DOWN UNDER - Snowball Earth - page 70

roughly once every few hundred thousand years. If you could slow down your frenetic human lifespan
until it matched the stately passage of geological time, you could study a compass and watch these
flips in action. Flip, and the needle would swing halfway around the dial to point south; flip, and it
would point north again; flip, back to south. The magnetic minerals trapped in Linda's rocks had be-
haved exactly like that geological compass needle. Each ancient flip had been frozen in. 1
These flips are useful to those who study the ancient Earth, for two reasons. First, if they appear
in a sequence of rocks, the magnetic field there has to be original. Neither subsequent heating of the
rocks nor the influx of new magnetic material can produce this alternating pattern of fields. By finding
reversals in the Flinders ice rocks, Linda confirmed Joe's discovery that ice had been present near the
equator.
More important, Linda had also found evidence that the ice was extremely long-lived. Her rocks
contained perhaps as many as seven flips. If magnetic reversals in the Precambrian happened roughly
as often as they do today, Linda's ice rocks had to span at least a few hundred thousand years, and
probably several million. Here was the first tangible evidence that the Snowball glaciations really
were the longest ever known. It seemed as though they had lasted quite long enough to build up huge
amounts of carbon dioxide in the atmosphere, and trigger the events that Paul and Dan envisaged.
Paul was thrilled when he heard about Linda's reversals. The Snowball theory had survived its first
important challenge—the test of time. Nick was perfectly aware of the irony that his own student's
work had ended up supporting the Snowball, but he was also proud of Linda. She had made an in-
triguing observation, providing a potentially important clue about the conditions that prevailed during
the glaciations.
Still, just because the ice had stuck around at the equator for an inordinately long time, that didn't
mean the rest of Paul and Dan's theory was correct. Nick and Linda were both still determined to show
that the overall picture was nothing like Paul and Dan's dramatic total freeze-over.
The next serious test of the Snowball, however, wasn't from Nick's camp, though it did come with
another dose of irony. Once again the challenger had worked in the Flinders Ranges, and provided key
evidence that was later incorporated in Paul's theory. But this wasn't Linda, or anyone who worked
with her. The new challenger was George Williams, the Australian who had instigated the research
that caught Joe Kirschvink's attention back in the 1980s. George had worked in the Flinders Ranges
for decades. He's the person who studied the tidal rhythmites there, and whose work laid the founda-
tion for all the magnetic work that followed. He had done plenty of further magnetic work there, too,
and he was utterly convinced that there had been ice at the equator. But he was also convinced that this
had nothing to do with a global Snowball. George had an alternative explanation: the Earth, he said,
had tipped over on to its side. Sound crazy? Well, he had evidence that seemed to prove it, and that
was soon giving Paul Hoffman sleepless nights.
P ORT A UGUSTA is a small, grim coastal town a few hundred miles north of Adelaide, just west of the
Flinders Ranges. Its street signs proclaim it to be the “Gateway to the Outback”. At Port Augusta the
rough stuff begins. Though the roads to the south are smooth and civilized, many of the northern ones
are little more than dirt tracks.
From there, the driving instructions to Mount Gunson Mine are simple enough. One paved road
goes north, the Stuart Highway. Take it. After a hundred miles or so, turn right. Yes, this will be the
only right turn. You can't miss it. No, really, you can't possibly miss it. There's nothing else there.