How The Plate Tectonic Theory Evolved

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How The Plate Tectonic Theory Evolved

• There were two different concepts which were
  developed which preceded the current accepted
  theory of plate tectonics.
• Those two concepts which are now part of the
  theory of plate tectonics are
• Continental Drift
• Sea Floor Spreading

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Continental Drift
Alfred Wegener published this theory in 1915 in a
book called The Origin of Continents and Ocean.
He hypothesized that the earths crust floated
on top of a liquid core.
Wegner
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Some People Before Wegner
Wegners book tied together the work of other
scientist such as an Austrian geologist named
Eduard Suess and South African geologist named
Alexander du Toit. Both Suess and du Toit had
done extensive work independently with fossils
that pointed to similarities between the land
masses that we know as continents to be today.
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What Wegner Proposed

• 200 million years ago there was a massive single
  continent on planet earth called Pangaea. Pan
  which is Greek for all and gaea which means
  earth.
• Pangea split into two halves
• Gondwanaland
• Laurasia
• They were separtated by the Tethys Sea

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Pangea Breaks Up

• Gondwanaland splits up and forms
• Africa
• South America
• Antarctica
• Australia
• India
• Laurasia splits up and forms
• North America
• Eurasia (except India)

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Watch Pangea Break Up
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Time Line for Break Ups

• After Wegners work other people did work on the
  theory. Here is what people accept as a time
  line of events for how the continents have
  changed over time.

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Time Line for Continental Drift

• 1.1 Million Years Ago. (Ma) Super continent
  called Rodinia (predates Pangea)
• ? Rodinia splits into Laurasia and Gondwanaland.
  Laurasia goes north and Gondwanaland goes south.
• 514 Ma. Laurasia drifts south toward
  Gondwanaland.
• 425 Ma. Laurasia crashes into Gonwanaland.
  (North America smashed into Northwestern Africa)
• Pangea is formed.
• 200 Ma. Pangea breaks apart forming a valley
  called the Atlantic Ocean.
• 152 Ma. The continents as we know them today
  started to form
• 65 Ma. All the present continents were in place
  except India. India eventually drifted north and
  hit Asia forming the Himalayas.
• 250 to 300 Ma. Pangea will re-form?

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Sea Floor Spreading

• In 1962 Harry Hammond Hess introduced the concept
  call Sea Floor Spreading.
• His idea was that cracks open along the crest of
  mid-ocean ridges and that new seafloor forms in
  those area.

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The Earth 200 Million Years From Now?
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What Hess Said

• New floors on the ocean form when magma rises
  from the mantle where there is a large amount of
  pressure. This is underneath the crust. The
  magma wells up through the crack in the ridge of
  the crust, runs down the sides, and solidifies to
  form the new ocean floor.

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What Sea Floor Spreading Looks Like
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Plate Tectonics(The Whole Thing Comes Together)
In 1965 a Canadian geologist named Wilson coined
the term plates to describe the pieces of the
earth crust. Wilson went so far as to map out
faults in the earth which showed where these
plates were. In 1967 D.P. Mackenzie of England
and R.L. Parker introduced and tied together all
the theories that had preceded this and started
using the word plate tectonics.
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What We Know Today

• Today scientist believe there are 9 major plates
  and 12 minor plates.
• Scientist have been able to verify this theory in
  a number of ways. The most convincing one is
  satellite imaging and remote sensors
• The plates are still moving today. Some as slow
  as 1.2 inches a year and some as fast as 4 inches
  a year. (10 feet in 100 years!

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A Look at the Plates of the Earth
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How Does This Work?
To understand Plate Tectonics as a theory you
have to understand the layers of the earth. Lets
talk about that!
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Layers of the earth are called

• The four layers of the earth are
• The inner core(gray)
• The outer core (red)
• The mantel (orange/brown)
• The crust (brown)

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How Big Are We Talking
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The Inner Core

• The inner core is a solid ball.
• This ball is made of iron and
• nickel.
• The center is believed to be solid because of the
  extreme pressure. (45,000,000 per square inch)
• 800 miles thick
• The solid inner core was discovered in 1936 by
  Inge Lehmann

Lehmann
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The Outer Core

• The outer core is made of melted metals
• mostly iron and nickel.(metals in liquid state)
• The outer core is 4000F to about 9000F
• The outer core is about 1400miles thick

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Mantle

• The mantle is 1800 miles thick.
• The temp varies from 1600 at the top to 4000 at
  the bottom. (The rocks are plastic like. It is
  suspected you can bend and mold them.)
• The mantle is composed of hot and dense rocks.
• The rock flows are like asphalt under weight and
  move with temperature change.

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The Crust

• The crust is approximately 30 to 75 km in size.
• At the bottom of the crust layer where it meets
  the Mantle it is about 1600. Even at this
  temperature rock still melts.
• The crust floats on the mantle.
• The crust is broken into 9 major plates.
• The plates are made of continental crust and some
  ocean crust.
• Continental crust can vary from 50 to 5 km.
  Oceanic crust is varies from 10 to 5 km.

