Geomagnetic reversal

A geomagnetic reversal is a major change in the Earth's dipole magnetic field. During this event, the positions of magnetic north and magnetic south are interchanged. It is important to note that magnetic poles are not the same as geographic poles. The Earth's magnetic field moves between periods of normal polarity and reverse polarity. Scientists refer to these specific periods of steady magnetic direction as chrons.

These reversals happen deep within our planet. The magnetic field is generated by the liquid outer core. Sometimes, the field undergoes an excursion rather than a full reversal. An excursion is a short episode where the field inverts only for a few hundred years. During these excursions, the field reverses in the liquid outer core, but it does not reverse in the solid inner core. This happens because diffusion in the liquid outer core occurs on timescales of 500 years or less. In contrast, the inner core has a longer diffusion timescale of around 3,000 years.

Full geomagnetic reversals take much longer to complete. Most estimates suggest these transitions take between 2,000 and 12,000 years. Some individual reversals may last as long as 70,000 years. Research by Clement in 2004 suggests that the duration of a reversal depends on latitude. Shorter durations are found at low latitudes. Longer durations occur at mid and high latitudes. The most recent full reversal is called the Brunhes–Matuyama reversal. It occurred approximately 780,000 years ago.

Humans have been uncovering this history for over a century. In the early 20th century, geologist Bernard Brunhes noticed volcanic rocks with opposite magnetization. In the late 1920s, Motonori Matuyama provided systematic evidence. He observed that rocks with reversed fields were of early Pleistocene age or older. In 1959, Allan Cox and Richard Doell created the first magnetic-polarity time scale. They worked with geochronologist Brent Dalrymple at the United States Geological Survey. Later, Neil Opdyke showed that these same patterns appeared in deep-sea sediment cores.

One of the most significant discoveries involves the ocean floor. In 1963, Frederick Vine and Drummond Matthews explained magnetic stripes on the seafloor. They combined seafloor spreading theory with the known time scale of reversals. As new seafloor rock forms at a central ridge, it is magnetized by the current field. As the seafloor spreads, it creates pairs of magnetic stripes parallel to the ridge. This provided a key scientific test for the theory of continental drift. These stripes allow scientists to estimate when most oceanic crust developed.

While reversals are common, some eras are remarkably stable. These long periods with no reversals are called superchrons. A superchron is a polarity interval lasting at least 10 million years. The Cretaceous Normal Superchron lasted for 37 million years. It began about 120 million years ago. Another major event was the Kiaman Reverse Superchron. It lasted for more than 50 million years from the late Carboniferous to the late Permian. A third candidate, the Moyero Superchron, is more controversial. It may have lasted over 20 million years in Siberia.

Statistically, the pattern of these reversals appears to be random. There is no known correlation between the lengths of different polarity intervals. There is also no preference for normal polarity over reversed polarity. Scientists use different mathematical models to study this randomness. Some models suggest the reversals follow a Poisson process. Others suggest a gamma process, which shows a reduced probability of reversal for tens of thousands of years after one has occurred. Some physicists even suggest the process follows a Lévy distribution. This means the reversals are part of a complex, potentially chaotic system.