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Inside Earth
Chapter 2 Section 1
Forces Within Earth
Section 19.1
Forces Within Earth
Faults form when the forces acting on
rock exceed the rock’s strength.
Review Vocabulary
fracture: the texture or general appearance of
the freshly broken surface of a mineral
Stress and Strain
Along the boundaries between two
tectonic plates, rocks in the crust
often resist movement.
Over time, stress builds up.
I. Types of Stress
• Forces that act on rock per unit
volume (changes its shape)
Effects of
Stress
(Taiwan)
A. Compression
1.
2.
3.
4.
Squeezes rock
Folds or breaks rock
Convergent boundary
Results in mountains
B. Tension
1.
2.
3.
4.
Pulls on crust
Stretches rock
Divergent boundary
Results in valley
C. Shearing
1. Rocks slip past each other
2. Slip apart or break
3. Transform boundary
Section 19.1
Stress and Strain
Forces Within Earth
•Compression causes a material to
shorten.
•Tension causes a material to
lengthen
•Shear causes distortion of a
material.
The deformation of materials in
response to stress is called strain.
Even though rocks can be
twisted, squeezed, and stretched,
they fracture when stress and
strain reach a critical point. At
these breaks, rocks can move,
releasing the energy built up as a
result of stress. Earthquakes are
the result of this movement and
release of energy.
II. Types of Strain
A. Elastic deformation
1. Low stress
2. material is compressed, bent, or
stretched
3. When the stress is removed,
material returns to its original
shape.
B. Plastic deformation
1. When stress builds up
past a certain point
a. elastic limit
2. permanent deformation
a. Failure of rock
Most materials exhibit both
elastic and plastic behavior.*
As pressure increases, rocks
require greater stress to
reach the elastic limit. At high
enough temperatures, solid
rock can also deform,
causing it to flow in a fluidlike
manner. This flow reduces
stress.
* pg. 529
III. Kinds of faults
Crustal rocks fail when stresses
exceed the strength of the rocks. The
resulting movement occurs along a
weak region in the crustal rock
called a fault, which is any fracture
or system of fractures along which
Earth moves.
A. Reverse fault
1. Where rocks converge
2. Horizontal and vertical
compression
3. Shortening of the crust
4. One side pushes up relative to
other side
B. Normal Fault
1. Partly
horizontal,
partly vertical
movement
2. Pulls rock apart
3. One side moves
downward
4. Occurs where
plates diverge
C. Strike-Slip fault
1. Very little up & down motion
2. Rocks move past each other
(horizontal shear)
Section 19.1
Forces Within Earth
Visualizing Fault Movement
Types of Faults
Table 19.1 Page 531
IV. Earthquake Waves
Irregular surfaces in rocks can
snag and lock along faults when
movement occurs. As stress
continues to build in these rocks,
they undergo elastic deformation.
Beyond the elastic limit, they
bend or stretch. Before that limit,
an earthquake occurs when they
slip or crumble.
A. Seismic waves vibrations of the ground produced
during an earthquake
1. P waves
a. Primary waves
b. Compress & expand
c. Moves through solids & liquids
d. Fastest
B. S waves
1. Secondary
waves
2. Move side to
side and/or up
and down
3. Moves through
solids only
4. Slower than
P-waves
Primary and Secondary waves travel
through Earth’s interior and are,
therefore, known as body waves
3. Surface waves
a. Ground moves like ocean
waves
b. Slowest
B. Generation of seismic waves
1. Focus
a. point of initial
fault rupture
(failure)
b. usually
several
kilometers
below Earth’s
surface
2. Epicenter – the point on
Earth’s surface directly
above the focus
Faults form when the
forces acting on rock exceed
the rock’s strength.
 Stress is force per unit of
area that acts on a material
and strain is the deformation
of a material in response to
stress.
 Reverse, normal, and
strike-slip are the major
types of faults.
 The three types of
seismic waves are Pwaves, S-waves, and
surface waves.