Unit 4: Earthquakes & Volcanoes

AIM: How does stress in the crust change Earth’s surface?

Forces in Earth’s Crust• The movement of Earth’s plates creates enormous

forces that squeeze or pull the rock in the crust.

• Stress – a force that acts on a rock to change its shape or volume.– Some rocks can become brittle and snap– Some rocks can bend slowly

What are the 3 Types of Stress?• Tension, compression, and

shearing work over millions of years to change the shape and volume of rock.

1. Tension: stretches rock so it becomes thinner–Occurs where 2 plates

are moving apart.

What are the 3 Types of Stress?2. Compression: squeezes rock until it folds or breaks.–Occurs where 2 plates

are colliding.

What are the 3 Types of Stress?3. Shearing: pushes a mass of rock in two opposite directions–Can cause rock to break

and slip apart, or change its shape.

Kinds of Faults• Fault – a break in the crust where rock surfaces slip

past each other.– Most faults occur along plate boundaries.

Kinds of Faults1. Normal Faults: tension pulls crust apart – divergent• Fault is at an angle:–1 block of rock lies above – hanging wall–the other lies below – footwall

• Hanging wall slips downward

Animation

Normal Fault at Death Valley,

California

http://geotripper.blogspot.com/2008_02_01_archive.html

Kinds of Faults2. Reverse Faults: caused by compression– Same structure as normal fault, but rock moves in

opposite direction– Hanging wall slides upward

Animation

http://rst.gsfc.nasa.gov/Sect2/Sect2_7.html

Kinds of Faults3. Strike-Slip Faults: plates move past each other sideways• Caused by shearing formed at transform boundaries

Animation

http://comp.uark.edu/~sboss/ex4s02answers.htm

Changing Earth’s Surface1. Folding – rock bends without breaking– Upward folds – anticline– Downward folds – syncline

• Colliding plates can compress and fold crust over wide area– Produces largest mountain ranges– Ex: Himalayas in Asia

“Folding” Video Clip

Anticline-syncline pair in Silurian strata, Newfoundland, New Jersey.

http://www.rci.rutgers.edu/~schlisch/structureslides/rte23folds.html

Small-scale folding in stepping stones

2. Stretching Earth’s Crust - two normal faults cut into rock– Parallel formation creates fault-block mountains

Changing Earth’s Surface

Fault-Block mountains Video Clip

Basin and Range

3. Uplifting forces can raise plateaus– Large area of FLAT land elevated high above sea level– Forces in crust push up large, flat blocks of rock

Changing Earth’s Surface

Plateau near Zion National Park, Utah