Geoscience Reference
In-Depth Information
And then a whisper started in Brian's head. Perhaps the ice was global. Perhaps it had been every-
where in the world. At first his main interest in this was an arcane geological one. If the ice really had
once been everywhere, the ice rocks it had left behind could be a Precambrian global marker. This
might be a way of matching time-slice to time-slice for rocks all around the world, shedding light on
the Earth's otherwise obscure Dark Age.
5
Then Brian realized something else, something much more important. This ice came just before
one of the most dramatic periods in Earth's history: the great evolutionary explosion that created com-
plex life. Perhaps the ice was the trigger. It might explain why the Earth moved from the Dark Age to
the Age of Enlightenment. Brian knew that biologists couldn't explain this breakthrough. There were
simply no theories that made sense. And he realized that he might now have the answer. A climate
change as big as the one he was proposing—surely that would be enough to shake the Earth from
its slimy idyll, and jump-start the true beginnings of biodiversity. Excited, Brian marshalled his argu-
ments about this “Great Infra-Cambrian Glaciation” and set about writing them up.
B
RIAN
H
ARLAND
wasn't the first person to propose a global ice age. A Swiss researcher named Louis
Agassiz had done so nearly a hundred years earlier. Agassiz suggested that ice had run rampant around
twenty thousand years ago, much more recently than the Precambrian. In this he was partly right—ice
had indeed stretched beyond its polar bounds then. But Agassiz's ice age was nowhere near as extens-
ive as he'd imagined.
6
Agassiz came upon the idea of prehistoric ice in the 1830s, while studying the geology of the Alps.
He surmised that parts of the Rhone valley had been carved out by ice that had long since melted, and
found boulders transported far beyond the existing fringes of the Alpine glaciers. He then began to
discover that other parts of the world also showed signs of extensive ice. Scotland, for instance, bore
carved valleys similar to those in Switzerland, but no glaciers remained there. Putting the evidence
together, Agassiz proposed that there had once been a mighty ice age with glaciers stretching from the
North Pole down to the Mediterranean. Then he grew more ambitious. The ice, he declared, had been
everywhere. He even claimed to have found glacial traces in the Amazon rain forest.
Agassiz was the first ice champion. In the end he succeeded in convincing a sceptical world that ice
could
ever
stretch beyond its present polar bounds. Thanks largely to him, we now know that the polar
caps wax and wane on timescales of a hundred thousand years or so. When they stretch to their largest
size, the world enters an ice age. The most recent one, which finished just eleven thousand years ago,
is also the most famous, the time of woolly mammoths, mastodons and sabre-toothed tigers. And the
receding ice heralds an “interglacial”, the warm time between ice ages that we are experiencing today.
Though researchers still argue about what causes ice ages, most believe that they are driven by subtle
changes in the amount of heat reaching the Earth as it wobbles in its orbit around the sun.
But Agassiz's ice ages were nowhere near as extreme as the Snowball. In the last of his ice ages,
the sheet of ice that coated northern America reached only as far south as New York. Another ice sheet
blanketed northern Britain and Scandinavia, but scarcely made it into mainland Europe, let alone as far
south as the Mediterranean. Some pack ice spread outwards from Antarctica into the Southern Ocean,
and New Zealand felt the chill, but that's as far as it went. Ice didn't get anywhere near the equator.
Other than an occasional iceberg, most of the oceans were ice-free. Global white-out? It wasn't even
close.
Agassiz had a religious reason for overblowing the extent of his ice age. He felt it provided con-
crete proof that a providential God intervened in Earth's processes. God, so Agassiz thought, had de-
