Geoscience Reference
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that the Earth's entire surface is broken up into plates that shift around, some of them bearing contin-
ents on their backs. But still, in essence, Wegener's idea was vindicated.
It's ironic, then, that this vindication put yet another spoke into Brian's Snowball wheel. If the
continents truly did move, then there was a much easier way to explain Brian's ice rocks than the out-
rageous idea of global ice. Everyone knew that the poles were cold and the equator was hot. So each
continent must simply have drifted over to the polar regions to collect its ice, and then wandered away
again.
Of course, Brian had already thought of that. He was a
champion
of continental drift. As he'd made
clear in his papers about the ice rocks, he'd already tried to fit all the continents together in a huddle
around the pole. But there was simply no way to do it. However he arranged his geological jigsaw, he
couldn't cram all the continents into the polar regions. Some were always left out in the sun.
But plate tectonics was now on everyone's lips, and to many geologists, moving continents could
explain everything. Brian realized that he had only one option. He had to prove that at least one of the
continents was near the equator when the ice formed.
This, he figured, would be tantamount to proving that there had been a global freeze, since it's
extremely hard to freeze the equator without freezing everything else, too. Our sun's rays come to us
untrammelled, in single-minded parallel lines. At the equator, they strike the equator more or less full
on. At the poles they always hit at an angle. Shine a torch directly on to a piece of paper, and you'll
see a neat round circle. Now tilt the torch, and the circle will grow and distort into an ellipse—the
same amount of light spread out over a greater area. The same thing happens on our spherical planet.
Directly overhead at the equator, the sunlight is fiercely intense. But the further north or south you go,
the more it spreads.
The upshot of our celestial geometry is this: it's easy to freeze the polar oceans and to make gla-
ciers in Alaska and Antarctica, even at sea level where there's no thin mountain air to help. But the
closer you get to the tropics, the harder it becomes to make ice. If the temperatures had somehow
dropped low enough to freeze the equator, everything else must also have frozen.
To see if the equator really had been frozen, Brian decided to adopt the same technique that had
been used to vindicate Wegener: rock magnetism. Many rocks come with their own magnetic birth
certificate, because they adopt the local pattern of the Earth's magnetic field. This has a classic, char-
acteristic shape. Stick one end of a piece of wire into the top of an orange, bend the wire over, and push
the other end into the bottom of the orange. That will give you some idea of how the Earth's magnet-
ic field looks. It shoots straight upwards at the poles and passes horizontally over the equator. If you
were standing near magnetic north, the field would pass through your foot, say, and up out through
your head. If you were standing near the equator, the field would pass through you horizontally, across
your hips, waist and shoulders.
When they're young and soft, rocks are still impressionable; they can take on the stamp of the
Earth's field. The magnetic particles they contain line up the way the Earth's field does. As the rock
is compressed and hardens, these particles are fixed in place, and the direction they point in tells you
where they were born. If their field is vertical, they were born near the poles. If horizontal, they come
from the equator. The rock magnet is weak, to be sure, much weaker than one that you'd stick on your
refrigerator. But it is just measurable.
Brian decided to try to find out if his rock samples from Svalbard and eastern Norway had fields
that were horizontal. He built a new instrument, so sensitive that it could detect magnetic tremors from
the elevator as it rose and fell in its shaft, fifty yards away. He measured sample after sample. At
