Some of the highest quality images ever taken of the Earth's lower crust reveal that the upper and lower crust form in two distinctly different ways.
"This new way of studying the ocean crust is the equivalent of a new telescope in astronomy," said Bruce Malfait, head of the National Science Foundation (NSF)'s marine geosciences section, which funded the research. "It allows us to look at Earth processes and composition at a remarkable new level of detail."
To form the images, the researchers, led by a team from Columbia University's Lamont-Doherty Earth Observatory (LDEO), analyzed sound waves bounced off structures deep in the Earth, a process similar to creating an ultrasound image. They published the results of their work in the Aug. 25 issue of the journal Nature.
The Earth's oceanic crust is formed from molten rock, or magma, located beneath mid-ocean ridges. Magma chambers located in the mid-crust are known to be responsible for generating the upper-oceanic crust. But it has so far been unclear if the lower crust is formed from the same magma source, or if it arises primarily from "magma lenses"--smaller bodies located at or near the base of the crust.
The resulting images are the first of their kind to show solidified lenses and sills--narrow lateral intrusions of magma--embedded within the crust-mantle boundary known as the Moho transition zone. The presence of such lenses and sills deep near a mid-ocean ridge suggests the lower crust is at least partially formed from several smaller sources of magma rather than from a single large source located in the middle of the crust.
"This demonstrates that the process of crustal formation is more complex than believed," said LDEO scientist and lead author of the study, Mladen Nedimovic. "It also favors the emerging view that volcanoes have a complicated plumbing system consisting of many interconnected sills and magma conduits. We still have a lot to learn about what goes on beneath the surface of the Earth."
Researchers from LDEO as well as the Scripps Institution of Oceanography and the Woods Hole Oceanographic Institution collected seismic data from the oceanic crust beneath the sea floor off the coast of Wash., Ore., and northern Calif.
The study was also funded by a grant from the Doherty Foundation.
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