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Tying This All Together
Now that you understand the layers of the earth
lets get back to plate tectonics. Lets try to
figure out how these plates move. The whole
thing rests on the idea of convection currents in
the earth.
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Currents in the Mantle
As the rocks heat from the mantle which is
closest to the core heat up they move upward in
the fluid environment of the core. This forces
the cooler rocks at the surface to sink toward
the core where they are reheated and the cycle
back to the surface. These movement of rocks
magma causes the plates to move.
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What the Theory Explains

• Earthquakes What happens when plates collide or
  slide each others (boundaries or faults)
• Volcanoes Form when two plates converge. One
  plate may slide under the other plate. The
  pressure from the rock going under the other
  plate causes the rock to form magma and then
  rises to the earths surface.
• Vents Underwater volcanoes have been observed
  under water. This can be explained by two plates
  moving away from each other causing the magma to
  rise up between the plates.

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Types of bounderies

• Divergent Plates pulling apart
• Convergent Plates meeting
• Transform - Plates slipping along side each
  other

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Examples of What Happens At Plate Boundaries
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Diverging Plate Boundaries
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What are Diverging Plate Boundaries?

• Two plates moving away from each other.

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What are some examples?

• Mid ocean ridges such as the Mid Atlantic Ridge
• Continental rifts
• Great Rift valley in Africa
• Iceland
• The area around the Red Sea

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Red Sea

• Arabian plate moves away from African Plate.
• Diverging plates create a valley into which water
  flows.
• Several lakes and seas in the area of the two
  plates will eventually turn into an ocean.

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Iceland

• Iceland sits on the Mid- Atlantic Ridge.
• North American Plate and Eurasian Plate moving
  apart
• Iceland is being ripped in half

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How fast do diverging plates advance?

• Diverging plates do not advance very fast.
• About 2.5 cm/yr- 5 cm/yr.

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How do diverging boundaries form?

• Magma beneath the earths crust moves in a
  spinning path called convection currents.
• Convection currents may cause the plates to
  separate, called diverging plate boundaries.

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• This creates a gap in the earths crust which
  fills with magma from the earths interior.
• The magma hardens, creating a new crust.

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What is happening below the surface to affect the
plates?

• Convection currents determine the direction
  plates advance.
• In diverging plates, the convection currents
  beneath the surface are rotating in opposite
  directions.

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What is meant by Birth of an Ocean? How does
this occur?

• When a continent is split in half by moving
  plates, magma weakens the crust where it is being
  stretched.
• The weakened crust falls and creates a rift
  valley that will eventually fill with water,
  creating a new ocean basin.

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• The Red Sea is an active example of the birth of
  a new ocean.

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CONVERGING PLATE BOUNDARIES
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INTRODUCTION

• Converging plate boundaries occur when two plates
  of Earths core slide towards each other, this
  may cause earthquakes and the growth of mountains
  and or volcanoes. There are three kinds of
  converging plate boundaries oceanic-continental,
  oceanic-oceanic, and continental- continental.

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OCEANIC-CONTINENTAL

• When an oceanic plate pushes into a continental
  plate, it makes a mountain range. When these
  plates start to move apart it generates an
  earthquake.

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OCEANIC-OCEANIC

• When oceanic plates push together one is usually
  pushed underneath the other, which forms an
  oceanic trench. This is a subduction zone.
• Also when oceanic plates are pushed together it
  can form undersea volcanoes.

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CONTINENTAL-CONTINENTAL

• When two continental plates push together they
  usually both go up and form a plateau or mountain
  range.

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RING OF FIRE
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RING OF FIRE

• The Ring of Fire is a zone of volcanic eruptions
  and earthquakes on the floor of the Pacific
  Ocean. On the map on the slide before, you can
  see how it wraps around. The Ring of Fire is the
  place of the largest plate reactions on the
  planet. 81 of the worlds largest earthquakes
  occur at the ring of fire. It is about 40,000 km
  in length.

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SUBDUCTION ZONES

• Subduction zones are places where the plates come
  together and one goes under the other.

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EXAMPLES OF SUBDUCTION ZONES

• The Marianas Trench is an example of an
  Oceanic-Continental subduction zone.
• The Chocolate Mountains fault in California is an
  example of a Continental-Continental subduction
  zone.

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MORE SUBDUCTION ZONES

• On the coast of South America, on the Peru-Chile
  trench, the Nazca Plate (oceanic) is being
  subducted under the continent of South America.
  Because of that, the South American Plate
  (continental) is being lifted up, making the
  Andes mountains. This is an example of
  oceanic-continental subduction zone.

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CONVERGING PLATE BOUNDARIES EXAMPLES

• The Juan de Fuca Volcanoes are an example of
  oceanic-oceanic convergence.

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CONVERGING PLATE BOUNDARIES EXAMPLES

• This is a picture of the San Andreas fault . The
  line you see is going through the Carrizo plain.
  This is continental-continental converging as
  well.

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CONVERGING PLATE BOUNDARIES EXAMPLES

• The Kilauea Volcano in Hawaii is an example of
  oceanic-oceanic convergence.
• The Rocky Mountains are an example of
  continental-continental convergence.

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Wrap Up