The primordial event that gave Europe its current form could have happened 105 million years ago below Madagascar. From the planet’s interior, hot rocky material then rose through the mantle and spread beneath the rigid, outer shell of the Earth. This mushroom-shaped “mantle plume” stretched the earth’s crust, separating India from Madagascar. Then a supervolcano erupted between the new continents.
Traces of this can still be found today in the form of flood basalts on both sides of the geological plate boundary that formed at that time – in Madagascar and in the south-west of India. “The original crime scene was this mantle plume,” says geologist Derya Gürer from the Australian National University in Canberra in a video call while she was in a yellow thermal jacket on the lower deck of the research vessel Joide’s resolution sits, which is traveling south of South Africa and thus not so far away from that place and that time when the Morondava volcano erupted. “I’m in the Upper Cretaceous,” she says, referring to the age of the seabed beneath her.
The outbreak of Morondava Supervolcano must not only have been a catastrophe for the then rulers of the planet – the dinosaurs – it probably also triggered a plate tectonic chain reaction that continues to have an effect today and has shaped the shape of the entire Mediterranean region. That’s what geoscientists around Gürer claim journal NatureGeoscience. “50 years after establishing the theory of plate tectonics, we have now discovered that individual tectonic events do not take place in isolation, but are connected with one another and can take place one after the other in a chain reaction,” says Gürer.
From time to time, the tectonic plates seem to suddenly change speed and even direction
Tectonic plates are constantly in motion, this has been known for decades. But only in the last few years have geologists speculated on the idea that the plates not only move evenly across the globe, but also suddenly change their speed or even their direction from time to time. “Suddenly” in geological terms means within just a few million years. Such realignment is more or less due to chance – for example, when hot rocky material rises to the surface, when tectonic plates collide, or when one submerges beneath the other. A single one of these events could even set off a chain reaction, with one tectonic event triggering another, reorganizing Earth’s lithospheric plates. “Although plate movements are ultimately driven by mantle convection, plates that are part of this convection probably drive themselves to a large extent,” says geophysicist Bernhard Steinberger from the Geoforschungszentrum Potsdam (GfZ). “In particular, when a plate is subducted and goes down into the mantle, it pulls the rest of the plate with it.”
It is a self-reinforcing mechanism: the deeper the plate descends, the stronger the pulling force. And that creates forces that can set in motion new tectonic events in far-flung places around the world. Sounds understandable – but this theory has not yet been proven.
Derya Gürer wanted to change that with two colleagues from the Netherlands and Israel. To play through the theory with an example, they chose one of the most discussed reorganizations of the lithospheric plates, which is said to have taken place over 100 million years ago in the Upper Cretaceous. However, they first had to solve a stubborn problem that all their predecessors had failed to solve.
Typically, geologists can reconstruct plate tectonics from ages long past by scanning the ocean floor with a magnetometer. When hot molten rock rises from the earth’s interior at the submarine volcanic chains, the so-called mid-ocean ridges, and cools on the sea floor, new oceanic crust forms, which eventually magnetizes. Approximately every 500,000 years, the magnetic field on Earth reverses, which is why the polarity of the new sea floor is reversed as a result. If this now migrates on both sides of the mid-ocean ridges like on a conveyor belt, a magnetic stripe pattern forms on the ocean floor. And this allows conclusions to be drawn about earlier plate shifts.
A journey through the earth’s magnetic field
However, nobody knew exactly what happened between 126 and 83.6 million years before our time. Because there was no magnetic reversal during this period – and therefore no magnetic barcode that the geologists could read. More than 40 million years of active plate tectonics remained hidden – until the Israeli geoscientist Roi Granot made a discovery. In 2005 and 2008, the professor from Ben-Gurion University in Negev spent a month on board the research ship Le Suroitwith which already the wreck of titanic had been found across the Atlantic. From Africa to America, always perpendicular to the striped patterns on the ocean floor. It was a journey through the earth’s magnetic field: from older to younger ocean floors, from the beginning to the end of the Cretaceous quiet zone.
Behind it, the ship pulled what looked like an oversized screwdriver on a 300-meter-long cable. Normally, Granot would tow this magnetometer close to the water’s surface to measure the ocean floor’s magnetic orientation free of interfering signals. However, since that in the Upper Cretaceous calm zone had not changed, he let the magnetometer slide deeper into the ocean. There, as it turned out, he was able to determine the magnetic strength of the ocean crust, which is constantly changing over the course of the earth’s history. Derya Gürer’s colleague Granot took advantage of this: If the rashes were particularly strong, he used drill cores to date the basalt stones on both sides of the mid-ocean ridge – on the African and on the North American side. With knowledge of the age and the magnetic pattern, it was now possible to calculate much more precisely how fast and in which direction Africa had moved relative to Eurasia in those 40 million years.
With the help of these fixed points, the geoscientists were able to sequence the tectonic events of that time, put them together in models like pieces of a puzzle and trace a chain reaction. “We now understand how tectonic events are related,” says Granot. He compares the knowledge he gained about plate tectonics with entering a dark room. “You see some furniture without being quite sure what it is,” he explains. “And then someone turns on the light and you see that there are books on the shelf that you could only guess at before. Red and blue and yellow books. And then you start to understand: Ah, this room must be a library. Man begins to understand the dynamics that shaped this space.”
This is how Gürer and her colleagues realized what shaped the Mediterranean region. From their detective search for clues, they conclude that the magmatic “plume head” once separated the African and Indian plates along a weak point in the earth’s crust and pushed them apart at their southern foothills. The plates then rotated against each other around their respective fixed points, while the oceanic crust of the Indian plate submerged under the Eurasian plate to the north. This subduction zone still stretches from Turkey to India and is responsible for numerous earthquakes.
“The chain reaction has propagated from East to West.”
According to Gürer’s model, the new subduction zone then caused the next domino to tip over in the Upper Cretaceous: After around ten million years, the subducting plate in north-eastern Africa had developed enough tensile force to rotate the African plate counterclockwise, accelerating it and turning it to smash into Europe. That back then Tethys Sea, once a vast ocean south of Eurasia, was pulled underground along the new subduction zone and turned back into liquid rock – “consumed,” as geologists say. Gürer’s calculations show that the Tethys Sea was closing faster than previously thought. Only relics in the Mediterranean basin and in mountains such as Cyprus and Anatolia remain today. Based on the analysis of rock series, Gürer was able to reconstruct the history of the forerunner of the Mediterranean.
Probably due to the change of direction in Africa 85 million years ago, a new subduction zone developed from the Iberian Peninsula to the western Alps. “The chain reaction spread from east to west,” says Gürer. “Along the old seam where the ocean disappeared.”
First, the African plate dipped below the Eurasian in the western Mediterranean, but the Eurasian oceanic lithosphere is now beginning to subside beneath North Africa and the traction generated could drive new “dynamic responses” in the future, the geoscientists write in their paper. So the chain reaction continues.
Next, Gürer wants to test the model of the plate tectonic chain reaction using the Pacific and relate it to changes in the climate system. “Plate tectonics plays a very important role in life on Earth,” says the geologist. “It controls the cycles on earth and causes the planet to reinvent itself again and again.”