Transcript The Earth`s Crust - mrgsearthsciencepage

The Earth’s Crust
© Lisa Michalek
Earthquakes
 Any
vibrating, shaking, or rapid motion
of the Earth’s crust.
 Most occur when stress builds along a
zone of weakness or a break in the
rock known as a fault.
 When the crust shifts,
energy is released.
 The energy radiates
in all directions
through vibrations.
Earthquakes
 The
place underground where the
break occurs is the focus of the
earthquake.
 The epicenter is the location at the
Earth’s surface just above the focus.
Measuring Earthquakes
 Magnitude
(Richter Scale)
– Seismographs are the most reliable
measures of earthquakes.
– Each increase in one unit of magnitude
means a ten-fold increase in shaking.
 Intensity
(Mercalli Scale)
– Based upon the reports of people who
experienced the earthquake and
observed the destruction.
Mercalli vs. Richter Scale
Seismic Waves
 P-Waves
– Primary (they arrive first), Pressure, or PushPull.
– Material expands and contracts and particles
move back and forth in the path of the wave.
– Sound waves that travel through solids, liquids
or gases.
 S-Waves
Seismic Waves
– Secondary (arrive later), Shear, or Side-toside.
– Material shears out of shape and snaps back.
– Travels only through solids.
Seismic Waves
 Surface
Waves
– Travel along the earth's surface.
– The slowest waves but the ones that damage in
large earthquakes.
An Earthquake’s Epicenter
 Seismologists
– Scientists who study earthquakes
 Use
the difference in the speeds of P
and S-Waves from three seismic
recording stations to locate the
epicenter
Earthquake Origin Time
 To
find the
origin time,
a seismologist
needs to know
the arrival and
travel time of
the P-waves.
The Earth’s Layers
 The
Crust
– Varies from 5-60 KM
– In most places, a thin layer of
sedimentary rocks covers the mostly
granite-like rocks of the continental
crust
– The oceanic crust,
under layers of
marine sediments,
is composed of
darker and denser
rocks similar to
basalt
The Earth’s Layers
 The
Mantle
– Extends to a depth of about 2900 KM.
– Earthquake waves travel faster in the
mantle than they do in the crust.
– Composed mostly
of dense, the dark
mafic minerals
olivine and
pyroxene.
The Earth’s Layers
 The
Core
– Composed of iron and nickel.
– Outer Core is thought to be liquid
because S-waves are unable to pass
through the outer core.
– The Inner Core
seems to be solid.
The
Earth’s
Layers
Earthquake Shadow Zones
When a major earthquake occurs, both
P-waves and S-waves are received over
most of the earth.
 The opposite side of the earth will receive
P-waves but no S-waves.

– S-waves cannot penetrate the liquid outer core.
There is also a region where neither
P-waves or S-waves are received.
 Refraction (bending) of the waves at the
mantle-core boundary causes this ringshaped region known as the shadow zone.

Earthquake P-wave Shadow Zones
Earthquake P-wave Shadow Zones




A P-wave traveling through the outer core is
labeled K.
A bounce off the
core is labeled c.
A P-wave in the
inner core is I.
S-waves do not
pass through the
core, because the
outer core is fluid.
The “Ring of Fire”
A
large number of the world’s
volcanoes and seismic events occur
around the edges of the Pacific Ocean.
 Japan, the western coast of the United
States are
on the
Ring of Fire.
 These areas
are damaged
frequently by
earthquakes
and volcanoes.
Seismic Hazards
Earthquakes can cause damage by shaking,
movement of the crust, or large waves in
oceans, called tsunamis.
 Seismic Risk Level Map for the U.S.

– Probability of damage in 100 years.
Blue = none
green = minor
yellow = moderate
red = major
Volcanic Hazards
When volcanoes erupt
they may spew hot
lava, hot ash, and/or
toxic gases.
 The lava and ash can
bury cities, and the
toxic fumes can
suffocate people.
 Volcanoes can also
provide fertile soil
that is composed of
weathered volcanic
material.

Continental Drift
 In
1912, Alfred Wegener,
proposed that in the
distant past, Earth’s
continents were all joined as a
single landmass.
 He said that the continents have
separated and collided as they have
moved over Earth’s surface for
millions of years.
Continental Drift
Wegener proposed that if the land areas
were brought back together, the move
would line up ancient mountain ranges,
similar continental rock formations and
evidence of ancient glaciers.
 There are even similar
fossils on both sides
of the Atlantic that
would be brought
back together by
the re-assembly
of Pangaea.

Continental Drift
Evidence from the Oceans
 In
the 1950’s both fossils and the
analysis of radioactive material
showed the age of the oceanic crust
increases with distance from the
mid-ocean ridges.
– Some oceans were growing wider from
the middle.
 Scientists
also
used magnetic
measurements
of the oceanic
crust.
Plate Tectonics
 The
surface of Earth is composed of about
a dozen major rigid, moving crustal plates
and several smaller plates.
 These plates contain areas of light
continental
rock and
dense
oceanic
bottoms.
Plate Boundaries
 The
lines along which plates meet
and interact.
 Convergent boundaries occur from
converging plates.
– As a result of this collision mountains
rise as the crust crumples.
Plate Boundaries
 Subduction
occurs when a dense
oceanic plate dives beneath a lighter
continental plate.
 Subduction forms ocean trenches that
are linear
fractures and
are the
deepest parts
of the oceans.
Plate Boundaries
 When
a plate slides past another
plate, they meet at a transform
boundary.
San Andreas Fault in CA
Plate Boundaries
A
divergent boundary (rift) is
found at the mid-ocean ridges where
upwelling material creates new crust
that moves away from the ridge in
both directions.
What Moves the Plates?
 Convection
currents within Earth
enable heat to escape from the
Earth’s interior.
 These currents create and expand the
ocean bottoms,
and they carry
the continents
as “rafts” of
lighter rock.
Hot Spots
In several places on earth, hot plumes of
magma pierce the crust.
 As a crustal plate moves over this source of
magma, volcanoes form at the hot spot.
 This movement of a
plate leads to the
formation of a chain
of volcanoes of
differing ages.
 One example is the
Hawaiian Islands.
