The asthenosphere is a layer of the Earth's interior that includes part of the lower crust and part of the mantle. The upper boundary of the asthenosphere is roughly 50 miles below the ground's surface. Its lower boundary, however, is inconsistent and depends primarily on temperature. Experts at the United States Geological Survey estimate its average thickness as 112 miles.
Asthenosphere comes from the Greek word "asthenos," which means "weak." This is a reference to the asthenosphere's fluidity and mechanical weakness. According to geologists at San Diego State University, the upper portion of the asthenosphere is semi-solid, but in many places its lower regions are molten. The asthenosphere is composed of iron-magnesium silicates and has an overall consistency akin to hot tar. Molten rock in the Earth's deeper layers is much hotter and behaves as a true liquid.
The region directly above the asthenosphere is the lithosphere, the entirely solid uppermost region of the Earth's crust. Earthquakes that occur in the lithosphere are potentially catastrophic because of the severe jarring forces created when two tectonic plates interact. Because the asthenosphere is more malleable, the deep-zone earthquakes that occur in it often go undetected. The United States Geological Survey reveals earthquakes also occasionally occur even deeper in the layers below the asthenosphere.