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17.1 ~ Drifting Continents


17.1 ~ Drifting Continents Did Pangea Exist? The Theory of Continental Drift Wegener s idea that the continents slowly moved over the earth became known as ... – PowerPoint PPT presentation

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Title: 17.1 ~ Drifting Continents

17.1 Drifting Continents
  • Did Pangea Exist?

The Theory of Continental Drift
  • Wegeners idea that the continents slowly moved
    over the earth became known as continental drift.
  • Unfortunately Wegener could not provide a
    satisfactory explanation for the force that
    pushed or pull the continents. Therefore most
    geologists rejected his idea.

Evidence of Continental Drift
  • From Rock Formation
  • From Fossils
  • From Ancient Climate

Evidence of Continental Drift
  • Evidence from landforms/rock formations
  • Mountain ranges and other features
  • Example - Mountain ranges in South Africa line up
    with mountain ranges in Argentina.

Evidence for Continental Drift
  • Evidence from fossils
  • A fossil is any trace of an ancient organism that
    has been preserved in rock.
  • Glossopteris (a seed fern) fossils have been
    found in rocks in Africa, South America,
    Australia, India, and Antarctica.

The Theory of Continental Drift
  • A German scientist named Alfred Wegener formed
    the hypothesis that the continents had moved!
  • He proposed that all the continents had once been
    joined together in a single landmass and have
    since drifted apart.

Evidence for Continental Drift
  • Evidence from climate
  • Fossils of tropical plants have been found too
    far south to have survived.
  • Glacial deposits are found too far north to have
  • These clues provide evidence that continental
    drift really happened.

Even with the evidence, most scientists rejected
Alfred Wegeners theory for about a half a
century, from the 1920s to the 1960s.
17.2 Sea-floor Spreading
  • Is the ocean floor flat?

Technology allowed scientists to study the ocean
  • American geophysicist Harry Hess helped develop
    the theory of plate tectonics.
  • Echo-sounding methods
  • Sonar uses sound waves to measure water depth
  • Magnetometer detects small changes in the
    magnetic fields

Topography of Ocean Floor
  • Mid Ocean Ridges underwater mountain chains
    that run through the Earths Basins
  • Deep-sea trenches a narrow, elongated
    depression in the seafloor with very steep sides
  • Magma rises to the surface and solidifies and new
    crust forms
  • Older Crust is pushed
  • farther away from the ridge

Ocean Rocks and Sediments
  • Analysis
  • Age of rocks vary in place, and the age of
    oceanic crust consistently increases with
    distance from a ridge
  • Sediments are deposited on the ocean floor
    thickness of the sediments increases with
    distance from the ocean ridge

  • Rocks contain iron-bearing minerals that provide
    a record of Earths magnetic field.
  • Paleomagnetism studies this magnetic recorded
    paleoAncient prehistoric old
  • For example Basalt, it is rich in iron-bearing
    minerals, provides and accurate record of ancient

  • Basaltic lava cools, the iron becomes oriented
    parallel to Earths magnetic field.
  • Therefore, these minerals are compass needles

Geomagnetic Time Scale
  • Magnetic reversal is a change in Earths
    magnetic field. Long terms changes are called
    epochs short terms are called events
  • If is has the same orientation as the present
    field normal polarity ()
  • If it is opposite reversed polarity (-)

Seafloor Spreading
  • Theory states that new ocean crust is formed at
    ocean ridges and destroyed at deep-sea trenches
  • What actually happens during seafloor spreading?
  • magma is forced toward the crust along an ocean
    ridge and fills the gap that is created. When it
    hardens a new ocean floor is added

Are the continents moving or are they passengers
that ride with the ocean crust?
  • Sea floor spreading is the final answer Wegner
    needed to complete his model of continental drift.

Lab Activity
  • In this lab you will build a model to help
    understand the process of sea-floor spreading and
    how magnetic reversals are recorded in the ocean

Sea-floor Spreading Model
  • Rubric Project grade
  • Model is labeled Title/ Name/ Period- 10 pts
  • Lines are measured accurately 10 pts
  • Label magnetic polarity 10 pts
  • Label age of rocks 10 pts
  • Lines vary in thickness 10pts
  • Student Data Write up packet 50pts
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