By Richard Black Environment correspondent, BBC News, Vienna
source : http://www.bbc.co.uk
12 April 2011
Europe may be starting to burrow its way under Africa, geologists suggest.
The continents are converging; and for many millions of years, the northern edge of the African tectonic plate has descended under Europe.
But this process has stalled; and at the European Geosciences Union (EGU) meeting last week, scientists said we may be seeing Europe taking a turn.
If they are correct, this would signal the start of a new subduction zone – a rare event, scientifically fascinating.
Beneath the Mediterranean Sea, the cold, dense rock at the extreme north of the African plate has virtually all sunk under the Eurasian plate on which Europe sits.
But the African landmass is too light to follow suit and descend.
“Africa won’t sink, but Africa and Europe continue to move together; so where is this taken up?” asked Rinus Wortel from the University of Utrecht.
“It looks possible that on the appropriate timescale, we are witnessing the beginning of subduction of Europe under Africa,” he told BBC News.
The Mediterranean Sea’s geological structure and history are quite complex.
The Utrecht group’s analysis goes like this:
The slow convergence – just a few centimetres per year – was obstructed partially by collision of the two plates further East, in Turkey, and then by the fact that the lightness of the African continent prevented further subduction.
As a result, bits of the African plate that did subduct have broken off and are descending to the Earth’s mantle.
To fill this gap, bits of the Eurasian plate have been pulled southwards across the Mediterranean, such as the Balearic Islands, Corsica and Sardinia. The same thing is happening with Crete.
And computer modelling suggests the end product of all this could be the initiation of subduction in the opposite direction from the past.
Additional evidence comes from observations of earthquakes.
“We see what motion occurs in the earthquakes, and we see that the fault planes dip towards the South,” said Professor Wortel.
Although the power of subduction zones to generate enormous events has been thrown into stark light once more by the Tohoku earthquake and tsunami off the Japanese coast, the geology of the Mediterranean is very different.
Nevertheless, there is some concern among scientists that European countries are putting only small amounts of money into building a tsunami warning system for the region.
At the EGU, Stefano Tinti from the University of Bologna, Italy, said the EU collectively had put only about 8m euros into tsunami research over the last five years.
Over the same period, Germany alone funded the Indonesian early warning system to the tune of about 55m euros, he said.
“There was interest just after [the Asian tsunami of] 2004, and then interest rapidly decreased again,” Professor Tinti, who until recently chaired an intergovernmental co-ordination group on tsunami in Europe, told reporters.
“It’s very political; and putting together all these member states in order to co-operate and put money in – even though the sums are very small, was a very difficult task.”
Although tsunamigenic earthquakes in the Mediterranean are smaller than the biggest ones around the Pacific rim, Magnitude 8 events have been recorded, such as the 1303 Crete quake whose tsunami devastated Heraklion and Alexandria.
Confirmation that European subduction had started could allow scientists to model the region better, and so make better assessments of earthquake and tsunami risk.
But the long timescales involved in geological processes make this a challenge.
“We’ll keep track of the seismic activity to see whether it continues to indicate this underthrusting of the Mediterranean subsurface underneath North Africa,” said Rinus Wortel.
“But it’s not going to mature in the lifetime of a scientist.”