On the day I’m typing this, the magnetic north pole is in the far north of Canada, about 270 miles from “true north,” which is the real North Pole defined by the Earth’s spin axis. Since this time last year, the magnetic north pole has moved 26 miles—it’s on its way across the Arctic Ocean toward Russia. This sounds spectacularly unhelpful for navigators, although since the world is a big place it’s not as bad as it seems. But the magnetic field moves because of where it comes from, and it’s a reminder that the innards of our planet are more than just a static ball of rock.
Deep down beneath our feet, the iron-rich outer core of the Earth is churning slowly. It’s shifting heat from the center out toward the surface, and the rotation of the planet forces the molten rock to rotate, too. Because of the iron, the sluggish outer core is an electrical conductor, and that means it can behave like the electromagnet in the toaster. It’s thought that the currents running through the Earth’s outer core as it turns are responsible for generating our planet’s magnetic field. The process is based on the slow shifting of molten rock; and because the details of the rock movements change with time, the magnetic poles meander. They stay approximately aligned with the spin axis of the Earth because the rotation of the iron-rich rock is caused by the rotation of the whole planet, but the alignment is only approximate.
So if you really care about accurate navigation, you need to correct for the current position of the magnetic pole, because it’s not the same as the true North Pole. Today’s maps show the direction of both poles. I just had a look at an Ordnance Survey map of part of the south coast of the UK, and both magnetic and spin north are marked at the top. I can see that if you followed a compass directly north for 40 miles, you’d end up about 1 mile west of the line toward true north. A map seems like such a permanent record, and yet the magnetic field that you may use to help you navigate with it is fickle. Modern technology means that you and I won’t often get lost because of this. But the aviation industry, with one of the most sophisticated modern navigation systems humans have developed, certainly pays attention. For a start, it has to keep relabeling its runways.
Next time you’re at or near an airport, take a look at the large signs at the start of each runway. Every runway around the world is labeled by a number, which is its direction in degrees from north, divided by ten. So the runway at Tampa International Airport in Florida was given the number 18, because a plane landing on it will fly in on a heading of 180 degrees. Each runway will have a specific designation that’s a number between 01 and 36.§§ But this heading is relative to magnetic north, because that’s what a compass is telling you. So in 2011, runway 18 at Tampa became runway 19, to keep up with the movement of the magnetic pole. The runway hadn’t moved, but the Earth’s magnetic field had. Aviation authorities keep an eye on it all, and correct the runway designations when it becomes necessary. Since the poles move relatively slowly, the changes are manageable.
But the meandering of the poles is only the start. The Earth’s fickle magnetic field has far more to offer than navigational assistance. And the clues it leaves behind have provided the final confirmation of one of the most controversial, simple, and profound ideas that geologists have ever had. The continents, the immense rocky masses that dominate Earth’s surface, are moving.
IN THE 1950S, human civilization was whooshing into a new technological and scientific era. The foundations of our modern society were being laid: Microwave ovens, Legos, Velcro, and the bikini had all arrived and were working their way into popular use. Humanity was coming to terms with the arrival of the atomic age, social rules were being completely rewritten, and credit cards had just been invented. And yet, in the midst of all this galloping modernity, we couldn’t make sense of the planet we were living on. Geologists had been fantastic at cataloging the Earth’s rocks, but they couldn’t explain the Earth itself. Where did all these mountains come from? Why is this volcano here? Why are some rocks so old and some so new? Why are the rocks different everywhere you look?
One of the many observations crying out for a satisfactory explanation was that the east coast of South America and the west coast of Africa looked as though they had once fit together like jigsaw-puzzle pieces. The rocks matched, the shapes matched, and the fossils matched. How could all that possibly be coincidence? But most scientists just saw it as an unimportant curiosity; it was almost unthinkable that anything that big could go anywhere. In the early 1900s, a German researcher named Alfred Wegener finally gathered together all the evidence and proposed the idea of “continental drift.” Wegener suggested that South America and Africa had once been connected, and that one of these huge land masses had broken away from the other and drifted across the face of the planet. Very few scientists took this idea seriously, because the idea of something as gigantic as a continent just drifting 3,000 miles to the west seemed ludicrous. If that was true, what was doing the pushing? Wegener himself suggested that the continents plowed through the oceanic rock, but couldn’t provide any evidence. There was no “how” and no “why,” and the theory was quickly shelved. No one else had any better ideas, and the question was left alone.
By the 1950s, there still weren’t any better ideas around, but there were some new measurements. The lava spewed out by volcanoes had iron-rich compounds in it, and it was discovered that each speck of one of those compounds could act like a compass needle, twisting around to line up with the local magnetic field. The really useful part was that when the lava cooled down and formed solid rock, the tiny iron minerals couldn’t move anymore, so they were locked in position. These tiny frozen compasses meant that a record of the Earth’s magnetic field was built into the rock at the moment it formed. When geologists used this record to look at the changes in the magnetic field through the ages, something even more curious came to light. The direction of the Earth’s magnetic field seemed to reverse every few hundred thousand years. It completely flipped, so that south became north and north became south. It didn’t seem to matter too much, but it was very odd.