An unexpected find in tiny crystals hints that the Earth's mantle may not quite behave as has been believed for nearly a century.

Geography textbooks describe the Earth's mantle beneath its plates as a well-mixed viscous rock that moves along with those plates like a conveyor belt. But that idea, first set out some 100 years ago,

Traditionally, textbooks have described the mantle as a viscous, well-mixed layer that shifts along with tectonic plates, much like a conveyor belt. This theory has been a cornerstone of geology for nearly a century but has been notoriously difficult to prove.