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https://www.ethz.ch/en/news-and-events/eth-news/news/2014/09/mantelplumes-brechen-kontinente.html
Mantle plumes crack continents
04.09.2014 | Peter Rüegg | Research
Using a simulation with an unprecedentedly high resolution, Earth
scientists from University of Paris VI and ETH Zurich have shown that magma
columns in the Earth's interior can cause continental breakup – but only if
the Earth's skin is already taut.
The volcano Erta Ale in Ethiopia indicates the upwelling of the Afar
plume. (Image: filippo_jean / flickr.com)
In some parts of the Earth, material rises upwards like a column from the
boundary layer of the Earth's core and the lower mantel to just below the
Earth's crust hundreds of kilometres above. Halted by the resistance of the
hard crust and lithospheric mantle, the flow of material becomes wider,
taking on a mushroom-like shape. Specialists call these magma columns
"mantle plumes" or simply "plumes".
Are mantel plumes responsible for the African rift system?
Geologists believe that plumes are not just responsible for creating
volcanoes outside of tectonically active areas – they can also break up
continents. The scientists offer the Danakil Depression (the lowlands in
the Ethiopia-Eritrea-Djibouti triangle) as an example of this. This "triple
junction" is extremely tectonically and volcanically active. Geologists
believe that the so-called Afar plume is rising up below it and has created
a rift system that forks into the Red Sea, the Gulf of Aden and Africa's
Great Rift Valley. However, the sheer length of time required, geologically
speaking, for this process to take place, means that nobody is able to
confirm or disprove with absolute certainty that the force of a plume
causes continental breakup.
Simulations becoming more realistic
Evgueni Burov, a Professor at the University of Paris VI, and Taras Gerya,
Professor of Geophysics at ETH Zurich, have now taken a step closer to
solving this geological mystery with a new computer model. Their paper has
recently been published in the journal Nature. The two researchers
conducted numerical experiments to reproduce the Earth's surface in
high-resolution 3D.
These simulations show that the rising flow of material is strong enough to
cause continental breakup if the tectonic plate is under (weak) tensile
stress. "The force exerted by a plume on a plate is actually too weak to
break it up," says Gerya. In experiments using simple models, the
researchers allowed the plumes to hit an unstressed plate, which did not
cause it to break, but merely formed a round hump. However, when the
geophysicists modelled the same process with a plate under weak tensile
stress, it broke apart, forming a crevice and rift system like the ones
found around the world.
At the triple junction, the Afar plume might have been involved in the
creation of the huge rift system which comprises the Red Sea, the Gulf of
Aden an the African Rift Valley. (Image: wikimedia)
"The process can be compared to a taut piece of plastic film. Weak, pointed
force is enough to tear the film, but if the film is not pulled taut, it is
extremely difficult to tear." This mechanism has already been proposed in
the past as a possible model for explaining continental breakup, but had
never been outlined in plausible terms before now.
First high-resolution simulations
"We are the first to create such a high-resolution model which demonstrates
how a plume interacts with a plate under tensile stress," says Gerya. Fast
and powerful computers and stable algorithms programmed by the scientists
themselves were required for the simulations. The researchers benefited
from technical advances made and experience accumulated by the ETH
professor in this field over the past ten years.
In the model, the deformations are created quickly from a geological point
of view. Rift systems several kilometres deep and more than a thousand
kilometres long can form after "just" two million years. The processes are
therefore up to ten times faster than tectonic processes such as subduction
and 50 times faster than the Alpine orogeny, for example.
Disputed idea
The idea of mantel plumes is widely disputed, with some researchers denying
that they even exist. "I think it is much more likely that they do exist,"
says Gerya. As is often the case in geology, especially when researching
the Earth's interior, such processes and phenomena like the existence of
plumes cannot be observed directly. Furthermore, the periods over which
geological processes take place are far too long for humans to experience
first-hand. "So far, we have only been able to observe the effects that
plumes have on the Earth's surface and on the propagation of seismic waves
in the Earth's interior."
The scientists are therefore reliant on good, realistic models that show
the processes in a geological time lapse. How realistic the calculated
simulations are depends on the parameters used. The plume-plate interaction
model incorporated physical laws, the characteristics of materials in the
Earth's crust and mantle, and temperature and pressure conditions. "We know
the rules, but humans generally lack the intuition to identify how they
interact on geological timescales."
Reference
Burov E, Gerya T. Asymmetric three-dimensional topography over mantle
plumes. Nature 513, 85–89 (04 September 2014), published online 3rd Sept
2014. DOI:10.1038/nature13703
Received on Fri Sep 05 2014 - 08:03:16 EDT