Paleomagnetism

Sequences of rocks can thus act like a magnetic tape. Unfortunately, the original record is usually altered secondarily through time and various weathering processes. Paleomagnetic methods have to be employed to remove this magnetic noise and extract a true primary magnetization.

The results of paleomagnetic studies over the past four decades have had a revolutionary influence on our understanding of Earth history. The most significant single finding is that the orientation of magnetic minerals in rocks is often very much out of phase with the earth's present magnetic field. At least two possible explanations for this phenomenon are possible and, in fact, have been proposed by scientists.

First, Earth's magnetic field itself changes over time. Differences in orientation result from changes in the magnetic poles, not in the orientation of the minerals.

Second, variations in the orientation of magnetic minerals have been caused by the movement of the minerals themselves. Since the minerals are now—and have for a long time been—frozen into the rocks, this theory would suggest that it is the rocks themselves that are moving across the earth's surface.

In fact, scientists now know that both of these explanations are correct; the earth's magnetic poles have wandered from place to place over time and the rocks in which magnetic minerals are found have traveled across the earth's surface. In addition, there is strong evidence that the polarity of the earth's magnetic field has shifted (the north pole changing to the south pole, and vice versa) at least 171 times in the past 76 millions years. These reversals of polarity take place rather slowly, over a period of 5,000-10,000 years. They then remain fixed for a period of up to a million years.

Earth's magnetic field has been a dipole field for more than 99.9% of Earth's history. Its shape resembles that of the field of a bar-magnet. The field lines emerge at one pole and re-enter at the other pole. The earth's magnetic field however is not caused by a huge mass of iron with a remanent magnetization, but its origin lies in the outer fluid core where convective motion generates the magnetic field in a self-sustaining dynamo action. This dynamic origin of the geomagnetic field is the main reason why its shape and orientation are not constant but subject to temporal variations on time scales that range from millions of years to days. Recently, for example, the dipole axis is inclined by about 11 (against the spin axis). Averaged over time spans greater than 100,000 years, the dipole axis is parallel with Earth's spin axis.

In addition to these dramatic reversals of polarity, Earth's magnetic poles have also wandered. About 300 million years ago, for example, the north magnetic pole was located in the eastern region of Siberia. It then traveled northward to the northern coast of Siberia, along to the coastline to Alaska, and then northward to its present location.