A vent site caused by the African fissure. Credit: Cindy Ebinger
Twenty-five kilometers from the nearest road in the Afar Desert in northeastern Ethiopia, geologists on camel-back are converging on a 60-kilometer fissure in the ground. The crack opened just three weeks after a series of earthquakes in September, and scientists believe the eight-meter-wide fissure marks one dramatic step in a million-year process that could lead to a new ocean basin.
"As geologists, normally we measure time-averaged movements of the Earth’s plates," said Cindy Ebinger, of the Royal Holloway University in London. "But in this case we were able to watch one of the punctuated events that lead to the formation of an ocean basin."
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Researchers working in the area have long suspected that the same forces that generated other ocean basins were shaping the Afar Depression, which is home to the lowest point in Africa, Lake Asal (153 m below sea level). The depression is formed by a triple junction of tectonic plates and is one of two places on Earth where a mid-ocean ridge can be found on land. The only other place is Iceland.
“We’ve been trying to understand rift zones from looking at the long-term record," Ebinger said, "but as far as the process that creates the ocean basin, [until now] we haven’t been able to capture it."
When the land unzipped along the fault line a volcanic vent opened. Ebinger said it is believed that in the course of three weeks magma was injected into the fault through dykes.
"[The dykes are] a series of thin vertical sheets, that are shooting down the rift," she said. "They don’t quite reach the surface. We don’t see them, and they are silent, but they allow the surface opening to happen.”
Working with Ethiopian researchers Dereje Ayalew and Gezahegn Yirgu and other scientists from Britain and the United States, Ebinger was part of a rapid response team that placed instruments in and around the fissure immediately after the earthquakes.
Ebinger took a fly-by in a helicopter shortly after the rift opened. “We could quickly see the narrow zone of intense deformation," she said. "It felt like I was flying down the axis of a mid-ocean ridge, with birds rather than fish as my companions."
Ebinger is now preparing to return to the area for a camel-based expedition to set up more instrumentation and to gather more data to be used in conjunction with satellite imagery.
The Afar Desert is a remote and challenging environment. Shade is scarce and daytime temperatures regularly reach 35° C (95° F). Though the area has little infrastructure to damage, the resident herders left as a result of the spate of earthquakes, which included one day when 54 separate quakes registered on seismographs around the world.
Among other things, the researchers will be trying to figure out if this recent seismological activity will trigger subsequent movement; the opening of the crack is likely to have increased pressure at its north and south ends.
The Afar rift offers a snapshot of a process that has happened many times around the world. Witnessing the early stages allows geologists to make direct comparisons with Iceland, where the mid-ocean ridge spreading has been ongoing for tens of millions of years.