• An earthquake is the shaking and
trembling that results from the sudden
movement of part of the Earth’s crust.
• The most common cause of earthquakes
is faulting. During faulting, energy is
released. Rocks continue to move until
the energy is used up.
San Andreas Fault
• The San Andreas fault extends 960 km
from Mexico to the north of California.
• The land to the west is moving north. The
land to the east of the fault is moving
south. All the rocks do not move at the
same time so earthquakes occur in one
area and then another.
San Francisco Quake
• In 1906, movement along the San Andreas
fault caused an earthquake in San
• Earthquakes which occur on the ocean
floor produce giant sea waves called
tsunamis. Tsunamis can travel at speeds
of 700 to 800 km per hour. As they
approach the coast, they can reach
heights of greater than 20 meters.
• Most faults occur between the surface and
a depth of 70 kilometers.
• The point beneath the surface where the
rocks break and move is called the focus.
The focus is the underground origin of an
• Directly above the focus, on the Earth’s
surface is the epicenter. Earthquake
waves reach the epicenter first. During an
earthquake, the most violent shaking is
found at the epicenter.
• There are three main types of seismic
• Seismic waves that travel fastest are P
waves. They travel through solids, liquids
• They move at different speeds depending
on the density of the material through
which they are moving. As they move
deeper in the Earth they move faster.
• P waves are push-pull waves.
• Seismic waves that do not travel through
the Earth as fast as P waves do are called
secondary or S waves.
• S waves travel through solids but not
liquids or gases.
• S waves cause particles to move from side
to side. They move at right angles to the
direction of the wave.
• The slowest moving seismic waves are
called surface waves or L waves.
• L waves originate on the Earth’s surface at
the epicenter. They move along the
surface the way waves travel in the ocean.
The Earth’s surface moves up and down
with each L wave.
• L waves cause most of the damage.
• Invented in 1893 by John Milne, a
seismograph detects and measures
• A weight attached to a spring remains
nearly still even when the Earth moves. A
pen attached to the weight records any
movement on a roll of paper on a
constantly rotating drum. The drum moves
with the Earth and affects the line.
• Seismologists study earthquakes. They
can determine the strength of an
earthquake by the height of the wavy line
recorded on the paper.
• The seismograph record of waves is called
• The Richter scale is used to calculate the
strength of an earthquake.
The Richter Scale
• The amount of damage created by an
earthquake depends on several factors.
• The earthquake’s strength
• The kind of rock and soil that underlie an
• The population of the area
• The kind of buildings in the area
• The time at which the earthquake occurs
• Scientists have identified warning signals
the help predict earthquakes with greater
• Often changes occur in the speed of P and
S waves before an earthquake occurs.
• Sometimes slight changes in the tilt of the
Earth’s surface can be detected.
• Some scientists believe animals behavior
Formation of a Volcano
Deep within the Earth, under tremendous
pressure and at great temperatures, rock
exists as a hot liquid called magma. This
molten rock is found in pockets called
• When the magma reaches the surface, it
is called lava. The place in the Earth’s
surface through which magma and other
materials reach the surface is called a
volcano. In some places, lava can build
up to forma cone-shaped mountain.
• The opening from which lava erupts is the
vent. Volcanoes often have more than
• There are four types of lava.
• One is dark-colored and contains a lot of
water. This is rich in iron and magnesium
and cools to form igneous rocks such as
basalt. This lava is thin and runny and
most tends to flow. The islands of Hawaii
and Iceland were formed by many lava
• The second type of lava is light in color.
This lava, contains little water and is rich in
silicon and aluminum. Light-colored lava
causes explosive eruptions. Silicon tends
to harden in the vents and form rocks.
Steam and new lava build up under the
rocks. When the pressure becomes great,
a violent explosion occurs. When this type
of lava cools it form the igneous rock,
rhyolite, which resembles granite.
