Plate Tectonics

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Plate Tectonics
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Continents in Motion

• Alfred Wegener was a German meteorologist that
  proposed the theory (1912) that the Earth was not
  fixed - it "moved" on plates. Come to find, in
  the 1960s, Alfred Wegners theory was proven
  correct. This theory is known as Continental
  Drift.

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Book written by Wegener

• In 1915, Wegener published his evidence and
  conclusions in a now classic book, Die entstehung
  der kontinente und ozeane (The origin of
  continents and oceans). Wegener proposed that all
  modern continents were once assembled together in
  a supercontinent he named Pangaea.

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Continental Drift

• Theory that the continents had once been part of
  one or more landmasses that had separated and
  moved apart.
• They are supposed to have broken up about 200
  million years ago.

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Pangaea Means All Earth

• Wegener proposed that all continents had once
  been joined in a supercontinent he named Pangaea.
  Pangaea, he thought, had existed from the
  primordial earth until the Mesozoic, when it
  began to break up. Wegener at first considered
  mantle convection as a possible driving
  mechanism, but later rejected that in favor of
  Tidal forces as the cause for Pangaea's breakup
  and continental drift.

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Panthalassa Means All Seas

• 245 Million years ago  Pangaea existed when some
  of the earliest dinosaurs were roaming around
  this big earth. The land Pangaea was surrounded
  by a sea called Panthalassa.

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Pangaea breaks up Forming

• Pangaea started to break up into two smaller
  supercontinents during the Jurassic period. By
  the end of the Cretaceous period, the continents
  were separating into land masses that look like
  our modern-day continents.

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3 continents associated with Laurasia ?
4 continents associated with Gondwana ?
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Evidence Supporting Continental Drift
1. Continental Margins Giant Jigsaw Puzzle.
These two widely separated continents seem to
resemble two separate pieces of a larger jigsaw
puzzle. North America, too, seems to mirror the
western coastline of Europe.
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Evidence Supporting Continental Drift

• 2. Fossils of Mesosaurus A small reptile that
  lived 270 million years ago that was found in
  Eastern South America and western Africa.
  Mesosaurus lived near swamps and rivers and it
  would have been impossible for this reptile to
  have swam across the Atlantic.

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Evidence of Continental Drift

• 3. Glossopteris Fern Fossils
• Fern fossils have been found in Africa,
  Australia, India, and Antarctica.

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Evidence of Continental Drift

• 4. Glacial Deposits
• Evidence of Glacial Deposits was found on 5
  different continents.

Glacial striations on rocks show that glaciers
moved from Africa toward the Atlantic Ocean and
from the Atlantic Ocean onto South America. Such
glaciation is most likely if the Atlantic Ocean
were missing and the continents joined.
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Evidence of Continental Drift

• 5. Geologic Evidence
• The age and type of rocks in the coastal
  regions of widely separated areas, western Africa
  eastern Brazil, matched closely.
• Mountain chains such as the Appalachians and
  Scandanavian Mtns. Of Greenland and Northern
  Eupope seem to fit closely in age and structure.

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Wegeners Theories Rejected

• Most scientist rejected Wegeners theories
  --------- Why ? ----------
• It was difficult to conceive of large continents
  plowing through the sea floor to move to new
  locations. What kind of forces could be strong
  enough to move such large masses of solid rock
  over such great distances?
• While in Wegeners lifetime, he never found the
  WHY to why the continents moved.

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• Wegener suggested that the continents simply
  plowed through the ocean floor, but Harold
  Jeffreys, a noted English geophysicist, argued
  correctly that it was physically impossible for a
  large mass of solid rock to plow through the
  ocean floor without breaking up. Recent evidence
  from ocean floor exploration and other studies
  has rekindled interest in Wegener's theory, and
  lead to the development of the theory of plate
  tectonics.

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The WHY is Answered

• The WHY came in 1967, when a group of scientist
  set out to map the ocean floor along the
  Mid-Atlantic Ridge. This was an undersea mountain
  chain with a steep, narrow valley running down
  its center. (65,000 km or 40,625 miles long)
• Glomar Challanger Sent out to collect rock
  samples at the bottom of the sea for age dating.
  Results Seafloor 70 to 150 m.y.a. and
  Continent 3 to 4 Billion.

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Intro to Plate Tectonic Theory

• Plate tectonics is the theory that Earth's outer
  layer is made up of plates, which have moved
  throughout Earth's history.
• The theory explains the how and why behind
  mountains, volcanoes, and earthquakes, as well as
  how, long ago, similar animals could have lived
  at the same time on what are now widely separated
  continents.

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Definite Proof

• But the purpose of the Glomar Challenger was
  scientific exploration. One of the most important
  discoveries was made during Leg 3. The crew
  drilled 17 holes at 10 different sites along a
  oceanic ridge between South America and Africa.
  The core samples retrieved provided definitive
  proof for continental drift and seafloor renewal
  at rift zones. This confirmation of Alfred
  Wegener's theory of continental drift
  strengthened the proposal of a single, ancient
  land mass, which is called Pangaea.

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Harry Hess
Navy submarine commander during World War II.
Hess proposes sea-floor spreading1960

• Suggested that there was actually a break at the
  center of the ridge known as a RIFT ZONE.
• Magma from deep within the Earth was coming to
  the surface at this Rift Zone and creating new
  sea floor.

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Earths Layers

• Earth built by distinct layers inner core,
  outer core, mantle, crust the crust is the
  thinnest layer

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Lithosphere

• The rigid blocks of the crust and upper mantle
  that extend downwards to about 100 km deep.

