Earth before Pangaea [Preview]

The North American continent may be more nomadic than most of its inhabitants

Over the past decade, interest in the Rodinian supercontinent that preceded Pangaea has spawned research centers and international programs to study this supercontinents assembly, geography and fragmentation. One related result of this scientific ferment is the "snowball Earth" hypothesis, which proposes that Earth was covered with ice at sea level all the way to the equator 600 million to 700 million years ago, at the time of Rodinias fragmentation and the formation of the Pacific Ocean basin.

The snowball Earth hypothesis posits an extreme global environment that challenges our understanding of climate past, present and future. If confirmed, it would mean that a dramatically chilly period directly preceded the explosion of multicellular life that occurred approximately 545 million years ago. Because forecasters rely on the distribution of continental landmasses in designing computer climate models, our rather esoteric study of ancient supercontinents has clearly taken on added significance in recent years.

With the absence of ocean floor predating Pangaea and the fragmentary nature of evidence from the continents, opinions regarding this period of Earth history inevitably differ. Some experts even doubt the very existence of the late Precambrian Rodinian supercontinent described in this article--doubts difficult to reconcile with the thousands of kilometers of preserved late Precambrian rifted continental margins.

Other researchers have used the same data we have relied on to reach radically different notions of the way this pre-Pangaean supercontinent may have looked. Instead of a connection between the southwestern U.S. and East Antarctica, for example, some experts propose that the U.S. Southwest and Mexico were connected to southeastern Australia. And an older idea has been revived, to the effect that Siberia was rifted from the proto-Pacific margin of North America. Nevertheless, two lines of evidence persuade me that our concept of the way Earth looked before Pangaea is the correct one.

First are the fruits of our 19931994 trip to Antarctica: the rock specimens we obtained from Coats Land. The paleomagnetic data obtained from those rocks do indeed show that this part of Antarctica could have been adjacent to the core of present-day North America when the rocks formed as volcanic deposits some 1.1 billion years ago. Extensive lava flows of this age lie exposed near Lake Superior and extend in the subsurface through Kansas to Trans-Pecos Texas, the Keeweenawan province. Although identical deposits exist throughout southern Africas Umkondo province, my colleagues Jim Connelly here at Austin and Staci Loewy of the University of North Carolina at Chapel Hill have demonstrated that our Coats Land rocks contain lead isotopes that match those of North America's Keeweenawan province--but are quite distinct from the isotopic composition of the Umkondo lavas of Africa.

Second, evidence increasingly suggests that lower Paleozoic limestones of the Precordillera of northwestern Argentina originated in North America--yet another geologic calling card revealing North America's former presence off the Pacific margin of South America. Workers from both continents who have analyzed the rocks of Argentinas Precordillera have shown unequivocally that they originated in North America.

It remains unclear whether this ancient North American limestone arrived in South America as a Madagascar-like microcontinent or through transfer resulting from a continent-continent collision--as Italy was much later transferred from Africa to Europe when those two continents collided. Yet however they were transferred to South America, these limestone rocks offer the strongest possible evidence that North America did indeed make an end run around the Pacific margin of South America [see illustration on page 17] during Paleozoic times and that ancestral North America probably originated somewhere between the present Antarctic-Australian and South AmericanAfrican parts of a pre-Pangaean supercontinent.

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