Science High School Reviewer | Grade 4 Science | Earth Science | Shaping Earth | Lesson 17: The Moving Crust

You have learned that Earth is made up of
layers. The crust is the thin outer layer. The
mantle lies beneath the crust.

Plates
Earth’s surface is broken into several huge
plates of rock. Plates are made of crust. The crust
is solid. The upper mantle can flow. When the
mantle flows, Earth’s plates move.
Earth’s plates move about as slowly as your
fingernails grow. The edges of plates are where
changes to the crust happen. You cannot see or
feel most of the changes. Others you cannot miss!

Faults
Some plates slide past each other
from side to side. The place where
they rub is called a fault. A fault is a
long, narrow crack in the crust.
A plate on one side of a fault
can slide up while the other slips
down. Rising plates can form faultblock
mountains. If the lifting is
spread over a wide area, a plateau
(pla•TOH) may form. A plateau is a
high landform with a flat top.

Folds
Some plates meet at the edges of
continents. If the land scrunches up
between them, a fold forms. A fold
is a bend in the rock layers.
If the land keeps scrunching,
a fold becomes a mountain. A
mountain is a tall landform that rises
to a peak. As time passes, wind and
rain can break off bits and pieces.

What causes
earthquakes? (diagram)

An earthquake is a sudden
shaking of Earth’s crust. It is
caused by plates moving along a
fault. When the plates slide against
each other, energy builds up in the
rock. Rocks may store this energy
for many years in Earth’s crust.
Then suddenly, they break apart!
Earthquakes are common in places
with active faults, like parts of
Alaska and California.

How Earthquakes Travel
An earthquake begins below
the ground. The energy released by
a sudden plate motion shakes the
crust. Vibrations, or waves, move
through the crust in all directions.
Did you ever drop a pebble in
water? The waves of an earthquake
travel like the ripples of water. As
they move away from the center of
the earthquake, the waves weaken.
Even so, you may feel them at the
surface hundreds of miles away!

Philippines – Pacific Ring of Fire

Earthquake Safety
Most earthquakes are too weak
to notice. Others can cause extreme
damage. During a major earthquake,
buildings and roads may break
apart. Bridges might collapse.
Do you know what to do if the
ground below you starts to shake?
You can stay safe in an earthquake
by following a few simple rules. If
you are indoors, duck under a table
or doorway. Keep away from walls
and windows. If you are outdoors,
stay away from trees, power lines,
and anything that might fall down.
Earthquakes in the Ocean
Some earthquakes strike below
the ocean. If an earthquake is strong
enough, it can cause the ocean crust
to lift suddenly. When this happens,
look out! A giant ocean wave, or
tsunami (soo•NAH•mee), might hit
the shore. Tsunamis cause the most
damage along coastlines. They can
destroy everything in their path.

How do scientists study
earthquakes?
Any movement can cause a
vibration. Seismic waves are the
vibrations caused by earthquakes.
When an earthquake strikes,
seismic waves travel out from
the source in all directions. The
waves move at different speeds.
Some of the waves travel along or
near Earth’s surface. Others travel
through Earth’s interior.
Measuring Seismic Waves
Scientists measure seismic
waves with an instrument called
a seismograph (SIZE•muh•graf). A
seismograph detects and records
earthquakes. It shows seismic
waves as curvy lines along a
graph. The lines show how much
the ground shakes. The stronger
the quake, the steeper the lines.

Seismic Networks
When an earthquake hits, one of
the first questions is “Where was
it?” Earthquake scientists have a
network of seismographs around the
world. They collect data from each
seismic station. Then they calculate
the location and depth of the quake.

What is a volcano?
A volcano is a mountain that
builds up around an opening in
Earth’s crust. Sometimes a volcano
will force materials from Earth’s
interior out of its opening. Scientists
call this event an eruption.
A volcanic eruption can send out
melted rock, gases, ash, or small
rocks into the air. Melted rock is
called magma. Once magma reaches
Earth’s surface, it is called lava. By
erupting often, a volcano can build a
large mountain. Each eruption adds
a layer of lava and ash. The lava and
ash cool and harden into rock.
Some volcanoes rest quietly
for many years until they erupt
suddenly. Others erupted often in
the past, but will never erupt again.
Where Volcanoes Form
Most volcanoes occur at the
edges of plates. When two plates
meet up, one can sink below the
other. As it sinks, the plate gets
hotter. The rock melts into magma.
The magma rises and forms a volcano.
Volcanoes also form where
Earth’s plates move apart. The space
between the moving plates allows
magma to rise to the surface.
Some volcanoes form far away from
plate edges. These hot spots are places
where Earth’s crust is very thin. Magma
can easily break through to the surface.
The islands of Hawaii formed over
a hot spot in the Pacific Ocean. The
islands are the tops of huge volcanoes
that rose from the ocean floor.