An ancient slab of Earth's crust, buried deep beneath the Midwest, is sucking huge chunks of North America's crust into the mantle, researchers say.
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The impact of the plate has created giant "leakage" that extends from the underside of the continent to about 400 miles (640 kilometers) deep into the mantle, according to a new study. These drops lie beneath an area stretching from Michigan to Nebraska and Alabama, but their presence appears to affect the entire continent.
The area showing the leak resembles a large funnel, with rocks from all over North America being pulled horizontally into it before being sucked in. As a result, large parts of North America are losing material from the lower part of its crust, the researchers say.
The researchers found that the drops result from the pulling force of a piece of oceanic crust, which was broken off from an ancient tectonic plate called the Farallon plate.
The Farallon Plate and the North American Plate once formed a subduction zone along the continent's west coast, with the former sliding beneath the latter and recycling its material into the mantle. The Farallon Plate broke apart due to the advance of the Pacific about 20 million years ago , and the remnant plates that were subducted beneath the North American Plate were slowly removed.
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One of these plates is currently crossing the boundary between the mantle and lower mantle transition zone about 410 miles (660 km) beneath the Midwest. This piece of oceanic crust is responsible for a process known as “cratonic thinning,” according to the new study, published March 28 in the journal Nature Geoscience.
Cratonic thinning refers to the wear and tear of regions of Earth's continental crust and upper mantle that have remained mostly intact for billions of years. Despite their stability, these regions can undergo changes, but this has never been observed, due to the vast geological timescales involved, according to the study.
Now, for the first time, researchers have documented cratonic thinning as it happens. The discovery was made possible thanks to a larger project led by Junlin Hua to map what lies beneath North America using a high-resolution seismic imaging technique called “full-waveform inversion.” This technique uses different types of seismic waves to extract all the available information about the physical parameters underground.
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This seepage will not lead to changes at the surface anytime soon, the researchers said, adding that it may even stop as the Farallon plate sinks deeper into the lower mantle and its influence diminishes.
Source: livescience
