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The Warsaw Voice » Society » March 31, 2015
Institute of Geophysics Polish Academy of Science
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Tracing Continental Drift
March 31, 2015   
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Paleomagnetism is a relatively new field in Earth sciences that offers a unique insight into how continents were formed and how they have drifted over hundreds of millions of years.

The principles and methods of paleomagnetic research are based on analyzing the natural magnetic orientation of rocks, both sedimentary and igneous. Such analysis is aimed at identifying primary magnetization—the magnetization of rock when it was formed. All rock contains magnetically active materials. When rock forms, these arrange themselves according to geomagnetic field lines, the same field that affects a compass needle, for example. Minerals behave exactly the same: they arrange themselves towards the Earth’s magnetic pole.

When rock changes its location due to continental drift, for example, the paleomagnetic direction fixed within it also changes its location.

“Many millions of years later, when we identify the rock’s fixed magnetization direction, it is completely different and no longer indicates the direction of today’s magnetic poles,” says Prof. Marek Lewandowski from the Paleomagnetic Laboratory at the Institute of Geophysics of the Polish Academy of Sciences. “The difference between the primary and present direction is a measure of the path traveled by a continent or a piece of continental or oceanic crust,” he adds.

Step by step, by studying the natural magnetization of increasingly older rocks, scientists can trace how the magnetization direction has changed and on this basis calculate the pole’s coordinates in different geological eras. The result is a map charting the movement of the Earth’s magnetic poles. Such maps are drawn for different continents.

This is the first step to reproducing the paleogeographic situation in early geological eras.

“That is the essence of the method and also its most classic and spectacular application. It was the basis for proving that continental drift, as posited by German geophysicist Alfred Wegener in the early 20th century, did actually take place,” Lewandowski explains.

Such studies brought evidence that the Atlantic is a relatively young ocean as far as geological history goes: it was formed about 200 million years ago. That was the beginning of the disintegration of Pangea, a super-continent that still existed in the early Jurassic period. With the help of the paleomagnetic method, it is possible to trace step by step how the Atlantic opened up, and also the changing configuration not only of America in relation to Europe but also Africa in relation to South America and other continents. That is how the paleogeographic history of the Earth’s surface is re-created.

That is not all, though: the continental drift that scientists discover is always a result of the convection of rocks, which slowly migrate, drifting from deep in the Earth’s mantle all the way to the surface. Continental drift does not happen because continents wander of their own accord; it reflects what is going on inside the Earth. Paleomagnetic records also reflect the state of the Earth’s magnetic field which today protects people from solar wind and high-energy ionized particles and is formed in the Earth’s liquid core.

“Paleomagnetism enables us to trace the movement of continents, the dynamics of the Earth’s interior and the history of its core. That’s what is unique about this method: it was the cause of a revolution that took place in Earth sciences in the mid-20th century,” Lewandowski says. “When we found out that the continents were drifting and in addition that the geomagnetic field can switch its polarity and the poles can trade places, it opened our eyes to the fact that the Earth is an extremely dynamic planet,” he adds.

Paleomagnetic research uses two kinds of methods: physical and chemical. The former consist of placing a rock sample in a magnetometer in a zero geomagnetic field. Next, extremely sensitive sensors pick up the natural remnant magnetization. The piece of rock is treated like a very weak magnet—its direction of magnetization is identified.

Using chemical methods, on the other hand, scientists treat a rock sample with different substances, which enables them to find out what materials are magnetization carriers in the rock. On this basis they learn about the rock’s history, for example if it was ever subjected to high temperature.

For many years the Institute of Geophysics’ Paleomagnetic Laboratory has been studying rocks from the Apennines, through the Dinaric Mountains (Croatia), the Ardennes and Vosges, the Carpathians, all the way to Norway and Spitsbergen, in research conducted in collaboration with local scientific partner institutions. “Without international cooperation we would have nothing to do. For paleomagnetic research to make sense it has to be conducted globally,” Lewandowski says.

Research is also being conducted in environmental paleomagnetism. This largely involves linking the amount of magnetically active materials to changes in the natural environment. For example, scientists study changes in the number of magnetically active molecules in very fine dust that forms pollution in the Earth’s atmosphere.

“This is fundamental research, but it has a practical aspect to it,” according to Lewandowski. “If we didn’t know the history of continental drift, if we didn’t understand the Earth’s history, practical commercial operations, such as mining and oil and gas exploration and extraction, would be much less effective,” he adds. An example is the drilling for shale gas that is being conducted at many sites around the world. This prospecting is often carried out on a wild-guess basis—as a result of insufficient knowledge of deep geological structures and insufficient understanding of geological processes that have continued in a given area for hundreds of millions of years. And this is the kind of knowledge that paleomagnetic research provides, along with other geophysical methods.
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