<P> A brief complete reversal, known as the Laschamp event, occurred only 41,000 years ago during the last glacial period . That reversal lasted only about 440 years with the actual change of polarity lasting around 250 years . During this change the strength of the magnetic field weakened to 5% of its present strength . Brief disruptions that do not result in reversal are called geomagnetic excursions . </P> <P> In the early 20th century, geologists first noticed that some volcanic rocks were magnetized opposite to the direction of the local Earth's field . The first estimate of the timing of magnetic reversals was made by Motonori Matuyama in the 1920s; he observed that rocks with reversed fields were all of early Pleistocene age or older . At the time, the Earth's polarity was poorly understood, and the possibility of reversal aroused little interest . </P> <P> Three decades later, when Earth's magnetic field was better understood, theories were advanced suggesting that the Earth's field might have reversed in the remote past . Most paleomagnetic research in the late 1950s included an examination of the wandering of the poles and continental drift . Although it was discovered that some rocks would reverse their magnetic field while cooling, it became apparent that most magnetized volcanic rocks preserved traces of the Earth's magnetic field at the time the rocks had cooled . In the absence of reliable methods for obtaining absolute ages for rocks, it was thought that reversals occurred approximately every million years . </P> <P> The next major advance in understanding reversals came when techniques for radiometric dating were developed in the 1950s . Allan Cox and Richard Doell, at the United States Geological Survey, wanted to know whether reversals occurred at regular intervals, and invited the geochronologist Brent Dalrymple to join their group . They produced the first magnetic - polarity time scale in 1959 . As they accumulated data, they continued to refine this scale in competition with Don Tarling and Ian McDougall at the Australian National University . A group led by Neil Opdyke at the Lamont - Doherty Geological Observatory showed that the same pattern of reversals was recorded in sediments from deep - sea cores . </P>

Earth's magnetic field seems to reverse orientations about every