This means anything that happened more than 150 million years ago has very little influence today. In some areas of the globe, old tectonic plates sink toward the interior, making their way to the core-mantle boundary. This subduction often occurs at the boundary of two or more tectonic plates. When slabs of cold plate sink during subduction, they are reheated gradually by the mantle. The old plates that make their way toward the boundary of the mantle and the core are cooler than the mantle rock.
Seismic waves travel faster through the cooler material than they do through warmer material.
Consequently, seismic waves can be used to indicate where the cooler material resides inside Earth. Their model showed how portions of Earth's surface sink due to gravity. The most notable similarity is a large cooler mass sinking beneath the Eastern United States. The seismic data indicate that a large cooler mass exists where we would expect to find the present-day location of the Farallon plate. Once material reaches that boundary, it exerts little influence on mantle fluid-flow behavior.