ByRichard Kemeny
Published June 23, 2023
• 8 min read
Perched atop the Anatolian tectonic plate, wedged between three larger plates, Turkey is in one of the most seismically active regions on the planet. The magnitude 7.8 and 7.5 earthquakes that devastated Turkey and Syria in February 2023 occurred when the Anatolian plate slid against the Arabian plate to the south. But something has perplexed scientists about this part of the planet for years: Why does Turkey have volcanoes in the interior, far from the tectonic boundaries where volcanic activity generally occurs?
In a study published in the journal Geochemistry, Geophysics, Geosystems, a team of scientists think they’ve found an answer. By studying seismic waves underground as well as clues in rocks at the surface, they discovered evidence of a channel of molten rock flowing horizontally just below the Anatolian plate. This magma is hotter and moving faster than the surrounding material in Earth’s upper mantle, causing it to stick close to the surface and drive volcanism.
The team also traced the source of this magma flow: the East African Rift, a series of fractures in Earth’s crust over 1,250 miles away. The findings suggest a plume of molten rock rising within the rift, where the African plate is splitting apart, propels the horizontal magma channel, which barely cools as it travels underground and feeds volcanoes along its path.
“That plume material can travel laterally along the base of the tectonic plate quickly and over large distances is consistent with observations, for example, from around the Icelandic plume,” says Fergus McNab, a geophysicist at the GFZ German Research Center for Geosciences in Potsdam who wasn’t involved in the study. “The distances involved here are larger, though, and the fact that volcanism is still being generated at such distances is unique.”
Horizontal plume travel has been modeled elsewhere, including beneath Hawaii and parts of the Pacific Ocean. These collected findings suggest mantle material can travel much farther than previously thought without losing much heat, offering a possible explanation for some volcanic activity in unexpected places.
Volcanism beyond borders
Turkey has a long history of intermittent volcanism, with the most recent eruption occurring on July 2, 1840, when magma heated water and caused an explosion within Mount Ararat. The blast triggered a landslide that swept over nearby villages, killing around 1,900 people.
The eruption has long puzzled scientists, since Mount Ararat is several hundred miles from a tectonic boundary. Most volcanoes cluster around hotspots at the edges of Earth’s tectonic plates—slabs of rock that drift slowly atop the planet’s mantle like giant pieces of cracked egg shell. When these plates collide, one normally sinks below the other, releasing molten rock that drives volcanoes above.
But there are several volcanic fields that lie in the middle of tectonic plates. Such intraplate volcanoes, as researchers call them, are sometimes fed by plumes of hot rock flowing up vertically from the mantle. But others occur where no such plume seems to exist, as is the case beneath Mount Ararat in Anatolia.
Previous research that investigated the volcanism around the Anatolia plate has led some scientists to suggest local tectonic processes drive the activity, such as the crumbling of the lower plate into the mantle. But these explanations alone don’t quite match up with the high temperatures seen spread throughout the region. So Junlin Hua, a geologist at the University of Texas at Austin, and colleagues dug a little deeper.
Retracing origins
The researchers combined seismic and geochemical clues to study the temperature and profile of the mantle below eastern Anatolia. Seismic imaging showed a channel where waves slow down—indicating higher temperatures and a partially molten mantle—roughly 60 to 90 feet deep in a region of the planet’s interior known as the asthenosphere.
The team then analyzed data from 117 basalt samples found in Turkey’s Karacadağ volcanic field. Erupted magma crystallizes in a specific way that can reveal details of its formation. Using this information, they determined a temperature in the channel of around 2,600 degrees Fahrenheit, 95 degrees hotter than the ambient mantle.
The researchers then looked at chemical isotopes in basalt samples taken from sites along the channel’s 1,250-mile route between East Africa and Turkey. With data from 1,004 rock samples, they found overlapping traces of strontium, neodymium, and lead isotopes that pointed to a common origin.
“The magmas are telling us they’re consistent,” says Karen Fischer, a seismologist at Brown University and co-author on the study. “They’re also telling us that they’re consistent with the same source in the mantle.”
Further modeling of these rocks revealed that magma in the channel travels fast enough to maintain a higher temperature than the rest of the asthenosphere. In order to maintain this heat, the models suggest, the magma is traveling roughly 24 centimeters per year, taking just shy of 11 million years to arrive in Anatolia. This may seem slow, but for magma working its way through the dense mantle, it’s quite fast.
“These flows can be among the fastest mantle movements on Earth,” says Maxim Ballmer, a geodynamicist at University College London who wasn’t involved in the study.
This speed, the authors propose, is driven by pressure from the upwelling plume at the East African Rift and the lower viscosity of the hotter magma. “What’s really important is that it’s still hot, so it can generate these volcanoes,” Hua says.
How exactly the channel started remains an open question that could be explored in future work. “One possibility … is that plate spreading in the Red Sea encouraged northward flow, though this is not explored in any detail,” McNab says.
One clue was found in the isotopes: a shift in their composition around 10 million years ago, around the time the Anatolian and Arabian plates collided. This suggests the channel, which may have already extended as far as Jordan, could have found a new opening during the tectonic collision, Hua says.
Otherworldly plumes
The new findings are forcing scientists to rethink how far the material from a rising plume can spread before triggering volcanic eruptions. “Material from mantle plumes can reach and alter parts of the Earth at much larger distances than maybe one conventionally thinks,” Fischer says. “There do seem to be these corridors where plumes actually can affect the upper mantle thousands of kilometers away.”
Plumes were thought to radiate as a disk on reaching the surface, though the new research suggests they could also disperse in thin channels, quickly and over long distances. “If you think of a plume sending out tendrils in many different directions, that starts to give you an explanation for these phenomena,” Fischer says.
This could be one mechanism to explain some of the mysterious volcanic activity in the past, such as across the Central Atlantic magmatic province, an area of widespread volcanism that coincided with the breakup of Pangea roughly 200 million years ago. The runaway volcanic activity is thought to have caused a mass extinction at the end of the Triassic.
The work could also inform future volcanism research on other planets, like Venus, which has no plate tectonics but does appear to have plume-like activity. And studying the churnings of our planet’s interior and the movements of its tectonic plates can help us understand the environments that form on the surface.
“Only recently, we have begun to understand how the very processes that trigger volcanic eruptions and earthquakes also help to stabilize the ocean volume and climate over millions or billions of years,” Ballmer says,” thereby sustaining conditions on the surface over timescales that are needed for the evolution of higher life.”
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