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Lesson 4 Summary
Use with pp. 228–233
Lesson 4: What causes earthquakes
and volcanoes?
Earthquakes are caused by the sudden
shifting of rock as tectonic plates shift
position. Plate movement is so slow that we
don’t feel it. Sometimes, plates don’t move
for years because jagged rock edges in the
lithosphere have stopped their movement.
But over time, pressure builds up. When
it gets too strong the rocks suddenly move
forward and earthquakes occur.
The underground point where the
earthquake occurs is called the focus. This
is where energy first builds up. The point
on Earth’s surface directly above the focus
is called the epicenter.
Energy from an earthquake is released
in waves. Vibrations in the form of waves
spread out from the focus and the epicenter.
Waves cause the ground to move back and
forth, up and down, or in a circular motion.
As the waves spread out, they lose energy.
For this reason, the worst damage usually
occurs closest to the epicenter.
Most major earthquakes in the United
States have occurred in California and
Alaska. These states are on the plate
boundary between the Pacific Plate and the
North American plate. Most earthquakes
take place near the edges of plates. The
strength of an earthquake is measured on
a magnitude scale. Each increase of 1 on
the scale represents 31 times more energy
that is released. The effect of an earthquake
depends on its size and strength, distance
from the epicenter, kind of rock in the area,
types of buildings in the area, and the
number of people who live there.
Volcanoes
A volcano occurs when magma from the
mantle comes through the crust of Earth.
The magma comes from the melted layer
of the mantle. Like earthquakes, most
volcanoes occur near plate boundaries.
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Chapter 8, Lesson 4 Summary
When one plate sinks beneath another
at a plate boundary, the sinking crust melts
into magma. Pressure builds up from gasses
trapped in the magma. When the crust of
the overlying plate can no longer withstand
the pressure, magma explodes through it as
a volcano. Magma that reaches the surface
is called lava.
Volcanoes also take place where plate
boundaries are moving away from each
other on the ocean floor. This type of volcano
is much more common and less destructive.
Magma quietly flows out onto the ocean
floor, and new crust forms from the lava.
Predicting Volcanoes and
Earthquakes
Scientists use many tools to predict
earthquakes and volcanoes. Seismometers
detect tremors, or shaking movements.
Tremors may mean that magma is rising
or that Earth’s plates are shifting. Another
instrument, the tiltmeter, detects changes
in the tilt of the land. This can mean that
magma is rising within a volcano. Scientists
also study patterns of volcanic eruption.
Predicting earthquakes is harder.
Scientists can find the location of faults, but
they can’t easily predict when the plates
will shift. They listen for tremors with a
seismograph. The pen of the seismograph
records movements of the ground on a
sheet of paper. Scientists also look for small
movement between plates. Laser beams
directed along fault lines can detect small
changes in ground level. Some ways to stay
safe during an earthquake include:
• Prepare a safety plan beforehand and
follow it during an earthquake.
• If indoors, get under a table or desk.
• Stay away from objects that might fall.
• If outdoors, keep away from buildings,
moving to an open area.
Quick Study
© Pearson Education, Inc. 6
Earthquakes
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Lesson 4 Checkpoint
Use with pp. 228–233
Lesson 4 Checkpoint
1. How is an earthquake’s epicenter related to its focus?
2. Why are volcanic eruptions generally more violent where boundaries
are moving together than where they are moving apart?
3. What causes an earthquake?
© Pearson Education, Inc. 6
4. What should you do during an earthquake?
Quick Study
Chapter 8, Lesson 4 Checkpoint
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