Most schoolchildren learn that the Earth has three (or four) layers: a crust, mantle and core, which is sometimes subdivided into an inner and outer core.
Most schoolchildren learn that the Earth has three (or four) layers: a crust, mantle and core, which is sometimes subdivided into an inner and outer core. That’s not wrong, but it does leave out several other layers that scientists have identified within the Earth, including the transition zone within the mantle.
In a study published this week in Science, Princeton geophysicists Jessica Irving and Wenbo Wu, in collaboration with Sidao Ni from the Institute of Geodesy and Geophysics in China, used data from an enormous earthquake in Bolivia to find mountains and other topography on the base of the transition zone, a layer 660 kilometers (410 miles) straight down that separates the upper and lower mantle. (Lacking a formal name for this layer, the researchers simply call it “the 660-km boundary.”)
To peer deep into the Earth, scientists use the most powerful waves on the planet, which are generated by massive earthquakes. “You want a big, deep earthquake to get the whole planet to shake,” said Irving, an assistant professor of geosciences.
Big earthquakes are vastly more powerful than small ones — energy increases 30-fold with every step up the Richter scale — and deep earthquakes, “instead of frittering away their energy in the crust, can get the whole mantle going,” Irving said. She gets her best data from earthquakes that are magnitude 7.0 or higher, she said, as the shockwaves they send out in all directions can travel through the core to the other side of the planet — and back again. For this study, the key data came from waves picked up after a magnitude 8.2 earthquake — the second-largest deep earthquake ever recorded — that shook Bolivia in 1994.
Read more at Princeton University
Image: Princeton seismologist Jessica Irving, an assistant professor of geosciences, sits with two meteorites from Princeton University's collection that contain iron thought to be from the interiors of planetesimals. Irving uses seismology to investigate the interior of our own planet, recently finding mountain-sized topographic roughness on the 660-km boundary at the base of the mantle's transition zone. (Credit: Denise Applewhite, Princeton University)
