The polarity of earth's magnetic field reverses at irregular intervals.
Before humanity had the luxury of a Global Positioning System (GPS), navigation relied in large part on the compass and the direction of the earth's magnetic poles. We tend to think that the earth's magnetic poles are fixed, but according to scientists they drift over long periods of time and actually reverse at irregular intervals. According to an article in The Scientific American in 1999, the intervals range from 10,000 to 25,000,000 years. The last pole reversal occurred about 780,000 years ago.
Paleomagnetism
Many rocks in general and basaltic rocks in particular contain magnetite, and when the rock cools to a point where the magnetite becomes magnetic, called its Curie temperature, its magnetic polarization freezes to match the current polarization of the earth. Geologists can therefore know that if a sample of rock has reverse polarity it must have solidified when the earth's polarization was reversed. Paleomagnetism is the study of these various polarities to determine the age of rock and to make inferences about the earth's geologic past.
An Example of Paleomagnetic Banding
Basaltic rock contains magnetite.
The sea floors of both the Atlantic and Pacific oceans are spreading. New sea floor is being created, and, in the Atlantic, the new material is being pushed to the east and west of the mid-Atlantic ridge. The new material is primarily basaltic rock and therefore contains a clear record of the earth's polarization at the time it solidified. As the spreading moves slowly away from the ridge over long periods of time, it produces bands of sea floor with opposite magnetic polarization. This type of alternating polarity reversal in rock is called paleomagnetic banding.
Paleomagnetic Banding and Plate Tectonics
When the German meteorologist and geologist Alfred Wegener first proposed continental drift in 1915, his ideas were rejected and even ridiculed by the world's geological establishment. Today, plate tectonics is supported by much and varied evidence, not the least of which is paleomagnetism and paleomagnetic banding. Alfred Wegener died in 1930, 20 years before scientists discovered measure paleomagnetism. The majority of today's geologists, if not all, now accept this explanation of how the continents came to be where they are.
Tags: earth magnetic, Alfred Wegener, earth magnetic poles, earth polarization, irregular intervals, long periods
