What did Earth's ancient magnetic field look like?

What did Earth's ancient magnetic field look like?
Illustration of ancient Earth's magnetic field compared to the modern magnetic field. Credit: Peter Driscoll

New work from Carnegie's Peter Driscoll suggests Earth's ancient magnetic field was significantly different than the present day field, originating from several poles rather than the familiar two. It is published in Geophysical Research Letters.

Earth generates a strong extending from the core out into space that shields the atmosphere and deflects harmful high-energy particles from the Sun and the cosmos. Without it, our planet would be bombarded by cosmic radiation, and life on Earth's surface might not exist. The motion of liquid iron in Earth's outer core drives a phenomenon called the geodynamo, which creates Earth's magnetic field. This motion is driven by the loss of heat from the core and the solidification of the inner core.

But the planet's inner core was not always solid. What effect did the initial solidification of the inner core have on the magnetic field? Figuring out when it happened and how the field responded has created a particularly vexing and elusive problem for those trying to understand our planet's geologic evolution, a problem that Driscoll set out to resolve.

Here's the issue: Scientists are able to reconstruct the planet's magnetic record through analysis of ancient rocks that still bear a signature of the magnetic polarity of the era in which they were formed. This record suggests that the field has been active and dipolar—having two poles—through much of our planet's history. The geological record also doesn't show much evidence for major changes in the intensity of the ancient magnetic field over the past 4 billion years. A critical exception is in the Neoproterozoic Era, 0.5 to 1 billion years ago, where gaps in the intensity record and anomalous directions exist. Could this exception be explained by a major event like the solidification of the planet's inner core?

In order to address this question, Driscoll modeled the planet's thermal history going back 4.5 billion years. His models indicate that the inner core should have begun to solidify around 650 million years ago. Using further 3-D dynamo simulations, which model the generation of magnetic field by turbulent fluid motions, Driscoll looked more carefully at the expected changes in the magnetic field over this period.

"What I found was a surprising amount of variability," Driscoll said. "These new models do not support the assumption of a stable dipole field at all times, contrary to what we'd previously believed."

His results showed that around 1 billion years ago, Earth could have transitioned from a modern-looking field, having a "strong" magnetic field with two opposite poles in the north and south of the planet, to having a "weak" magnetic field that fluctuated wildly in terms of intensity and direction and originated from several poles. Then, shortly after the predicted timing of the core solidification event, Driscoll's dynamo simulations predict that Earth's magnetic field transitioned back to a "strong," two-pole one.

"These findings could offer an explanation for the bizarre fluctuations in magnetic field direction seen in the geologic record around 600 to 700 million years ago," Driscoll added. "And there are widespread implications for such dramatic field changes."

Overall, the findings have major implications for Earth's thermal and magnetic history, particularly when it comes to how magnetic measurements are used to reconstruct continental motions and ancient climates. Driscoll's modeling and simulations will have to be compared with future data gleaned from high quality magnetized rocks to assess the viability of the new hypothesis.

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Consistency of Earth's magnetic field history surprises scientists

Journal information: Geophysical Research Letters

Citation: What did Earth's ancient magnetic field look like? (2016, June 24) retrieved 25 August 2019 from https://phys.org/news/2016-06-earth-ancient-magnetic-field.html
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Jun 24, 2016
Purely theoretical, based on the - I think - fringe idea that the core solidified at 1 Ga. There are phase diagrams of iron that suggest it solidified right away.

Also, Earth's geodynamo field doesn't stop cosmic rays much (but the vast solar magnetic field is believed to stop 90 %). It is the atmosphere that does, and then the geodynamo field that shields the atmosphere, mostly its water. C.f. Venus.

Jun 24, 2016
" fringe idea that the core solidified at 1 Ga" It is not so much a fringe idea. Really it is a bit of an unknown when it started to solidify, and the belief is that this solidification powered movement of the outer core material that sustains a magnetic field. I suspect that before iron started to solidify there would have been other solid minerals denser than molten core that would have settled to start with at the centre. But more research in high pressure experiments is needed to verify possibilities. There would not have to be much of a thermal gradient to produce highly complex convection.

Jun 25, 2016
Yeah, as can be seen I prevaricated a bit. Maybe I should have said that it is a new hypothesis, and the reigning consensus - such as it is in an open question - is a much earlier solid core formation, 4 - 2 Ga. [ https://en.wikipe...ner_core ]

Jun 27, 2016
Graeme raises an interesting question: given relatively heavy elements other than iron in the core, we should expect some to precipitate out earlier than iron.

But those elements are probably present only in trace amounts - very small compared to the amount of iron there. They would have little impact on the overall magnetic field, which is generated by convection of liquid iron.

Where my skeptical soul stumbles is on the idea that iron solidification didn't begin until less than a billion years ago.

I don't know how good the simulation reported by this article is, but it's reasonable to assume that the newly-formed Earth had a very steep thermal curve from center to surface, much steeper than today. We would expect internal convection currents to be quite forceful in the early years. A steeper thermal curve also implies faster heat transport and faster heat radiation into space. Stronger magnetic field, likely dipolar because of the strength of the currents.

Jun 27, 2016
As Earth aged, the heat gradient should have dropped and the currents should have become less forceful.

Slowing currents might account for Earth's acquisition of a weaker, multipolar field - meandering currents could produce that - but then why, at .5 or .6 billion years ago, did the field suddenly strengthen and resume dipolarity again? What caused the currents to speed up?

One possible answer is, the thermal gradient rose again. But it's hard to imagine a cause. (Fission in the core? But why then and not earlier?)

Another possible answer has to do with fluid dynamics around the growing core - regardless of when the core began to form, it was certainly growing from the moment of its formation. This fluid dynamics question is what the simulation reported in Phys.org purports to explore. But I'm skeptical that they had enough data to make an accurate simulation.

There's a lot about Earth's interior that is still mysterious.

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