The study of the Earth's magnetic field as recorded in the rock record was an important key in reconstructing the history of plate motions. We have already seen how the recording of magnetic reversals led to the confirmation of the seafloor spreading hypothesis. The concept of apparent polar wander paths was helpful in determining the speed, direction, and rotation of continents.
Apparent Polar Wander
To illustrate the idea of polar wander, imagine you have a composite volcano on a continent like the one in the sketch below. I assure you that the sketch will be better understood if you also watch the screencast in which I talk while I draw it.
This volcano erupts from time to time, and when its lava solidifies and cools, it records the direction of the Earth's magnetic field. A geologist armed with a magnetometer could sample down through the layers of solidified lava and thus track the direction and intensity of the field over the span of geologic time recorded by that volcano. In fact, geologists did do this, and they discovered that the direction of the north pole was not stationary over time, but instead had apparently moved around quite a bit. There were two possible explanations for this:
- Either the pole was stationary and the continent had moved over time, or
- The continent was stationary and the pole had moved over time.
Seafloor Spreading Saves the Day!
Before plate tectonics was accepted, most geologists thought that the pole must have moved. However, once more and more measurements were made on different continents, it turned out that all the different polar wander paths could not be reconciled. The pole could not be in two places at once, and furthermore the ocean floors all recorded either north or south, but not directions in between. So how could lavas of the same age on different land masses show historic directions of the north pole differently from each other? Once seafloor spreading was recognized as a viable mechanism for moving the lithosphere, geologists realized that these "apparent polar wander paths" could be used to reconstruct the past motions of the continents, using the assumption that the pole was always in about the same place (except during reversals).
Calculating a paleomagnetic latitude
The example in my fabulous drawing gives a rather vague description of the idea behind using paleomagnetic data to reconstruct the former positions of the continents, but how is it actually done? We use magnetometers. A magnetometer can measure the angle between the direction of the Earth's magnetic field and horizontal. This is called the magnetic inclination. Because the Earth is a round body in a dipole field, the inclination is directly dependent on latitude. In fact, the tangent of the angle of inclination is equal to twice the tangent of the magnetic latitude, which is the latitude at which the permanently magnetized rock was sitting when it became magnetized. Therefore, given knowledge of your present location and a magnetometer reading of the inclination of your geologic item of interest, such as a basalt flow, you can calculate the magnetic latitude at the time of its formation, compare it to your present location, and determine how many degrees of latitude your present location has moved since that rock cooled.