The asthenosphere – which is derived from the Greek asthenes, meaning weak – is the uppermost part of the Earth’s mantle, right below the tectonic plates that make up the solid lithosphere. Traditionally, the asthenosphere has been viewed as a passive region that separates the moving tectonic plates from the mantle.
Studies suggest that the asthenosphere may play a more active role as the source of heat and magma responsible for volcanoes occuring within the interior of a tectonic plate. In addition, the asthenosphere may have a major impact on plate tectonics and the pattern of mantle flow.
Convection simulations show that this region is hotter than expected and this has renewed interest in the asthenospheres role in Earth’s engine. Evidence also suggests that the asthenosphere is hotter than the mantle below it. This increase in temperature can reduce the mechanical strength and eliminate the need to bring heat directly from the Earth’s core to the asthenosphere to explain volcanic activity at hot spots. The hottest regions in the asthenosphere could be as much as 200 degrees Celsius warmer than beneath mid-ocean ridges and even hotter than the core-mantle boundary. This is important because the asthenosphere is at or near its melting point.
Anderson, D., King, S. (2014). Driving the Earth machine? Science, 346(6214), 1184-5