Asthenosphere

The asthenosphere is the layer of Earth situated at an average depth of about 62 mi (about 100 km) beneath Earth's surface. It was first named in 1914 by the British geologist Joseph 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 internal structure traditionally known as the mantle. Scientists have not seen the asthenosphere of Earth, but its existence has a profound effect upon the planet and the manner in which the Earth's crust behaves. For anyone living near a plate boundary on Earth, the asthenosphere contributes mightily to the uneasy geologic conditions which may plague the area.