Various theories have attempted to explain the origin and positioning of the earth' continents. A new hypothesis now suggests that there, perhaps, was only one huge continental land mass instead of the two supercontinents thought to exist before
IT WAS believed that about 500 million
years ago in the Palaeozoic era, there
were two major land masses - Gondwana and Laurentia. While the southern
supercontinent of Gondwana comprised of present day Africa, Australia,
Antarctica, South America and India,
North America and most of Europe and
Asia, excluding India, made up the
northern supercontinent of Laurentia.
But at the meeting of the Geological
Society of America, held recently in New
Orleans, us, researchers including Ian
Dalziel, from the University of Texas,
Austin, suggested otherwise. According
to them, a chunk of North America
appears in western Argentina, inclicating that Laurentia was not a separate
continent. The new theory places North
America opposite to the position it
occupied earlier. Dalziel, who first proposed the new geography, says that the
chunk which was dropped in western
South America 500 million years ago,
pms down the errant Laurentia continent to within a few thousand km off
South America's west coast. The
Argentine connection gives a clue to
pilaeogeographers about how all the
continents could have been either one
large landmass, called the Pangea,
which formed 250 million years ago
in the Permian era or else close enough
to appear as one (Science, Vol 270,
No 5242).
Partial support was offered to
Dalzieps hypothesis by three other
speakers at the meeting. According to
them, 490 million years ago, Laurentia
and South America had been, if not in
contact, then close enough to exchange
an 800 kni long chunk of plate. William
Thomas of the University of Kentucky
had in 1991 theorised that 540 million
years ago, a block of Laurentian crust
had sneaked away from the present Gulf
Coast, never to return. 'Ibomas had no
idea where it had got away to. This
conundrum was later explained in a
paper authored by Ricardo Astini of
the National University of Cordoba,
Argentina. The paper spoke of a crustal
block Precordillera, now locked in the
Andes, north of Mendoza in far western
Argentina, which distinctly contains
North American fossils of 540 million
years ago. Astim drew further conchtsions from the similarities between rock
types and the stratigiaphic (sequence of
strata) records in the Precorditlera and
the southern us. "They are identical,"
adds Robert Hatcher of the University
of Tennessee.
Fossil records suggest that before
Precordillera finally docked in its present position, it spent some time on its
own floating around as an island. This
indicates that the two continents of
South America and Laurentia were not
in direct contact, but were 1,000-2,000
km apart. Direct contact would eliminate all doubts about the position of
Laurentia, and Dalziel is continuing further studies on the subject. Another
hypothesis is that before colliding,
Precordillera could have been attached
to North America by a submarine
plateau, thus not detaching itself from
its parent body totally.
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