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Asthenosphere

• Below 100 km a plastic like zone of molten magma
  extends down to about 200 km thick called the
  Asthenosphere.
• The less dense Lithosphere floats on the more
  dense Asthenosphere.

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Video Review
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Types of Boundaries

1. Divergent
2. Convergent
3. Transform

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D i v e r g e n t Boundary

• A divergent boundary occurs where 2 plates are
  pulling apart.
• The force associated with this is called
  Tension.
• Example Mid-Ocean ridges where Sea Floor
  spreading is occuring.

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Convergent Boundary 3 Types

• A convergent boundary is a boundary between 2
  colliding plates. When 2 plates collide one plate
  may dive under the other plate at a Subduction
  Zone.
• There are 3 types.
• 1. Ocean Ocean Convergence
• 2. Ocean Continent Convergence
• 3. Continent Continent Convergence

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Force associated with Convergent Boundaries
What type of force is associated with convergent
boundaries?
Compression
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Ocean Ocean Convergence

• When two oceanic plates collide creating deep sea
  trenches.
• Example Marianas Trench the deepest part of the
  Pacific Ocean near the coast of Japan at at 11033
  meters (36201 feet) deep.
• Magma rises to form volcanoes or Island arcs on
  the ocean floor parallel to the trench.

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Ocean to Continent Convergence

• The Denser Oceanic plate descends into the less
  dense Continental Crust and SUBDUCTS its way down
  to the Asthenosphere.
• Volcanoes may form
• Ex The Andes in South America were created when
  the Pacific plate ran into the South American
  plate creating the Andes Mountain Range.

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New crust is continually being pushed away from
divergent boundaries (where sea-floor spreading
occurs), increasing Earth's surface. But the
Earth isn't getting any bigger. What happens,
then, to keep the Earth the same size?
Subduction Zone
In locations around the world, ocean crust
subducts, or slides under, other pieces of
Earth's crust. The boundary where the two plates
meet is called a convergent boundary. Deep
trenches appear at these boundaries, caused by
the oceanic plate bending downward into the
Earth.
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Continent to Continent Conv.

• Continental rocks have low densities in which
  neither is more dense than the other. This causes
  them to buckle up and create Mountains when they
  collide.
• Very little volcanic activity.
• Earthquakes are CoMmoN.
• Ex Appalachians and Himalayas

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Convergent Convergent boundary
When two land masses meet neither will slide
under the other. Instead, the two crush together
at what is known as a convergent boundary. They
crumple and fold. Some pieces of land are thrust
over or under other pieces. The result is a
mountain range.
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The Himalayas, the highest mountains in the
world, were created this way. (In fact, they're
still growing.) So were the European Alps.
Even the Appalachian Mountains formed when two
land masses came together. Although with the
Appalachians, the crushing ended long ago -- all
that's left now are the eroded remnants of a once
high mountain range.
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Transform Fault Boundary

• Boundaries where plates are sliding past one
  another in opposite directions or in the same
  direction but at different rates.
• The Force associated with this boundary is
  Shearing.
• Ex San Andreas Fault in California.
• YOU GET EARTHQUAKES !!!

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Slippin' and a Slidin'
Transform boundaries neither create nor consume
crust. Rather, two plates move against each
other, building up tension, then releasing the
tension in a sudden and often violent jerk. This
sudden jerk creates an earthquake.
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Convection Currents

• Convection currents in the mantle appear to be
  the driving force that provides the energy
  necessary to move the Earths plates.

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Convection
is the process by which currents are driven by
temperature differences within fluid bodies. When
fluid is heated, it expands, lowering the density
of the heated material, causing it to rise
through the cooler fluid. As it rises, leaving
the vicinity of the heat source, in this case,
hot magma near the earth's surface, it will cool.
When it becomes more dense (because it is cooler)
than the surrounding fluid, it will begin to
sink. This behavior sets up circulation currents
in the mantle that cause plate movement.
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Arthur Holmes     believed a fluid mantle
possessed convection currents created by heat
trapped beneath the Earth's surface. Holmes
hypothesized that convection currents welled up
toward the surface and then drug continents
across the surface.
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Paleomagnetism
based on the principle that magnetic particles
will align themselves with the Earth's magnetic
field
is a technique used to cross date ocean cores and
to establish major intervals over wide areas. By
measuring the polarity of the magnetism in
samples it is possible to determine the layer in
which the Earths polarity reverses. Between
periods of normal and reversed polarity.
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What is Sea-Floor Spreading?
In the early 1960s, Princeton geologist Harry
Hess proposed the hypothesis of sea-floor
spreading, in which basaltic magma from the
mantle rises to create new ocean floor at
mid-ocean ridges. On each side of the ridge, sea
floor moves from the ridge towards the deep-sea
trenches, where it is subducted and recycled back
into the mantle
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Ocean Floor Map
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Making of a Mountain?

1. How tall is Mt. Everest (feet)?
2. How many people have reached the top?
3. A Syncline is a downward fold in a rock. What is
   an Anticline?
4. How many Himalayan peaks are there?
5. What is Orogenesis?
6. When did India hit Asia?
7. How thick is the Lithosphere
8. How many major plates are there?
9. What sea once separated India from Asia?
10. How much are the Himalayas rising each year?

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Answers

1. 29,035 ft
2. 1500 or 1502 people
3. An upward fold
4. 30
5. Mountain Building
6. 40 mya
7. 100 Km or 62.1 mi.
8. 15 slabs
9. Tethys Sea
10. 5 mm/yr or .2 in.s