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’smagnetic fieldis like a bar magnet with the ends of the bar sticking out at
each pole (Figure6.8). 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 tinymagnetitecrystals 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’smagnetic
polarity. In the 1950’s, scientists began usingmagnetometersto look at the magnetic
properties of rocks in many locations.
Figure 6.8: Earth’s magnetic field is like a magnet with its north pole near the geographic
north pole and the south pole near the geographic south pole. ( 32 )
Geologists noted that magnetite crystals in fresh volcanic rocks pointed to the current mag-
netic 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 con-
tinent 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