It's what's inside that counts, so the saying goes, and the earth is no exception. Solid rock in its mantle, hot enough to flow like warm taffy, sculpts the planet from the inside out by pushing tectonic plates across the surface. Crashing plates crumple into mountain ranges or plunge into the sticky rock below, only to rise again millions of years later as bits of the lava that billows from mid-ocean ridges. Without this rocky recycling program, the earth would be as sterile and pockmarked as the moon. After three decades of heated debate, an emerging hypothesis may quiet the conflict.
Tectonic slabs sink through the top layer but never penetrate a hypothesized denser layer below.
What is more, a layer of radioactive elements could explain why the planet makes more heat than it otherwise should. But with better seismic data to focus the picture, seismologists began to see the mantle as one giant boiling pot of soup. They saw hints of tectonic slabs diving deep below that boundary. About two years ago van der Hilst noticed that seismic patterns tend to break up below about 1,700 kilometers. But van der Hilst suspected that as these blobs heated up they would seep into the surrounding rock and disappear.