EARTHQUAKES AND
VOLCANOES
Ms. Pollock
Earth and Space Science
Spring 2008
Earthquakes
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Shaking and trembling that results from sudden
movement of part of Earth’s crust
Most common cause faulting
 Break
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in Earth’s crust along which rocks move
On sea floor can cause tsunamis
Ground rises and falls with seismic waves
Energy released during rock movements
Seismic Waves
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Faults possible at Earth’s surface or deep within –
most less than 74 km deep
Point where rocks break and move focus
(underground point of focus)
Surface point directly above focus epicenter; source
of most violent shaking
Earthquakes known as seismic waves
 Tell
about interior of Earth
 Three main types
Seismic Waves
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Primary (P) travel fastest; travel through solid,
liquid, and gas; push-pull waves; cause rock
particles to move back and forth
Secondary (S) travel through solids, but not liquids
or gases; not recorded at all locations, since cannot
pass through molten interior; cause rock particles to
move side to side
Surface (L) waves slowest; travel from focus directly
up to epicenter; most damaging, as they bend and
twist Earth’s surface
Seismographs
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Instrument that detects and measures seismic waves;
weight attached to spring or wire
Not attached directly to Earth, so only moves when
Earth quakes
Creates waves on paper
Seismologists determine strength of earthquake based
on heights of wavy lines
Measured according to Richter scale – measure of
energy released by earthquake
Any number above 6 very destructive
What determines the amount of damage done?
Predicting Earthquakes
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Earthquakes studied in hope of accurate prediction in
future
Useful prediction must be reliable and complete.
Prediction far enough in advance could make city
planning easier.
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San Francisco’s earthquake-proof buildings
Warning signals discovered by scientists
Change in speed of S and P waves
 Small rise or fall of land near faults
 Changes in water levels in wells

Formation of a Volcano
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Magma deep within Earth under intense heat and
pressure
Located in pockets called magma chambers
Magma constantly in motion through cracks or by
melting rocks
Called lava at Earth’s surface
Can build up to form cone-shaped mountain
Location where lava reaches Earth’s surface called
a volcano
Volcanic Eruptions
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Different types of eruptions for different types of
volcanoes
Some quiet, some very violent
Location of lava eruption called vent
Often multiple vents
Chemical composition of magma determined by
analyzing mineral makeup of lava
Types of Lava
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Dark with lot of water
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Light with little water
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Rich in iron and magnesium with silica
Forms rhyolite
Intermediate composition
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Forms basalt
Rich in iron and magnesium
Composition between dark and light
Forms andesite
Large amounts of gases

Forms rock with many holes, such as pumice and scoria
Types of Lava and Eruptions
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Dark lava – quiet flows
 Runny
and smooth
 Islands of Hawaii and Iceland
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Light lava – violent eruptions
 Hardens
in vents
 Steam and new rocks build up beneath vents
 Great pressure to cause explosions
Volcanic Eruptions
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Rock fragments blown into air
Smallest particles volcanic dust; very fine
Particles size of rice grains volcanic ash; falls to
Earth and forms small rocks
Large rock particles volcanic bombs; can be size of
large boulders
Smaller bombs called cinders
Molten when leaving volcano; harden in air
Types of Volcanoes
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Type of eruption affected by type of volcano
Some quiet and over large area
Some violent
Some combinations of quiet and violent
Cinder Cones
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Made mostly of cinders and other rock particles
Formed from explosive eruption
Not high; narrow base with steep sides
Paricutin in Mexico
Shield Volcanoes
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Quiet lava flows
Large area
Gently sloped, dome shaped mountain
Mauna Loa in Hawaii
Composite Volcanoes
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Alternating layers of rock particles and lava
Beginning violent eruption with bombs, cinders, and
ash from vent
Followed by quiet eruption with lava flow that
covers rock particles
Large cone-shaped mountain result of many
alternating eruptions
Mount Vesuvius in Italy and Mount Etna in Sicily
Volcanic Structure
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Often crater at top – funnel-shaped pit or
depression
If the crater becomes larger due to the collapse of
walls, it is called a caldera.
Volcano and Earthquake Zones
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Often volcano and earthquake zones in same areas
of world
Sometimes one is the result of the other.
Most major occurrences in three zones of world.
Volcano and Earthquake Zones
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Ring of Fire
 Around
edge of Pacific
 New Zealand, the Philippines, Japan, Alaska, western
coast of America affected
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Near Mediterranean
 Asia,
India, Italy, Greece Turkey
 Site of many volcanic eruptions
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Iceland to middle Atlantic
 Long
ridge of under-ocean volcanic mountains
 Formation of new parts of Earth’s crust
Continental Drift
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Early 1900s, Alfred Wegener
Earth’s continents once joined in giant landmass,
called Pangaea
Parts of supercontinent “drifted” to current positions
Theory called continental drift
50 years of study and gathering evidence before
acceptance
Evidence From Fossils
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Glossopteris – seed fern that grew in South Africa,
Australia, and India 250 million years ago
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Seeds to large to be carried over ocean to different
continents
Mesosaurus – freshwater reptile found in South America
and Africa
Only able to survive in shallow fresh water; could not survive
to swim across Atlantic
 When the landmass separated, some of the animals were
left on each part.
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Evidence From Rocks
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Rock structure
Cape Mountains of South Africa folded mountains –
formed by crumpling of Earth’s crust
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Glacial deposits
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End abruptly at Atlantic Ocean, matching rocks in Buenos
Aires, Argentina
Rocks left behind by rock sheet; America, Africa, India,
Australia, Antarctica similar
Mineral deposits
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Salt, coal, limestone derived from coral reefs in unusual
locations
Ocean Floor Spreading
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Some scientists not accepting of continental drift until
1950s and 1960s
Better observations of ocean floor and new mapping
techniques
Discovery of midocean ridges and rift valleys
Much volcanic and earthquake activity at midocean
ridges
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Lava erupting from rift valleys pushes ocean floor away as
new seafloor forms
Deep-sea drilling
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Rocks near midocean ridges found to be younger than rocks
farther away from the ridge
Magnetic Stripes
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Magnetic stripes in ocean-floor rocks
Magnetic minerals lined up in molten rock, becoming
permanent magnets as the rocks harden
Discovered shifts in polarity of Earth
Pattern identical on both sides of midocean ridge –
half moving in each direction
Plate Tectonics
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Combination of continental drift and seafloor
spreading
Lithosphere composed of seven major plates,
named after surface features
Edges of continents not always boundaries; most
boundaries on ocean floor
Three types of plate boundaries
Plate Boundaries
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Divergent – plates moving apart; also called
constructive; new material being made; midocean
ridges
Convergent – plates coming together; also called
destructive; material being subducted; trenches;
violent reactions; Ring of Fire
Transform fault – usually found at right angles to
midocean ridges; plates grinding and sliding; site of
many earthquakes
Plate Motion
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Believed that convection currents within Earth cause plate
movements
Density of material altered due to extreme temperatures
within Earth; hotter, less dense matter rises and pushes down
cooler matter
Plates move on top of molten material
Oceanic crust more dense than continental crust; oceanic
plates subducted beneath continental plates
Plate collisions cause folding with volcanoes, mountains, and
trenches
Undersea volcanoes rising above surface of ocean create
island arcs, like the Aleutians of Alaska
Past and Future Drifting
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Plates moving different speeds and directions
Plates believed separate 510 million years ago,
then formed Pangaea
Pangaea surrounded by single large ocean,
Panthalassa
200 million years ago breaking of Pangaea
Current drift rate 1 to 5 cm per year