• Question: Why does the Earth's magnetic field change?

    Asked by abichohan to Davie, Gemma, James P, James V, Nuala on 29 Jun 2012.
    • Photo: James Pope

      James Pope answered on 29 Jun 2012:

      Hi abichohan,

      Our gravitational field is caused by the movement of molton iron moving around in the outer core, this fluid movement causes the magnetic field. The main changes of the magnetic field I have learnt about are the geomagnetic reversals. These are periods when the magnetic poles of the earth switch round, so if at the moment the North Pole of the Earth and the North Pole of the magnetic field were the same, in a reversal, the North Pole of the magnetic field, flips to the South Pole of the Earth. These reversals occur every 100,000 to 50 million years apart, but most of them occur on 800,000 to 1million year intevals. The last one was about 750,000 years ago. We can measure these in rocks that have magnetic minerals in them, mainly in rocks that contain iron, mostly in lava flows. We can drill into these rocks and work out the field direction at a time (which direction the positive magnetic pole is pointing) in the geological record and from that date the rocks on a timescale.

      When reconstructing past climates, geomagnetism is one of the best ways of dating how old the sediments are, that we are getting the fossil plankton from to reproduce the climates!

    • Photo: James Verdon

      James Verdon answered on 1 Jul 2012:

      H abichohan,

      The magnetic field is caused by the circulation of the earth’s molten iron outer core. This circulation effectively creates a giant electromagnet. However, scientists aren’t exactly sure why the field reverses. It must have something to do with changes in the core circulation, but because we have no way of measuring the circulation directly, we’re not sure what those changes are.

      The earth’s magnetic field is quite important for us (and not just so we can use compasses). The field can deflect charged, radioactive particles that are created by the sun and are heading right for us. So it acts as a shield. Without it, we would be bombarded by an increase in radiation, which could have effects on the earth’s plants and animals, including us.

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