In Times of Ur

By Carl Zimmer|Wednesday, January 01, 1997
Geologists have long been playing a game of leapfrog back through time. In 1912 Alfred Wegener claimed that the continents had reached their current positions after the disintegration of a single giant supercontinent that he called Pangaea (Greek for all Earth). By the 1980s enough rocks had been hammered and sectioned to confirm Wegener’s theory and even to map the 200-million-year-old landmass. This success inspired speculative geologists to leap further back in time and suggest that Pangaea had been assembled from pieces of another supercontinent. They named this older landmass, which they claimed had broken up more than 500 million years ago, Rodinia (Russian for motherland). In recent years research has shown that Rodinia was real.

But this past year John Rogers, a University of North Carolina geologist, leapfrogged practically all the way off the playground when he published a paper bluntly entitled A History of Continents in the Past Three Billion Years.

Rogers was first inspired to write his speculative saga while working in southern India. The rock there belongs to a craton, the ancient core of a continent, and it is ancient indeed--about 3 billion years old. Recently several other cratons were well dated for the first time, and Rogers noticed something puzzling: when southern India was part of Pangaea, all the cratons surrounding it were also 3 billion years old. It seemed an unlikely coincidence. You can do whatever statistical games you want, but you can’t take these cratons, mix them up, and then put all the 3-billion- year-old ones in the same place, he says. It’s like an E-mail going out that says, ‘All cratons of 3 billion years of age, please report to such- and-such place.’ Instead, Rogers thinks these cratons were always together as a single, 3-billion-year-old continent, the first continent on Earth. He calls it Ur.

Other cratons, it turned out, are also clumped together by age, which led Rogers to wonder whether a few other continents formed after Ur. A surprisingly straightforward history of continental drift began to fall into place. Finally, I just said, ‘Okay, let’s let the neck hang out there and let people take a whack at it,’ says Rogers.

For 500 million years, he claims, Ur wandered alone, growing slowly as its volcanoes spewed magma and as it occasionally smashed into smaller stray cratons. Meanwhile other parts of Earth’s crust were thickening and cooling, and by 2.5 billion years ago a new continent appeared that Rogers calls Arctica, made up of Canada, Greenland, and a large part of Siberia. Then, 2 billion years ago, came two more continents: Baltica, representing most of western Europe, and Atlantica, made of eastern South America and western Africa.

The first big coming together of the continents occurred 1.5 billion years ago, when Baltica and Arctica collided, forming a landmass called Nena. The next crash was 1 billion years ago, when Ur and Atlantica plowed into Nena, producing the supercontinent Rodinia. It survived for 300 million years and then fell apart again into Ur, Atlantica, and Nena, which continued to expand with stray cratons and new crust. Pangaea formed and then broke up. Only then did rifts form within the original cratons, scattering the present continents across the oceans.

How is it that Ur, after surviving for almost 3 billion years, happened to be torn apart during the breakup of Pangaea, such that its remnants are found today in South Africa, Madagascar, southern India, and Australia? That’s just one of the questions other geologists are asking about Rogers’s hypothesis, which is a long way from being verified. Yet the fact that any geologist would dare to assemble such a history is itself a milestone of sorts. It means there’s nowhere left for the next leapfrogger to go.
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