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Chapter 33. Our Restless Planet. The Theory of Continental Drift. Scientists of the early 20 th Century thought the oceans and continents were fixed The exterior didn’t move but the interior was cooling and contracting (Called Shrinking Earth Theory)

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Chapter 33

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Chapter 33 l.jpg

Chapter 33

Our Restless Planet

Presented by April Senger


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The Theory of Continental Drift

  • Scientists of the early 20th Century thought the oceans and continents were fixed

  • The exterior didn’t move but the interior was cooling and contracting (Called Shrinking Earth Theory)

  • They assumed that with the Earth shrinking and the outer part staying the same size, it caused the skin of the planet to contort and wrinkle into valleys and mountains

Presented by April Senger


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Alfred Wegener

  • Many people including Wegener noticed that the shorelines of the continents fit together like puzzle pieces

  • He thought that at one time all the continents were one super continent named “Pangaea” meaning all land (300 mya)

  • Evidence such as continental shelf borders, fossils, trees, paleoclimatic data etc help support his ideas

  • His theories were not accepted due to lack of evidence for “Why” the plates moved

Presented by April Senger


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Paleomagnetism

  • Magnetic Bands were discovered on the ocean floors

  • The polarity of the magnetic fields was solidified in the rocks when they formed

  • It told us the direction to the magnetic pole from the rock’s location at the time

  • It also gave us the magnetic latitude of the rocks location when it was formed

  • It showed that either the poles moved or the continents did

  • The continents made more sense so Continental Drift’s interest was rekindled with a search for how it moved

Presented by April Senger


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H.H. Hess

  • Presented sea floor spreading

  • The ocean floor is not permanent and is constantly being renewed

  • The mid-oceanic ridges were located above convection cells in the mantle

  • The plates move in conveyor belt fashion from the ridge

  • Older lithosphere is pushed from the ridge crest and recycled back into the mantle at an ocean trench

Presented by April Senger


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The Mechanism

  • Paleomagnetic studies supported that they was sea floor spreading and now scientists had a “Why” the plates moved

  • Wegener’s theory of continental drift was taken more seriously with a means for the continents to move

  • This lead to the Theory of Plate Tectonics

Presented by April Senger


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Plate Tectonics

  • TPT state that the Earth’ outer shell, the lithosphere, is divided into 8 relatively large plates and a number of smaller ones

  • The plates ride atop the asthenosphere

  • Most earthquakes, volcanoes, and mountains occur along plate boundaries

  • The creation and destruction of the lithosphere also occurs at these locations

Presented by April Senger


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Divergent Boundaries

  • Divergent boundaries are when two or more plates move apart, creating a gap between them Magma is the molten rock within the Earth that comes to the surface at a divergent boundary creating new oceanic floor

  • The mid-oceanic ridge is a mountain range that occurs at a divergent boundary that runs down the center of the Atlantic Ocean from the Artic to the tip of South America

  • A rift valley is the central valley surrounded by high mountains that occur at the boundary

  • For the last 160 million years the Mid-Oceanic Ridges has moved 3200 km (2 cm a yr) = Width of A Ocean

Presented by April Senger


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Convergent Boundaries

  • Convergent boundaries are when older lithosphere is destroyed as the more dense oceanic plate dives beneath the continental or oceanic plates

  • The Andes occur at a convergent boundary in an area called a subduction zone. Trenches, mountains and volcanoes are common at these boundaries

  • Some of the trenches can be 6.8 (Mariana Trench by Asia) to 4.3 miles (Peru-Chile Trench Pacific Ocean side of S. America) deep

  • The oceanic crust is forced deep into the mantle where it is melted into magma. The magma near the surface often rises due to density as a volcano

Presented by April Senger


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3 Kinds of Convergence

  • Oceanic-Oceanic Convergence can create an island arc

  • Oceanic-Continental Convergence causes the more dense ocean plate to dive beneath the continental plate

  • Both have subduction zones, earthquakes and trenches

Presented by April Senger


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Concept Check

  • Erosion wears mountains down and yet the Andes mountains grow taller each year. Why?

  • Subduction is still occurring which causes the uplift of the mountains

  • The uplift is greater than the erosion

Presented by April Senger


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Looking for the 3rd Kind?

  • Continent-Continent Convergence does not have clear subduction because the density of the plates is similar

  • You don’t see the volcanic activity here

  • The formation of metamorphic rocks is common

  • Expect many earthquakes

  • Notes the Himalayas, European Alps & The Appalachian Mountains

Presented by April Senger


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Transform-Fault Boundaries

  • Transform fault boundaries are when the lithosphere breaks up and moves horizontally past each other

  • The rocks move in horizontal directions

  • Earthquakes are common at transform fault boundaries

  • The San Andreas fault is an excellent example of a transform fault (1500 km long)

  • It is moving at 5 cm a year (N.A. Plate & Pacific Plate)

  • In 1906, it moved 6 m causing the San Francisco EQ

  • Faults can run in the same directions at different speeds or in opposite directions

  • Faults can occur on the continent or in the ocean

Presented by April Senger


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What Does All This Mean?

  • The study of geology has shown us that “The present is the key to the past”

  • Past clues show us what may happen in the future

  • The Earth is a dynamic planet and we should be prepared for changes

  • Many scientists are working on those predictions

Presented by April Senger


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