Science High School Reviewer Earth Science: Forces That Shape Earth

What forces change
Earth’s crust?
The forces that move continents can also change
the continents’ shapes. Plates slide past each other
at transform boundaries, and the pieces of rock rub
together. This force, called shearing, works like the
blades of a pair of scissors and causes the rock to
break. Plates collide at convergent boundaries. The
force of this collision, called compression, squeezes
the rock. At divergent boundaries, plates separate
as new crust forms between them. The force of this
separation is called tension. Tension makes the crust
longer and thinner. When force exceeds the rock’s
strength, the rock breaks, forming a fault. A fault is
a break or crack in the rock of the lithosphere along
which movements take place. Faults are usually
located along the boundaries between tectonic plates.

Three Kinds of Faults
Forces cause different kinds of faults. Shearing
forms strike-slip faults. Tension produces normal
faults. In a normal fault, the rock above the fault
moves down. Look at the diagram on the next page.
Can you see how this lengthens the rock layers?
Compression produces reverse faults. In a reverse
fault, the rock above the fault moves up.

Types of Faults
A strike-slip fault is produced at a
transform boundary. The plates slide past
each other without moving up or down.
Slabs of rock move past each other in
different directions. The San Andreas
Fault is an example of a strike-slip fault.

A reverse fault is produced at a
convergent boundary. The plates push
together. Rock above the fault surface
moves upward. The Himalayas in central
Asia were formed at a reverse fault.

A normal fault is produced at a divergent
boundary. The plates pull apart. Rock
above the fault surface moves down. The
Sierra Nevada in California were formed
at a normal fault.
Uplifted Landforms
Mountains form where plates
push against each other. Sometimes
the plates compress rock. Mountains
made up mostly of rock layers folded
by being squeezed together are folded
mountains. At other times, the rock
breaks. Mountains made by huge,
tilted blocks of rock separated from
the surrounding rock by faults are
fault-block mountains.
A large area of high, flat land that
was formed by movement of Earth’s
crust is called a plateau. The Colorado
Plateau formed when rock layers were
pushed upward. The Colorado River,
cutting through part of that region,
eventually formed the Grand Canyon.

What are earthquakes?
Forces at plate boundaries
stretch, push, and bend large sections
of rock. Energy can build up in the
rock for years or even decades. When
the rock breaks or slips, energy is
released, and Earth’s crust moves.
Earthquakes can also occur away
from plate boundaries. Here the
condition of rocks and soil may cause
movements and shifting that can
produce earthquakes.
The point below the surface of
Earth where an earthquake begins
is called the focus. Many smaller
earthquakes, called aftershocks,
can follow a major earthquake.
Aftershocks can be almost as strong
as the original earthquake. They can
continue for days, weeks, or months
after the first earthquake.

Earthquake Waves
The sudden movement of an
earthquake causes rock to vibrate.
A vibration that travels through Earth
and is produced by an earthquake or
volcanic eruption is called a seismic
wave (SIGHZ•mik). Seismic waves
spread out in all directions from an
earthquake’s focus. The location on
the surface directly above the focus is
called the epicenter (EP•i•sen•tuhr).
People located at or near the epicenter
are the first to feel the earthquake.

What can we learn from seismographs?
Earthquakes cause different kinds
of seismic waves. There are two main
types of seismic waves: surface waves
and body waves. Each type vibrates
and travels in a different way and at
a different speed.
Waves near Earth’s surface are
called surface waves. They are generally
the most destructive type of seismic
wave. They move more slowly than
body waves and travel along the surface
of Earth like ripples on a pond.
Body waves travel through the
interior of Earth. There are two types of
body waves: primary waves (P waves)
and secondary waves (S waves).
P waves are the fastest seismic
waves. They travel through gases,
liquids, and solids by pushing and
pulling against the material they pass
through.

