Science Spin November 2008

How thick is Europe's crust?

By Tom Kennedy

Some parts of Europe are going up, and other regions are going down. Terra is not so firma as we usually like to assume, and if the Earth could be compressed to the size of an egg, the crust we stand on would be a lot thinner and more fragile than the shell. The Earth's diameter is 12,800 km, and the solid crust is, on average, just 40 km thick.

Last month, one of the world's leading geoscientists, Prof Sierd Cloetingh, from the Vrije Universiteit in Amsterdam, was in Ireland to talk about the deep structure of Europe, and as he explained, the continental crust is far from uniform, and while some parts are on the rise, other regions are sinking under their own weight.

The study of the Earth's crust began in the oceans where geoscientists first discovered a great cycle of renewal as molten material welled up from below to fill the voids left by plates drifting apart. Plate techntonics, said Sierd Cloetingh was quite a revolution, but it took a while before geoscientists began at investigate how similar forces are working on land. As he explained, the situation on land is a lot more complex, and by comparison, the ocean floor is a lot thinner and younger.

Parts of the continental crust could be up to 4 billion years old, and on average Europe would be about 32 km thick. Ocean crusts are typically just 6 km thick, and the older edges might be just 200 million years, so in some respects they can represent less of a challenge to a geoscientist than land.

Revolutionary discoveries at sea, said Prof Cloetingh, have now been followed by another revolution in understanding the deep structure of Europe and other continents. That understanding has come about through the amalgamation of data from a variety of sources. For generations, geologists have been mapping and observing near surface features of Europe, and in recent years this information has not just been added to, but enhanced, by satellite imagery, deep drilling, and charting of magnetic variations.

The Deep Earth project now involves input from geoscientists from 23 countries, including Alan Jones from the Dublin Institute of Advanced Studies.. The picture that emerges now is one of high contrast, with the continent being stretched apart in some areas to create thin skinned basins, as in the southern Pannonian Basin, and elsewhere, as in the far north, mountain ranges rest on a much thicker crust.

One of the significant findings from this ongoing study is that crustal movements occur in three dimensions, and sometimes features that we have long assumed to be due exclusively to weathering and erosion, such as the steep sides through which the Danube flows on its long journey east, may have, in fact, been shaped more by uplift. Out in the oceans most of the action is concentrated along the plate boundaries, but in a continental mass caught between colliding masses, a bewildering number of forces are at work, and as Prof Cloetingh commented, "that makes Europe a great place for geologists to work."

Deep basins

In places where the crust has been pulled apart large basins, such as the massive Pannonian Basin, through which the Danube and Tisza rivers flow, have been created. This opened up about 18 million years, it once was a great lake or sea, and as Professor Cloetingh observed, "the basin is not just a hole in the ground." For starters, the extension was far from uniform, and while around the rim sinking was followed by a marked vertical bounce back movement, the deeper parts are still being pulled down. "Uplift," remarked Prof Cloetingh, "can be just as dramatic as subsidance."

After formation back in Miocene times, geo forces were far from spent. Folding resulted in 50 km long waves, creating huge synclines and the anticlines that serve as the traps for oil and gas that are now being drained.

Quakes and volcanoes

In what is known as the European Cenozoic Volcanic Province, there is a lot of stress caused by sinking of the Alpine mass as it is pushed down by the the advancing Adriatic plate. Sometimes it takes an earthquake to relieve the stress, such as one that destroyed the city of Basil in the Middle Ages. Earthquakes, apparantly, can be easy enough to trigger, and Prof Cloetingh recalled how deep drilling for geothermal energy managed to set off an earthquake that registered 4 on the Richter scale.

One of the odd geological features in Europe is that it has an area of volcanism in the middle, and this appears to be be related to the Alpine subduction zone. However, unlike thin crust areas such as Hawaii, the volcanism has a different origin. Fingers of hot magma flow up from relatively a shallow depth of 400 km, whereas the upwelling in Hawaii comes from much deeper to penetrate a much thinner crust.

Nothing similar has been found on other continents, said Prof Cloetingh, adding, "but maybe that's because no one has yet looked." Features like this are more likely to occur in Europe, he explained, because with a large continental land mass, the crust can become decoupled from the upper mantle. In oceans, he said, there is just one layer, but in the continental situation there can be lateral flow between upper and lower levels.

At first, he said, its all like we learned from the book, but then when upward plumes starts to interact with middle layers, the situation becomes a lot harder to describe, and instead of uniform motion, we can get a wave, perhaps several hundred kilometres long, and out of tune with the upper layer.

To understand some of what's going on, a sandwich model was constructed in Prof Cloetingh's Amsterdam laboratory. As in the field, each layer was made up to have different consistencies and flow properties. By applying different forces, the researchers were able to make observations that helped them explain what was happening on a giant scale.

Sierd Cloetingh was at Trinity College Dublin recently to talk about the Deep Earth project as one of the Planet Earth lectures organised by GSI, RIA and other partners.