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Today – 2/9

Today – 2/9. Weather report Review The magnetic field and the sea floor! Workshop Review session: Mon 5-7, Chavez 110. Last Time. Continental drift rejected despite good evidence because no mechanism conceivable Seismic profiling reveals internal structure of the Earth

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Today – 2/9

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  1. Today – 2/9 • Weather report • Review • The magnetic field and the sea floor! • Workshop • Review session: Mon 5-7, Chavez 110

  2. Last Time • Continental drift rejected despite good evidence because no mechanism conceivable • Seismic profiling reveals internal structure of the Earth • Convecting outer core powers geodynamo, creating Earth’s magnetic field

  3. Earth’s Magnetic Field • Diverts solar wind, which would destroy ozone layer and ruin electronics • Guides migrating animals from bacteria to bees to sea birds to toothed whales

  4. Earth’s Magnetic Field • Reverses it polarity occasionally! • Interacts with solar storms to produce the aurora borealis • Leaves its signature in magnetic rocks like magnetite

  5. “Halloween” Solar Storm www.nasa.gov

  6. Solar Storm www.nasa.gov

  7. Aurora from Space www.nasa.gov

  8. Magnetism in Minerals • Each electron has “spin”, i.e. acts as a tiny bar magnet • Electrons pair up in orbitals around nuclei; the electrons in a pair have opposite polarity • Lone electrons in an orbital impart a net magnetic moment to an atom • If the magnetic atoms in a mineral are all oriented identically, the mineral is magnetic

  9. Magnetite • The crystal structure of magnetite allows magnetic iron atoms to retain their magnetic orientation • Above “Curie point”, 580 °C, atoms too energetic to be permanently magnetized • Below Curie point, atoms line up with Earth’s magnetic field, the mineral becomes permanently magnetized and records the orientation of Earth’s magnetic field at the time the magnetite cooled through the Curie point • Early 1900’s scientists recognized that magnetism in Earth’s rocks comes in two flavors: same polarity as today or opposite

  10. Old Assumptions re Ocean Floor • 4 Ga • Flat and featureless • Deeply buried in sediment

  11. Mid-Ocean Ridges (MORs) • Naval survey, mid-1800’s • Sonar developed 1906 for icebergs, adapted for submarine detection WWI • Mid-ocean ridges: giant mountain chains running down the center of the oceans • After WWII, Atlantic sea floor sediment found to be a thin veneer

  12. Magnetic Striping of the Sea Floor • Magnetometers developed WWII for submarine detection • Used to explore the magnetic properties of the sea floor during the 1950’s • Magnetic striping: found alternating bands of reversed magnetic polarity in the rocks running parallel to the MOR’s! • Pattern symmetrical about the MOR’s!

  13. Sea Floor Spreading! • 1968 – Glomar Challenger drills the ocean floor across the S Atlantic from S America to Africa • Retrieved rocks show that ages decrease to zero at the MAR, then increase, so new oceanic crust is being created at the MAR! • The core has revealed to us the mechanism for continental drift!

  14. Age of the Atlantic Sea Floor www.noaa.gov

  15. Age of the Sea Floor www.pubs.usgs.gov

  16. pubs.usgs.gov

  17. Workshop • Distance = rate  time, d = rt • t = d/r • r = d/t • Rate of sea floor spreading = 2  distance from rock to ridge / age of rock

  18. Example 90 km atlas.geo.cornell.edu

  19. Back in the Lab • Isotopic analysis reveals that the rock is 3 Ma • What is the rate of sea floor spreading?

  20. Clear Unclear

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