Earth crust in Africa rips

Researchers are witnessing a dyking event at the East African Rift Valley

Relic of a "Dyking Event" © Public domain
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For the first time, scientists have witnessed the earth breaking up in the middle of a continent and molten rock being pushed to the surface. Such a "dyking event" is considered one of the key factors for plate tectonics and thus for the processes that shape the face of the earth to this day.

The crust of the earth is divided into several different sized plates, which constantly move against each other. In the course of the history of the earth, large continents broke apart several times. But how can this happen? For the thinner oceanic crust, the mechanism is well-known: in so-called "Dyking Events" molten rock swells to the surface and pushes itself between two plates. As so-called "dykes", such intrusions are still visible millions of years later. For the much thicker continental crust, however, this has never been seen before - until now.

Magmawall slides between two plates

"In order to break a continent, it is first necessary to overcome the strength of the lithosphere, " explains Eric Calais, professor of geophysics at Purdue University and one of the scientists who have now witnessed the event in Tanzania and about it in the current "Nature". Report issue. "But if we calculate the forces of plate tectonics, we find that they are not strong enough to do that. But we know that continents can split and that this has happened several times in the past. But how can that happen? One way is to give the system a little push again and again - and that's exactly what Dyke intrusions do. "

Researchers at work: Sarah D. Stamps and Elifuraha Saria installing a GPS instrument in Tanzania. © Eric Calais

During such an intrusion, magma penetrates from deep reservoirs through the surrounding rock and ascends towards the surface. It pushes the rock apart and over time swells the lithosphere at this point due to its heat. The magma fills up rifts and cracks in its ascent. Gradually, this creates a vertical Magmawall, which presses the earth's crust at this point apart.

"When these events recur for millions of years, an ocean eventually forms between the two plates, " explains Calais. "Such dyking events are part of the theories, but scientists have never been in the right place at the right time and with the right tools to record them." The first stages of such ocean formation are the However, researchers now first observed in Tanzania. display

Minibeben as an indication of the unusual

In the summer of 2007, a series of smaller earthquakes in northern Tanzania attracted the attention of the international research team that was in Nairobi, Kenya at the time. It appeared that the quakes originated from Lake Natron, a depression near the east branch of the East African Rift Valley. French researchers had previously designed seismometers in the region, which registered up to 600 quakes within two weeks. Tanzanian geologists provided GPS measurements of the area, which Calais compared with previous values ​​to identify possible shifts.

In fact, the subsoil in the Natron area had shifted too strong, however, to be explained by the earthquakes alone. Something unusual had to happen. "After these earthquakes, one of the volcanoes in the area entered into an explosive eruptive phase, indicating that magma was involved, " said Calais. "It occurred to us that it could be a dyking event."

Map with earthquake and "Dyking Event" from the year 2007 (black circles symbolize earthquakes, red line shows the location of the "dyking event"). Purdue University / Calais laboratory

Only the data combination occupies the event

Together with colleagues in Luxembourg, the scientist then evaluated radar interferometry data, which provided detailed recordings of ground movements. At the same time, a Belgian team investigated the ground and mapped more than 20 kilometers of open columns confirming the InSAR data. Overall, the event left behind a wall of magma, which had pushed to almost ten kilometers long and one and a half meters in width between the plates.

"Only after we had all the measurements, especially the InSAR data, was it clear that the combination of all values ​​could only be explained by the penetration of a dyke, " says Calais. The event was initiated by a slow gliding of the tectonic plates along a fault even something like this had never been observed before. Dislocations usually move suddenly, creating earthquakes, but this was a very quiet seismic sequence of events that lasted about a week. "

Dyking on Grabenbruch for decades?

The researcher does not exclude that there have been several such dyking events along the East African Rift in recent decades, as they are difficult to detect. "If there is evidence that such events have occurred in the recent past, there is no reason to believe that they have not occurred in the last million years, " said the researcher. "This could be an important contribution to the dynamics of the East African Rift Valley System."

The scientists now want to continue to observe the region in order to record possible consequences of the event. "When a big event like this happens, the stress state of the upper layers of the Earth changes and we expect it to be followed by more events, " says Calais. "Other Magmareservoire could have been touched and trigger another Dyking event. It will take a while for the system to relax and retire. "

(Purdue University, 11.12.2008 - NPO)