by Bethany Augliere Wednesday, February 8, 2017
About 200,000 years ago, modern humans evolved in East Africa, including in what’s now Ethiopia. They — like earlier hominins who had preceded them — likely encountered occasional explosive eruptions spewing ash and lava into the air and onto the landscape, according to a new study in Nature Communications.
Geologists from Ethiopia, the U.K. and the U.S. examined a 200-kilometer-long stretch of the active East African Rift in Ethiopia. There, warm underlying mantle is raising the regional landscape and slowly splitting the African Plate apart, leading to persistent volcanic activity along the rift. But the size and frequency of past explosions are uncertain, and scientists know very little about volcanic activity during the emergence of modern humans. Studying rock samples and geologic structures, the researchers found evidence for a 150,000-year pulse of explosive eruptions large enough to alter landscapes and, therefore, influence early human behavior and evolution.
Currently, more than 10 million people live within this active rift zone, and many of its volcanoes are poorly studied. “We wanted to figure out when the last eruption took place and how frequent explosive volcanism was, so we can make estimates as to the likelihood of future volcanic activity,” says William Hutchison, a volcanologist now at the University of St. Andrews in Scotland and lead author of the new study.
To understand the eruptive history of the region, the researchers spent weeks in the field examining ancient lava flows. They also used satellite imagery and airborne laser scanning to map lava flows and other structural features, such as faults and calderas, from above.
To calculate the age of caldera collapse, Hutchison and his team identified the thickest pyroclastic sequence at each volcano. At two volcanoes, Aluto and Corbetti, they analyzed ratios of argon isotopes in crystals of the mineral sanidine, which tell when the crystals — and thus the rock they are in — cooled. To date two other volcanoes, Shala and Gedemsa, they used previously published data.
For post-caldera collapse deposits less than 50,000 years old, burnt vegetation — which was alive before being singed and preserved by flowing lava — from immediately beneath the rocks was dated using radiocarbon dating.
The team found that the volcanoes they studied — four of Ethiopia’s largest — had explosive caldera-forming eruptions between 320,000 and 170,000 years ago. Eruption rates along the rift during that period were also five times above the average rate over the last 700,000 years, which came as a surprise, Hutchison says. “We expected that major explosive eruptions would take place at regular intervals, but, instead, found evidence for a burst of explosive activity,” he says.
Hutchison and his team provided the first dated history for these volcanic centers, says Christine Lane, a geographer at the University of Cambridge in England who was not involved in the study. “It’s an area we have so little data about, so it’s really great to see some good chronology and good dating.” The eruptions from the study were potentially similar in scale to the deadly 1883 explosion of Krakatoa in Indonesia, which spewed at least 10 cubic kilometers of material. Like Krakatoa, these East African eruptions would have blanketed the ground in ash and ejecta, disrupting habitats and water sources, and they would have left massive craters in the rift floor, Hutchison says.
The changes to the landscape raise the question of how the burst of volcanism impacted human evolution, migration and behavior. For example, if explosive volcanism is typical of the East African Rift, it’s possible this activity pushed some early hominins out of Africa, Hutchison says. While previous studies have linked climate change and human evolution, little work has focused on the potential effects of volcanic activity, he notes.
“Evidence of this particular phase of volcanism — at this time that was so critical to hominin evolution — opens up the idea that we should be looking into this as well,” Lane says. This work will also help scientists date archaeological sites in the region by matching ash found at the sites to ash deposits examined in this study, she says.
Large, explosive eruptions pose little risk to modern inhabitants of the area, Hutchison says. But Aluto and Corbetti, two silicic volcanoes, have both experienced frequent small eruptions over the past 20,000 years. The team also found lava and pyroclastic flows from events in the last 1,000 years, so there might still be hazards to the region, he says.
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