When P waves push, they compress,
or bunch up, the material. When P
waves pull, they stretch or expand the
material. This pushing and pulling
causes the material to vibrate forward
and backward in the direction in which
the waves are moving. During an
earthquake, P waves move in the same
direction as the shaking rock.
S waves are much slower than
P waves and travel only through
solids. If an S wave is moving ahead,
the vibrations move either up and
down or from side to side. This causes
the material that the wave is passing
through to shake up and down or from
side to side. S waves vibrate at a right
angle to their direction of travel.
Sensitive instruments on Earth’s
surface record these vibrations. A
seismograph (SIGHZ•muh•graf) is an
instrument that detects, measures,
and records the energy of earthquake
vibrations at a given location. Scientists
can also use seismographs to find an
earthquake’s epicenter.
By using instruments such as these
to study waves, scientists have learned
a great deal about the different
layers of Earth.
Locating the epicenter of an earthquake
The height of a wave on a
seismograph indicates the magnitude,
or the measure of the energy released
during an earthquake. The strength
of an earthquake can be measured
in several ways. One measure is
magnitude, and another is the extent of
damage in an area.
Two Measures of Earthquakes
The Richter (RIK•tuhr) scale is
a set of numbers that describes an
earthquake’s magnitude on a scale of
1 to 10. An increase of 1 on the scale
means a tenfold increase in magnitude.
The strength of an earthquake can
also be measured by its intensity, or the
strength as it is felt on Earth’s surface.

The Mercalli (mer•KAH•lee) scale rates
what people feel and observe when
an earthquake occurs. It is based on
observed effects, not on mathematics.
Because of this difference, the Mercalli
scale is less reliable than the Richter
scale.
Tsunamis
In December 2004, an earthquake
in the Indian Ocean launched a
tsunami (tsew•NAH•mee), a series of
huge waves caused by an earthquake
or volcanic eruption beneath the
ocean floor. The tsunami broke over
the coasts of several nations. It caused
extensive damage and loss of life.
Water in a tsunami moves away
from the epicenter of the earthquake
in all directions. Tsunamis have long
wavelengths and low amplitudes, or
wave heights. The speed of a tsunami
depends on the depth of the water.

Diagrams Richter scale and Mercali scale, pg. 275, SACL6

Protecting against earthquake hazards
How volcanoes form, pg. 276 SACL6
Types of volcanoes
cinder cone volcano, a landform mainly
made up of small rock particles, or cinders.
A second kind of volcanic landform is
a shield volcano, a landform made up of
many layers of rock.
The third kind of volcanic landform is a
composite volcano, a landform made up of
layers of thick lava flows alternating with
layers of ash, cinders, and rocks. These layers
form a symmetrical cone with steep sides that
are concave, or curving inward.

Sometimes a volcano’s crater collapses into
the vent. This forms a very wide crater called
a caldera (kal•DER•uh).
Volcanoes that have erupted recently
are active volcanoes. Some volcanoes are
dormant, or sleeping. They have not erupted
for a long time, but they have erupted in
recorded history. If a volcano has never been
observed to erupt, it is said to be extinct.

Other volcanic landforms
A string of island volcanoes, or
an island arc, can form where one
oceanic plate is driven under another. Part of the sinking plate melts, and
magma moves up through the crust
along a line parallel to where the
plates meet. The Aleutian Islands and
the Philippine Islands are volcanic
island arcs. Where plates move apart,
volcanoes can form at gaps along the
plates’ edges. These volcanic landforms
are called rift volcanoes.
The Black Hills
of South Dakota are dome mountains.
If magma hardens in vertical cracks
across horizontal layers, a dike forms.
When the rocks around a dike are
worn away, the dike looks like a long
ridge. When magma hardens between
horizontal layers of rock, a flat sill is
formed. Sometimes, a sill’s magma is
thick and does not spread out very far
horizontally. Instead, it pushes upward.
This forms a dome-shaped laccolith
(LA•kuh•lith). The largest and deepest
magma formation is a batholith
(BA•thuh•lith). Batholiths are large
pockets of magma that reach deep into
the crust.

Forces That Shape Earth
Weathering is the breaking down of rock
into smaller pieces by natural processes.

Physical Weathering
Physical weathering (also called mechanical
weathering) is the breaking down of rock by physical
changes. It can be caused by freezing water, moving
water, plants, or animals.

Chemical Weathering
Some forces that cause weathering
are chemical in nature. Chemical
weathering is the breaking down
of rock by changes in its chemical
composition. Oxygen and acids are
powerful agents of chemical weathering.

Which forces carry and drop?

