The next time you shut off the water at the sink, stick around for a minute and peer at the faucet spout. You might see a droplet of water form there and then pull away until, at last, it breaks free and falls through the air.
This same process, more or less, is happening to Earth’s continental lithosphere beneath a region of the Eurasian tectonic plate known as the Hindu Kush mountains, which straddle Afghanistan and Pakistan. There, a pair of geophysicists found a “drip,” or a “blob” of continent, that is, like our water droplet, pulling away from the lithosphere and descending into the mantle.
The blob, the team reported in April in the AGU journal Tectonics, looks like a droplet that is just about ready to break off of its spout. The mass is pulling away from the crust at a rate as fast as about 100 millimeters per year, and as it moves, it triggers earthquakes that scientists have been unable to explain the origin of—until now.
“We never quite understood the Hindu Kush earthquakes,” said geophysicist Peter Molnar of the University of Colorado Boulder, who led the research.
That’s because these earthquakes defy convention: They do not happen along an obvious path, or lineament, as is often the case in seismic zones elsewhere on the planet. Instead, the pattern of the Hindu Kush earthquakes resembles something akin to a “round patch” on the planet’s surface, said Rebecca Bendick, a geophysicist at the University of Montana in Missoula who coauthored the new research alongside Molnar.
One part of the mystery relates to the fact that there is no obvious tectonic feature in that part of the world that researchers might point to as a driver behind the region’s quakes. Nevertheless, “there’s a long catalog of earthquakes happening underneath the Hindu Kush to very unusual depths,” Bendick said.
To unravel what might be going on, Molnar and Bendick, employing a team of Afghan scientists, gathered seismic data around the Hindu Kush area over the past several years. A picture of a dripping blob began to emerge on the basis of where quakes happened at different depths. Although the team does not know for sure just how big the blob is, the seismic data suggest that it spans a 300-kilometer-deep zone that is about 150 kilometers north to south and about 100 kilometers east to west—not quite as large as the continent-scale deep-Earth blobs that Eos reported on in February.
Geoscientists thought before that the only way that rocks of Earth’s lithosphere could cycle into the planet’s interior was via subduction, whereby oceanic lithosphere dives beneath continents at tectonic plate boundaries. But now that there is what Bendick sees as definitive proof that dripping blobs of continental lithosphere exist, the subduction-only paradigm may be infirm.
Before, it was thought that continental crust never was really lost over time, and researchers rely on this assumption whenever they rewind the clock and try to figure out where ancient continents were and how they fit together in Earth’s deep history. But if the continents can lose parts of themselves over time, those reconstructions may not be entirely accurate.
The Hindu Kush blob, Molnar explained, likely began dripping, at the earliest, only about 10 million years ago. “It’s a relatively recent process, and you’re stretching out this material so fast that pretty soon it’s all gonna drip off and go away,” he said.
The dripping is happening relatively quickly, but, Bendick explained, there is no great understanding of how common such blobs are over geologic time. So for now, how much mass continents might be losing, over what timescale, and, in turn, how inaccurate reconstructions of ancient continents may be will remain unknown.
Nevertheless, one mystery that researchers can consider solved is the fact that blobs do indeed exist, explained Philip England, a geophysicist at the University of Oxford in the United Kingdom who was not involved in the work.
“Molnar and Bendick’s elegant analysis identifies one such drip,” England said. The find, he added, should help with identifying and studying blobs that exist elsewhere within the planet.
—Lucas Joel ([email protected]), Freelance Journalist