• The stage was set
• The major breakthrough was to come from a field called paleomagnetism
• To understand paleomagnetism we need to understand a bit about the Earth’s magnetic field
• Dipolar
– Like a bar magnet not quite aligned with the rotation poles
• Declination
– Difference between magnetic pole (magnetic north) and rotation pole (true north)
– Varies with position on Earth
– Changes with time as magnetic pole moves
• Components
– Dipole is about 80%
– Non-dipole is about 20%
• Variations
– Diurnal
• about 5%
• Caused by atmospheric effects
– Magnetic storms—irregular in frequency & size
– Intensity Decay
• Currently decreasing
• At current rate, will be zero in 1700 years
– Dipole Wander
• True dipole wander or wobbling of poles
• Limited in scope
• Over periods of ~10,000 years, the average magnetic pole position coincides with the rotation pole
– Westward Drift of non-dipole field
• About 0.18°/yr.
• Will complete a cycle in about 2000 years
– Reversals
• We now know the Earth’s magnetic field reverses
• North becomes South
• Irregular intervals
• Causes of Earth’s Field

– We really don’t know
– Must be internal
– Can not be a permanent magnet
– Self-exciting dynamo
• Convection currents in the outer core
• Moving conductor in a magnetic field
• Generates electrical current
• Electrical current produces a magnetic field
• Sum of all convection cells produces Earth’s field
• Rocks become magnetized parallel to the Earth’s magnetic field at a particular time and place
• Rocks that acquire magnetization quickly (like lava flows) record instantaneous pole positions
• Intrusions and sedimentary rocks acquire a magnetization much more slowly, averaging out short term polar movements
• Although the magnetic pole moves, when its position is averaged over periods on the order of 10,000 years, the average position of the magnetic pole coincides with the rotation pole
• This means that paleomagnetic latitude is the same as geographic latitude
• Thermal Remanent Magnetization
– Due to cooling
– Magnetic minerals acquire a magnetization parallel to the Earth’s magnetic field when they cool below the Curie Temperature
– Important in igneous rocks
• Detrital Remanent Magnetization
– Grains settle and become magnetized parallel to the Earth’s field
– Important in sedimentary rocks
• Chemical Remanent Magnetization
– New minerals grow parallel to Earth’s magnetic field
• Isothermal Remanent Magnetization— lightning strikes
• Viscous Remanent Magnetization—just sitting around
• Many rocks acquired their magnetization at a latitude different from their present latitude
• Rocks of the same age from anywhere on a single continent generally agree
• Either the poles were shifting or the continents were drifting
– Hold continents fixed and move poles for several continents
– If all paths are the same, the poles moved; otherwise, the continents moved
• Some rocks came up reversely magnetized

• Discovered that rocks of certain ages anywhere in the world were reversed
• Concluded that the Earth’s magnetic field reverses
• Oceanic Data
– Since WWII magnetometers were towed routinely
– Found a series of magnetic highs and lows
• Correlated from traverse to traverse
• Produced a series of “magnetic stripes” on the ocean floor
• Knew about magnetic stripes and reversals that had been found on land
– Combined that information to conclude that highs and lows represented normally and reversely magnetized rock
– Suggested new material generated at mid-ocean ridges
– Moved, as if on a conveyor belt away from ridge
– Replaced by new material
– This became known as sea-floor spreading
• Lawrence W. Morley and Andre Larochelle had been studying reversely magnetized rocks and aeromagnetic maps
• Morley saw the map showing the stripes in August of 1961
– The map was published by Mason and Raff
– Like the authors, Morley could not explain the stripes
• In early 1963 he read Robert Dietz’s description of seafloor spreading
• Morley says that the explanation for the stripes suddenly dawned on him and he wrote an article
– Submitted it to Nature in February of 1963
– It was not published because of a a lack of space
– It also did not have a lot of data to support the conclusion
• Morley then submitted his paper to the Journal of Geophysical Research in April of 1963
– While awaiting reviews, Morley presented his ideas at two meetings (one in June and one in August)
– It was rejected, with the comment, “His idea is an interesting one—I suppose—but it seems most appropriate over martinis, say, [rath er] than in the Journal of Geophysical Research”
– Morley planned to submit the paper to another journal
• The September 7, 1963, issue of Nature contained the paper by Vine and Matthews
– Vine & Matthews get all the credit

– Morley gets to be a footnote
• The paper by Vine and Matthews really marked the beginning of the general acceptance of the idea of plate tectonics