Earth Science

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Earth Science

• Historical Introduction
• Sea Floor Spreading the Earths Magnetic Field

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During WWII

• Maps of ocean floor using
• Sonar
• Magnetometers

Harry Hess, a Captain during WWII, gathered data
while cruising from battle to battle
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Surprising Data

• Deepest areas (trenches) were close to the
  continents
• Underwater mountain ranges ran through every
  ocean
• Each mid-ocean range had a central valley running
  down the middle (rift valley)

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Dark blue deepest areas Yellow shallowest
areas
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Surprising Data

• Symmetrical patterns of magnetic reversals
  centered on ocean ridges

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The Earths Magnetic Field Reverses!

• Reversals are recorded as iron-rich lava cools
• Reversals occur every 200,000 years
• We havent had one for 700,000 years!

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Reversals recorded in lava flow
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Age Evidence

• Ocean crust is much younger than continental
  crust
• Youngest rocks are found closest to ocean ridges

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Data provides support for Sea Floor Spreading

• Lava erupts at mid-ocean ridges and cools,
  creating new sea floor
• Sea floor dives back under continental crust at
  trenches (Subduction Zones)

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Process is driven by convection currents within
asthenosphere