• The third type of lava has a chemical
composition similar to that of both the
dark-colored type and the light-colored
type. Different varieties of igneous rocks
in the Earth’s crust, such as andesite, are
form from this type of lava.
• The fourth type of lava contains large
amounts of gases such as steam and
carbon dioxide. When this lava hardens, it
forms rocks with many holes in them, due
to gas bubbles. Pumice and scoria are
igneous rocks formed from this type of
• During volcanic eruptions, many rock
fragments are blown into the air. The
smallest particles are called volcanic dust.
(less than 0.25 mm)
• Volcanic Ash ( 0.25 -5mm) falls to the
Earth and forms small rocks.
• Volcanic bombs (a few cm to several
meters) are molten and harden as they
travel through the air.
Types of Volcanoes
• Different types of volcanic eruptions form
different types of volcanoes.
• Cinder cones
• Shield volcanoes
• Composite volcanoes
• Volcanoes made mostly of cinders and
other rock particles that have been blown
into the air are called cinder cones.
Cinder cones form from explosive
eruptions. Because the material is loosely
arranged, the cones are not high. The
have a narrow base and steep sides such
as Paricutin in Mexico.
Cinder Cone Volcano
• Volcanoes composed of quiet flows are
called shield volcanoes. Because it is
rummy, the lava flows over a large area.
After several eruptions, a dome-shaped
mountain is formed such as Mauna Loa
(4km over sea level) in the Hawaiian
• Volcanoes built up of alternating layers of
rock particles and lava are called
composite volcanoes. During the
formation of a composite volcano, a violent
eruption first occurs, hurling volcanic
bombs, cinder and ash out of the vent.
Then a quiet eruption, produces lava flow
that covers the rock particles. After
alternating eruptions, a cone-shaped
mountain forms such as Mount Vesuvius.
• Images of victims in
eruption of Vesuvius
in 79 AD. Most died
as a result of
• There is often a funnel-shaped pit or
depression at the top of a volcanic cone.
This pit is called a crater. If the crater
becomes very large as a result of the
collapse of its walls, it is called a caldera.
A caldera may also form when the top of a
volcano explodes or collapses.
• Volcanoes are rather unpredictable .
Some erupt regularly, others have not
erupted in modern history. Scientists
classify them as active, dormant or extinct.
• An active volcano is one that erupts wither
continually or periodically such as Mount
Katmai in Alaska and Mount St. Helens in
the Cascade Range.
• A volcano that has been known to erupt
within modern times but is now inactive is
classified as a dormant volcano. Mount
Rainier in Washington state are example
of dormant volcanoes in the United States.
• A volcano not known to have erupted
within modern history is classified as an
extinct volcano. They have been worn
away almost to the level of their magma
chamber. Scientists can be wrong. Mount
St. Helens was considered to be dormant
but erupted after long periods of inactivity.
Volcano and Earthquake Zones
• Most major earthquakes and volcanic
eruptions occur in three zones of the
world. Scientists believe that there is a
great deal of movement and activity in the
Earth’s crust in these three zones.
Ring of Fire
• One major earthquake and volcano zone
extends nearly all the way around the
edge of the Pacific Ocean. This zone
goes through New Zealand, the
Philippines, Japan, Alaska and along the
western coasts of North and South
America. The San Andreas fault is part of
• A second Major earthquake and volcano
zone is located near the Mediterranean
Zone and extends across Asia into India.
Many countries in the zone, including Italy,
Greece and Turkey, have violent
earthquakes. Many volcanic eruptions
also occur in this zone.
Mid-Atlantic Ridge Zone
• The third major earthquake and volcano
zone extends through Iceland and to the
middle of the Atlantic Ocean. There is
under the ocean a long range of volcanic
mountains called the Mid-Atlantic Ocean
Range. Scientists believe that the volcano
and earthquake activity are due to the
formation of new parts of the Earth’s crust
along the ridge. The volcanic island of
Iceland is part of this zone.