Is plate tectonics real or not
Planet in motion: five questions about plate tectonics
Which questions are still open?
Some of these modern people, like Alfred Wegener in the past, are still puzzling over a few phenomena that cannot easily be explained by continental drift. The tectonic plates slowly migrate over the hotspots, where hot material slowly swells upwards from the depths of the earth's interior and ultimately causes volcanoes to erupt.
In the sea, these mountains grow to a considerable size and after a while they can even rise a few thousand meters above the water level. However, these volcanoes continue to migrate with the tectonic plate, while many researchers believe that the hotspot does not move. After many millennia, the lava supply dries up and the volcano does not continue to grow. Rainfalls wash material down from the crater rim, on the coast the waves gnaw the island and the extinct volcano shrinks until it disappears below sea level. The hotspot has long since welded its way through the tectonic plate several kilometers away and a new volcano erupts.
Over many millions of years, a long chain of volcanoes is created, geologists are familiar with a number of such hotspot tracks in the world's oceans. Probably the most impressive is the 6100 kilometer long Hawaii-Emperor chain. Its oldest volcano is the Detroit Seamount near the Kamchatka Peninsula, which was active around 80 million years ago. At the same time, the Loihi volcano, southeast of Hawaii, is pushing up from the sea floor, the summit of which is still 900 meters under water today. The only flaw in this theory is the kink on the 47 million year old Daikakuji Seamount: While the Emperor chain stretches from north to south, the Hawaii chain bends there at a 60-degree angle to the southeast.
Geologists originally explained this kink with the fact that the movement of the Pacific plate changed abruptly 47 million years ago. This is exactly what the model calculations by GFZ researcher Trond Torsvik show. This turning could have been triggered by the Izanagi plate, which once separated the Pacific and Eurasian plates between Hawaii and Japan. "The last remains of this Izanagi plate finally disappeared into the depths around 47 million years ago," says GFZ researcher Bernhard Steinberger. When that resistance was gone, another plate may have rerouted the Pacific plate. This is how the kink in the volcanic chain came about. It is still unclear why the Pacific plate changed direction so abruptly.
On top of that, this turning is far from enough to understand the location and age of the volcanoes in the kinked chain. It only fits better if the hotspot under the Hawaiian Islands has moved at the same time. "Model calculations suggest that hotspots can move a few millimeters a year," explains Bernhard Steinberger. Together with the change in direction of the Pacific plate, this hotspot hike explains the kink in the volcanic chain quite well, the GFZ researchers showed in 2017 in the journal Nature Communications.
But perhaps these hotspots also play an important role in the formation of sea plates and thus one of the foundations of plate tectonics, in addition to the formation of long volcanic chains in the sea? So far, the researchers don't really know which event in the middle of a thick continental plate will trigger the creation of a mid-ocean ridge. Hotspots may play a decisive role in this, suspect scientists such as Reinhard Werner at the Helmholtz Center for Ocean Research Kiel (GEOMAR). If the plume head accidentally encounters a weak zone in the continental plate, it could not only release extremely rich lava flows there, but also practically tear the plate apart as it drifts on. That is exactly what could have given the starting signal for the Central Ocean Ridge in the south of present-day Africa 135 million years ago, which is now driving Africa and South America apart.
A similar event could have created the Red Sea between Africa and Arabia and further south at the East African Rift Valley separating the east from the rest of Africa. "Exactly between these two zones there is a hotspot under today's Eritrea and Ethiopia that could play a major role in the birth of a new sea," says GFZ researcher Bernhard Steinberger. If this consideration is correct, in a few million years, similar to how the Red Sea separates Africa from Asia today, a new arm of the sea could split off East Africa.
A few millions of years ago, the situation apparently looked similar where the Upper Rhine Graben now separates the Black Forest from the Vosges. In the north this rift continues to the North Sea and in the south to the Mediterranean. Geophysicists are still puzzling about why the forces from the interior of the earth that once created this rift have obviously weakened.
When does plate tectonics end?
Today the Upper Rhine Graben is an isolated case. In the past three billion years, however, the drive of plate tectonics seems to have stalled more than once around the world, only to start up again after a while. Such cycles probably last several hundred million years, and it does not look as if they only depend on the gigantic currents in the earth's mantle. Rather, GFZ researchers Stephan Sobolev and Michael Brown from the University of Maryland in College Park in the US suspect in the journal Nature that erosion on the surface of the planet also plays an important role.
This influence can still be observed today on the South American coast, to the chagrin of the people affected. There, sea plates submerge under the South American plate and thus complete a gigantic cycle: while magma rises from the earth's mantle on the mid-ocean ridges, forms new seabed and dissipates heat from the deep inside of the planet, a plate is swallowed there, which is also the interior of the earth cools a little. However, it doesn't always seem to work completely smoothly. Again and again, very strong earthquakes devastate the region between the Chilean island of Chiloé and northern Peru, while further south the tremors are usually much less severe.
The reason for this difference seems to be the climate: in the Andes further to the south, abundant precipitation falls, which supplies a rainforest in the temperate latitudes with water, while further to the north there is a thousand-kilometer-long desert on the coast. The researchers also see this difference in the bottom of the Pacific: While the heavy rainfall in the south is severely eroding the Andes and sending a lot of sediment into the sea via the rivers, hardly any sediment accumulates further north. This sediment and especially the water it contains seems to lubricate the area between the two plates well and thus allows the sea plate to slide down into the depths fairly undisturbed. Further to the north, however, the sediment and thus the lubricant is missing. The two plates get caught there again and again. If these hooks loosen after many decades, the plates suddenly catch up with the neglected, slow gliding past each other with one blow, triggering a violent earthquake.
Stephan Sobolev and Michael Brown suspect that in the history of the earth this lubricant sediment has always been in short supply. As a result, the submersion of the sea plates slowed down and slowed down the entire plate tectonics. When large parts of the earth, or perhaps even the entire globe, were buried under a gigantic ice sheet around 750 to 630 million years ago, the glaciers scraped away thick layers of rock at their bottom and, like oversized conveyor belts, transported huge amounts of sediment into the thick ice sheets buried seas. Like a kick starter, this lubricant really got the plate tectonics going again.
Such huge glaciers seem to have restarted the convection currents in the Earth's mantle more than once. However, the high water content of the sediments seems to be particularly important for good lubrication. "If the warming sun has blown most of the water off the earth in perhaps one to three billion years, the plate tectonics could end irrevocably," muses Bernhard Steinberger. However, if there are still people, the end of the continental drift should be one of the least of our worries.
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