Summary
As a rule, rocks don't have much vertical mobility in the planet. But a report in this issue (p. 1841) breaks that rule by presenting evidence that a chunk of rock in the Swiss Alps once rested 400 kilometers or more down in the mantle. The shape and composition of tiny mineral grains in the half-kilometer rock mass imply that they were sweated out of the surrounding material as the rock rose from deep in the Earth. The researchers speculate that the ascent took place when the deep rock was caught in a region of light crustal rock that had been forced deep into the planet during a continental collision, then bobbed back to the surface.
