Geoscience Reference
In-Depth Information
spread, any high-latitude continents will switch loyalties. Because their rocks become covered with
ice, they can no longer soak up carbon dioxide. Instead it stays in the atmosphere to do its greenhouse
thing, warming up the Earth and melting the excess ice. So if ever the polar caps start to grow, high-
latitude continents will force them to shrink again.
Now imagine what would happen if all the continents were arranged in a band around the Earth's
equator. In that case the polar ice caps could spread with impunity. There would be no high-latitude
continents to cover, and hence nothing to stop the ice going all the way. By the time ice reached the
equatorial continents, it would be too late to prevent a Snowball.
2
This idea also neatly explains why, at least in Paul's period, there was a series of Snowballs rather
than just one. Between 750 and 590 million years ago, the continents could simply have stayed near
the equator. A Snowball would begin when some trivial cooling trigger set the ice moving. With no
high-latitude continents to stop it, the ice would continue until the Earth was encased. Over the next
10 million years or so, carbon dioxide gas pouring out of volcanoes would build the atmosphere into
a furnace, until it became so hot that the ice melted back. Gradually, then, the carbon dioxide levels
in the atmosphere would drop, until the whole process started again. As long as the continents stayed
near the equator, another cooling trigger would set another Snowball rolling. And another. And anoth-
er. Until, eventually, the continents moved on and the world was spared.
There aren't enough outcrops from Joe's earlier Snowball to know whether it was a series of events
or just one. But researchers believe that the sun was much feebler then, and that an individual Snow-
ball would have lasted much longer. Perhaps Joe's single early Snowball lasted so long that the con-
tinents had begun to move away from the equator again by the time the ice receded.
So equatorial continents could provide the rare but reasonable recipe for a Snowball. If this explan-
ation is right, that's encouraging news for our own future. Right now we have plenty of continents at
high latitudes. Most of the world's landmasses are way up in the north—think of Canada, Europe and
Russia. Presumably, these far northern lands are protecting us all from the ice. Well, not necessarily. It
turns out that in spite of this reassuring continental arrangement, the Earth may even now be preparing
for another descent into ice.
D
AVE
E
VANS
used to be a graduate student of Joe's. He's the one who prodded Joe into measuring the
South African samples, and proving that the older ice rocks had been close to the equator. (Dave found
the samples collecting dust, and resurrected them.) Now, in his early thirties, he's a professor at Yale
University. He is thin and gangly, with thick, wavy hair and a pleasant smile, and looks younger than
most of his students. Though he is organized and careful, from working in Joe's lab he also has this
legacy: the capacity to consider crazy ideas that might just be true.
While he was at Caltech, Dave didn't just work with Joe on the ancient Snowball. He also invest-
igated another of Joe's “nutty” ideas. As the Earth's tectonic plates creep over its surface, they usually
travel at a sluggish few inches a year—the same speed that your fingernails grow. But Joe and Dave
believe that at certain times in the past, the continents let rip, travelling at what for them was the break-
neck speed of several feet a year. They did so, according to this theory, because they had an inexorable
urge to reach the equator.
Spinning objects always prefer to have most of their weight around their middles. Think of a
child's spinning top: tall, thin ones are much easier to knock over than short, fat ones, because they're
more unstable. If the instability is too much to handle, the object will try to readjust. Suppose you
dropped a large lump of clay on to the top of a spinning basketball. If the lump was heavy enough, the
