Gondwana Supercontinent Underwent Massive Shift During Cambrian Explosion

New Haven, Conn. — The Gondwana supercontinent underwent a 60-degree rotation across Earth’s surface during the Early Cambrian period, according to new evidence uncovered by a team of Yale University geologists. Gondwana made up the southern half of Pangaea, the giant supercontinent that constituted the Earth’s landmass before it broke up into the separate continents we see today. The study, which appears in the August issue of the journal Geology, has implications for the environmental conditions that existed at a crucial period in Earth’s evolutionary history called the Cambrian explosion, when most of the major groups of complex animals rapidly appeared.

The team studied the paleomagnetic record of the Amadeus Basin in central Australia, which was part of the Gondwana precursor supercontinent. Based on the directions of the ancient rock’s magnetization, they discovered that the entire Gondwana landmass underwent a rapid 60-degree rotational shift, with some regions attaining a speed of at least 16 (+12/-8) cm/year, about 525 million years ago. By comparison, the fastest shifts we see today are at speeds of about four cm/year.

This was the first large-scale rotation that Gondwana underwent after forming, said Ross Mitchell, a Yale graduate student and author of the study. The shift could either be the result of plate tectonics (the individual motion of continental plates with respect to one another) or “true polar wander,” in which the Earth’s solid land mass (down to the liquid outer core almost 3,000 km deep) rotates together with respect to the planet’s rotational axis, changing the location of the geographic poles, Mitchell said.

The debate about the role of true polar wander versus plate tectonics in defining the motions of Earth’s continents has been going on in the scientific community for decades, as more and more evidence is gathered, Mitchell said.

In this case, Mitchell and his team suggest that the rates of Gondwana’s motion exceed those of “normal” plate tectonics as derived from the record of the past few hundred million years. “If true polar wander caused the shift, that makes sense. If the shift was due to plate tectonics, we’d have to come up with some pretty novel explanations.”

Whatever the cause, the massive shift had some major consequences. As a result of the rotation, the area that is now Brazil would have rapidly moved from close to the southern pole toward the tropics. Such large movements of landmass would have affected environmental factors such as carbon concentrations and ocean levels, Mitchell said.

“There were dramatic environmental changes taking place during the Early Cambrian, right at the same time as Gondwana was undergoing this massive shift,” he said. “Apart from our understanding of plate tectonics and true polar wander, this could have had huge implications for the Cambrian explosion of animal life at that time.”

Northern Crater Shows Prehistoric Deep Impact
by Ned Rozell
To the rhinos and crocodiles of the far north, the day was like any other. They ate, swam and napped, unaware a celestial body was headed their way at 60,000 miles per hour. Suddenly, a wayward comet screamed into the atmosphere, struck Earth and created a bowl a mile deep and 15 miles in diameter.
Mars On Earth: Arctic Crater Reveals Martian Secrets (pt 2)
Haughton Crater is the remaining scar from a high-speed collision between Earth and some heavy object from space about 23 million years ago. The comet or asteroid that created the crater was perhaps more than a mile (up to 2 kilometers) across and slammed into the forest that existed on Devon Island. Everything was annihilated for scores of miles in all directions. The impact churned up rock from more than a mile below the surface, vaporizing much of it. It's estimated that between 70 and 100 billion tons of rock was excavated from the crater in the moments just after the impact. While clouds of dust and gas filled the air, rock rained down from the sky, much of it in the form of what geologists now call breccia, which simply means "broken up." Scattered within the breccia are pieces of a rock called gneiss that normally is dark and dense. In Haughton Crater breccia, the "shocked gneiss" resembles pumice stone -- it's ash-white, porous and very lightweight.
Voices of the Rocks
by Robert Schoch
and Robert Aquinas McNally
(pp 1-3)
other supplier
"Yet, as it will, life returned to this site of complete devastation... The world those fossils described, the one that flourished on the order of 20 million years ago, during the early Miocene epoch, was strikingly different from today's Arctic... Devon Island was covered with a forest of birch trees and conifers, a landscape that one now finds about 2,000 miles to the south, in Minnesota, Wisconsin, and Maine. Now-extinct forms of rhinoceros and mouse deer browsed among the trees; shrews and pika-like relatives of modern rabbits darted through the shadows; and freshwater fish swam the lakes and streams...

"Even farther back, on the order of 45 to 65 million years ago, during the Paleocene and Eocene epochs, the fossil record shows Devon Island to have been still more profoundly different. Back then, what is now the Arctic was a region of swampy lowlands, slow-moving rivers, and towering forests of dawn redwood, kadsura, and ancestral forms of hickory, elm, birch, sycamore, and maple. Primitive fishes, crocodiles, salamanders, newts, and turtles inhabited the rivers and marshes, while the forests and meadows supported flying lemurs, early primates, forerunners of today's cats and dogs, and ancestors of the rhinos, tapirs, and horses."

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