P4+Asthenosphere

The asthenosphere is located right under the lithosphere and is part of the upper mantle. It also has fairly low density. The lower boundary is around 180-220km under the surface. Seismic waves travel through the asthenosphere relatively slowly when compared to the lithosphere. The asthenosphere was suspected to exist around 1926. This word came from the Greek language and means "weak sphere." It mainly lies at 100-200km below sea level. It may possibly extend to 700km below the surface.