The Asthenosphere (a:without; stheno:strength; sphere:layer) What is the asthenosphere? The asthenosphere is the layer of the Earth just below the lithosphere that has a texture comparable to warm tar. The tectonic plates of the lithosphere float above the asthenosphere. It is composed of molten rock, or magma, that is weak and can easily be deformed. Where is the asthenosphere located? The asthenosphere is located directly below the tectonic plates of the lithosphere. The asthenosphere takes up about 6% of the mantle, but it is still capable of holding up the tectonic plates. The asthenosphere starts about 100 km into the Earth, and is about 400 – 700 km thick. What happens in the asthenosphere? The temperature of 1,400 degrees to 3,000 degrees Celsius and pressure conditions cause the asthenosphere to have a texture similar to weak plastic. The currents in the asthenosphere are like convection currents; they flow outward from the Earth’s interior. Due to heat and pressure, igneous rocks can form inside the asthenosphere. How do other parts of the Earth interact with the asthenosphere? In the asthenosphere, the rocks flow because of the stresses of the churning motion from the Earth’s deep interior. The seismic waves coming from the Earth’s deep interior slow down as they pass through the asthenosphere. This is how we know that the asthenosphere has a ductile, or not brittle, manner. The Discovery of the Asthenosphere The asthenosphere was discovered by geologists in the 1960s. It was discovered because seismic waves slow down as they pass though the asthenosphere.