Forces that Shape the Earth

Forces that Shape the Earth

8th Grade Science

Continental crust constantly changes over time due to plate tectonics. Forces at plate boundaries are strong enough to break rocks or change their shape.

Stress

Force that acts on rock to change its shape or volume, adds potential, or stored energy, to rock until it changes shape or breaks.

Three different types of stress can occur in Earth's crust including:

Tension

Compression

Shearing

Tension

Pulls on the crust, stretching rock so it becomes thinner in the middle.

Compression

Squeezes rock until it folds or breaks.

Shearing

pushes a mass of rock in two opposite directions.

Strain

a change in the shape of a rock caused by stress.

Two Types of Strains are:

• Elastic Strain - change in rock that is not permanent, when stress is removed the rock will go back to its original shape.

• Plastic strain - creates a permanent change in the shape of a rock, usually occurs when rocks are weak or hot.

Each type of stress mentioned above produces different landforms.

Landforms Created by Compression

• Mountain Ranges - collision between two continental plates

• Ocean Trenches - one plate goes under another during collision forming a deep trench where the two plates meet.

• Volcanic Arcs - curved line of volcanoes that form parallel to plate boundaries.

Landforms Created by Tension:

• Mid-Ocean Ridges - tension causes oceanic crust to spread allowing hot rock from the mantle to rise creating high ridges.

• Continental Rifts - When divergent boundaries occur within a continent, they cause enormous splits in the crust.

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Landforms Created by Shearing

• Transform Faults - when plates slide horizontally past each other they form a fault, or a break in the rock of the crust.

• Fault Zones - an area of many fractured pieces of crust along a large fault.

Multiple Choice

Plastic strain __________________ permanently change or deform rocks.

• does

• does not

Multiple Choice

Elastic strain _________________ permanently change or deform rocks.

• does

• does not

Multiple Choice

Force acting on a surface (such as rock) is called ......?

• density

• mass

• stress

• volume

Multiple Choice

The theory of ____________ proposed that Earth's continents had once been joined together in a single landmass.

• continental drift

• plate tectonics

• landmass

• evolution

Multiple Choice

Identify this picture

• no stress

• compression

• tension

• shear

Multiple Choice

Identify this picture

• no stress

• compression

• tension

• shear

Multiple Choice

This picture shows

• no stress

• compression

• tension

• shear

Multiple Choice

This picture shows

• no stress

• compression

• tension

• shear

Multiple Choice

A pulling apart force

• compression

• shear

• tension

Multiple Choice

Squeezing force that acts from opposite sides

• compression

• shear

• tension

Multiple Choice

Parallel forces acting in opposite directions

• compression

• shear

• tension