Laypeople think there are only two fundamental kinds of plate tectonic movements: the kind where two continental plates collide and the kind where one slips under another. Not so, it seems. The enigmatic uplift of the entire central Turkish plateau, and the volcanoes along the plateau margins, were the result of classic plate collision – and a vast bit of upper mantle and crust breaking off and sinking deep into the bowels of the earth, releasing the upper crust to leap upward. By a lot.
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The scientists call the phenomenon “lithospheric drip,” lithosphere referring to the upper mantle and drip referring to drip.
Think of it like a massive drip, and then detachment, of a layer of continental plate, they explain.
Turkey isn’t best known for exploding mountains, but the fact is that it has both active and extinct volcanoes, one of which is none other than the famed Mount Ararat. (Which scholars do not think is the resting place of Noah’s Ark.) Anyway, Ararat last erupted in the year 1840, exactly 19 years after the mountain’s first known ascent. Most of Turkey’s other volcanoes are dormant, or at least have not erupted in millennia. But another mystery is why the volcanoes are where they are, which isn’t necessarily on the borders of continental plates.
Turkey is however very well known for earthquakes as Africa converges on Eurasia. At least some of the devastating temblors that have rocked Turkey over the millennia are due to the central Turkish plateau in Anatolia – which is a distinct geological region – rising by as much as a kilometer over the last 10 million years or so. The southern margin of the plateau seems to have risen by as much as 2 kilometers. That is quite the brisk clip. And why did it do that? Because vast bits from the crust and upper mantle of the planet thickened and “dripped down,” into the molten rock below, explains the team.
As the thickened material from the upper mantle and crust sank into the lower mantle, first it formed a basin, say the scientists from universities in Toronto and Istanbul. And then that weight broke off entirely from the neighboring crust and upper mantle, sinking further into the depths of the mantle. That, in turn, seems to have created a massive gap in the plate beneath Central Anatolia, explains Prof. Oğuz Göğüş of Istanbul Technical University. The basin, freed of the weight that broke off and sank into the deeper mantle, sprang upwards like a very ungainly impala and the result is central Turkey.
Evidence for their theory includes seismic data showing missing lithosphere (this is based, in part, on anomalous seismic waves) and a telltale "dropping root arc" structure. Further evidence is that the whole plateau’s massive uplift and volcanism in Cappadocia and Galatia have no obvious explanation according to classic plate tectonics.
A whole land mass bobs up
“Essentially, by dropping this dense lithospheric [mantle] anchor, there has been an upward bobbing of the entire land mass across hundreds of kilometers,” says Göğüş, lead author of the study in Nature Communications.
The new theory involving lithospheric drip, drop and uplift began to percolate in geophysical minds because they couldn’t explain the elevation of central Turkey, and the absence of expected lithospheric matter, as well as geological enigmas such as volcanoes where plate tectonics didn’t predict them.
What would cause the lithosphere to thicken to the point that it actually sinks into the mantle below? Plate shortening caused a mountain belt to form and fold, triggering the thickening and then the dripping, they postulate.
“It’s a new variation on the fundamental concepts of plate tectonics,” commented Prof. Russell Pysklywec, chairman of the Department of Earth Sciences at the University of Toronto and one of the study’s co-authors. “It gives us some insight into the connection between the slow circulation of near-solid rock in Earth’s mantle caused by convection currents carrying heat upwards from the planet’s interior, and observed active plate tectonics at the surface. This is part of the holy grail of plate tectonics - linking the two processes to understand how the crust responds to the mantle thermal engine of the planet.”
Central Turkey isn’t the only place where the lithosphere under the crust simply seems to be missing, and Pysklywec kindly reassures that our homes and cities aren’t going to suddenly sink into the mantle or jump like gazelles upward. “Our results show that the Central Anatolian Plateau rose over a period of millions of years. We’re talking about mantle fluid motions and uplift at the pace at which fingernails grow,” Pysklywec said.