Earthquakes and Volcanoes
Elastic Limit
• There is an elastic
limit to the amount a
rock can bend and
stretch before it will
break.
• Once the limit is
passed, the rock will
move along a fault
line.
Earthquakes
• An earthquake is the shaking and
trembling that results from the sudden
movement of the rock along the fault.
• Energy is released. The earthquake will
continue until the energy is used up.
Fault Line
Three Types of Force
• Compression is the
force that squeezes
rock together.
• Tension is the force
that pulls rocks apart.
• Shearing is the force
that causes rocks on
either side of a fault to
slide past each other.
Faults
• Tension causes rocks
to be pulled apart and
the rocks move down
along the fault line.
• Compression causes
rocks to squeeze
together and rocks
move up along a fault
line.
• Strike slip is side by
side.
Himalayan Mountains
• The Himalayan
Mountains which
were formed by the
convergence
(compression) of
Indo-Australian Plate
and the Eurasian
Plate has many
reverse and thrust
faults.
Seismic Waves
• The waves of energy generated by an
earthquake are called seismic waves.
• There are three main types of seismic
waves.
• Primary (P waves)
• Secondary (S waves)
• Surface (L waves)
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.
Focus
• 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 earthquake.
Epicenter
• 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.
Primary Waves
• Seismic waves that travel fastest are P
waves. They travel through solids, liquids
and gases.
• 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.
Secondary 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.
P and S Waves
Surface Waves
• 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
and side to side with each L wave.
• L waves cause most of the damage.
The Seismograph
• Invented in 1893 by John Milne, a
seismograph detects and measures
seismic waves.
• 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.
Seismograph
Seismologists
• 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
a seismogram.
Scales Used to Measure
Earthquakes
• The Richter scale is used to calculate the
strength (magnitude) of an earthquake.
• The Modified Mercalli is used to measure
the intensity (amount of destruction) of an
earthquake.
Seismograph Stations
• Each type of wave reaches a seismograph
station at a different time based on its
speed.
• Primary waves arrive first. Secondary
waves travel slower and arrive later.
• The difference in arrival time is used to
estimate the distance from the station to
the epicenter.
Locating an Epicenter
• Scientists need readings
from three or more
stations to determine the
location. A circle is
drawn around each
station. The radius of
each circle is equal to the
station’s distance from
the epicenter.
• The point of intersection
is the location of the
epicenter.
Interior of the Earth
• Seismic waves
change speed as they
move through the
earth’s layers. They
speed up when they
pass through the
bottom of the crust
and enter the mantle.
This boundary is
known as the Moho.
The Moho
• The boundary is
named for the
scientist, Andrija
Mohorovicic who
discovered this
information.
• It is called the
Mohorovicic
discontinuity or Moho.
Seismic Speed in the Mantle
• The mantle is divided into layers based on
changes in seismic wave speed. Both P
and S waves slow down again when they
reach the asthenosphere.
• They speed up when they move through a
more dense part of the mantle.
Predicting Earthquakes
• Scientists need to know the amount of
stress applied as well as the elastic limit of
the rock to determine when an earthquake
will occur.
Predicting Earthquakes
• Scientists have identified warning signals
the help predict earthquakes with greater
accuracy.
• 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
is affected.
Seismic Speed in the Core
• The core is divided into two layers based
on how they travel through it.
• Primary waves slow down and secondary
waves do not travel through the liquid
outer core.
• Primary waves speed up again when they
reach the solid inner core.
Seismic Risk Map of the U.S.
The Richter Scale measures the
strength or magnitude
• An increase of one
magnitude on the
Richter scale means
that 32 times more
energy is released.
• An earthquake of
magnitude 6 is 32 x
32 x 32 times greater
than an earthquake
with a magnitude of 3.
Magnitude
• The height of the lines
traced on the paper of
a seismograph is a
measure of the
energy that is
released, or the
magnitude of an
earthquake.
The Modified Mercalli Scale measures the
amount of damage
• Earthquakes can also
be described by the
amount of damage
they cause. The
Modified Mercalli
scale describes the
intensity of an
earthquake using the
amount of structural
and geologic damage
in a location.
Earthquake Damage
• The amount of damage created by an
earthquake depends on several factors.
• The earthquake’s strength
• The kind of rock and soil that underlies an
area
• The population of the area
• The kind of buildings in the area
• The time at which the earthquake occurs
Earthquake Belts Worldwide
Tsunamis
• 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.
• The Tsunami Warning Center is located in
Hilo, Hawaii.
Volcanoes
• A volcano is an
opening in the Earth
that erupts gases, ash
and lava.
• Kilauea is the world’s
most active volcano.
Formation of a Volcano
Magma
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
magma chambers.
Viscosity of Magma
Lava
• 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 form a cone-shaped mountain.
• The opening from which lava erupts is the
vent. Volcanoes often have more than
one vent.
Dark-Colored Lava
• 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
flows.
Light-Colored 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.
Combination Lava
• 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.
Gaseous 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
lava.
Volcanic Eruptions
• 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
Cinder Cones
• 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
Shield Volcanoes
• 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
Islands.
Shield Volcanoes
Composite Volcanoes
• 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.
Mount Vesuvius
Mount Vesuvius
• Images of victims in
eruption of Vesuvius
in 79 AD. Most died
as a result of
suffocation.
Composite Volcano
Crater
• 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.
Volcanic Crater
Volcanic Activity
• Volcanoes are rather unpredictable .
Some erupt regularly, others have not
erupted in modern history. Scientists
classify them as active, dormant or extinct.
Active Volcanoes
• 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.
Dormant Volcano
• 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.
Extinct Volcano
• 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
this zone.
Ring of Fire
Mediterranean Zone
• 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.
Earthquake Belts