Continental Drift, Sea Floor Spreading and Plate Tectonics

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Continental Drift, Sea Floor Spreading and
Plate Tectonics

• 11/22/2009

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Noticing the Continents

• About 400 years ago explorers were sailing the
  oceans and examining the coastlines. These
  explorers brought back enough information for
  mapmakers to create reliable maps.
• In 1620, Francis Bacon noticed that the shapes of
  some of the continents looked as if they
• could fit together. He proposed that they
• once fit together but he didnt have a way
• to explain it.
• Everyone rejected his idea!

REJECTED
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• What continents look like they fit together best?

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Continental Driftthe beginning
Everything was once connected!

• In 1912, a German meteorologist came up with a
  theory that was similar to Francis Bacons.
• His name was Alfred Wegener. He called his
  theory, the Theory of Continental Drift.
• In this theory, he said that not just Africa and
  South America were once connected but, all the
  continents were once connected and have since
  moved apart.
• He called this giant land mass (supercontinent)
  PANGAEA. This word means All land.

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Evidence for Continental Drift

• The continents of South America and Africa look
  like they fit together.

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Evidence for Continental Drift

• Fossils of animals and plants were found on
  multiple continents.

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Evidence for Continental Drift

• Mountain ranges matched up between different
  continents.
• The mountain ranges on the East Coast of North
  America and Northern Europe were made of the same
  rock and the rock was the same age.

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Evidence for Continental Drift

• When the continents are put together to form
  Pangaea, the remains of glacial materials fit
  together to form a pattern like the large ice
  sheets that cover our poles today. 

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Evidence for Continental Drift

• Coal usually forms in tropical climates. Coal can
  be found in areas of North America which could
  not support coal formation.
• Coal forms in Tropical climates.
• Similar rock layers were found across multiple
  continents.

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Theory of Continental Drift
REJECTED
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A New Idea Sea Floor Spreading

• After Wegeners death, more clues were discovered
  that provided the reason why the plates moved.
• It was the invention of echo sounding devices
  (sonar) that gave insight as to what the ocean
  floor looked like.

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Echo Sounding
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Sea Floor Spreading

• Scientist found that the ocean floor contained
  mountain ranges with a steep, narrow valley
  running down its center. These underwater
  mountain ranges were all connected and spread
  through the center of most oceans. The range when
  all connected were 65,000 km long.
• In 1947, scientists went to map the Mid-Atlantic
  ridge. While studying the ridge, the scientists
  collected and studied rock samples. They found
  something astonishing.
• The ocean floor was younger than the continents.

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The Mid-Ocean Ridge
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Sea Floor Spreading

• The scientists wanted to know two things
• How did the mid-ocean ridges form?
• Why where the rocks younger in the ocean crust
  when compared to the continental crust?
• Write a hypothesis as to why the ocean crust is
  younger than the continental crust.

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Sea Floor Spreading

• In 1960, Dr. Harry Hess came up with some
  answers.
• A driving force, convection currents, moved the
  plates. The ocean floor, and the rock beneath it,
  are produced by magma that rises from deeper
  levels. The hot, less dense magma rises and
  cools, pushing the older crust out from the
  center.

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Convection Currents
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Sea-Floor Spreading

• At the mid-ocean ridge, molten material rises
  from the mantle and erupts. The molten material
  spreads out, pushing older rock to both sides of
  the ridge.
• This process that continually adds to the ocean
  floor is sea floor spreading.

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Evidence for Sea Floor Spreading

• In the 1960s scientists found evidence that new
  material is erupting along the mid-ocean ridge.
• The presence of pillow shaped rocks has showed
  that molten material has erupted again and again.

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Pillow Lava rocks
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Evidence for Sea-Floor Spreading

• Evidence in the rocks shows that Earths magnetic
  poles have reversed.
• The iron particles lined up in the opposite
  direction.

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Evidence for Sea-Floor Spreading
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Evidence for Sea-Floor Spreading
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Evidence for Sea-Floor Spreading

• In the 1968, scientists aboard the Glomar
  Challenger gathered information about the Sea
  floor.
• They found rocks no older than 180 million years.

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Evidence of Sea-Floor Spreading

• Evidence from drilling into the sea floor shows
  evidence of the ages of rocks.
• The closer you get to the mid-ocean ridge the
  younger the rock. The farther you get, the older
  the rock.

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Subduction

• The processes of subduction and sea-floor
  spreading can change the size and shape of the
  oceans.
• Because of this processes, the ocean floor is
  renewed about every 200 million years.

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Subduction in the Oceans

• The Pacific Ocean is shrinking! This is due to
  the fact that a deep-ocean trench is swallowing
  more crust than the mid-ocean ridge can produce.
• The Atlantic Ocean however, is expanding!

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Continental vs. Oceanic Crust
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Plate Tectonics

• The theory of plate tectonics combines the
  theories of Continental drift and Sea floor
  spreading.
• The earths crust is not all in one piece. It is
  divided in about a dozen major plates that ride
  on the earths asthenosphere.

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Divergent Plate Boundary
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Convergent Plate Boundary
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Transform Plate Boundary
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Ocean-Continent Convergent
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Ocean-Ocean Convergent
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Continent-Continent Convergent
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