Geology Reference
In-Depth Information
To garner even a vague sense of Earth's changing face, every one of the three dozen
cratons is being scrutinized by armies of geologists. Decades of meticulous fieldwork and
lab studies are being undertaken. Data from every part of the planet are being integrated.
Then all the cratons will have to be juxtaposed like bumper cars on a globe—and the ima-
ginarymovieoftheirwanderings,startingfromtheknowngeographyofthemodernworld,
will be played slowly backward. Inevitably, that movie becomes fuzzier and more spec-
ulative the further back we go. Nevertheless, the picture that is slowly emerging is ex-
traordinary. According to the latest interpretations, Earth has experienced a repeated cycle
of at least five supercontinent assemblies and breakups, extending back perhaps three billi-
on years.
The story of Earth's earliest landmasses is still emerging, and more than a few contro-
versiesswirlaroundthetopic.Noonehashadthenervetodrawmorethanasketchmapof
Earth's surface 3 billion years ago, at least not yet, but one well-vetted idea has named the
first continent-size landmass Ur, formed about 3.1 billion years ago from earlier scattered
cratonic bits of what are now South Africa, Australia, India, and Madagascar. (An even
earlier postulated large landmass, Vaalbara, may have existed about 3.3 billion years ago,
but evidence is slim.) According to comparisons of paleomagnetic data from all these Ur-
forming regions, what are now separate cratons were sutured together for most of Earth
history—their global perambulations appear to have been virtually parallel and thus prob-
ably linked. Indeed, magnetic data suggest that the continent of Ur persisted for almost 3
billion years and began to split apart only about 200 million years ago.
Theearliesttrue
super
continent,dubbedKenorlandorSuperia(afterassociatedrockloc-
alities in North America), is thought to have formed about 2.7 billion years ago from Ur
and lots of other smaller pieces. Each time one craton collided with another, a suture zone
was formed, while epic compressional forces pushed up a new mountain range. A pleth-
ora of such features can be determined from rocks 2.7 to 2.5 billion years old, suggesting a
sequential growth of the supercontinent. Paleomagnetic data reveal that Kenorland was at
low latitude, probably straddling the Equator, for most of its relatively brief existence.
With those early tracts of land came Earth's first episodes of large-scale erosion and the
first great pulses of sediments into the shallow ocean margins. Most early-Earth modelers
posit an ancient atmosphere quite different from that of today. Oxygen was totally absent,
while carbon dioxide levels may have been hundreds or thousands of times greater than
in our time. Rain would have fallen as drops of carbonic acid that ate away at the land
and transformed hard rock to soft clays. Rivers carried their muddy cargo into the shallow
coastal slopes of the encircling oceans, where thick deltalike wedges of soft sediments ac-
cumulated.
By about 2.4 billion years ago, about the same time that oxygen began to accumulate in
theatmosphere,Kenorlandexperiencedtheflipsideofsupercontinentformation.Geomag-
