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Plate Tectonics. Drifting Continents Chapter 17.1. Vocabulary . Continental Drift – the continents were joined as a single landmass that broke apart 200 mya Still drifting Pangaea – ancient landmass made up of all the continents

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plate tectonics

Plate Tectonics

Drifting Continents

Chapter 17.1

  • Continental Drift – the continents were joined as a single landmass that broke apart 200 mya
    • Still drifting
  • Pangaea – ancient landmass made up of all the continents
  • Alfred Wegener – found evidence to support the theory of continental drift
evidence of the drift
Evidence of the Drift
  • Rock formations
    • Large geologic structures, such as mountain ranges fractured as the continents split
    • There should be similar rock types on opposite sides of the Atlantic
      • Rocks on the Appalachians are identical to rocks in Greenland and Europe
evidence of the drift1
Evidence of the Drift
  • Fossil formations
    • Similar fossils of several different animals and plants that lived on or near land had been found on several different continents
      • Land dwelling animals could not possibly have swum the great distances between continents
      • Trilobites
      • Ages of fossils predated the breakup of Pangaea
evidence of the drift2
Evidence of the Drift
  • Climatic evidence
    • Fossils of plants indicating the same type of climate have been found
      • On different continents
      • In current climates where they wouldn’t have survived
evidence of the drift3
Evidence of the Drift
  • Coal deposits
    • In Antarctica show that the land must have been at one time closer to the equator
  • Glacial deposits
    • 290 million year old deposits found in warm climates
    • Land must have at one time been located near the south pole
evidence of the drift4
Evidence of the Drift
  • Wegener’s idea was generally rejected
    • Most scientists believed in the early 1900’s that the continents were fixed
    • 2 flaws in the theory
      • What force was strong enough to move the continents?
      • How could the continents move through solids?
plate tectonics1

Plate Tectonics

Seafloor Spreading


  • Sonar – use of sound waves to detect and measure objects under water

Magnetic reversal – when Earth’s magnetic field changes polarity between normal and reversed

Magnetic field demo


Sometimes the magnetic field of the earth completely flips.

    • The north and the south poles swap places.
    • Such reversals, recorded in the magnetism of ancient rocks, are unpredictable.
    • They come at irregular intervals averaging about 300,000 years;
      • the last one was 780,000 years ago. Are we overdue for another? No one knows.

Isochron – imaginary line on a map that shows points of the same age; formed at the same time

  • Seafloor spreading – ocean crust is formed at mid-ocean ridges and destroyed at deep-sea trenches
    • Continuous cycle of magma intrusion and spreading

Mid-Atlantic Range – chain of underwater mountains that run throughout the ocean basins

    • Lenth of 65,000 km
    • Contains active and extinct volcanoes
tectonic plates

Tectonic Plates

Plate Boundaries


tectonic plates1
Tectonic plates
  • Huge pieces of crust and upper mantle that fit together at their edges to cover Earth’s surface
    • 12 major plates and several minor plates
    • Move slowly
      • In different directions and at different speeds in relation to each other
      • Edges are called boundaries
types of boundaries
Types of Boundaries
  • Divergent (divide) Boundary
  • Convergent (collision) Boundary
  • Transform Boundary
divergent boundaries
Divergent Boundaries
  • Definition – place where two of Earth’s tectonic plates are moving apart
    • Associated with volcanism, earthquakes, and heat flow
    • Found primarily in the seafloor

Rift valley – long, narrow depression that forms when continental crust begins to separate at divergent boundaries

convergent boundaries
Convergent Boundaries
  • Three types
    • Oceanic – Oceanic
    • Oceanic – Continental
    • Continental – Continental
  • Subduction – process by which one tectonic plate slips beneath another tectonic plate


Chapter 17.4

back to wegener
Back to Wegener
  • Remember the two flaws to his theory of continental drift?
    • What type of force could possibly move the continents?
    • How do the continents move through solids?
convection is the answer
Convection is the answer!!!
  • Large scale motion in the mantle
  • Transfer of thermal energy by the movement of heated material
let s talk about states of matter
Let’s talk about States of Matter
  • As matter cools
    • It contracts
    • Becomes denser
    • Cooled matter than drops due to gravity
  • Warmer matter
    • Is displaced
    • And then rise
so how s it work in the earth
So how’s it work in the Earth?!?
  • Up and down flow produces a pattern of motion called a convection current
  • Convection currents aid in the transfer of thermal energy
    • From warmer to cooler regions

Earth’s mantle is composed of partially molten material

    • Radioactive decay heats up the molten material in the mantle
    • Causes enormous convection currents to move material throughout the mantle
convection in the mantle
Convection in the Mantle
  • Driving mechanism of plate movements
    • Stiff part of mantle attached to the crust (cool)
    • Farther below, the mantle is hot and pliable
    • So…
    • The cool drops and is heated
    • The warm rises and cools
    • And the cycle continues

So… how does it all get started?

    • Set in motion by subduction
    • Move just a few centimeters per year

So how are convergent and divergent movement related to mantle convection?

    • Rising material spreads out as it reaches the upper mantle
    • Causes both upward and sideways forces
    • Downward part of convection occurs where sinking force pulls tectonic plates downward