Bellringer
Describe how Plate Tectonic forces and
earthquakes might be related.
Forces in Earth’s
Crust
Notes
Types of Stress
• When Earth’s plates move, rocks are
pushed and pulled. The pushes and
pulls are called stress.
• Stress adds energy to rocks. Rocks
keep storing the energy until they
cannot stand any more more stress.
Then the rocks break or change shape.
• If a rock only changes shape, this is
known as a fold. If a rock breaks, this is
a fault.
Fold
Fault
Forces in Earth’s
Crust
• Tension is stress that pulls and
stretches rocks. Tension makes rocks
thinner in the middle. Tension happens
when two plates move apart, acting like
two suction cups.
Forces in Earth’s
Crust
• Compression is stress that squeezes
rocks. Compression makes rocks fold
or fault. Compression happens when
two plates push together.
Forces in Earth’s
Crust
• Shearing is stress that pushes rocks in
opposite directions. Shearing makes
rocks break, slip apart, or change
shape. Shearing happens when two
plates slip past each other in opposite
directions.
180
Kinds of Faults
• A fault is a break in Earth’s crust where
rocks are under stress.
• In many faults, the fault line is slanted.
So the block of rock on one side of the
fault is above the bock of rock on the
other side of the fault. The overhead
wall is called the hanging wall. The
downward block is called the footwall.
Kinds of Faults
• There are three different types of faults
(one for each type of stress): normal,
reverse, and strike-slip.
• A normal fault happens when tension
pulls rocks apart (divergence). In a
normal fault, the hanging wall appears
to move down relative to the footwall.
• Normal faults are found all along the
Rio Grande rift valley in New Mexico.
Kinds of Faults
• A reverse fault happens when
compression pushes rocks together
(convergence). In a reverse fault, the
hanging wall slides up and becomes
higher then the footwall.
• Movement along reverse faults
produced part of the northern Rocky
Mountains in the western United States
and Canada.
Kinds of Faults
• A strike-slip fault happens when
shearing pushes rocks in opposite
directions (transform). In a strike-slip
fault, two blocks of rock move past each
other, but neither block moves up or
down.
• The San Andreas fault in California is
an example of a strike-slip fault.
Changing Earth’s
Surface
• Stresses in Earth’s crust cause the
surface to change. Different stresses
cause different changes.
• Compression causes folding. Folding is
the bending of rock without breaking.
Folding is like a rug getting wrinkled up
when it is pushed across the floor.
Changing Earth’s
Surface
• Folds that bend upward into ridges are
called anticlines. Folds that bend
downward into valleys are called
synclines.
• Anticlines = think how “A” points
upward.
• Synclines = “sync” or “sink” down.
• The central Appalachian mountains, the
Himalayas, and the Alps are all folded
rocks.
Changing Earth’s
Surface
• Tension causes stretching. When crust
stretches, many normal faults form.
• Sometimes a block of rock moves
upward between two normal faults. The
block forms a mountain called a faultblock mountain.
• When the block between two normal
faults moves downward, this is called a
rift valley.
Changing Earth’s
Surface
• Stresses in the crust can also form
plateaus. A plateau is a large area of
flat land that has been lifted up above
sea level.
• Forces deforming the crust uplifted the
Colorado Plateau in the “Four Corners”
region of Arizona, Utah, Colorado, and
New Mexico. Much of the Colorado
Plateau lies more than 1,500 meters
above sea level.
The flat land on the horizon is the Kaibab Plateau, with forms
the North Rim of the Grand Canyon in Arizona. The Kaibab
Plateau is part of the Colorado Plateau.
This uplift of the crust is what allowed the Colorado River to
erode its way down over hundreds of millions of years to its
current position.