Asthenosphere

The mechanically weak and ductile region of Earth's upper mantle, lying below the lithosphere from a depth of about 80 to 200 kilometers and extending as deep as 700 kilometers, its lower boundary not well defined. It is almost solid, with a slight amount of melting, less than a tenth of a percent of the rock, contributing to its weakness; where it wells upward, more extensive decompression melting makes it the most important source of magma on Earth, feeding the mid-ocean ridges. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/