How do we know what the asthenosphere is made of?
Scientists make various assumptions about the composition of Earth’s layers through careful analysis of seismic data, a method called subsurface detection. At the base of the lithosphere, a depth of about 40 miles (65 kilometers), there is another distinct seismic transition called the Gutenberg low velocity zone. At this level, all seismic waves appear to be absorbed more strongly than elsewhere within Earth. Scientists interpret this to mean that the layer below the lithosphere is a zone of partially melted material. This “soft” zone is called the asthenosphere, from the Greek word asthenes, meaning “weak.” It extends to a depth of about 155 miles (250 kilometers). Underlying the crust is the mantle, which is composed mainly of minerals containing magnesium, iron, silicon, and oxygen. The uppermost section of the mantle is a rigid layer.
