Deforming Earth’s Crust

1. Deforming Earth’s Crust Pgs. 103 - 109

2. Stress • The amount of force per unit area that is put on a given material. • When you have something like a piece of uncooked spaghetti and apply a stress to it, it will bend. • If you apply enough stress to it, it will break. • The same concept can apply to rocks and the Earth’s crust.

3. Rocks get Stressed • Rocks can change their shape due to stress and this process is called deformation. • Rock layers can bend when stress is placed on them and if enough stress is applied it can break. • A type of stress that occurs when an object is squeezed is compression. • This occurs when two tectonic plates collide and form mountains. • Tension is the stress that occurs when forces act to stretch an object. • This occurs when two tectonic plates diverge.

4. Folding of Rock Layers • Folding occurs when rock layers bend due to stress in the Earth’s crust. • Several types of folds occur depending on how the rock layers deform. • Synclines (folds down), anticlines(folds up), monocline (leaves ends of rock layers horizontal) • Folds can be large or small • They can be the entire side of a mountain or the size of a pocket knife.

5. Faulting in Rock Layers • The surface along which rocks break and slide past each other is called a fault. • The blocks of crust on each side of the fault are called fault blocks. • If the blocks are not split vertically, they can be determined either the hanging wall or the footwall. • The footwall is the sloping wall that you could place a foot on to walk up. • The hanging wall is the sloping wall that you could hang down from.

6. Types of Faults • Depending on how the hanging wall and footwall move, determines the type of fault. • A normal fault is when the hanging wall moves down in relation to the footwall due to tectonic plate moving apart. • Tension pulls the plates apart. • A reverse fault is when the hanging wall moves up in relation to the footwall due to tectonic plates moving together. • Compression pushes the plates together.

7. Telling the difference • Normal and reverse faults are easy to tell apart when you look at the different rock layers. • First you need to find the hanging wall and footwall. • Look at the angle of the break. • Next, you need to determine if the hanging wall moves up or down in relation to the footwall.

8. Strike-slip Faults • These occur when opposing forces cause a rock to break and move horizontally. • If you were standing on one side of a strike-slip fault looking across the fault when it moved, the ground on the other side would appear to move to your left or right. • These faults are associated with transform boundaries.

9. Mountain Building • Folds and faults are the tiny steps that begin the process of mountain building with plate tectonics. • As tectonic plates collide into each other compression and tension forces shape the Earth’s crust. • Eventually, the fold and faults turn into mountains. • The three most common types of mountains formed are: folded mountains, fault-block mountains, and volcanic mountains.

10. Folded mountains • They form when rock layers are squeezed together and pushed upward. • This is similar to pushing on opposite edges of a pile of paper. • When you push them together, the middle rises. • This is how folded mountains form. • Examples: • Appalachian Mountains • Himalayan Mountains

11. Fault-block Mountains • These form when tectonic forces put enough tension on the Earth’s crust, a large number of normal faults can result. • Large blocks of Earth’s crust drop down relative the other blocks and mountains are formed. • This faulting can cause jagged, sharp peaks as well. • Examples: Tetons in Wyoming

12. Volcanic Mountains • They form when molten rock erupts onto the Earth’s surface. • They actually form from new rock being added to the Earth’s crust. • They form at convergent boundaries of tectonic plates. • There are a large number of volcanic mountains in the pacific rim and it has been nicknamed the Ring of Fire.