The asthenosphere represents a crucial layer within Earth’s mantle, positioned directly beneath the more rigid lithosphere. It extends from approximately 80 to 200 kilometers (50 to 120 miles) below the surface and can reach depths of up to 700 kilometers (430 miles).
Primary Material Composition
The asthenosphere is primarily composed of solid rock, specifically a type known as peridotite. This rock is rich in minerals such as olivine and pyroxene. Despite its solid state, the material in the asthenosphere is capable of flowing over geological timescales. This behavior occurs due to the high temperatures and immense pressures present at these depths. The composition is largely similar to the rest of the upper mantle.
Physical State and Behavior
The asthenosphere exhibits unique physical properties. It behaves as a ductile and plastic material, meaning it can deform and flow slowly without fracturing. This enables slow convection currents within the layer. A small percentage of partial melting, typically ranging from 0.1% to 5%, contributes to its reduced viscosity and ability to flow. It is important to note that the asthenosphere is not a liquid layer, but rather a “soft” solid.
Distinction from the Lithosphere
The boundary between the asthenosphere and the overlying lithosphere is primarily defined by their mechanical behavior, not a significant difference in chemical composition. The lithosphere, which includes Earth’s crust and the uppermost part of the mantle, is rigid and brittle. In contrast, the asthenosphere is ductile and flows. Both layers are largely composed of similar peridotite rock, but variations in temperature and pressure at different depths dictate their mechanical properties. The lithosphere-asthenosphere boundary (LAB) is often considered a rheological transition, where the rock shifts from behaving rigidly to behaving more plastically.
Role in Plate Tectonics
The asthenosphere’s unique composition and physical state are central to its role in plate tectonics. Its ability to flow allows the rigid tectonic plates of the lithosphere to move across Earth’s surface. Convection currents within the asthenosphere are a primary driving force for plate movement. Heat from Earth’s interior causes material in the asthenosphere to rise, spread, cool, and then sink, creating a slow, continuous circulation. This convective motion provides the force for the lithospheric plates to slide and interact, leading to geological phenomena like earthquakes, volcanoes, and mountain building.