Earth before Pangaea [Preview]

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

Making our way up a ridge toward the top of the cliff, we saw that the lowest layers of the Cambrian strata--which lie below the limestone--were made of pink conglomerate and coarse sandstones. As the sea advanced over the deepening rift and the subsiding margin, it had ground the Precambrian rocks into boulders, pebbles and sand grains. The deposits became more fine-grained as we climbed, and the quartz sandstones immediately underneath the Nelson Limestone had the appearance of old friends. They were full of vertical worm burrows known as Skolithus.

These tubes are the only traces of ancient filter feeders, which extracted nutrients from sediments and left a clayey residue around their burrows. "Just like western North America," I noted out loud, "but then just like the Durness rocks of northwestern Scotland, too." Indeed, strata deposited by the seawater that advanced to cover most of the continents 540 million years ago--as shown by the presence of Cambrian seashores in such places as Wisconsin--are remarkably similar on all continents.

Matching Mountains...
THERE IS NOTHING like personal experience with rocks, however, to set a geologist thinking. My first impressions of the Transantarctic Mountains in 1987 raised a question that stayed near the forefront of my mind: Could the continent from which Antarctica rifted apart at the end of the Precambrian possibly have been western North America? Or were their margins at that distant time merely in similar environments on either side of an even more ancient Pacific Ocean basin?

The answer has far-reaching implications. The global paleogeography of the time ("paleo" is a prefix that geologists use to indicate "historical") is currently a mystery. To know how the continents were distributed could provide clues to the vast environmental alterations that preceded the Cambrian period. Late in Precambrian times there were several ice ages, and the oceanic and, presumably, atmospheric chemistry changed greatly. Multicellular animals evolved, heralding a biological profusion that included the far-distant ancestors of vertebrates and, hence, of human beings [see "End of the Proterozoic Eon," by Andrew H. Knoll; Scientific American, October 1991].

It is clearly difficult to map out with much certainty the geography of an ancient time on a dynamic planet with continents that move. Alfred Wegener and other pioneers of the theory of continental drift had noted that several North and South American mountain ranges truncated at the Atlantic margins match up neatly across the ocean with mountain ranges in Europe and Africa. Nowadays magnetic data and satellite images of the ocean floor showing fractures--appearing rather like railway tracks, along which the continents slid apart--allow us to reconstruct the supercontinent Pangaea very accurately.

A number of lines of evidence indicate that Pangaea was not the original configuration of the continents. When iron-bearing rocks solidify from lava, they become magnetized in the direction of Earth's magnetic field. The magnetization of rocks that congealed from pre-Mesozoic lava is quite different in North America and Africa, suggesting that in an earlier era these continents moved separately. Volcanic rocks that were fragments of ancient ocean floor have also been found in mountain ranges of Pangaea such as the Famatinian belt (Argentina), the Mozambique belt (Africa) and the older Appalachians. These early Paleozoic and Precambrian ophiolites--as the rocks are called--demonstrate that former ocean basins closed when the supercontinent amalgamated. Struck in the 1960s by the presence of early Paleozoic ophiolites in the Appalachian Mountains of the Maritime provinces in Canada, the imaginative Canadian geophysicist J. Tuzo Wilson asked: "Did the Atlantic Ocean open, close and then reopen?"

In reconstructing continental configurations prior to Pangaea, we get no help from the ocean floors. Although the Pacific Ocean basin already existed, ocean floor of such antiquity has long been thrust under the continents bordering the basin. Geologists therefore have no oceanic "railway map" for continental drift before Pangaea. We have to fall back on evidence from the continents themselves, just as Wegener did when attempting to reconstruct Pangaea before modern oceanography and satellites.

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