6.3 Magnetic Polarity Evidence for Continental Drift

"The Wegener hypothesis has been so stimulating and has such fundamental implications in geology..."

"...as to merit respectful and sympathetic interest from every geologist. Some striking arguments in his favor have been advanced, and it would be foolhardy indeed to reject any concept that offers a possible key to the solution of profound problems in the Earth's history.” - Chester R. Longwell, "Some Thoughts on the Evidence for Continental Drift," 1944

Wegener and his supporters did all they could do to find evidence to support continental drift. But without a mechanism the idea would not be accepted. What was needed was the development of technologies that would allow scientists to find more evidence for the idea and help them describe a mechanism. But first, they would find still more evidence that the continents had moved.

Magnetic Polarity Evidence

The next breakthrough in the development of the theory of plate tectonics came two decades after Wegener’s death. Magnetite crystals are shaped like a tiny bar magnet. As basalt lava cools, the magnetite crystals line up in the magnetic field like tiny magnets. When the lava is completely cooled, the crystals point in the direction of magnetic north pole at the time they form. How do you expect this would help scientists see whether continents had moved or not?

As a Wegener supporter, (and someone who is omniscient), you have just learned of a new tool that may help you. A magnetometer is a device capable of measuring the magnetic field intensity. This allows you to look at the magnetic properties of rocks in many locations. First, you’re going to look at rocks on land. Which rocks should you seek out for study?

Magnetic Polarity on the Same Continent with Rocks of Different Ages

Geologists noted important things about the magnetic polarity of different aged rocks on the same continent:

• Magnetite crystals in fresh volcanic rocks point to the current magnetic north pole (Figure below) no matter what continent or where on the continent the rocks are located.

• Older rocks that are the same age and are located on the same continent point to the same location, but that location is not the current north magnetic pole.
• Older rocks that are of different ages do not point to the same locations or to the current magnetic north pole.

In other words, although the magnetite crystals were pointing to the magnetic north pole, the location of the pole seemed to wander. Scientists were amazed to find that the north magnetic pole changed location over time (Figure below).

Can you figure out the three possible explanations for this? They are:

1. The continents remained fixed and the north magnetic pole moved.
2. The north magnetic pole stood still and the continents moved.
3. Both the continents and the north pole moved.

Magnetic Polarity on Different Continents with Rocks of the Same Age

How do you figure out which of those three possibilities is correct? You decide to look at magnetic rocks on different continents. Geologists noted that for rocks of the same age but on different continents, the little magnets pointed to different magnetic north poles.

• 400 million-year-old magnetite in Europe pointed to a different north magnetic pole than magnetite of the same age in North America.
• 250 million years ago, the north poles were also different for the two continents.

Now look again at the three possible explanations. Only one can be correct. If the continents had remained fixed while the north magnetic pole moved, there must have been two separate north poles. Since there is only one north pole today, what is the best explanation? The only reasonable explanation is that the magnetic north pole has remained fixed but that the continents have moved.

Wegener Was Right!

How does this help you to provide evidence for continental drift? To test the idea that the pole remained fixed but the continents moved, geologists fitted the continents together as Wegener had done. It worked! There has only been one magnetic north pole and the continents have drifted (Figure below). They named the phenomenon of the magnetic pole that seemed to move but actually did not apparent polar wander.

This evidence for continental drift gave geologists renewed interest in understanding how continents could move about on the planet’s surface.

Summary

• Using magnetic evidence found on a single continent in the 1950s, scientists showed that either the north magnetic pole was in a different spot in Earth’s past or that the continents had moved.
• When they added magnetic evidence from a second continent, they showed that in the past there had either been two magnetic north poles or the continents had moved.
• Since there is only one magnetic north pole today, they concluded that the simplest explanation is that the continents have moved.

Review

1. What is apparent polar wander?
2. How does magnetic evidence from one continent show that either the north magnetic pole has moved or the continents have moved?
3. How does magnetic evidence from two continents show that the continents have moved?