Asthenosphere is zone of Earth’s mantle (upper part of the mantle)lying beneath the lithosphere and believed to be much hotter and more fluid than the lithosphere. The asthenosphere extends from about 100 km to about 700 km below Earth’s surface.
It was first named in 1914 by the British geologist J. Barrell, who divided Earth’s overall structure into three major sections: the lithosphere , or outer layer of rock-like material; the asthenosphere; and the centrosphere, or central part of the planet. The asthenosphere gets its name from the Greek word for weak, asthenis, because of the relatively fragile nature of the materials of which it is made. It lies in the upper portion of Earth’s structure traditionally known as the mantle.
Heat from deep within Earth is believed to keep the asthenosphere malleable, lubricating the undersides of Earth’s tectonic plates and allowing them to move. Convection currents generated within the asthenosphere push magma upward through volcanic vents and spreading centres to create new crust. Convection currents also stress the lithosphere above, and the cracking that often results manifests as earthquakes. According to the theory of plate tectonics, the asthenosphere is the repository for older and denser parts of the lithosphere that are dragged downward in subduction zones.
Asthenosphere boundary is usually referred to as LAB. The asthenosphere is almost solid, although some of its regions could be molten (e.g., below mid-ocean ridges). The lower boundary of the asthenosphere is not well defined. The thickness of the asthenosphere depends mainly on the temperature. In some regions the asthenosphere could extend as deep as 700 km . It is considered the source region of mid-ocean ridge basalt (MORB).