The asthenosphere (Greek: ἀσθενός without force) is a layer in the interior of the Earth between 80 and 300 km depth, which is characterized by acting as a "weak" layer under the more rigid lithosphere. The asthenosphere is part of the Earth's mantle, and is located above the mesosphere.
The lower and upper limits of the asthenosphere are gradual transitions caused by the deformation behavior of rocks and therefore cannot be determined absolutely.
Behavior and composition of the asthenosphere
The asthenosphere is believed to be largely composed of rock with the composition of lherzolite. Under the pressure and temperature conditions, this rock in the asthenosphere plastically deforms. This is in contrast to the lithosphere, where brittle and elastic deformation can also take place.
Seismic waves travel at low speed through the asthenosphere, due to the "weakness" of the layer. This is why it is also called "low speed zone".
In the asthenosphere, heat flow occurs by mantle convection. The rising material only loses its heat slowly, making the rising of the warm material in the asthenosphere an adiabatic process. When hot material rises rapidly from the interior of the Earth, partial melting can occur due to the drop in pressure.
Discovery of the asthenosphere
The asthenosphere has been suspected since 1926, but the analyzes of the Great Chilean Earthquake of May 22, 1960 confirmed for the first time that the asthenosphere exists.
Tomographic research shows that the asthenosphere is less warm and more rigid under the continents. It may even be that the weak zone is only present under oceans.
At mid-ocean ridges, the asthenosphere extends to a few kilometers from the surface. The lithosphere here is extra thin due to the upward movement of the asthenosphere.