I’ve seen pictures of Pangaea, the giant landmass that eventually separated into the continents we know today. But why were the continents smushed together like that in the first place? What made the land higher on that one side of the Earth? Were there other continents we can no longer account for? Is it related to the asteroid that may or may not have smashed into the Earth and helped form the moon?
Careful, bud. Thinking outside the box is
great, but we don’t want to cross the border
into the completely insane. That’s a chronic
risk with continental drift, talk of which
was a sure way to clear out your end of the
bar at scientific conferences until the 1950s
and which still inspires wacky theories.
Asteroids don’t figure in any of those I’ve
wait till you get a
load of the expanding
The most famous
early proponent of
was received skeptically
when he proposed
his theory in
1912, partly because
he couldn’t explain
what might cause
to move around.
they could. They
posited that once
upon a time the Earth had been much
smaller and was completely encased in
the supercontinent we now call Pangaea.
Volcanic activity caused the planet to
expand, cracking Pangaea apart like the
shell of a boiled egg and leading to the
eventual scattering of the continents.
Obvious objection: Where was all the
extra volume that went into the expanding
Earth supposed to be coming from? Was the
Earth rising like a cake in the oven? Some
proponents claimed the expansion was a
result of a reduction in the universal gravitational
constant or of the creation of new
matter in the planet’s core by some strange
subatomic process; others just insisted by
various proofs that the Earth was expanding
for reasons unknown. But it wasn’t, and isn’t.
Precise measurements have now established
that the Earth hasn’t enlarged appreciably
since the era of the dinosaurs. Claims to the
contrary aren’t taken seriously by scientists.
Or most scientists, anyway. I notice that
one James Maxlow was awarded a Ph.D.
by a seemingly respectable institution in
Australia on the strength of a 2001 thesis in
which he claims, among other things, that
the present-day continents fit together
with 99 percent accuracy if projected onto
a smaller sphere. One admires Maxlow for
his persistence in pursuing this notion.
(He’s got a book and a Website, attends
conferences, etc.) But one also remains
pretty confident it’s nuts.
If a once-smaller Earth doesn’t explain
why the continents were all smushed together
at one point, what does? We’ll get to that.
The main thing to understand is that the
Earth has been in a constant, if extremely
slow, froth for much of its 4.6 billion-year
existence—Pangaea, thought to have existed
250 million years ago, wasn’t the first supercontinent
and won’t be the last. Conjectured
predecessors include Ur (3 billion years ago),
Kenorland (2.7 to 2.5 billion), Columbia (1.9 to
1.8), Rodinia (1.1), and Gondwana (540 million
years ago). The constant shuffling arises from
the fact that the hard outer shell of our planet
floats atop a region of flowing molten rock,
allowing the continents to skate along at the
rate of 1 to 2 inches per year. The chief engine
of plate tectonics, as this process is called, is
the seafloor. At the midocean ridges, molten
rock pushes up from below, causing the floor
to expand laterally.
to the coasts, the
edges of the floor
get shoved below
plates in a process
Because of this,
very little of the
seafloor is more
than 200 million
years old, while
parts of the continents
are older than
4 billion years.
Why do we get
Some suggest that
the continents are drawn together by zones
in the Earth where the seafloor is pulled
down into the lower mantle in a process
called superdownwelling, drifting toward
the suction like rubber ducks in a draining
bathtub till they collide. Why do supercontinents
later break apart? One theory is that
the oversize landmass traps so much heat
beneath it that the crust ultimately cracks
open. Another idea is that crust-rending
“superplumes” of hot magma roil up from
the spots where the superdownwelling
occurred. Same result either way: the big
continent splits back into smaller ones.
What next? I found maps offering one vision of the future on the Website of Christopher Scotese, a geologist at the University of Texas at Arlington. The highlights: About 50 million years from now, Africa plows into Europe; about 150 million years from now, Australia becomes one with Antarctica; and, by about 250 million years from now, another supercontinent has formed, with North and South America, Eurasia, and Africa in one giant clump. In short, the Earth will stay lively— not that it’ll matter to us.
Comments, questions? Take it up with Cecil on the Straight Dope Message Board, StraightDope.com, or write him at the Chicago Reader, 11 E. Illinois, Chicago 60611.