A schematic of Earth’s hot inner layers. New research finds that the uppermost layer of the mantle is partially melted
Most of Earth’s mantle is hot but solid, with rocks that deform slowly rather than cracking like the cooler rocks of the crust do. But new research finds that around 93 miles below Earth’s surface, there’s a worldwide layer of melted rock. The discovery of this gooey layer will help researchers better understand how the tectonic plates ‘float’ on top of this mantle layer. The melted rock is in the asthenosphere, the upper layer of the mantle that sits between about 50 and 124 miles below Earth’s surface. The only way to peer into this layer of the mantle is with seismic waves from earthquakes. Researchers can detect the waves at seismic stations set up around the world, looking for subtle changes in the waveforms that indicate what kinds of materials the waves travelled through. Previously, researchers knew from these types of studies that some parts of the asthenosphere were hotter than others, and patchy areas of melt had been detected. But little was known about how deep and widespread the melt was.