Utrecht University: Geologists unravel plate tectonic chain reaction

Geologists at Utrecht University are working hard to unravel the secrets of plate tectonics, the mechanism that continuously shapes Earth’s crust and is causing earthquakes and volcanic eruptions. This time, another mystery has been dissected. In the Earth’s geological past, there were ‘short’ periods of a few million years during which many tectonic plates around the world suddenly changed their speed and direction. What caused these abrupt changes in plate movements? Earlier research showed that changes in movement between two plates can result from continental collisions or rising mantle plumes. But could such collisions or mantle plumes set off a global chain reaction? Now geologists have succeeded in finding evidence that supports this. “With this discovery, we are able to better understand the driving forces behind plate movements, and thus processes such as mountain formation or volcanism.”

This paper, published in Nature Geoscience, was a collaboration between geoscientists from Utrecht University, Australian National University, and Ben-Gurion University of the Negev. To test their hypothesis, the researchers asked themselves the following question: did the formation of a new subduction zone north of Arabia that was triggered by a mantle plume that caused a super volcano near Madagascar ~100 million years ago set off a chain reaction? Utrecht professor of plate tectonics and paleogeography Douwe van Hinsbergen, geologist, former Utrecht PhD student and first author Derya Gürer, and geophysicist Roi Granot, analysed the consequences step by step. “If our hypothesis is correct, the new subduction zone that formed north of Arabia should have caused forces that accelerated, and rotated the African Plate in the 10 million years after subduction initiation. However, to analyse this, we had to solve a major problem,” says Gürer.

Quiet zone
Like a tape recorder, the chronicle of past plate motions is archived in the oceanic crust’s magnetic. As soon as rock form during cooling of the magma beneath the ocean floor at mid-ocean ridges, they record and store the Earth’s magnetic field. When the magnetic field reverses, it points in the opposite direction in younger rocks than in older rocks. The crust of the oceans thus contains a magnetic archive, reconstructed in a bar code of changing black and white patterns. This archive allows us to reconstruct the movement of tectonic plates in the distant past.

But in the period between 125 and 83 million years, there were no reversals of the magnetic field. “In the oceanic crust that formed in this period, the magnetic field is therefore everywhere pointing in the same direction. It forms the so-called magnetic Quiet Zone,” Van Hinsbergen explains. Until now it was impossible to reconstruct plate movement changes within this time interval. But 10 years ago, Roi Granot discovered that the rocks in the Central Atlantic Quiet Zone did record abrupt changes in magnetic noise. “By applying the methods that we normally use for magnetic reversals to magnetic noise variations, we were able to greatly improve the plate model. It showed that the African plate did indeed accelerate and rotate when the new subduction zone started pulling,” says Granot. This rotation in turn caused a sequence of events, including a new subduction zone in the Western Mediterranean, which in turn pulled the western Mediterranean apart. “This is the first time that evidence has been found for a plate tectonic chain reaction. With this research, we have dissected a mechanism to explain why there are short periods of time in which plates suddenly change direction. These plate movements affect mountain formation, marine gateways, volcanism, and even the global climate,” says Gürer.

Practical applications
The research by the international team of geoscientists contributes to unravelling the mechanisms behind plate tectonics. The plate tectonic chain reaction triggered by a super volcano illustrates how the dance of tectonic plates is ultimately driven mainly by the forces of subducting plates that sink into the Earth’s mantle – the so-called ‘slab pull’. Geoscientists study these mechanisms because they govern earthquakes, volcanism, mountain building, and the formation of ore deposits and other resources. Previously, Van Hinsbergen’s team investigated, for example, the previously mentioned super volcano as a possible underlying trigger of plate tectonics, the limited role of mantle convection on plate motion and reconstructed the lost continent of Greater Adria in the Mediterranean region. “I have been studying these mechanisms for 20 years, and although we are finding more and more pieces of the puzzle to unravel the drivers behind plate tectonics, it is also becoming increasingly clear what we do not yet understand,” says Van Hinsbergen.

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