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Plate Tectonics
The Slowly Changing Crust
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A Crash Course in Seismic Waves
Why?
Studying them produced the model of the Earths interior that
seems to best fit multiple geologic phenomena
What?
P-waves
P = primary, because these are the fastest moving, and first to arrive after a
quake
Compressional waves that are able to move through both solids and liquids
S-waves
S = secondary; slightly slower than P-waves
Only able to move through solids
Surface waves
Well discuss these when we get to earthquakes
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The Earths Interior
Structure
Lithosphere: Crust and upper mantle solid and rigid
Asthenosphere: lower mantle semi-solid, semi-fluid
Boundary between them is known as the Mohorovoi
Discontinuity, or Moho
Discovered by studying changed in velocities of seismic waves
Outer Core: molten iron and nickel
Inner Core: solid iron and nickel S-waves cant travel through liquid; absence of S-waves on other side
of the world after earthquakes lead to idea of a molten interior
Different parts to the core helps explain the presence of Earths
magnetic field, as well
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Structure
Lithosphere: Crust and upper mantle solid and rigid
Asthenosphere: lower mantle semi-solid, semi-fluid
Boundary between them is known as the Mohorovoi
Discontinuity, or Moho
Discovered by studying changed in velocities of seismic waves
Outer Core: molten iron and nickel
Inner Core: solid iron and nickel S-waves cant travel through liquid; absence of S-waves on other side
of the world after earthquakes lead to idea of a molten interior
Different parts to the core helps explain the presence of Earths
magnetic field, as well
The Earths Interior
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Alfred Wegener
Theory of
Continental Drift:
The continents were
once together as one
giant landmass calledPangaea, and have
slowly been drifting
apart throughout time.
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Alfred Wegener
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Wegeners Influences
James HuttonGeologic Time
Credited with determining that the age of theEarth was millions of years, rather than thethousands of years that was widely believed atthe time.
Charles Lyell Law of Uniformitarianism: the processes
that affect the Earth now are the sameprocesses that have affected the Earth
throughout time.
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Wegeners Evidence
Shape of the continents
He noticed that Africa and South Americaseemed to fit together like a jigsaw puzzle,
as well as other continents
Wegener wasnt the first to notice this, but
he was the first to support his ideas of
moving continents with evidence
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Wegeners Evidence
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Wegeners Evidence
Geologic Evidence Types of Rocks
Rock layers and rock formations ondifferent coastlines matched in thickness,rock type, and age.
Geologic Evidence Glacial Deposits
Wegener also found glacial till depositsand striations on the bedrock in Africa andSouth America, suggesting glacial activityon continents currently in hot climates.
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Wegeners Evidence
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Wegeners Evidence
Geologic Evidence Coal Deposits
Wegener pointed out that coal deposits inEastern Europe, North America, and
Northern Asia could have only formed in
warm, humid climates, rather than in the
cool climate areas they are found today.
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Wegeners Evidence
Geologic Evidence Coal Deposits
Wegener pointed out that coal deposits inEastern Europe, North America, and
Northern Asia could have only formed in
warm, humid climates, rather than in the
cool climate areas they are found today.
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Wegeners Evidence
Geologic Evidence Fossils
Wegener found fossils of non-swimmingorganisms on separate continents
Fossils came from a specific time period,
when there was no evidence of land bridges
between continents
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Wegeners Evidence
Geologic Evidence Fossils
Wegener found fossils of non-swimmingorganisms on separate continents
Fossils came from a specific time period,
when there was no evidence of land bridges
between continents
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Wegeners Evidence
Geologic Evidence Broken Mountain
Chains On different continents there are the geologic
remains of ancient mountain ranges that
have been weathered down. The rock
composition and age would suggest that theyused to be part of the same range.
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Wegeners Evidence
Geologic Evidence Broken Mountain
Chains On different continents there are the geologic
remains of ancient mountain ranges that
have been weathered down. The rock
composition and age would suggest that theyused to be part of the same range.
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Wegeners Critics
Didnt think the theory could possibly be
true because:
Wegener died in Greenland on an
expedition trying to prove his theory.
