What is the Asthenosphere?

The asthenosphere is a layer in the Earth below the lithosphere and above the lower mantle. It is a rheological layer, defined by its physical properties, not a compositional layer, defined by chemistry. The layer is continuous, but it not found at a constant depth (fig 1). The asthenosphere is the layer from 70-100km to a depth of around 700km. At Mid-Ocean Ridges the asthenosphere comes near to the surface and is the source of the magma. Under the oldest continents the asthenosphere is at its thinnest.

	Figure 1: Comparison of the chemical (left) and rheological (right) layering of the Earth. Redrawn from Kearey and Vine (1996).

The asthenosphere is found using seismic waves. As the asthenosphere is partially molten (temperatures range from 55% of the melting temperature to 85% of the melting temperature) it is a low velocity zone. The temperature also means that the asthenosphere is weak compared to the lithosphere and lower mantle. The partial melt status of the asthenosphere is due to its location on the melt curve on a temperature-depth curve (fig 2). This figure indicates that the asthenosphere is not completely molten or solid, but is probably a solid with small patches of melt.

	Figure 1: The intersection of the asthenosphere temperature with the mantle solidus at Mid Ocean Ridge indicates melting and the closeness of the asthenosphere curve indicate partial melting occurring. Redrawn from Kearey and Vine (1996).

The asthenosphere plays a critical role in plate tectonics, acting as a lubricating layer.

Come and disuss in the forums.

References

P. Kearey and M. Brooks, 1991. An Introduction to Geophysical Exploration.

P. Kearey and F.J Vine, 1996. Global Tectonics.

W. Lowrie, 1997. Fundamentals of Geophysics.