Ancient lava reveals secrets of Earth’s magnetic field cycle

A study from the University of Liverpool has added to the growing corpus of evidence that Earth’s magnetic field is subject to a roughly 200-million-year-long cycle of highs and lows.

The researchers used a suite of techniques from the field of palaeomagnetism – the study of past variations in Earth’s fluxing magnetic field – to analyse rock samples from ancient lava flows found in eastern Scotland. They determined that between 332 and 416 million years ago, the strength of the geomagnetic field preserved in the rocks was less than a quarter of its strength today – a time period they’ve termed the Mid-Palaeozoic Dipole Low (MPDL). That’s similar to a previously identified period of low magnetic strength that began around 120 million years ago.

“This comprehensive magnetic analysis of the Strathmore and Kinghorn lava flows was key for filling in the period leading up the Kiman Superchron, a period where the geomagnetic poles are stable and do not flip for about 50 million years,” says lead author Louise Hawkins.

“Our findings, when considered alongside the existing datasets, support the existence of an approximately 200-million-year-long cycle in the strength of the Earth’s magnetic field related to deep Earth processes.” 

Fluctuations in Earth’s magnetic field are important because they have material implications for life: the magnetic field shields us from blasts of deadly solar radiation. But Earth’s magnetism is not stable, and even has the ability to completely flip or reverse with potentially seismic implications. Understanding these implications may give us clues about the impacts of future fluctuations on life.

A recent UK study, for example, found that the mass extinction event that occurred at the Devonian-Carboniferous boundary 359 million years ago was associated with elevated UV-B radiation, which occurred around the same time as the weakest field measurements from the MPDL.

Read more: Magnetic field locked in by ancient crystals

Hawkins says the identification of these signals of magnetic fluctuation is particularly valuable because it’s so rare: “As almost all of our evidence for processes within the Earth’s interior is being constantly destroyed by plate tectonics, the preservation of this signal for deep inside the Earth is exceedingly valuable as one of the few constraints we have.

“Our findings also provide further support that a weak magnetic field is associated with pole reversals, while the field is generally strong during a Superchron, which is important as it has proved nearly impossible to improve the reversal record prior to approximately 300 million years ago.”

The new study, published in Proceedings of the National Academy of Sciences, builds on previous research from the university into these periodic fluctuations in Earth’s magnetism. The research is part of the university’s Determining Earth Evolution from Palaeomagnetism (DEEP) group, bringing together researchers from geophysics and geology to probe the Earth’s geological processes across deep timescales.

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