PALEOMAGNETISM
Unexplained Observations
• Wegener’s Data
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Fit of continents
Distribution of fossils
Ancient climate data
Correlation of rock sequences
• Newer Data
– More recently discovered fresh-water reptiles
– Mountain belts that extend beyond the margins of continents
– Thickness of seafloor sediments—If the oceans are old, the sediments on the ocean floor
are not thick enough
– Age of oceanic rocks—Why are the oldest rocks on the ocean floor less than about 180
million years old?
– Distribution of earthquakes
Where Earthquakes Occur
• Three general belts
– Circum-Pacific
– Alpide or TransAsiatic or Mediterranean- TransAsiatic
– Mid-ocean ridges
• Depth of Focus
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Shallow (Normal)—0-70 km (red)
Intermediate—70-300 km (green)
Deep—>300 km (blue)
In fact, no earthquakes are known from depths much greater than 700 km
Circum-Pacific Belt
• Virtually all deep focus earthquakes
• Most larger, intermediate focus earthquakes
– >90%
• 90% of the large normal focus earthquakes
• At least 80% of the world’s seismic energy released each year
Trans-Asiatic Belt
• The rest of the large intermediate focus earthquakes
• The rest of the large normal focus earthquakes
Mid-ocean Ridges
• Small, normal focus earthquakes
• Occur along the ridge
• Also occur on transform faults offsetting ridge segments
Vertical distribution
• Most earthquakes are shallow (normal focus)
• In places in the ocean Hugo Benioff noted a pattern of increasing
focal depth
Conclusions
• Earthquake foci get deeper under volcanic island arcs
• Earthquake foci also get deeper under continents where trenches
are adjacent to continents
• Volcanoes are present at all of these sites
• There must be a connection among ocean trenches, earthquakes
of increasing focal depth, and volcanism
Introduction
• 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
Earth’s Magnetic Field
• Dipolar
– Like a bar magnet not quite aligned with the rotation poles
The Earth’s Magnetic Field
• Declination
– Difference between magnetic pole (magnetic north) and rotation pole (true
north)
– Varies with position on Earth
– Changes with time as magnetic pole moves
The Earth’s Magnetic Field
• 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
The Earth’s Magnetic Field
– 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
The Earth’s Magnetic Field
• Causes of Earth’s Field
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We really don’t know
Must be internal
Can not be a permanent magnet
Self-exciting dynamo
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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
How Rock’s Become Magnetized
• 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
How Rocks Become Magnetized
• 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
Paleomagnetic Results
• 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
Reversals
• 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
Meanwhile…..
• 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
Ocean-Floor Anomalies
Vine and Matthews
• 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
Recognition of Magnetic Stripes
Magnetic Time Scale
Seafloor Magnetic Anomalies
Age of the Ocean Floor
Historical Notes
• 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
Historical Notes
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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, [rather] than in the Journal of Geophysical Research”
– Morley planned to submit the paper to another journal
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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
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The paper by Vine and Matthews really marked the beginning of the general acceptance of the
idea of plate tectonics