Magnetic Polarity Evidence - The puzzling new evidence came from studying Earth’s magnetic field and how it has changed. If you have ever been hiking or camping, you may have used a compass to help you find your way. A compass uses the Earth’s magnetic field to locate the magnetic North Pole. Earth’s magnetic field is like a bar magnet with the ends of the bar sticking out at each pole.
Currently, the field’s north and south magnetic poles are very near to the Earth’s north and south geographic poles. Some iron-bearing minerals, like tiny magnetite crystals in igneous rocks, point to the north magnetic pole as they crystallize from magma. These little magnets record both the strength and direction of the Earth’s magnetic field. The direction is known as the field’s magnetic polarity. In the 1950’s, scientists began using magnetometers to look at the magnetic properties of rocks in many locations.
Geologists noted that magnetite crystals in fresh volcanic rocks pointed to the current magnetic north pole. This happened no matter where the rocks were located, whether they were on different continents or in different locations on the same continent. But for older volcanic rocks, this was not true. Rocks that were the same age and were located on the same continent pointed to the same point, but that point was not the current north magnetic pole. Moving back in time, rocks on the same continent that were the same age pointed at the same point. But these rocks did not point to the same point as the rocks of different ages or the current magnetic pole.
In other words, although the magnetite crystals were pointing to the magnetic north pole, the location of the pole seemed to wander. For example, 400 millon year old lava flows in North America indicated that the north magnetic pole was located in the western Pacific Ocean, but 250 million year old lava flows indicated a pole in Asia, and 100 million year old lava flows had a pole in northern Asia. Scientists were amazed to find that the north magnetic pole changed location through time.
There were three possible explanations for this puzzling phenomenon:
- the continent remained fixed and the north magnetic pole moved
- the north magnetic pole stood still and the continent moved
- both the continent and the north pole moved.
The situation got stranger when scientists looked at where magnetite crystals pointed for rocks of the same age but on different continents. They found these rocks pointed to different magnetic north poles! For example, 400 million years ago the European north pole was different from the North American north pole at that same time. At 250 million years, the north poles were also different for the two continents.
The scientists again looked at the three possible explanations. If the correct explanation was that the continents had remained fixed while the north magnetic pole moved, then there had to be two separate north poles. Since there is only one north pole today, they decided that the best explanation had to involve only one north magnetic pole. This meant that the second explanation must be correct, that the north magnetic pole had remained fixed but that the continents had moved.
To test this, geologists fitted the continents together as Wegener had done. They discovered that there had indeed been only one magnetic north pole but that the continents had drifted. They renamed the phenomenon of the magnetic pole that seemed to move but actually did not apparent polar wander. This evidence for continental drift gave geologists renewed interest in understanding how continents could move about on the planet’s surface. And we know that the magnetic pole wanders, too, so the correct explanation was that both the continents and the magnetic poles move.
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