What did Earth’s ancient magnetic field look like?

Posted: June 27, 2016 by oldbrew in Geomagnetism, research
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Credit: NASA

Credit: NASA


Whether or not dynamo theories are sound, this research highlights historic variability in Earth’s magnetic field behaviour and could have ‘widespread implications’. Phys.org reporting.

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 magnetic field 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.

Source: What did Earth’s ancient magnetic field look like? | phys.org

Comments
  1. oldbrew says:

    Then there’s polar wander: both ‘true’ and ‘apparent’.

    ‘Polar wander is the motion of a pole in relation to a fixed reference frame. It can be used, for example, to measure the degree to which Earth’s magnetic poles have been observed to move relative to the Earth’s rotation axis.’
    http://en.wikipedia.org/wiki/Polar_wander

  2. Graeme No.3 says:

    oldbrew:
    Standby for the new scare. “Polar wander causes climate change – worse than we thought”.
    Everybody must go back to living in the Bronze age (or before) as iron is bad.

  3. tom0mason says:

    Graeme No.3 you have that the wrong way round, the shout surely will be —
    “Climate change causes polar wandering with possible positive feedback — worse than we thought!”
    🙂

  4. oldmanK says:

    Wasn’t going to say anything, but……

    According to Plato the Egyptian priests said four time the sun changed its place of rising——-. Maybe therefore much has shifted around, and maybe therefore the magnetic field was always in the same place.

    For one or two places that shifting is a dead certain.

  5. husq says:

    oldmanK says:
    June 28, 2016 at 1:26 pm

    The Papyrus Harris speaks of a colossal catastrophe of fire and water when “the North becomes the South. The Earth turned round on itself”
    The priets of Luxor “Twice in past ages the Sun used to rise in a different quarter from where it rises now and twice it was wont to set in the east instead of the west”

    Plato wrote about this in Politicus: “In certain periods, the universe has its present rotating movement and in other periods it turns in the opposite direction… Of all the changes that take place in the sky, this reversal is the biggest and the most complete.”

  6. Curious George says:

    The solidification of a nickel-iron ball 2400 km in diameter is not an overnight event. Even half a billion years seem too short.

  7. oldmanK says:

    @ husq: thank you for the references. Plato said something else, in similar vein, in ‘timaeus’. Now that is important for the hard evidence for that exists. It may serve us well to look into the matter sensibly.

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