Folds Asymmetrical – rock layers that bend into folds that are not symmetrical

Overturned fold – A fold in which one limb is tilted beyond 90 degrees.

Recumbent fold – A fold appears to be lying on its side.

Faulting The surface along which rocks break and slide past each other is called a fault. The footwall is the block of rock that lies below the plane of the fault. The hanging wall is the block that lies above the plane of the fault. Footwall Hanging wall

How do faults form? Faults form when rock is placed under so much stress that it no longer stretches or flows, but breaks.

Normal Fault When rocks are pulled apart because of tension (stress that pulls rocks apart). The hanging wall moves down relative to the footwall.

Reverse Faults When rocks are pushed together by compression (stress that pushes rocks together). The hanging wall moves up relative to the footwall.

Strike-Slip Faults When rocks are moved horizontally by opposing forces due to shearing. The two fault blocks move past each other horizontally.

Compare how the three types of faults form. Normal faults form from tension that pulls rocks apart at divergent boundaries. Reverse faults form from compression that pushes rocks together at convergent boundaries. Strike-slip faults form when shear stress pushes rocks in opposite directions at transform boundaries.

Recognizing Faults Slickensides are grooves, striations, or polished surfaces to indicate where rocks have moved. Scarp –a row of cliffs formed by faulting.

How do mountains form? Mountains form when tectonic plate motions crumple and push up the margins of plates.

Mountain Building Folded Mountains Formed when rock layers are squeezed together and pushed upward. Form at convergent boundaries where continents have collided.

Folded Mountain

Fault-Block Mountain Forms when tension in Earth’s crust causes the crust to break into a large number of normal faults.

Fault-Block Mountain

Volcanic Mountains Formed when molten rock erupts onto Earth’s surface. Most volcanic mountains are located at convergent boundaries.

Volcanic Mountains

Review Questions Why do rocks deform? Rocks deform when stress is placed on them. What causes faults to form? Faults form when so much stress is placed on rock layers that they break.

For each of the three types of faults, explain the forces that cause the faults to form. Normal faults form when tension causes rock to pull apart and break. Reverse faults form when compression pushes rocks together and causes the rock to break. Strike-slip faults form when shear stress causes rocks to break and to be pushed in different directions.

4. How are mountains related to tectonic plate motion? Folded mountains occur at convergent plate boundaries, where compression folds and uplifts rock. Fault-block mountains occur where divergent plate motion causes Earth’s crust to break into a series of normal faults. Volcanic mountains occur at convergent plate boundaries, where subduction causes the surrounding mantle rock to melt and magma to rise to the surface.

Quiz 6.3 review Anticline Syncline Folded mountains Tension Compression Fault-block mountains Normal fault Reverse fault Molten rock Strike slip fault