2.2.5 Palaeomagnetic evidence and 'polar wander'

The Earth has the strongest magnetic field of all the terrestrial planets, with similar properties to a magnetic dipole or bar magnet. As newly erupted volcanic rocks cool, or sediments slowly settle in lakes or deep ocean basins, the magnetic minerals within them become aligned according to the Earth's ambient magnetic field. This magnetic orientation becomes preserved in the rock. The ancient inclination and declination of these rocks can then be measured using sensitive analytical equipment.

As a continent moves over the Earth's surface, successively younger rocks forming on and within that continent will record different palaeomagnetic positions, which will vary according to the location of the continent when the rock was formed. As a result, the position of the poles preserved in rocks of different ages will apparently deviate from the current magnetic pole position (Figure 4a). By joining up the apparent positions of these earlier poles, an apparent polar wander (APW) path is generated. It is now known that the Earth's magnetic poles do not really deviate in this manner, and the changes depicted in APW paths are simply a result of the continent moving over time (Figure 4b).

Figure 4 Two methods of displaying palaeomagnetic data: (a) assumes that the continent has remained fixed over time, and records the apparent polar wandering path of the South Pole; (b) assumes the magnetic poles are fixed over time, and records the latitude drift of a continent. (Adapted from Creer, 1965)

Nevertheless, APW paths remain a commonly used tool because they provide a useful method of comparing palaeomagnetic data from different locations. They are especially useful in charting the rifting and suturing of continents.

Figure 5a shows North America and Europe have individual apparent polar wander paths. However, they are broadly alike in that they have similar changes in direction at the same time. Figure 5b shows the APW paths if the Atlantic Ocean is closed by matching the continental shelves.

Figure 5 (a) Apparent polar wander paths for North America and Europe, as measured, (b) Apparent polar wander paths for North America and Europe with the Atlantic closed. Poles for successive geological periods are shown. (c) The apparent polar wander paths for Europe and Siberia. (Adapted from Mussett and Khan, 2000)

Conversely, if the APW paths of two regions were different to begin with, but became similar later on, one explanation would be that the two regions were originally on independent land masses that then collided and subsequently began to move together as a single continental unit.

Despite Wegener's amassed evidence and the increasing body of geological, palaeontological and palaeomagnetic information, there remained strong opposition to his theory of continental drift, leaving just a few forward-thinking individuals to continue seeking evidence to support this theory (Box 1).

The scientific opposition reasoned that if continents move apart, then surely they must either leave a gap at the site they once occupied or, alternatively, must push through the surrounding sea floor during their movement. The geophysicists of the day quickly presented calculations demonstrating that the continents could not behave in this way and, more importantly, no one could conceive of a physical mechanism for driving the continents in the manner Wegener had proposed. Consequently, the theory of continental drift did not gain scientific popularity at the time and became increasingly neglected for several decades. To gain a wider scientific acceptance, Wegener's ideas had to await a greater understanding in the internal structure of the Earth and the processes controlling the loss of its internal heat.