During recent years, geologists have proposed that the Wilson Cycle has taken
place earlier in the earth's history and that Pangaea is only the youngest
supercontinent. Canadian geologist, now Harvard professor, Paul Hoffman has
described an earlier cycle of formation and breakup of supercontinents, which
he likens to a dance. The dancers are continental fragments and newly created
crust that come together and later separate. Each dance cycle takes about
a billion years and involves the building and dispersal of a supercontinent.
Based on his studies of the Canadian Shield, Hoffman has described how the
North American craton evolved through one "dance" cycle. The cycle
is set to a symphony in four movements.It began two billion years ago (b.y.)
and ended a billion years ago.
In the first movement (2.0-1.8 b.y.), seven microcontinents came together
to form the beginnings of the North American (and part of what is now Europe's)
craton. (Hoffman calls this the theory of the united plates of America.) New
crust formed along some of the margins of the new continent through plate
tectonic accretionary processes. Hoffman speculates that Precambrian shields
now in distant continents (e.g., Australia) were part of this supercontinent.
The second movement took place between 1.8 and 1.6 b.y. This was characterized
by tectonic accretion of new crust along the edge of the supercontinent. The
third movement (1.6-1.3 b.y.) featured silicic magmatism in the interior of
the continent, away from convergent boundaries. In the fourth movement, basalt
floods and rifting heralded the breakup of the supercontinent. The symphony,
or cycle, ended when the supercontinent fragmented-the "dancers"
broke away.
The breaking away of the "dancers" may have been vigorous enough
to cause SWEAT. SWEAT stands for Southwest United States-East Antarctic connection.
According to the SWEAT hypothesis, proposed in 1991 by geologist Eldridge
Moores, the North American and East Antarctic cratons were adjoining as part
of the Precambrian supercontinent. The North American craton fled to the northern
hemisphere, while the East Antarctic craton, along with the other Gondwanaland
continents, remained in the southern hemisphere.
The cycle would be repeated as the fragments came together during the Paleozoic
to form the supercontinent Pangaea. Pangaea, of course, split apart into the
present continents.
Hoffman also cites evidence that a similar cycle took place in the earlier
Precambrian, prior to 2 b.y.
How is the cycle explained? The gathering of continental fragments is attributed
to downwelling of cold mantle. The continental fragments are carried to the
area of sinking mantle where they collect, much like leaves washed along a
gutter collect over a drain. Once the supercontinent has formed, downwelling
mantle (and subduction) moves to the edges of the continent. The mantle beneath
the interior of the supercontinent now heats up, due to the insulation provided
by the continental crust and the fact that the downwelling cold mantle is
now at the periphery of the large continent. Once the mantle beneath the interior
of the continent heats sufficiently, hot spots develop resulting in magmatism
and eventual rifting and breakup of the supercontinent.
