concept

Seismic tomography

The technique that builds a three-dimensional image of a planet's interior by combining arrival times from tens of thousands of crisscrossing seismic rays. Where waves arrive earlier than expected the rock is colder and denser; where they arrive late it is hotter. One caveat matters: the method does not image chemistry directly, only velocity anomalies, and converting those into temperature or composition is always a separate interpretive step. Even so, it is what showed us subducted slabs traceable all the way into the lower mantle.