FORCES IN EARTH’S CRUST

Chapter 5 Section 4Grade 8 Science

FORCES IN EARTH’S CRUST Key Concepts of CH 2 Section 1

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

• Where are faults usually found, and why do they form?

• What land features result from the forces of plate movement?

What does stress look like?

• A teacher under stress! • Earth’s crust under stress!

What is stress?

• Stress is a force • Stress acts on rock to change its shape or volume• Deformation of rock occurs (“de”- undo “form”-

shape/appearance)• Examples of stress are TENSION, COMPRESSION, and

SHEARING• Stress adds energy to the rock• Energy is released when rock changes shape or breaks

(ex. Such as during an earthquake!)

Diagram of Stress in Earth’s Crust

Comparing Types of Stress

Type of Stress Direction of movement

Effect on crust

TENSION PULLS CRUST APART

STRETCHES ROCK-BECOMES THINNER

COMPRESSION PUSHES CRUST TOGETHER

SQUEEZE & COMPACTS ROCK

SHEARING PUSHES IN OPPOSITE DIRECTIONS

ROCK BREAKS, TWISTS, OR CHANGES SHAPE

- Forces in Earth’s Crust

TENSION• The stress force called tension pulls on the crust,

stretching rock so that it becomes thinner in the middle.

COMPRESSION

• The stress force called compression squeezes rock until it folds or breaks.

- Forces in Earth’s Crust

SHEARING

• Stress that pushes a mass of rock in two opposite directions is called shearing.

- Forces in Earth’s Crust

Where does stress come from?

• The movement of Earth’s plates creates enormous stress that squeezes or pulls rock in the crust.

• Since plates move slowly, changes in Earth’s crust also occur slowly.

• Over millions of years, stress will cause crust to bend, stretch, break, tilt, fold, & slide.

Folding & Faulting

• Folding occurs as rocks bend slowly, like road tar softened by the sun.

• Folding is caused mainly by compression.

• Folds are bends in rock that form when compression shortens and thickens the crust.

• Faulting occurs when rocks break

• Faulting is caused by extreme stress- tension, compression, or shearing.

• Faults are breaks in the crust where rock surfaces slip past each other.

How folding changes Earth’s surface

• Folding of rock is caused by compression forces as one plate pushes against another plate squish!

• Compression makes parts of crust shorter and thicker.• Folds can bend upward like an arch (anticline), or

downward like a valley (syncline).• Compression can form folded mountains in the middle

of a continent (like Appalachians) or at the edges of a convergent boundary (like Himalayas and Alps)

Folded Mountains- in the middle of a continental plate

Appalachian Mountain Range, USA

Folded Mountains- at convergent plate boundary

Himalayas Mountain Range, Nepal

How faulting changes Earth’s surface

• When enough stress builds up in rock, it breaks, creating a fault.

• Rocks on either side of a fault move up, down, or sideways• Most faults occur along plate boundaries, but can be found

anywhere stress builds up in the crust (ex. There are small faults in central Oklahoma, this is why they had EQ activity recently!)

• Three kinds of faults: normal, reverse, strike-slip- each differs in the direction rock moves on either side of fault

• Movement along a fault usually results in seismic activity…. earthquakes!!!

Comparing FAULTSType of Fault Stress Effect on crust

NORMAL Tension Pulls rock apart- up/down motion (forms at divergent boundaries

REVERSE Compression Pushes rock together- down/up motion

STRIKE-SLIP Shearing Plates move past each other- sideways motion (transform boundaries)

Your task: Show your understanding of forces in Earth’s crust using various forms of graphic organizers.

1. Complete Chart for types of stress. Use information from the slides & textbook.

2. Fill in a Venn Diagram comparing Folding & Faulting. You should include AT LEAST 3 similarities and differences- be specific!

3. Compile all of the material in an original acrostic using the word “STRESS”. For each letter, write a descriptive sentence. Add at least 1 colored drawing that shows an important fact about stress.