Minerals in Rock Mass Hold Clues to 400-Kilometer Ascent

摘要

Rocks, as a rule, know their place. In the planet's layered structure, the lightest rocks float on top as a crust; below are denser mantle rocks, and at the very bottom is the weighty iron core. Here and there a slab of ocean crust cools and becomes dense enough to sink into the mantle, but otherwise rocks don't seem to have great vertical mobility. That's why a group of mineralogists was so shocked recently to find evidence that a chunk of rock in the Swiss Alps had once been 400 kilometers or moredown in the mantle.