New research has found that variations in rock composition within oceanic plates, caused by ancient tectonic processes, can significantly affect the path and speed of these plates as they sink into Earth’s mantle.

At depths between 410 and 660 kilometers lies the mantle transition zone (MTZ), a key boundary layer that regulates the movement of material into the planet’s deeper interior. When subducting plates, those that dive beneath others, encounter large concentrations of basalt within the MTZ, their descent can slow down or even stall, rather than continuing smoothly into the lower mantle. While basalt-rich regions in the MTZ have been observed before, their origins have remained uncertain until now.