Scientists have created the most detailed image of the underlying geology beneath Earth’s Southern Hemisphere, showing previously unknown features such as an old ocean bottom that may wrap around the core.
This thin yet thick layer is located approximately 2,900 kilometers (1,800 miles) below the surface, where the molten, metallic outer core meets the rocky mantle above it. The CMB is the core-mantle boundary.
Understanding what’s beneath our feet in as much detail as possible is critical for studying everything from volcanic eruptions to changes in the Earth’s magnetic field, which shields us from solar radiation in space.
“Seismic investigations like ours provide the highest resolution imaging of our planet’s interior structure, and we are discovering that this structure is far more complicated than previously thought,” explains geologist Samantha Hansen of the University of Alabama.
Over a three-year period, Hansen and her colleagues employed 15 monitoring stations buried in Antarctica’s ice to map seismic waves from earthquakes. The movement and bouncing of such waves shows the composition of the material inside Earth. Because sound waves move more slowly in these places, they are referred to as ultralow velocity zones (ULVZs).
“After analyzing [thousands] of seismic recordings from Antarctica, our high-definition imaging method discovered thin anomalous zones of material at the CMB everywhere we probed,” says Arizona State University geophysicist Edward Garnero.
“The thickness of the material ranges from a few kilometers to [tens] of kilometers.” This means that we are seeing mountains on the core that are up to five times the height of Mt. Everest in some areas.”
These ULVZs are most likely oceanic crust that has been buried for millions of years, according to the experts.
While the sunken crust isn’t close to recognized subduction zones on the surface, where moving tectonic plates push the rock down into Earth’s deep, the study’s models reveal how convection currents could have transported the old ocean floor to its current location.
Making inferences about rock kinds and movement based on seismic wave movement is difficult, and the researchers aren’t ruling out alternative possibilities. However, at the moment, the ocean floor idea appears to be the most likely explanation for these ULVZs.
There’s also the possibility that this ancient ocean crust is wrapping around the entire core, though because it’s so thin, it’s difficult to tell. Future seismic studies should be able to improve on the current picture.
One use of the discovery is in determining how heat from the hotter and denser core escapes up into the mantle. The composition differences between these two layers are higher than the differences between the solid surface rock and the air above it in the area we live on.
“Our research establishes critical links between shallow and deep Earth structure and the overall processes that drive our planet,” Hansen explains.
The research has been published in Science Advances.