Seismic mapping of North America has revealed that an ancient slab of crust buried beneath the Midwest is causing the crust ...

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

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.

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.

One of these slabs currently straddles the boundary between the mantle transition zone and the lower mantle roughly ... of Earth's continental crust and upper mantle that have mostly remained ...