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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.