Chapter 17 – Earthquakes and Volcanoes
17A – Earthquakes
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Plate tectonic theory: states that the earth’s crust is made up
of several large, flat pieces called plates.
fold and fault block mountains are thought to be a result of
colliding plates.
What is an Earthquake
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earthquakes consist of several shockwaves traveling through the
earth.
seismologist: someone who studies earthquakes
seismographs: instruments used to measure earthquakes
Richter scale: scale of which earthquakes are measured
each increase in number on the Richter scale is an increase in
31.6 times the energy as the last number
if an earthquake of 10 occurred anywhere, everyone would feel it
most earthquakes last less than a minute
center of an earthquake is the focus (it is underground)
place on earth’s surface above the focus is the epicenter
earthquakes near water can cause tsunamis (large waves of
water)
Earth Waves
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P waves: first to be detected since they are the fastest waves.
Travel through all of the earth’s interior including the core and
are the weakest wave
S waves: second to be detected (second fastest). Travel
through the interior except for the core and are stronger than P
waves.
L waves: slowest and largest waves. Travel only along the
surface
Seismologists can tell the distance of an earthquake by
measuring the difference in time from the arrival of P and S
waves. With three or more stations, the epicenter can be found
Faults
 Modern thought is that earthquakes cause faults and not the
other way around
 Movement can be horizontal or vertical
 normal fault: wider-topped landmass moves down
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reverse fault: wider-topped landmass moves up
strike-slip fault: horizontal motion fault
Attempted Prediction
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scientists are still years away from this goal
9/10 (90%) of all earthquakes occur in Circum-Pacific and
Alpine-Himalayan volcano belts
erupting volcanoes often trigger earthquakes
can attempt to predict by studying the history of earthquake
activity in the area
plate tectonics do not accurately indicate when an earthquake
will occur or when a volcano will erupt
San Andreas fault is located in California and threatens to toss a
section of California into the sea
17B – 1 Volcanoes
What is a volcano
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word “volcano” came from Vulcano Island of the Mediterranean
Sea and was named after the Roman god of fire Vulcan.
volcano: land form built up by molten rock that has come to the
earth’s surface through a vent
vent: a cylindrical opening that connects the surface of the earth
with a source of magma far below
volcanoes emit lava, cinders, and volcanic ash
crater: depression at the top of a volcano cone
caldera: enlarged depression at the top of a volcano
Emissions
Gases
 lava emits gases and is usually a sign that lava has reached the
surface
 glowing avalanche: a hot mixture of solid particles suspended
in water vapor or other gases
 can emit steam and water vapor can have negative
consequences if the volcano is snow topped.
 lahars: mudslides caused by melting of snow on a volcano
Lava
 lava flows slower than a person can walk and is easy to avoid
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a thin, runny magma has more of a chance as flowing as lava
than thick magma
Solid Materials
 ash: tiny angular, glassy fragments of solidified magma
 people can suffocate on ash and is a common way to die
concerning volcanoes
 cinders: similar to ash except for larger
 bombs: blobs of lava which solidify as they fly though the air
 volcanic eruptions can be very loud
17B – 2 Classification
By Structure
 volcano’s shape and structure depends on what it emits
 shield volcanoes: volcanoes that emit mostly lava and are built
up by smooth flows of lava (have large surface area)
 Hawaiian Islands example of shield volcanoes
 cinder cones: volcanoes that emit mostly ashes, cinders, and
bombs and are usually small with steep sides
 Parícutin in Mexico is a cinder cone volcano
 composite volcano: volcano that emits lava and solid debris
 most volcanoes in the world are composite
By Activity
 active volcano: volcano that has erupted within 50 years ago
 dormant volcano: volcano that has erupted in the past, but not
within 50 years ago
 extinct volcano: volcano that has no historical record of
erupting
 these name can be misleading because volcanoes can erupt
whenever without warning
Locations
 there are over 500 active volcanoes in the world
 Circum-Pacific belt: (ring of fire) of around the Pacific is known
for its highly active volcanoes
 Alpine-Himalayan belt: belt of volcanoes including
Mediterranean area eastward to the Indonesian states
17C – Heated Ground Water
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Thermal gradient: average increase of temperature with every
kilometer of depth in the earth (30o C/km)
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ground water can be heated in two ways, either by the thermal
gradient or by being near a magma channel
Types of Heated Ground Water
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hot spring: where hot water rises to the surface
travertine: a mineral from hot water springs which forms
terraces on hillsides
fumaroles: vents in the ground where steam and other gases
escape
fumaroles can give off carbon dioxide which can collect in low
areas suffocating the wildlife
geyser: thermal spring that ejects water from the ground in
intervals
geysers openings are often surrounded by a whitish deposit
called geyserite (which consists of silica dissolved from igneous
rocks)
Harnessing Geothermal Energy
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geothermal energy: energy from hot ground water (can be
used as both a source of heat or for generating electricity)
natural steam can also be used to heat homes and water for
bathing
using geothermic energy for electricity began in 1904 in
Larderello, Italy
geothermal electric plants can generate only 1/3 the energy of
nuclear power plants