Geomagnetic reversal
[5] In the early 20th century, geologists such as Bernard Brunhes first noticed that some volcanic rocks were magnetized opposite to the direction of the local Earth's field.
In the absence of reliable methods for obtaining absolute ages for rocks, it was thought that reversals occurred approximately every million years.
Allan Cox and Richard Doell, at the United States Geological Survey, wanted to know whether reversals occurred at regular intervals, and they invited geochronologist Brent Dalrymple to join their group.
As they accumulated data, they continued to refine this scale in competition with Don Tarling and Ian McDougall at the Australian National University.
A group led by Neil Opdyke at the Lamont–Doherty Earth Observatory showed that the same pattern of reversals was recorded in sediments from deep-sea cores.
[7] During the 1950s and 1960s information about variations in the Earth's magnetic field was gathered largely by means of research vessels, but the complex routes of ocean cruises rendered the association of navigational data with magnetometer readings difficult.
Only when data were plotted on a map did it become apparent that remarkably regular and continuous magnetic stripes appeared on the ocean floors.
[6][7] In 1963, Frederick Vine and Drummond Matthews provided a simple explanation by combining the seafloor spreading theory of Harry Hess with the known time scale of reversals: sea floor rock is magnetized in the direction of the field when it is formed.
[7] Past field reversals are recorded in the solidified ferrimagnetic minerals of consolidated sedimentary deposits or cooled volcanic flows on land.
Most sedimentary rocks incorporate minute amounts of iron-rich minerals, whose orientation is influenced by the ambient magnetic field at the time at which they formed.
Through analysis of seafloor magnetic anomalies and dating of reversal sequences on land, paleomagnetists have been developing a Geomagnetic Polarity Time Scale.
The Jurassic Quiet Zone in ocean magnetic anomalies was once thought to represent a superchron but is now attributed to other causes.
They were once thought to represent a superchron called the Jurassic Quiet Zone, but magnetic anomalies are found on land during this period.
In 2006, a team of physicists at the University of Calabria found that the reversals also conform to a Lévy distribution, which describes stochastic processes with long-ranging correlations between events in time.
[25] During a transition, the magnetic field will not vanish completely, but many poles might form chaotically in different places during reversal, until it stabilizes again.
[26][27] Studies of 16.7-million-year-old lava flows on Steens Mountain, Oregon, indicate that the Earth's magnetic field is capable of shifting at a rate of up to 6 degrees per day.
[29] That said, paleomagnetic studies of other sections from the same region (the Oregon Plateau flood basalts) give consistent results.
[30][31] It appears that the reversed-to-normal polarity transition that marks the end of Chron C5Cr (16.7 million years ago) contains a series of reversals and excursions.
[32] In addition, geologists Scott Bogue of Occidental College and Jonathan Glen of the US Geological Survey, sampling lava flows in Battle Mountain, Nevada, found evidence for a brief, several-year-long interval during a reversal when the field direction changed by over 50 degrees.
For example, Gary Glatzmaier and collaborator Paul Roberts of UCLA ran a numerical model of the coupling between electromagnetism and fluid dynamics in the Earth's interior.
[41] Some scientists, such as Richard A. Muller, think that geomagnetic reversals are not spontaneous processes but rather are triggered by external events that directly disrupt the flow in the Earth's core.
[44] Supporters of this hypothesis hold that any of these events could lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field.
This proposed mechanism does not appear to work in a quantitative model, and the evidence from stratigraphy for a correlation between reversals and impact events is weak.
[45] Shortly after the first geomagnetic polarity time scales were produced, scientists began exploring the possibility that reversals could be linked to extinction events.
Possibly the first such hypothesis was that high-energy particles trapped in the Van Allen radiation belt could be liberated and bombard the Earth.
[48][49] Detailed calculations confirm that if the Earth's dipole field disappeared entirely (leaving the quadrupole and higher components), most of the atmosphere would become accessible to high-energy particles but would act as a barrier to them, and cosmic ray collisions would produce secondary radiation of beryllium-10 or chlorine-36.
