The asthenosphere is a layer of the Earth's mantle located beneath the lithosphere, which includes the crust and the uppermost part of the mantle. It extends from about 100 kilometers to 700 kilometers below the Earth's surface. This layer is semi-fluid and allows for the movement of tectonic plates, which float on its surface. The asthenosphere is composed of partially molten rock, making it less rigid than the lithosphere. This property enables the tectonic plates, such as the North American Plate and the Eurasian Plate, to shift and interact, leading to geological phenomena like earthquakes and volcanic activity.