PALEOMAGNETISM
INTRODUCTION
•The stage was set
–WEGENER’S OBSERVATIONS WERE STILL UNEXPLAINED
–NEWER DATA WAS STILL UNEXPLAINED
•Distribution
of newer fossils
•Distribution
of rocks
•Age of
oceanic rocks
•Thickness
of oceanic sediments
•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
–THERE ARE A VARIETY OF CHANGES UNIMPORTANT TO OUR PRESENT PURPOSE
•Diurnal
•Magnetic storms—irregular
in frequency & size
•Intensity Decay
•Westward
Drift of non-dipole field
THE EARTH’S MAGNETIC FIELD
–DIPOLE WANDER
•True dipole wander
•Limited
•Over
or wobbling of poles
in scope
periods of ~10,000 years, the average magnetic pole position coincides with the rotation
pole
–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
–WE REALLY DON’T KNOW
–MUST BE INTERNAL
–CAN NOT BE A PERMANENT MAGNET
–SELF-EXCITING DYNAMO
•Convection
•Moving
currents in the outer core
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
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
•Produced
from traverse to traverse
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
•Two Canadians by the names of Morley and LaRochelle came up with the same idea at
roughly the same time as Vine & Matthews
–THEY SUBMITTED THEIR RESULTS TO ANOTHER JOURNAL, WHERE IT WAS REJECTED AS TOO
SPECULATIVE
–VINE & MATTHEWS WORK WAS PUBLISHED IN NATURE AND THEY GET ALL THE CREDIT
•The paper by Vine and Matthews really marked the beginning of the general acceptance
of the idea of plate tectonics