Everyone is familiar with the concept of finding sunken treasure on the ocean floor. Now, scientists may have found an ancient ocean floor that is itself a type of geological buried treasure. Using seismic readings acquired in Antarctica over three years, a research team concluded that millions of years ago, the ocean floor journeyed deep towards the heart of our planet, where it landed as a relatively thin layer surrounding the Earth's core. The researchers say the finding could influence our understanding of how heat escapes from the core, or how oceanic material might return to the surface through volcanic eruptions.
When geoscientist Samantha Hansen and her team from the University of Alabama (UA) set up 15 seismographs in Antarctica in 2012, they were interested in using waves from earthquakes around the planet to image mountains that largely lay buried beneath the ice. While that research, which lasted three years, was successful and led to several papers, the data showed unusual energies showing up after the waves from earthquakes passed through the core-mantle boundary (CMB). This led Hansen and colleagues to conduct further investigations.
The CMB is found about 2,000 miles (3,219 km) below the surface of the Earth. There, the research team found that seismic waves slowed down when they hit a certain layer that, while measuring about three to 25 miles (40 km) thick, is extremely thin in terms of planetary composition. The slowing of the waves through this area characterized it as an ultra-low velocity zone (ULVZ).
"Analyzing thousands of seismic recordings from Antarctica, our high-definition imaging method found thin anomalous zones of material at the CMB everywhere we probed," said Edward Garnero, a professor in the School of Earth and Space Exploration at Arizona State University, who participated in the study. "The material’s thickness varies from a few kilometers to 10’s of kilometers. This suggests we are seeing mountains on the core, in some places up to five times taller than Mt. Everest."
The way in which the seismic waves slowed down when they hit the ULVZ led the researchers to conclude that it was composed of material from an ancient ocean floor that was pushed through the Earths' mantle over hundreds of millions of years. That's because such material would be even denser than the liquid rock comprising the mantle, and would slow seismic waves to the degree revealed in the data.
While measurements of the ULVZ have been taken before in patchy areas, the data from this study suggests that it might well surround the entire core of the Earth.
"Seismic investigations, such as ours, provide the highest resolution imaging of the interior structure of our planet, and we are finding that this structure is vastly more complicated than once thought," said Hansen. "Our research provides important connections between shallow and deep Earth structure and the overall processes driving our planet."
The research has been published in the journal, Science Advances.
Source: University of Alabama
