42 DISCOVERMAGAZINE.COM
As the World Turns
The theory of plate tectonics took shape in
the 1960s after more precise seafloor maps
and seismic activity monitoring revealed
signs of our planet’s shifting shell. But the
ideas built upon those of early 20th-century
German scientist Alfred Lothar Wegener,
who proposed the theory of continental
drift: The large landmasses we know today
were once joined together in a superconti-
nent. Although first to describe the idea sci-
entifically, Wegener was not the first to think
it. Late 16th-century mapmaker Abraham
Ortelius theorized that the Americas, Africa
and Europe had been torn asunder by cata-
strophic geological events.
Researchers now believe continental
plates have come together more than once
in the deep past. While the very definition
of a supercontinent remains under debate,
researchers agree that many of these mon-
s ter landmas ses were large enough to aff ec t
global climate by shifting ocean and air cur-
rents. These climatic changes, along with
the raising of mountains and other geologi-
cal events driven by plate tectonics, created
new ecological systems that encouraged
species to spread and diversify.
Vaalbara: The First
Supercontinent?
When: Formed as early as 3.6 billion
years ago; broke up as late as 2.1 to
2.7 billion years ago.
What went where: Uncertain; remain-
ing pieces of it are found today in the
Pilbara Craton of Western Australia and
Kaapvaal Craton of southern Africa.
What’s so super about it: Want to
start a fight among geologists? Say
Vaalbara is the oldest supercontinent.
The truth is, most of the crust from
Earth’s early days has been subducted,
eroded or deformed beyond recogni-
tion. So reconstructions of the shape,
location and time frame of the first
supercontinent, however you define it,
remain theoretical and hotly contested.
It’s not yet possible to determine how
much continental crust existed this
far back in the geological record, or
whether large pieces of it were con-
nected. Some researchers consider
Kenor, which may have formed around
2.7 billion years ago, as the true first
supercontinent; others argue for the
earliest mass-merging of crust around
2 billion years ago but can’t agree onwhether to call it Columbia or Nuna.
Rodinia: The
Empty World
When: Formed around 1 billion years
ago; broke up around 750 million
years ago.
What went where: The landmass
was likely centered around the
North American Plate’s craton,
which may have been smashed
against what’s now East Antarctica
and southern Africa.
What’s so super about it: While
Rodinia was not the first superconti-
nent, it’s the earliest one that every-
one seems to agree existed — just
don’t ask what it looked like. Most of
what’s known about the superconti-
nent comes from fragmentary pieces
of its orogens: the edges of individual
continental plates that crunched
up, forming mountains, during mas-
sive collisions. We know little of its
overall shape beyond these crumple
zones, though researchers believe it
covered about 18 percent of Earth’s
surface area. (By comparison, our
planet’s modern land masses cover
about 29 percent.) It was a lonely
place, however; complex terrestrialorganisms did not yet exist.
Get a Move On
It’s unclear why our planet’s plates can’t stay put — or
when they started moving. After Earth formed about
4.6 billion years ago, its surface was a hot mess of
magma for a long time. Isotopic analysis of some of
the oldest known rocks suggests that continental
crust may have started forming as early as 4.4 billion
years ago, but the details and timing of the process
are unknown. Some researchers believe that, when
the magma finally cooled enough to form a crust,
it was a single, contiguous shell. But the mantle
beneath, still hundreds of degrees hotter than it is
today, may have melted and fractured the fragile crust
into pieces, or plates.
What set the plates in motion is still debated, but
most researchers think that subduction — when one
plate overrides another — explains why they remain
in motion. The immense force of the cooler, denser
plate being pushed down into the mantle creates tre-
mendous energy, which then circulates in a convective
flow, pushing up hotter, less dense magma that breaks
through to the surface along divergent boundaries.
Salt crystals left impressions in 3.5 billion-year-old mudstone, part of Western
Australia’s Pilbara Craton. The craton, a stable interior of a continental plate, may
be one of two remaining pieces of Vaalbara, a theoretical supercontinent. The
other piece, the Kaapvaal Craton, is in southern Africa.R
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