Timing of the last reversals of the Earth's
magnetic field. Time goes from 5 Million
years ago (bottom) to present (top).
Periods in black match today's polarity;Source: Wikimedia Commons. A more complete scale here.
periods in white underwent reversed polarity.
One implication of the continental drift idea was that the oceans laying between continents that drifted away from each other should have gradually spread apart. This is known as the seafloor spreading hypothesis. But how to prove it?
Much earlier than that, the Earth's magnetic field had been studied scientifically since the beginning of the Spanish and Portuguese explorations of the Americas (Alvarez & Leitao, 2010, Geology, The neglected early history of geoscience). By the 17th century, maritime trading was dependent on the accurate mapping of magnetic intensity across the Atlantic Ocean. These studies culminated by the 19th century during the so-called Magnetic Crusade, leading to the realisation that the magnetic poles migrate significantly over historical time periods. And in fact, these rapid changes of the magnetic field soon became one of the theories proposed to explain why the magnetic orientation recorded in rocks depends on their geological age.
North Magnetic pole wander from 1590 to 2015. Click on the pins to see the year. From the GUFM and IGRF models. Via NOAA.
|Computer model based on Glatzmaier & Roberts. Magnetic field lines are in blue |
when the field points towards the center and yellow when pointing away from it. The
rotation axis of the Earth is centered and vertical. The dense clusters of lines are
within the Earth's core
Back in 1957, Marie Tharp found enigmatic alignments in the shape of the seafloor around the center of the Atlantic Ocean, roughly where seismicity was being detected. In 1963, both the geophysicist Frederick J. Vine and the geologist Lawrence W. Morley independently realized that if the seafloor spreading theory was correct, then the rocks surrounding mid-oceanic ridges should show symmetric patterns of magnetization reversals, recording the changes of the Earth's magnetic field in the volcanic rocks at the time when these erupted and cooled down at the mid-ocean ridges. This is now known as the Vine–Matthews–Morley hypothesis, and became a validation test for the seafloor spreading, and for the plate tectonics theory in general.
|Seafloor spreading at a mid-ocean ridge, recording time-changes of geomagnetic |
field polarity. Source: Wikimedia Commons.
|Map of the age of the seafloor based on the reversal of the magnetic field|
recorded in the oceanic crust during its formation at mid-oceanic ridges.
Red indicates a young seafloor, whereas blue is used for the oldest oceanic crust.
(Source: National Geophysical Data Center)
Update (2015-09): A Science News article on a recent study on core convection and the magnetic field.
Vine, F., & Matthews, D. (1963). Magnetic Anomalies Over Oceanic Ridges Nature, 199 (4897), 947-949 DOI: 10.1038/199947a0
Pétrélis, F., Besse, J., & Valet, J. (2011). Plate tectonics may control geomagnetic reversal frequency Geophysical Research Letters, 38 (19) DOI: 10.1029/2011GL048784
Glatzmaiers, G., & Roberts, P. (1995). A three-dimensional self-consistent computer simulation of a geomagnetic field reversal Nature, 377 (6546), 203-209 DOI: 10.1038/377203a0