Digging deeper into terrestrial magnetism
For centuries travellers used compasses for navigation without understanding how they worked. For many years it was assumed that the magnetic and geographic poles were the same, a misconception that led to the early discovery of North America by Christopher Columbus in 1492. At the magnetic poles the field lines point straight up and down and so a compass there is useless.
Like all forms of magnetism the Earth's magnetic field is produced by electric currents. One theory accounting for the production of these currents is that deep in the Earth's core, hot molten magma rises, cools and sinks. Then, the whole process repeats itself. It is thought that within these rising and falling masses of magma the rotation of the Earth creates organized patterns of circular electrical currents, called eddies. The interior of the planet in fact acts like a giant dynamo.
Geophysicists have found that some of the Earth's magnetic field reversals occurred within a few 10,000 years of each other, and others, tens of millions of years apart. Lately the field has been reversing on the average, every 200,000 years, although it hasn't happened in the last 800,000 years. It is not known whether this reversal occurs gradually, or whether there is a period of time when there is no magnetic field at all. This latter possibility could have devastating effects for life on Earth, as it is the magnetic field which protects the Earth from deadly solar radiation. In fact, there appears to be good correlation between magnetic field reversals in the past and extinction of certain species. It is not known why these reversals occur, but it is as if the 'dynamo' in the Earth's core is turned off and turned on again in the opposite direction.
Posted by Jay Roberts at 04:37 AM | Permalink