1. Nobody could see or feel the continents moving!
2. They couldnt think of any force that could
possibly be strong enough to move hugemasses of rock like continents.
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SEAFLOOR SPREADING
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SEAFLOOR SPREADING
Evidence
Age of RocksSamples of rocks taken from different areas of the
ocean floor showed that rocks closest to the mid-
ocean ridges are the youngest, and as you move
further away from the ridge, the rocks get older
PaleomagnetismAncient magnetic reversals are mirrored on either side
of the mid-ocean ridges
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SEAFLOOR SPREADING
Evidence
Age of RocksSamples of rocks taken from different areas of the
ocean floor showed that rocks closest to the mid-
ocean ridges are the youngest, and as you move
further away from the ridge, the rocks get older
PaleomagnetismAncient magnetic reversals are mirrored on either side
of the mid-ocean ridges
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Basics of Plate Tectonics
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Basics of Plate Tectonics Interactions of Plates(collisions, more specifically)
Continental-Oceanic Since the oceanic plate is made of basalt and is more dense, it
sinks beneath the continental plate (SUBDUCTION)
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Basics of Plate Tectonics Interactions of Plates(collisions, more specifically)
Continental-Continental Since they are relatively the same density, neither sinks immediately;
instead, they both crumple until one eventually subducts
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Basics of Plate Tectonics Interactions of Plates(collisions, more specifically)
Oceanic-Oceanic Similar to two continents colliding, except since these are more
dense, subduction happens more easily than with continentals
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Basics of Plate Tectonics Total Amount of Crust
Is relatively constant; for any crust that getsdestroyed during subduction, new crust is
created at mid-ocean ridges.
Rate of Movement Couple inches per year about how fast your
fingernails grow.
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Cause of Plate Tectonics CONVECTION
When a fluid (such as the asthenosphere) isheated, it becomes less dense, and rises
When it cools, it sinks because it becomes
more dense again
This creates currents in the asthenosphere,
which push the plates around
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Cause of Plate Tectonics CONVECTION
When a fluid (such as the asthenosphere) isheated, it becomes less dense, and rises
When it cools, it sinks because it becomes
more dense again
This creates currents in the asthenosphere,
which push the plates around
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Types of Plate Boundaries DIVERGENT
Means moving apart Creates TENSIONAL stress on the crust
RIFT VALLEY gap left between two partsof a diverging continental plate
MID-OCEAN RIDGES undersea mountainranges created where oceanic plates diverge
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Types of Plate Boundaries
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Rift Valley
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Mid-ocean Ridge
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Types of Plate Boundaries CONVERGENT
Means coming together Creates COMPRESSIONAL stress on crust
Several features are associated w/ these
boundaries:
TRENCHES Depressions left in the sea floorwhere one plate sinks beneath another
VOLCANOES As one plate sinks beneathanother, it melts, and the magma rises to the
surface, creating volcanoes
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Locations of Major Volcanic Eruptions
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Types of Plate Boundaries CONVERGENT
Features of convergent boundaries (cont.)EARTHQUAKES When one plate slides
beneath another, they snag on each other then
break apart again, causing earthquakes
These quakes help show where plate boundaries are
The depth of the quakes shows what kind of
boundary it is
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Earthquake epicenter locations
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Types of Plate Boundaries CONVERGENT
Features of convergent boundaries (cont.) FOLDED MOUNTAINS When continents
converge, the crust buckles before one piece
subducts. The buckling and folding creates
mountains like the Himalayas and Rockies. ISLAND ARCS When two oceanic plates
converge and one subducts, the resulting
volcanoes build up above sea level to become an
island chain known as an arc
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Types of Plate Boundaries TRANSFORM BOUNDARY
Plates move laterally, or slide sideways pasteach other
Crust is neither created nor destroyed here,
just deformed
As the plates slide past each other, they lock
up and build SHEARING stress often which isreleased in the form of shallow earthquakes
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Widespread Acceptance By combining continental drift and seafloor
spreading, the theory of plate tectonics has
provided a reasonable and useful
explanation for how the different geologic
features weve discussed came to be.
We can actually observe plate movementsnow with GIS sensors on separate plates,
measuring distances and speeds of plates.