Apparent Polar Wander

           By averaging natural remanent magnetization (which is the total natural magnetization retained by rocks as a record of the Earth’s magnetic field that existed when the rock formed) over periods exceeding 10,000 years, geophysicists are able to determine the positions of the geomagnetic poles. Since it is well known that magnetic minerals will align with the Earth’s electromagnetic lines when they are floating in magma, their orientation after crystallization can be studied and the location of the poles can be inferred at the time the rock was lithified. Consequently, the location of the rock, and the continental surface with which it was associated and therefore the significance of paleo-geography in determining the where and when of continental drift could be established as could the location of the poles when the rock hardened.

Author’s Note: Actually, the term polar wander is sort of a misnomer, since the poles typically do not move around . What’s happening is that the continents are drifting about. And how do we know that? Well, in a molten state, free-floating magnetic mineral grains and crystals will align themselves in response to the Earth’s electromagnetic field. As the magma cools and hardens, the positions of the magnetic mineral components become locked with respect to the Earth’s electromagnetic field. Once hardened into igneous rocks the magnetic orientation of those minerals can be measured and the geographic location of the poles existing at that time can be determined.

Prior to the late 1940s, the problem for geoscientists was that many times the mineral grains-crystals were not oriented in the same direction as the current magnetic field of the Earth, and that was a genuine puzzle that proved difficult to explain. During the early 1950s, working with the first reliable astatic magnetometers, geophysicists found unmistakable evidence that the poles had seemingly not been fixed through geologic time. Instead, they apparently had wandered around the globe. The magnetic evidence for that conclusion was contained in sea-floor rocks, in igneous continental rocks, and sedimentary deposits. But the real question facing geoscientists in general, and geophysicists in particular, was had the poles really been moving about or was something else afoot?

A determined and very talented group of geophysicists at Cambridge University under the guidance of Keith Runcorn, and aided by formulae created for them by the famous statistician, R. A. Fisher, managed to resolve a critical aspect of the fixed-mobile Earth debate through measurements of long-term motion of crustal blocks relative to the Earth’s spin axis. The first important discovery was made by Ken Creer in 1954 when, in his doctoral dissertation, he calculated a paleo-magnetic pole for North American’s Silurian Rose Hill Formation (which outcrops near Cumberland, Maryland) and argued that its location, somewhat east of the Silurian pole in Britain, was qualitatively consistent with Wegner’s Continental Drift Theory. Months later in 1954, research by Creer, Jan Hospers, Edward Irving, and Keith Runcorn (all at Cambridge) led to a reevaluation of the meaning of polar wander when intercontinental comparisons were made. When data were collected and analyzed from North America, largely by Runcorn, and from Australia, by Irving, it quickly became evident that the only scientifically valid way to explain the data required that continental movements must have taken place.

Creer is credited with being the first scientist to define apparent polar wander and Irving and Runcorn with demonstrating that the geomagnetic poles were fixed and did not wander but the continents had moved. In other words, the Cambridge University geophysicists established the field of paleo-geography. Their research, more than any previous work, paved the way to widespread scientific acceptance of the emerging theory of plate tectonics and the verification of Alfred Wegner’s then much maligned Continental Drift Theory. The important thing for students to realize is that if magnetic minerals in igneous rocks had been oriented to the Earth’s electromagnetic fields (poles) when they lithified and today they are not, that can only mean that something moved. That was the beginning of a cascade of scientific research through the 1960s (by scientists like Fred Vine, Drummond Matthews, Harry Hess, Robert Dietz, and J. Tuzo Wilson) that spelled doom for the fixed Earth proponents and supported those mobilists who found inspiration in Alfred Wegener’s concept of continental drift and in the pioneering work of the Swiss Alpinists. As a direct result of those efforts, what had been a theory almost universally criticized for its lack of hard evidence quickly morphed into what we know today as plate tectonics.

If scientists study the orientations of the magnetized mineral grains, they can infer the position of the poles when the rock solidified. The geologic clock can then run backward and the continents moved in conjunction with the changing position of the poles. Remember that when people use the term polar wander it’s similar to the saying, “The Sun rises in the east.” Although everyone knows the Sun doesn’t actually rise, not nearly as many realize that the poles didn’t wander around.