Earth before Pangaea [Preview]

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

The little airplane banked to the right. From my seat on the port side I could see its shadow crossing the ice. The skis made it look rather like a duck coming in to land on water, webbed feet outstretched. As the pilot leveled the aircraft, a huge cliff came into view, the dark brown of its rocks contrasting sharply with the pristine whiteness of ice and snow that faded into the horizon.

The steeply inclined layers of this Precambrian sandstone were distorted by concertinalike folds. I took several photographs. As we rounded the cliff, another came into view. Resting on top of the sandstone was a thin capping of rock almost as white as the background: Cambrian limestone. "Fascinating," I thought as I raised my camera again. "The basic geology here is very similar to that of western North America."

My colleagues and I had come to the Pensacola Mountains of Antarctica to study how the two geologic subdivisions--East and West--of the icy continent relate to each other. East Antarctica is an old Precambrian shield lying to the south of Australia, India and Africa; West Antarctica is part of the geologically young and active volcanic "ring of fire" that surrounds the Pacific Ocean. The uplifted rim of the East Antarctic shield meets West Antarctica along the Transantarctic Mountains, of which the Pensacolas form a northern extension.

It had been a long trip down: 14 hours from Los Angeles to New Zealand in a commercial jet, 10 hours from New Zealand to McMurdo Station in Antarctica in a ski-equipped Hercules transport and, finally, five hours across the continent to the Pensacola Mountains, bypassing the South Pole en route. Now, after setting up our base camp, we were finally at the mountains near the southern margin of the same ocean that laps the beaches of Los Angeles.

We still had to get to the rocks, however. In Antarctica such excursions take time. Having selected a possible crevasse-free landing site, our pilot brought the Twin Otter down for a "ski drag." That is, he put some weight on the landing gear but maintained enough airspeed to take off again. We circled and carefully examined these tracks. Crevasses can be hidden under snow, but here there were no telltale signs of blue cracks. Coming around again, we touched down and stopped quickly so as to reduce the chance of hitting rough ice beneath the snow. It was a bumpy landing, nonetheless, although the aircraft appeared to have suffered only superficial damage. We roped ourselves together for safety and started to walk across the windblown snow to the base of the cliff, leaving our anxious pilot to examine the plane.

Fossil Clues
THE BOUNDARYbetween the two rock types exposed in the Pensacola Mountains is one of the most fundamental in Earths history. After the birth of the planet 4.5 billion years ago came the four-billion-year-long interval of time known as the Precambrian. Toward the end of this era--about 750 million years ago, while the first soft-bodied, multicellular creatures were developing--the brown sandstones of the underlying Patuxent Formation we had just sighted were deposited. The strata were laid down in a rift valley that opened within the continental shield. As the rift deepened, rivers poured in, dropping their eroded soils onto the valley floor.

About 540 million years ago, an explosion of multicellular animal life ushered in the Cambrian period. Myriad cone-shaped skeletons of the creature Archaeocyatha collected in shallow seas that had advanced over the sandstone. These formed a reef along the rim of East Antarctica that was eventually transformed into limestone. (The cap on the Patuxent Formation is called the Nelson Limestone.) Because Archaeocyatha was a warm-water animal, what is now the western margin of the East Antarctic shield must have been situated in tropical latitudes during the Cambrian period.

The rifting event that led to the Patuxent sandstones' being deposited reflects the separation of East Antarctica from some other continental landmass. The divergence opened the Pacific Ocean basin about 750 million years ago. (Subsequently, igneous rocks from island volcanoes and material scraped off the subducting ocean floor accreted onto East Antarctica, forming West Antarctica.) This rifting occurred long before the supercontinent Pangaea--from which the present continents broke off--was formed. Pangaea was assembled only at the end of the Paleozoic era, approximately 250 million years ago. It started to fragment during the Jurassic period of the Mesozoic era, about 170 million years ago, creating the Atlantic and other young ocean basins.

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