Chapter 3 Plate Tectonics 3-1 Earth’s Drifting Continents Theory-guess A land bridge once...
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Transcript of Chapter 3 Plate Tectonics 3-1 Earth’s Drifting Continents Theory-guess A land bridge once...
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Chapter 3 Plate Chapter 3 Plate Tectonics Tectonics
Chapter 3 Plate Chapter 3 Plate Tectonics Tectonics
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3-1 Earth’s 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.
• Wegner’s –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 today’s 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 Earth’s Spreading Ocean Floor
• How could the continents plow through hard, solid ocean floor?
• In 1950’s and 60’s 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 earth’s magnetism is
recorded in rocks, when molten rock hardens a permanent record of the earth’s magnetism remains in the rocks
• As a result, scientist learned that the earth’s 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 Earth’s 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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63• 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 Alaska’s 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 Earth’s 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 Earth’s crust. 64
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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