Part Of North America Is Sinking Into Earth's Mantle: Study

A chunk of ancient oceanic crust buried deep beneath the Midwest is pulling parts of North America's crust down into the Earth's mantle, scientists have found.

The finding, published March 28 in Nature Geoscience, reveals that this underground process is creating massive "drips" beneath the continent, stretching from Michigan to Nebraska and Alabama. These rock drips hang as deep as 640 km into the mantle.

"We made the observation that there could be something beneath the craton," said Junlin Hua, lead author of the study.

The researchers say the cause is the Farallon slab - a leftover from the Farallon tectonic plate that once pushed under North America along the west coast. Though the plate broke apart around 20 million years ago, one large slab remains beneath the Midwest and is still pulling rock material downward.

This pull is thinning the base of the continent in a process called cratonic thinning. Cratons are ancient, stable chunks of the Earth's crust that have lasted billions of years. Until now, scientists had never witnessed cratonic thinning happening in real time.

"A very broad range is experiencing some thinning," Hua said in a statement. "Luckily, we also got the new idea about what drives this thinning."

The team used a cutting-edge imaging technique called full-waveform inversion, which uses seismic waves to create high-resolution underground maps. It revealed how rocks from across the continent are being funneled toward the dripping zone and pulled into the mantle.

"This sort of thing is important if we want to understand how a planet has evolved over a long time," said co-author Thorsten Becker, a geophysicist at UT Austin.

Computer simulations confirmed the findings. When the Farallon slab was included in the models, the dripping appeared. When it was removed, the dripping stopped.

Though dramatic in scale, scientists say there's no reason to panic. The process is extremely slow and won't cause any changes to the surface any time soon. In fact, it may stop altogether once the Farallon slab sinks deeper into the Earth.

This "helps us understand how do you make continents, how do you break them, and how do you recycle them," Becker said.

The study was supported by the National Science Foundation and UT Austin's Jackson School of Geosciences, with contributions from researchers at the University of Hawai'i, University of Nevada, and institutions in China.