Why does Earth’s magnetic field sometimes flip over? Does such a reversal affect living things?
Joe Kirschvink, paleomagnetist and professor of geobiology at Caltech, answers:
Earth’s magnetic field is produced by electrical currents flowing within the hot liquid metal of the outer part of the planet’s core. Like gases in the atmosphere and water in the oceans, this fluid metal undergoes random fluctuations in temperature and motion, analogous to the weather patterns we experience up top. Occasional “storms” in the core are strong enough to flip the polarity of the magnetic field, although the process of reversal is usually quite slow, taking 3,000 to 10,000 years. Reversals are also rather rare events: On average, only three or four of them happen every million years.
Many animals—including bees, fish, pigeons, and whales—possess a specialized set of neurological receptors containing tiny magnets, which allow them to sense and navigate by the geomagnetic field. During reversals, Earth’s field weakens (although it does not disappear entirely), and the direction of magnetic north may vary wildly. During a single animal’s lifetime, however, most of these changes are probably too small to be noticed. There have been several thousand magnetic reversals since the evolution of animal phyla about 600 million years ago, so any creatures that experience seriously detrimental effects when a flip occurs have probably been removed by natural selection.