The Earth’s internal structure is often described by its chemical composition, such as the crust, mantle, and core. A more functional classification, however, divides the planet’s outer layers based on their mechanical behavior and physical properties. This distinction separates the layers into the lithosphere and the asthenosphere, defining how materials respond to stress and heat at different depths. Understanding these two layers is fundamental to comprehending the geological processes that shape the Earth’s surface.
The Rigid Lithosphere
The lithosphere represents the planet’s outer shell, including the entire crust and the uppermost, rigid portion of the mantle. This layer is mechanically strong, behaving as a brittle solid under stress. Its thickness is variable, ranging from 50 kilometers beneath ocean basins to around 200 kilometers in stable continental interiors.
The lithosphere’s strength results from its relatively cool temperature compared to the deeper mantle. When subjected to pressure, the rocks do not flow; instead, they transmit stress and fracture. This brittle behavior causes earthquakes, which are largely confined to this strong, outer shell. The lithosphere is not a continuous shell but is fractured into approximately a dozen large segments known as tectonic plates.
The Plastic Asthenosphere
Lying directly beneath the lithosphere is the asthenosphere, a layer of the upper mantle extending down several hundred kilometers. The name is derived from the Greek word asthenos, meaning “weak,” which points to its defining characteristic. It is a mechanically weak layer that allows the overlying lithospheric plates to move.
The asthenosphere is composed of solid rock, primarily peridotite. It exists under immense heat and pressure that bring it close to its melting point, often cited around 1300°C. This thermal condition allows the rock to become ductile, or “plastic,” meaning it can deform and flow very slowly over geological timescales. This slow, creeping movement is a form of solid-state flow.
Comparing the Physical States
The difference between the lithosphere and the asthenosphere is defined purely by a change in physical behavior, not a change in chemical makeup. The boundary between the two layers is known as the Lithosphere-Asthenosphere Boundary (LAB). Above the LAB, the rock is cool and rigid enough to fracture when stressed.
Below the LAB, the rock is close enough to its melting temperature to be ductile and flow. The lithosphere transfers stress through mechanical fracture, while the asthenosphere transfers stress through viscous flow. This thermal boundary, where the rock transitions from brittle to ductile, exists entirely within the upper mantle.
How They Drive Plate Movement
The mechanical contrast between the two layers provides the mechanism for plate tectonics. The rigid lithospheric plates essentially “float” on the softer, deformable asthenosphere beneath them. The asthenosphere acts as a low-viscosity layer that facilitates the movement of the plates.
The force driving this motion is heat transfer within the asthenosphere, known as mantle convection. Hotter, less-dense material slowly rises, cools, and then sinks, creating massive convection currents. These currents drag and push the overlying lithospheric plates. This dynamic interaction causes continental drift, mountain building, and volcanic activity.