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The Basics of
Geography
Part 5
Earthquakes and Volcanoes
Earthquakes

As tectonic plates
grind or slip past
each other at a
FAULT or crack
in the earth, the
earth may shake
or tremble.
 Thousands
of earthquakes occur
every year, but most are not felt by
humans.
 A device called a SEISMOGRAPH
detects them by measuring the waves
from quakes.
 The
point directly above the surface
where an earthquake begins, is called
an EPICENTER.
A
man named C.F. Richter developed
the RICHTER SCALE, which
measures the energy released by a
quake. A 2.0 quake would not be
noticed, while a 4.5 would probably be
reported in the news. Major quakes
are those over a 7.0.
 In 1906, the quake that destroyed San
Francisco was likely at least an 8.0.
Earthquake Damage
Earthquakes can trigger landslides, create
fires from broken gas lines, and collapse
buildings on people. AFTERSHOCKS may
occur for days after a large quake.
Earthquakes can also produce deadly
TSUNAMIS, such as the one that struck
the Indian Ocean in December, 2004.
On April 18, 1906, a massive earthquake
struck San Francisco, California early in the
morning as most people still slept.
Fire killed far more people than the quake.
Some refugees lived in tents.
On December 26, 2004, a massive earthquake
over 9.0 struck in the Indian Ocean. It lasted
several minutes, created an enormous tsunami,
and killed hundreds of thousands.
Earthquake Clips

Earthquake Destruction

Earthquakes 101

2001 Seattle Earthquake

2004 Indian Ocean Earthquake and Tsunami
Volcanoes
MAGMA (liquid earth), gases, and water
from the lower crust or mantle collect in
underground chambers. When pressure
becomes too great, these materials pour
out onto the earth’s surface.
 Magma that reaches the surface is called
LAVA.

Until May 18, 1980, Mount St. Helens in
Washington had been dormant.
In spring 1980, the mountain began to show
activity, and it bulged.
The largest recorded
landslide in human
history takes place,
helping to trigger the
eruption of the
volcano.
An ash cloud darkened the Pacific Northwest,
and eventually circled the planet.
Mount St. Helens – June 2007
Just 5 miles from the volcano.