Scientists probing the Earth’s interior have found a large reservoir of water equal to the volume of the Arctic Ocean beneath eastern Asia. The left figure is a slice through the Earth, taken from the figure on the right, showing the attenuation anomalies within the mantle at a depth of roughly 620 miles. In both images, red shows unusually soft and weak rock believed to be saturated with water, and the blue shows unusually stiff rock (yellow and white show near-average values)
Looking down deep
Although they appear solid, the composition of some ocean floor rocks is up to 15 percent water. “The water molecules are actually stuck in the mineral structure of the rock,” Wysession explained. “As you heat this up, it eventually dehydrates. It’s like taking clay and firing it to get all the water out.”
The Beijing anomaly
Wysession has dubbed the new underground feature the “Beijing anomaly,” because seismic wave attenuation was found to be highest beneath the Chinese capital city. Wysession first used the moniker during a presentation of his work at the University of Beijing.
“They thought it was very, very interesting,” Wysession said. “China is under greater seismic risk than just about any country in the world, so they are very interested in seismology.”
Water covers 70 percent of Earth’s surface and one of its many functions is to act like a lubricant for the movement of continental plates.
“Look at our sister planet, Venus,” Wysession said. “It is very hot and dry inside Venus, and Venus has no plate tectonics. All the water probably boiled off, and without water, there are no plates. The system is locked up, like a rusty Tin Man with no oil.”
The crust thickness averages about 18 miles (30 kilometers) under the continents, but is only about 3 miles (5 kilometers) under the oceans. It is light and brittle and can break. In fact it’s fractured into more than a dozen major plates and several minor ones. It is where most earthquakes originate.
The mantle is more flexible – it flows instead of fractures. It extends down to about 1,800 miles (2,900 kilometers) below the surface.
The core consists of a solid inner core and a fluid outer core. The fluid contains iron, which, as it moves, generates the Earth’s magnetic field. The crust and upper mantle form the lithosphere, which is broken up into several plates that float on top of the hot molten mantle below.