Download forces in earth`s crust

FORCES IN EARTH’S CRUST
Chapter 5 Section 4
Grade 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
TENSION
COMPRESSION
SHEARING
Direction of
movement
PULLS CRUST
APART
PUSHES CRUST
TOGETHER
PUSHES IN
OPPOSITE
DIRECTIONS
Effect on crust
STRETCHES
ROCK-BECOMES
THINNER
SQUEEZE &
COMPACTS ROCK
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.
- Forces in Earth’s Crust
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.
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 FAULTS
Type of Fault
Stress
Effect on crust
NORMAL
Tension
Pulls rock apartup/down motion
(forms at divergent
boundaries
REVERSE
Compression
Pushes rock togetherdown/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.