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Seismic Waves
• When rock under Earth’s surface moves or breaks,
energy travels in the form of seismic waves, which
cause the ground to shake and vibrate—an
earthquake.
Study of seismic waves has led scientists to understand
that Earth is a layered planet consisting of:
• Crust
• Mantle
• Outer core
• Inner core
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Seismic Waves-Body Waves
Two main types of seismic waves:
• Body waves travel through
Earth’s interior
—Primary waves (P-waves)
—Secondary waves (S-waves)
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Primary Waves:
• Longitudinal motion:
—compress and expand the material through which
they move.
—occurs parallel to the wave’s direction of travel.
• Travel through any type of material—solid
rock, magma, water, or air
• Fastest of all seismic waves—first to register
on a seismograph.
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Secondary Waves:
• Transverse:
—Vibrate the rock in an up-and-down or
side-to-side motion.
—Occurs perpendicular to a wave’s direction of
travel.
• Travel through solids—unable to move
through liquids.
• Slower than P-waves—second to register on
a seismograph.
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Surface Waves:
• Slowest seismic waves and the last to
register on a seismograph.
– Rayleigh waves have a rolling-type of motion:
(similar to ocean wave movement)
Ground moves up and down.
– Love waves have similar motion to S-waves:
Horizontal surface motion (side to side)
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• Abrupt changes in seismic-wave velocity
reveal boundaries between different materials
within the Earth.
• The densities of the different layers can be
estimated by studying the various seismicwave velocities.
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Discovery:
Crust–Mantle Boundary
ˇ ´ had observed a
• In 1909, Andrija Mohorovicic
sharp increase in seismic velocity at a
shallow layer within Earth.
• Discovered the crust–mantle boundary.
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Discovery:
Mantle-Core Boundary
• Richard Oldham observed that
P-waves and S-waves travel together for a
distance, then encounter a boundary where
the S-waves stop and the P-waves refract.
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Mantle-Core Boundary
• In 1913, Beno Guttenberg
refined Oldham’s work by
locating the depth of the
core-mantle boundary
(2900 km).
P-wave shadow (where no
waves are detected) over part
of the Earth.
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Interpreting the
Core-Mantle Boundary
• In 1926 Sir Harold Jeffries determined
that part of the core must be liquid.
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Earth’s Internal Layers:
Inner Core-Outer Core
• In 1936, Lehmann
observed that Pwaves also refract at
a certain depth within
the core.
– At this depth, Pwaves show an
increase in
velocity, indicating
higher density
material.
• Lehmann discovered
the two parts: a liquid
outer core and a solid
inner core.
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Putting it together:
The discoveries of
everyone indicate
that Earth is
composed of three
layers of different
compositions: the
crust, mantle, and
core.
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The Crust:
• Oceanic crust –compact (10 kilometers in thickness)
— Composed of mafic rocks.
• Continental crust (20 and 60 kilometers)
– Composed of felsic rocks.
Low-density crust floats on the denser, underlying mantle
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Why are continents high and
oceans low?
• Isostasy!
Areas of continental crust stand higher
because it is thicker and less dense than
oceanic crust.
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The Mantle:
• 82% of Earth’s volume/65% of Earth’s mass.
• Earth’s thickest layer—2900 km
• Rich in silicon
and oxygen.
— Some iron,
magnesium, and calcium.
Divided into
two regions—upper mantle
and lower mantle.
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The upper mantle has two zones:
• Lithosphere - the
uppermost plus the
crust.
– cool and rigid.
—does not flow but rides
atop the lower portion
• Broken up into
individual plates.
• Movement of
lithospheric plates
causes earthquakes,
volcanic activity, and
mountain building.
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The upper mantle has two zones:
• Lower part-the
asthenosphere.
– behaves in a plasticlike manner, allowing
it to flow easily.
• The constant flowing
motion greatly
affects the surface
features of the crust.
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The Lower Mantle:
• Extends from a depth
of 700 kilometers to
the outer core.
• Under great
pressure the rock is
solid.
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The Core:
• Composed predominantly of
metallic iron.
• 2 layers—a solid inner core;
liquid outer core.
– The inner core is solid due
to great pressure.
– The outer core is under
less pressure and flows in
a liquid phase.
Earth’s magnetic field
generating flowing molten
core.
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Earthquakes
• Can occur on or
between plate
boundaries.
• Strain begins at
depth as elastic
deformation.
• When the build-up
of stress exceeds
the rock’s elastic
limit, the rock
breaks.
• This is how a fault
forms.
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There are three type of stress caused by
interactions between plate boundaries:
• Compressional stress—slabs pushed
together
• Tensional stress—slabs pulled apart
• Shear stress—slabs are both pulled and
pushed—sliding
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Continental Evidence for Plate
Tectonics: Faults
• Classified by relative
direction of movement
(displacement).
– Footwall
– Hanging wall
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Continental Evidence for Plate
Tectonics: Faults
In a normal fault, the hanging wall drops
down relative to the level of the footwall.
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Continental Evidence for Plate
Tectonics: Faults
A reverse fault occurs when the hanging wall
is pushed up relative to the footwall.
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Continental Evidence for Plate
Tectonics: Faults
In a strike-slip fault, blocks of rock slip past
one another with very little vertical
displacement.
(The San Andreas Fault is a strike-slip fault)
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Earthquake Measurement
• The Richter scale measures the energy
released in terms of ground shaking.
• Each increase of one unit on the scale
is a ten-fold increase in amplitude.
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Tsunami
• A giant sea wave, or series of sea waves,
generated by a powerful disturbance that
vertically displaces the water column.
• Reverse fault earthquakes thrust the seafloor
upward.
• Huge, displaced mass of water drops back
down to sea level and a large wave is
generated.
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Continental Evidence for Plate
Tectonics
Rocks respond to stress in 3 ways:
• Elastic deformation—returning to
original shape
• Brittle deformation—breaking
• Plastic deformation—flowing
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Continental Evidence for Plate
Tectonics: Folds
Syncline: Layers tilt in toward a fold axis.
Anticline: Layers tilt away from axis.
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