Scientists suggest portions of ancient continental rock can become more buoyant over time through a unique processs called mantle delamination. Photo by Lijun Liu/University of Illinois
Feb. 20 (UPI) -- The chance of a major earthquake in America's heartland remains extremely low, but new research suggests continental interiors aren't as tectonically stable as geologists thought.
Scientists recently discovered unusual signals of geologic activity under the tectonically stable interiors of South America and Africa. Their investigation of this activity suggests ancient continental rocks don't always behave as expected.
In their attempt to explain the unexpected geologic signals, scientists investigated the behavior of cratons, the oldest and most stable rock deposits in the continental lithosphere. These rocks have been slowly rising from deep beneath the Earth's surface for billions of years.
"We usually think of cratons as being cold, stable and low-elevation," Lijun Liu, a geology professor at the University of Illinois, said in a news release. "Cold because the rocks are far above the hot mantle layers, stable because their crusts have not been disturbed significantly by faulting or deformation, and their low elevation is because they have been sitting there, eroding down for billions of years."
There are places, however, where cratons buck expectations.
"There are regions of high topography within the cratons of South America and Africa," said graduate student Jiashun Hu.
Hu is lead author on the latest analysis of the unusual cratons of Africa and South America, published this week in the journal Nature Geoscience.
Hu and his colleagues used advanced imaging and supercomputer modeling to analyze the structures beneath these high-elevation cratons. Their work showed the cratons are much less dense than the underlying rock, which explains their rise.
But while the layers of cold mantle rock beneath are denser than the rock above, they're less dense than they used to be. Scientists believe a phenomenon known as mantle delamination may explain the changes. The process involves the peeling away of the lower, denser mantle layers as they're warmed by the heat of nearby mantle plumes.
"From several types of seismic imaging data, we can see what we think are delaminated mantle slabs sinking into the hot, viscous deep mantle," Liu said.
The craton's rocky roots are reformed as new rock cools. Scientists suggest the new rock is less dense, further amplifying the craton's buoyancy and pushing the ancient deposits to high elevations.
Scientists believe similar tectonic mechanisms could explain other geologic anomalies found among the otherwise quiet nature of continental interiors.
"The high topography of Africa and South America is only part of the story," Hu said. "There are many geologic phenomena such as the location of hotspot trajectories, continental volcanism, surface uplift and erosion, as well as seismically imaged deformation within the craton roots that all seem to correlate well with the proposed delamination event, implying a potential causal relationship."