6 - Continental drift and plate tectonics

Summary

WANDERING POLES OR WANDERING CONTINENTS?

Certain igneous rocks are magnetized when they cool according to the magnetic field of the earth at that time. Basalt preserves the field especially well, but many other rocks and some sediments do so too. The measurements yield the direction, the azimuth to the magnetic pole; with judiciously spaced samples, we can determine its past position by triangulation.

The rock also retains a magnetic record of the paleolatitude at which it formed. If one strides poleward carrying a bar magnet suspended on a string, the bar remains parallel to the lines of magnetic force and will thus point more steeply downward the closer we approach the magnetic pole (Figure 6.1). The angle, the inclination, depends only on the latitude. Alas, the azimuth to the pole and the paleolatitude are all we obtain; we know the direction of the meridian and the sample's position on it, but not the paleolongitude. With enough samples we can orient two ancient continents with respect to the pole and restore them to their initial latitudes, but we cannot know whether they were adjacent to one another or separated by a vast ocean.

By the late 1950s, curious facts had begun to emerge from a large number of measurements of this kind. If, for example, one obtained a set of magnetic pole positions for different moments in time from rocks on a single continent, the data would not cluster around a single point, but traced a line on the globe, a polar path.