A team of scientists led by Dr Leonardo Sagnotti of National Institute of Geophysics and Volcanology in Rome, Italy, has found evidence that the most recent Earth’s magnetic field reversal happened very quickly – over a period of less than 100 years.

Based on studies of old volcanic basalt, scientists know that the magnetic field of our planet has flipped many times throughout the planet’s history.

If you were alive about 800,000 years ago, and facing what we call north with a magnetic compass in your hand, the needle would point to ‘south.’ This is because a magnetic compass is calibrated based on Earth’s poles. The N-S markings of a compass would be 180 degrees wrong if the polarity of today’s magnetic field were reversed.

Some researchers have tried to take this natural geological occurrence and suggest it could lead to Earth’s destruction. But would there be any dramatic effects? The answer, from the geologic and fossil records we have from hundreds of past magnetic polarity reversals, seems to be ‘no.’

Reversals are the rule, not the exception. Earth has settled in the last 20 million years into a pattern of a pole reversal about every 200,000 to 300,000 years, although it has been more than twice that long since the last reversal.

A reversal happens over hundreds or thousands of years, and it is not exactly a clean back flip. Magnetic fields morph and push and pull at one another, with multiple poles emerging at odd latitudes throughout the process.

Scientists estimate reversals have happened at least hundreds of times over the past three billion years.

In a new study, published in the Geophysical Journal International, Dr Sagnotti and his colleagues have discovered something new about the Matuyama-Brunhes transition – the most recent magnetic reversal that occurred more than 700,000 years ago. They found that it happened very quickly – in less than a century.

“It’s amazing how rapidly we see that reversal. The paleomagnetic data are very well done. This is one of the best records we have so far of what happens during a reversal and how quickly these reversals can happen,” said co-author Courtney Sprain of the University of California and Berkeley Geochronology Center.

The finding is based on measurements of the magnetic field alignment in layers of ancient lake sediments now exposed in the Sulmona basin of the Apennine Mountains east of Rome, Italy.

The lake sediments are interbedded with ash layers erupted from the Roman volcanic province, a large area of volcanoes upwind of the former lake that includes periodically erupting volcanoes near Sabatini, Vesuvius and the Alban Hills.

The scientists measured the magnetic field directions frozen into the sediments as they accumulated at the bottom of the ancient lake.

They used argon-argon dating to determine the age of ash layers above and below the sediment layer recording the last reversal.

Because the lake sediments were deposited at a high and steady rate over a 10,000-year period, they were able to interpolate the date of the layer showing the magnetic reversal at approximately 786,000 years ago.

This date is far more precise than that from previous studies, which placed the reversal between 770,000 and 795,000 years ago.

“What’s incredible is that you go from reverse polarity to a field that is normal with essentially nothing in between, which means it had to have happened very quickly, probably in less than 100 years. We don’t know whether the next reversal will occur as suddenly as this one did, but we also don’t know that it won’t,” said co-author Dr Paul Renne of the University of California and Berkeley Geochronology Center.

“Whether or not the new finding spells trouble for modern civilization, it likely will help researchers understand how and why Earth’s magnetic field episodically reverses polarity.”

The magnetic record the scientists obtained shows that the sudden 180-degree flip of the field was preceded by a period of instability that spanned more than 6,000 years.

The instability included two intervals of low magnetic field strength that lasted about 2,000 years each.

Rapid changes in field orientations may have occurred within the first interval of low strength. The full magnetic polarity reversal – that is, the final and very rapid flip to what the field is today – happened toward the end of the most recent interval of low field strength.

_____

Leonardo Sagnotti et al. 2014. Extremely rapid directional change during Matuyama-Brunhes geomagnetic polarity reversal. Geophys. J. Int. 199 (2): 1110-1124; doi: 10.1093/gji/ggu287