There's a slow drama going on beneath your feet. 660 kilometers down is the boundary between the upper and lower mantle. Enormous slabs of colder, denser rock are slowly sinking through the upper mantle until they hit this boundary. These slabs are 30-100 kilometers thick and up to a thousand kilometers across!
Some punch straight through into the lower mantle and keep sinking. But many flatten out when they hit the boundary, sometimes lying there and piling up for tens of millions of years. You can see this in seismic images beneath Japan and the Marianas. Numerical models suggest that they pile up until they overwhelm the barrier and flush down in a comparatively sudden avalanche - lasting mere millions of years.
For why this boundary exists and more on what's going on down there, read my blog article. Geology rocks!
https://johncarlosbaez.wordpress.com/2026/08/22/the-mantle/
Some punch straight through into the lower mantle and keep sinking. But many flatten out when they hit the boundary, sometimes lying there and piling up for tens of millions of years. You can see this in seismic images beneath Japan and the Marianas. Numerical models suggest that they pile up until they overwhelm the barrier and flush down in a comparatively sudden avalanche - lasting mere millions of years.
For why this boundary exists and more on what's going on down there, read my blog article. Geology rocks!
https://johncarlosbaez.wordpress.com/2026/08/22/the-mantle/