![]() ![]() (Subduction is when one piece of crust pushes below another and dives into the mantle, as happens along the coast of California today.) If the cooling-magma-ocean theory is true and there is magnesiowüstite deep in the mantle, it could be pushed, squished and nudged into an anisotropic configuration by pieces of continental crust that have been pushed deep into the mantle in the process of subduction. No one has really looked to see if ultralow velocity zones have them, but there's reason to think they might. What does this mean for the real mantle? Well, Jackson said, anisotropies have been observed down there, too. These differences in how waves travel depending on the direction and the crystalline structure are called anisotropies. The fastest direction of travel at core-mantle pressures is across the face of the crystal in the lab. ![]() The fastest direction of travel for the waves at atmospheric pressure - along the edge of the crystal structure - is the slowest direction of travel for waves at core-mantle pressures, she said. A transverse wave traveling through the mineral moves at a little less than 1.8 miles per second (3 km/s) in one direction and a little more than 3.1 miles per second (5 km/s) in another, Jackson said. There can be up to a 60 percent difference in the speed of a wave going through the crystal depending on how it passes through. Īt core-mantle boundary pressures, though, the direction the waves travel matters a lot. At atmospheric pressure, Jackson said, waves exiting a magnesiowüstite sample are always the same, no matter what direction they travel through the crystal. They found that high pressures change everything. ![]()
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