Mon. Nov 28th, 2022
Earth's magnetic field nearly died during critical transition

You’ve almost certainly seen the diagram – the layers of the Earth have been exposed as a slightly more complicated hard-boiled egg. The crust we live on is actually a thin shell, with the hot (but still solid) mantle forming a thick layer underneath. In the middle – unlike Jules Verne – there are inner and outer core layers that are mainly composed of iron. The outer core is the only layer that is liquid, since the inner core is in fact solid.

Although you will never visit the core, it will have quite an impact on your life. The Earth’s magnetic field is produced by the convection of the liquid outer core, which sends compasses and protects us from the effects of the solar wind. The history of Earth’s magnetic field is a big question, not least because we don’t actually know for sure when the inner core solidified.

Magnets… well, you know

Over there to be actually geologic records of the magnetic field. Tiny crystals of magnetic minerals in cooling magma will align with the magnetic field before being frozen in place. This can be useful because Earth’s magnetic field often reverses poles (meaning compass needles would point to geographic south). The orientation of those mineral needles also indicates how close they were to the equator when they formed. The information captured by these minerals was the last piece to crack the case of plate tectonics, and it allows us to figure out where each continent was in the past.

From this data we can also infer how strong the magnetic field was. That’s what a team led by Richard Bono and John Tarduno of the University of Rochester was most interested in when they analyzed some 565-million-year-old rocks in Quebec.

We don’t have much data from that time period, which some researchers suspect may have been when the inner core finally started to solidify. In this case, the igneous rocks the researchers were working with slowly cooled underground, meaning their record likely spans about 75,000 years. That should be longer than the magnetic pole reversal usually takes, so any transient changes like that should be averaged out.

Weak sauce

The team found that the magnetic field was incredibly weak during the time studied. The late Jurassic is notable for an abnormally weak magnetic field, but this was only about a fifth as strong as that period. In addition, it appears that the magnetic poles rotated extremely frequently. It’s very weird behavior.

However, this data fits into the limited information we have from periods closest to this time, which is also quite odd. And the study gets really interesting when you compare it to simulations of the history of the Earth’s core. Some of those simulations have predicted that the solidification of the inner core was quite recent in geological terms and occurred around this time. In those models, the reorganization of the core causes the magnetic field to go wild for a while, floundering in a weakened state.

The researchers say their evidence fits a scenario where the inner core only started to solidify about 565 million years ago — nearly 4 billion years into Earth’s life. This adds to questions about what the magnetic field-producing “geodynamo” in the core looked like before that time and how it lasted so long. It also adds to the overflowing list of strange things going on in this chapter of Earth’s history, which just happens to include a remarkable evolutionary explosion of complex animal life.

In a summary with the paper in Natural GeosciencesCarnegie Institution for Science researcher Peter Driscoll (who was not involved in the study) explains the work needed to follow up on this juicy hypothesis: “Additional paleomagnetic observations, both directions and intensities, are the obvious next. step to provide a clearer picture of the state of the core around this time.Experiments on the solidification and conductivity of iron will further limit the core’s thermodynamics.Finally, numerical models of the evolving core can produce detailed predictions that test how all these components come together.”

And if this timeline of inner core formation is correct, Driscoll writes, “the inner core nucleation may have happened just in the nick of time to recharge the geodynamo and save Earth’s magnetic shield.”

Natural Geosciences2019. DOI: 10.1038/s41561-018-0288-0, 10.1038/s41561-019-0301-2 (About DOIs).

By akfire1

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