"Our new results suggest that for most of Earth's history, convection in the mantle was stratified into two distinct layers, namely upper and lower mantle regions that were isolated from each ...

Cross-section of the Earth's interior: crust, upper- and lower-mantle, and outer- and inner-cores. Journalists may use this image only with appropriate credit.

Moreover, they suggest that the environment in which they formed – a divide between the upper and lower mantle called the 660 ...

The potential compositional boundary was typically put at 660 km depth, corresponding to the major seismic discontinuity that marks the boundary between the upper mantle and lower mantle.