Some forces shape Earth’s surface
by moving materials from place to
place. Erosion is the picking up and
removing of rock pieces and other
particles. Particles moved by erosion
usually end up in a different place.
Deposition is the dropping off of
particles in another location.
Wind contributes to the erosion and
deposition that help change the land.
Wind may pick up tiny rock fragments
formed by weathering. When the wind
slows, the particles fall to the ground
Flowing Water
Water is a major cause of erosion.
Moving water carries particles as it
flows downhill. The faster a river
flows, the larger the particles it can
carry. Large particles roll, slide, or
bounce along the bottom and dig into
the riverbed, making the river deeper.
When the river slows down, some of
the particles are deposited as sediment,
or loose pieces of minerals, rock, and
organic material. The sediment forms
a barrier in the river, and water then
flows around the sediment.

Deposition can change the course of
a river and cause the river to turn, or
meander (mee•AN•duhr). Meandering
occurs in rivers with shallow slopes
and slow-moving water. Rivers with
steep slopes are usually straighter and
flow more swiftly.

Two other agents of erosion are
gravity and glaciers. Earth’s gravity
pulls materials from high places to low
places. This downhill movement, called
mass wasting, can happen slowly. After
an earthquake or a heavy rain, it can
happen quickly.

How can moving water change the land?
A river flowing from a high
elevation can make dramatic changes
to the land. As a river travels, it carves
a channel. The flowing water slowly
erodes the riverbed, cutting through
softer layers of rock over long periods
of time. Deep canyons, streams, and
valleys are formed by moving water.
Sediment is transported downhill,
causing additional erosion and making
more cuts in the bedrock. Eventually,
the sediment reaches the sea and is
then deposited offshore.

How Glaciers Form
Glaciers form when more snow falls
in winter than melts in summer. Over
the years, the snow builds up. The
weight of the new snow squeezes the
snow underneath, causing it to change
to ice. When the ice sheet is about 100
meters (328 feet) thick, it begins to
flow downhill because of its weight.

How is soil formed?
Weathering results in loose rock
pieces that can become part of the soil,
which supports rooted plants. Soil is a
mixture of weathered rock, air, water,
living things, and humus (HYEW•muhs).
Humus is material made of decayed
plant and animal remains. Bacteria,
fungi, worms, and insects all contribute
to the formation of humus.

Soil Layers, pg. 290 SACL6
Topsoil is the upper layer
of soil and is made mostly
of humus, water, air, and
minerals, which are naturally
occurring substances in
Earth’s soil. The humus in
topsoil is spongy and holds
water. This makes topsoil an
ideal material for supporting
the growth of plants.
The layer beneath the
topsoil is called the subsoil.
Some humus can be found
near the top of this layer.
As water seeps through the
topsoil into the subsoil, it
brings particles of clay and
other minerals with it.
Beneath the subsoil is partly
weathered parent rock,
the rock from which soil is
formed. Water seeps down
to continue weathering. No
humus exists at this depth.
Beneath this layer is solid
rock, or bedrock.
Different soils have distinct
properties. Clay soils are made of very
fine particles. Sandy soils are made of
particles that are coarser.
All soils have spaces called pores
between the rock fragments. If the
pores in the soil are connected, water
can pass through the soil easily.
This soil is said to be permeable
(PUR•mee•uh•buhl). Sandy soils
are permeable. If the pores are not
connected, or if there are few or no
pores, water cannot pass through
easily. This kind of soil is impermeable
(im•PUR•mee•uh•buhl). Clay soils are
nearly impermeable.

The uses and importance of soil
Wasteful practices

Changes in Geology Over Time

What is relative age?
Two ideas help scientists determine the age of
rock. One is original horizontality, the idea that
sedimentary rock forms in horizontal layers. The
second idea is superposition. This idea says that in a
series of rock layers, the bottom layer is the oldest,
and the top layer is the youngest.
Scientists use these two ideas to help them infer
a rock layer’s relative age. This is its age compared
to other rock layers. By looking at rock layers in an
exposed hill or canyon wall, scientists can tell which
layers are older than others.
By comparing rock layers across a
large region, scientists can make a geologic column.
A geologic column is a listing of Earth’s rock layers
ordered from oldest to youngest.

Fossil formation
What are fossils?
Rock layers often contain fossils.
Fossils are the remains, traces, or imprints
of living things preserved in Earth’s crust.
***diagram

What is absolute age?
was. Radioactive elements in rock
decay, or break apart, into other elements.
This decay occurs at a constant rate,
called half-life. Half-life is the time it takes
for half the mass of the original element
to change into the decay product.

Through this process, scientists are
able to determine the absolute age of
a rock layer. Absolute age is a rock
layer’s age in years.