EVER AGAIN - Snowball Earth - page 107

basketball would tip over until the excess weight was spinning around its waist, and the system was
safely back in balance.
Joe and Dave believe the same thing applies to our spinning planet. They think that if the shifting
continental lumps on its surface throw it off balance, the Earth will try to move them equatorwards.
This doesn't happen all the time, they say. There has to be a big enough imbalance before the Earth
will notice. But occasionally the random jitterbug of the continents brings them crashing together into
one massive “supercontinent”.
Even that's still not quite enough. All the continents in the world don't weigh much compared
to the Earth's massive innards: its thick mantle of plastic flowing rock and its hefty iron core. But
Dave believes that the supercontinent would act as an insulating cap over the mantle that lies beneath.
Gradually the mantle would heat up, and a great plume of rock would rise up like lava beneath the
supercontinent, lifting it up like a giant pustule. Now, with continents and mantle together, the lump
would tip the balance and the Earth would respond. Both supercontinent and underlying mantle would
go flying off to the equator, until the world became stable again. 3
This idea is every bit as controversial as the Snowball, and Dave—one of the few people in the
world to work on both—has now thought of an ingenious potential connection between the two the-
ories. What if slipping supercontinents make Snowballs? First the continents would collect together
into one enormous mass; then this mass would skid to the equator; then the hot plume of rock that still
lay beneath the supercontinent would blast it apart in a frenzy of volcanic activity that left fragments
scattered around the equator and tropics. And while this was going on, the polar caps could proceed,
unchecked, to cover the Earth.
At least some of the available evidence fits this idea. A supercontinent that geologists have named
Rodinia finally broke up around 750 million years ago, exactly when Paul's Snowball episodes began.
Nobody knows whether there was a supercontinent before the earlier Snowball, the one whose rem-
nants Dave and Joe measured in South Africa. But those same bulbous pillow lavas that provided their
samples might also contain clues about the state of the continents then. Those massive volcanic floods
didn't cover only South Africa; they also poured out on to many other parts of the world. And that's
exactly what you'd expect if a supercontinent was breaking up, and huge amounts of lava were spill-
ing through the cracks.
Were there any supercontinents that didn't produce Snowballs? Well, one called Pangaea existed
around 225 million years ago, without generating any notable ice. But Dave points out that Pangaea
broke up again relatively quickly. He suspects that it simply wasn't around long enough for that crucial
plume of hot rock to form underneath, and unbalance the Earth.
Now we're at the outer reaches of the Snowball idea, with speculation heaped on speculation. But
this idea of Dave's is intriguing. And if he's right, the corollary is also chilling. You see, the Earth is
making another supercontinent right now.
S IXTY MILLION years ago, not long after an asteroid slammed into the Earth to end the rule of the dino-
saurs, India began to sense the presence of Asia. The bulk of what is now the Indian subcontinent had
been drifting, footloose, ever since it broke away from Antarctica during the shattering of Pangaea.
Now it was moving steadily northwards at the rate of a few inches per year, and Asia was in its way.
There was only one possible outcome: a continental pile-up. When ocean basins collide, one or other
of the crusts tends to be forced downwards, back into the Earth's interior. But continents are not nearly
dense enough to sink. When two continents crash, the only way is up. 4