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Plate Tectonics. Chapter 3 (p 61-82). A New Understanding of Earth. Earth has a geologically active surface

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Plate tectonics

Plate Tectonics

Chapter 3 (p 61-82)


A new understanding of earth
A New Understanding of Earth

  • Earth has a geologically active surface

  • How do internal layering and heat contribute to mountain building, the arrangement of continents, the nature of the seafloors, and the wealth of seemingly randomly distributed geological features found everywhere?

  • Patterns?


The age debate
The Age Debate

  • Know to be 4.6 billion years old

  • Late 1700s most scientists believed that earth was about 6,000 years old

    • Based on interpretation of the Bible

  • Uniformitarianism – earth processes happening today are identical to those in the past

    • Going on for a very long time based on current rates


The age debate1
The Age Debate

  • Catastrophism – biblical catastrophic events shaped the young earth

    • Flood

    • Explained mountain-top fossils

  • Natural selection

    • Time was needed for many species to exist

      Age-of-earth arguments led to discoveries during the 1800s  ancient earth

      http://images.google.com/imgres?imgurl=http://www.cnn.com/2000/NATURE/09/13/great.flood.finds.ap/beam.ap.jpg&imgrefurl=http://www.cnn.com/2000/NATURE/09/13/great.flood.finds.ap/index.html&h=168&w=220&sz=12&hl=en&start=15&tbnid=bA5UiDTi_m_xCM:&tbnh=82&tbnw=107&prev=/images%3Fq%3Dgreat%2Bflood%26gbv%3D2%26svnum%3D10%26hl%3Den


A puzzling fit
A Puzzling Fit

  • Fit of South Atlantic continents

    • Evidence for single large landmass

    • Fossil evidence

    • Fit of continental shelves

      • All by chance?

      • No mechanism for continental movement suggested


Continental drift
Continental Drift

  • 1912 – Alfred Wegener

    • Pangaea (pan = all, gaea = earth)

      • Single supercontinent

        • Pieces broke apart 200 million years ago

        • Still moving today

      • Evidence

        • Fit of shorelines

        • Fossils of tropical plants in Antarctica



Seafloor spreading
Seafloor Spreading

  • 1960 - Mid-Atlantic ridge suggested as origin of new seafloor (spreading center)

    • Explains “fit” of continents

    • Mechanism for movement – convection currents in mantle

    • Then ridges should be hot  they are

    • New crust should become more dense over time  it does

    • Crust furthest from ridge should be oldest  it is


Where does the old crust go
Where does the old crust go?

  • Subduction zones

    • Crust plunges into the mantle in the Pacific

    • Creates a balanced system


Plate tectonics1
Plate Tectonics

  • 1965 - John Tuzo Wilson suggests that there are 12 plates that make up Earth’s lithosphere (crust)

    • These plates float on the asthenosphere

    • Moved by hot mantle becoming less dense  rising

      • Lifts and cracks the crust = plate edges

      • Avg. movement ~ 2 in per year


Plate tectonics2
Plate Tectonics

  • Plate movement caused by two forces:

    • Plates form and slide off the raised ridges of the spreading centers

    • Plates are pulled downward into the mantle by their cool, dense leading edges

  • This theory explains many previously unanswered questions




Divergent plate boundary
Divergent Plate Boundary

  • Line along which 2 plates are moving apart

    • Mid-Atlantic Ridge

    • Cooling magma creates new crust


Convergent plate boundary
Convergent Plate Boundary

  • Areas of violent geologic activity where plates are pushed together

    • Oceanic crust is destroyed


Convergent plate boundary1
Convergent Plate Boundary

  • What happens when 2 convergent plates are of equal density?


Transform plate boundary
Transform Plate Boundary

  • Plates moving laterally past one another

    • Necessary since Earth is a sphere

    • Does not create or destroy crust

    • Creates earthquakes



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