A group of scientists – led by Dr John Tarduno from the University of Rochester, NY – has recovered a magnetic field record from minerals for Iron Age southern Africa. The data suggest that the region of the planet’s core beneath southern Africa, referred to as the African large low shear velocity province (LLSVP), may be the birthplace of some of the more recent and future Earth’s magnetic pole reversals.
Schematic illustration of Earth’s magnetic field. Image credit: Peter Reid / University of Edinburgh.
“It has long been thought reversals start at random locations, but our study suggests this may not be the case,” said Dr Tarduno, first author of a paper published in the journal Nature Communications.
The scientists collected the data from five sites along South Africa’s borders with Zimbabwe and Botswana, near the Limpopo River.
That part of Africa belongs to a region called the South Atlantic Anomaly that today has an unusually weak magnetic field strength.
Earth’s magnetic field strength has decreased 16% since 1840 – with most of the decay related to the weakening field in the South Atlantic Anomaly – leading to much speculation that the planet is in the early stages of a field reversal.
“It’s only speculation because weakening magnetic fields can recover without leading to a reversal of the poles,” Dr Tarduno said.
The researchers believe they found the reason for the unusually low magnetic field strength in that region of the Southern Hemisphere.
“The top of the core beneath this region is overlain by unusually hot and dense mantle rock,” Dr Tarduno said.
That hot and dense mantle rock lies 1,865 miles (3,000 km) below the surface, has steep sides, and is about 3,730 miles (6,000 km) across.
The scientists hypothesize that the region, LLSVP, affects the direction of the churning liquid iron that generates Earth’s magnetic field.
“It’s the shift in the flow of liquid iron that causes irregularities in the magnetic field, ultimately resulting in a loss of magnetic intensity, giving the region its characteristically low magnetic field strength,” Dr Tarduno said.
He and his colleagues were able to get their data thanks to a knowledge of ancient African practices – in this case, the ritualistic cleansing of villages in agricultural communities.
“The villages were cleansed by burning down huts and grain bins. The burning clay floors reached a temperature in excess of 1,800 degrees Fahrenheit (1,000 degrees Celsius), hot enough to erase the magnetic information stored in the magnetite and create a new record of the magnetic field strength and direction at the time of the burning,” said co-author Dr Thomas Huffman of Witwatersrand University.
The researchers found a sharp 30% drop in magnetic field intensity from 1225 to 1550 CE. Given that the field intensity in the region is also declining today, they believe that the process causing the weakening field may be a recurring feature of the magnetic field.
“The new data cannot be used to predict with confidence that the present-day magnetic field is entering a reversal. However, it does suggest that the present-day pattern may be the latest manifestation of a repeating feature that occasionally leads to a global field reversal,” Dr Tarduno concluded.
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John A. Tarduno et al. 2015. Antiquity of the South Atlantic Anomaly and evidence for top-down control on the geodynamo. Nature Communications 6, article number: 7865; doi: 10.1038/ncomms8865
