Chapter 3 Plate Tectonics

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Chapter 3 Plate Tectonics
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3-1 Earths Drifting Continents

• Theory-guess
• A land bridge once stretched across the Atlantic
  Ocean and connected South America and Africa
• Evidence for this includes fossils of plants and
  animals that could not have possibly crossed the
  ocean.
• Fossils-Preserved remains of ancient organisms.

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• The earth once had a single landmass that broke
  up into large pieces, which have since drifted
  apart.
• The name of this giant landmass is Pangaea which
  means all earth.
• Wegners Theory of continental drift
• One supporting piece of evidence is the fossil
  Glossopteris( extinct now longer living plant)
  located in rocks about 250 M years ago have been
  found in South Africa, Australia, India and
  Antarctica.
• Presence in Antarctica indicates that at one time
  the climate for Antarctica was much different
  than it is today.---end 57

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Evidence from rocks

• Rocks from Africa and South America piece
  together.
• An ancient folded mountain chain that stretches
  across South Africa links up with an equally
  ancient folder mountain chain in Argentina.
• Coal fields with distinctive layers in Brazil
  line up with coal field with identical layers in
  Africa
• End 58

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3-1 Section Review

• What is continental drift?
• The theory that the Earth had one giant landmass
  that split to form todays continents.
• Who first developed a scientific argument for
  continental drift?
• Alfred Wegener

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• 2. How do scientists explain the existence of
  fossils of the same plants and animals on
  continents thousands of kilometers apart?
• Scientists use the theory of continental drift to
  explain this phenomenon.

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3-2 Earths Spreading Ocean Floor

• How could the continents plow through hard, solid
  ocean floor?
• In 1950s and 60s they discovered a large system
  of underwater mountains that have a deep crack,
  called a rift valley running through their
  center. These underwater mountains are known as
  midocean ridges
• End 60

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• Midocean ridges form the single largest mountain
  chain in the world. The chain is approx. 80,000
  kilometers long-roughly 20 times the distance
  from NYC to LAX and 3 KM high.
• A great deal of volcanic activity occurs at the
  mid-ocean ridges. Lava erupts from the rift
  valley that runs the length of the ridge.
• As the ocean floor moves away on either side of
  the ridge lava wells up and hardens

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• The hardened lava forms new ocean floor
• This process is called ocean-floor spreading.
• Ocean-floor spreading helps to explain how
  continents drift.
• Although sections of the midocean ridges are
  straight, the ridges as a whole curve because
  straight sections are offset by thin cracks
  called transform faults
• Rock samples from the ocean floor indicate that
  rocks next to a midocean ridge are younger than
  rocks farther away the youngest rocks are in the
  center of the ridge
• End 61

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Section 3-2 Pg 62

• Magnetic stripes in ocean-floor rocks confirmed
  ocean-floor spreading
• The history of the earths magnetism is recorded
  in rocks, when molten rock hardens a permanent
  record of the earths magnetism remains in the
  rocks
• As a result, scientist learned that the earths
  magnetic poles reverse themselves from time to
  time-during the past 3.5 M years they have
  reversed 9 times

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• The pattern of magnetic stripes is identical on
  both sides of a mid-ocean ridge.
• As magma hardens into rock at a midocean ridge
  half the rock moves in one direction and the
  other half moves in the opposite direction.
• The oldest rocks on land are 4 billion years old
  but the oldest rocks in the ocean are 200 million
  years old.
• Because the Earths surface remains the same
  size, the ocean floor is being destroyed as fast
  as it is being formed by ocean-floor spreading.
• End 62

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• Trenches-V shaped valleys that lie along the
  bottom of the oceans, they are the deepest parts
  of the oceans close to continents or near strings
  of islands such as Alaskas Aleutian Islands .
• Why is the location of these trenches
  significant?
• The trenches create the Ring of Fire.

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Subduction

• Older ocean floor moves away from the midocean
  ridges as new floor is formed.
• Eventually the older floor moves down deep into
  the Earth along trenches.
• Subduction-The process in which crust plunges
  back into the Earth.
• When the rocks are pushed deep enough they are
  melted by the heat of the earth.
• Some of the molten rock will rise up through the
  crust and produce volcanoes,, but most will
  become part of the mantle.
• As new rocks are formed along the ocean ridges,
  older rocks are subducted into trenches, the
  process balances itselt. End 63

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Section 3-2 Review

• 1. What process helps to explain how continents
  drift?
• The process of ocean-floor spreading helps to
  explain how continents drift

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• 2. Where are the youngest rocks on the ocean
  floor found?
• The youngest rocks on the ocean floor are found
  at the midocean ridges

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Section 3-2 Review

• 3. How can the magnetic orientation of rocks be
  used to trace the way a continent moved as it
  drifted?
• The magnetic orientation of rocks of different
  ages can be compared to get an idea of how the
  position of a continent has changed over time.

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The short, sad life of Kid Crusty
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The Life of Kid Crusty
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Crusty Celebrates his birthday

• Happy 200 millionth Birthday

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Crusty is attacked by subduction
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Crusty meets his maker and is born again
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3-3 Earths Moving Plates

• Plate- irregularly shaped slabs that fit together
  like paving stones to form the surface layer of
  the earth.
• Theory of Plate Tectonics-links together ideas of
  continental drift and ocean-floor spreading,
  explains how the Earth has evolved over time and
  helps to explain the formation, movements,
  collusions, and destruction of the Earths crust.
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Lithospheric Plates

• Lithosphere- The topmost solid part of the earth
• There are seven major lithospheric plates and
  many smaller plates, some too small to list on a
  map
• They move at different speeds, and in different
  directions, small plates that lack landmasses
  move as much as several centimeters a year-large
  plates that are weighted down with landmasses may
  only move a few millimeters a year.
• In a few cases, the edges of the continents are
  the boundaries of the plates, however most plate
  boundaries are on the ocean floor. 65

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Plate Boundaries

• There are 3 types of boundaries
• Divergent-Midocean ridges-aka constructive
  boundaries-
• Convergent-trenches-plates come together at the
  trenches-cause pressure and friction-
• Strike-slip boundaries-Two plates grind together
  and slip past each other horizontally-earthquakes
  usually occur here

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Plate Motion