The Big Questions by Michael Brooks

Author:Michael Brooks [Brooks, Michael]
Language: eng
Format: epub
Publisher: Quercus
Published: 2013-04-06T04:00:00+00:00

For a period of time, during a reversal there is no clearly defined field. So, could this happen with the Earth’s magnetic field, with potentially disastrous results? Unfortunately, even these simulations have not yet proved accurate enough for us to make forecasts for the Earth’s field. The best we can do, it seems, is to look at the evidence frozen into the planet’s rocky crust, and try to extrapolate our findings.

Written in the rocks

In the molten rock that pours from volcanoes and the gaps between tectonic plates in mid-ocean ridges, magnetic crystals—tiny grains of magnetite, for example—are free to move, and will orient themselves to the direction of the Earth’s magnetic field. When that rock cools, that orientation is frozen in, creating a rock whose magnetic field points toward its era’s magnetic north. By dating rocks and noting their magnetic orientation, researchers can build up a picture of how the direction of “north” has changed over millennia. This is how we gained the first evidence for a shield failure. In 1904, geomagnetic studies of the Massif Central mountains of southern France revealed that the orientation of the magnetic crystals in the rocks was significantly shifted from what it would be today. In the 1920s, similar observations were made across the world, and the field of paleomagnetism was born.

We now have evidence that, during the past 20 million years, the Earth’s field has collapsed and reversed more than 60 times. These reversals have occurred every half-million years or so, and can take thousands of years to complete. However, it is by no means a clockwork phenomenon. Sometimes, as happened during the age of the dinosaurs, no flips happen for tens of millions of years. We haven’t seen a reversal for 780,000 years now. So, does that mean we are due one? Is that why the Earth’s field is currently fading at what seems like an alarmingly fast rate?

We know, thanks to the logbooks kept during Captain Cook’s journeys in the South Seas, that the current failure only started relatively recently. We have mariners’ logs that date back to 1590, which record, amongst many other things, the direction of the Earth’s field and the angle at which the field lines enter the Earth. It was a useful navigational trick—in many ways the sailors’ lives depended on it. We have recorded a decline in field strength since Gauss began measuring it in 1840, but the ship’s logs show no change between the 1590 value and Gauss’s field strength.

Of course, it may be that we haven’t enough data to draw any firm conclusions; the “South Atlantic anomaly” may be leading us astray, for example. So should these strange measurements and discoveries give us cause for concern? Given the crucial role Earth’s magnetic field has played—and continues to play—in the development of life on Earth, the answer has to be yes.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.