Geoscience Reference
In-Depth Information
But for the Snowball story, the rhythmites have a more important role to play: their neat inscribed
lines show clearly where the rock slumped and folded as it was forming. Joe planned to use these folds
for a definitive test of whether the rock's magnetic field came from its birthplace.
Imagine a slab of something flexible—an eraser, say. Now take a pen and draw horizontal lines
along the side of the eraser, so it looks a bit like a layer cake seen from the side. If you bend the eraser
into an arch, the lines will bend too, following the curve of the arch. However, if you first bend the
eraser, and then draw lines horizontally across it, the lines won't follow the shape of the arch at all.
They'll cut right through it. A fold test works the same way. If the magnetic field in the rock formed
before it folded, the field lines will follow the curve. But if they were overprinted later in the rock's
history, they will cut through the curve, ignoring its shape completely.
So Joe took a slab of rock from Pichi Richi whose dark tidal lines had clearly folded into an arch,
and began to investigate. Did the lines follow the curve, showing they were original, or did they cut
through it, showing they were overprints?
A graduate student made the painstaking measurements and then presented Joe with the results.
Joe was astonished. The lines did seem to follow the curve. This changed the odds dramatically in his
mind. Perhaps the flat equatorial field really had come from the same time as the ice.
And then he had another idea.
On a field trip to Canada, he had noticed that among the Precambrian rocks there were thick red
layers of ironstones. That was mysterious. Ironstones belonged to a time much earlier in Earth's his-
tory, when oxygen first appeared in the atmosphere. Before there was any oxygen, the seas were full
of dissolved iron that had come from the Earth's interior, pouring out of underwater volcanoes and
deep-sea vents. But as soon as the air became oxygenated, the ocean's iron literally rusted. It turned
into solid iron oxide and was sprinkled on to the ocean floor to become the layers of ironstone you
can see in ancient rocks today. All that makes perfect sense. But the ironstones then stopped. Since the
air was full of oxygen, dissolved iron could never build up in seawater the way it once had, and there
were no more ironstone layers.
Until, that is, one bizarre iron blip a few billion years later, the blip that produced the ironstones
Joe saw in Canada. They appear elsewhere in the world, and always in the same geological time peri-
od—just towards the end of the Precambrian, just before the first complex animals emerged from the
slime, just around the time of the mysterious ice deposits. And then, shortly afterwards, they vanish
again. The very late Precambrian is the only time other than that very early period when ironstones
appear in the whole of Earth's history. The question is why.
Joe realized that he might now have the answer. Maybe the ice was the cause. If the oceans had
frozen over, perhaps seawater had been cut off from the air long enough to accumulate lots of dis-
solved iron from underwater volcanoes. If the ice then melted and exposed all this iron to the air,
then—boom!—it rusted again, and a new set of ironstones was born.
Everything was making sense. The magnetics from Pichi Richi seemed to show that there was ice
at the equator. That's the hottest place on Earth. If the equator freezes, everything else has to freeze,
too. And now the ironstones provided independent evidence that the oceans had frozen over at the
same time. Four years before Paul went to Namibia for the first time, Joe was looking, suddenly, at
total white-out.
He was thrilled, but also troubled. He wanted to believe in a global freeze, but what about the
contrary evidence from Budyko's ice catastrophe? The Earth couldn't possibly have frozen over, or it
would have remained frozen for ever. And yet Joe had seen the evidence for equatorial ice with his
