The asthenosphere is a layer of the Earth’s mantle located directly beneath the rigid lithosphere. Its name comes from the Greek word asthenós, meaning “without strength” or “weak,” which describes its defining mechanical characteristic. This layer is defined by its physical state, exhibiting relative mechanical weakness and ductility. It is a region where the rock is hot enough and under the right pressure to behave like a very viscous fluid over geological time scales. This property allows for slow, gradual movement that influences the planet’s surface.
Placement Within Earth’s Structure
The Earth’s interior is organized into layers based on chemical composition and physical properties. The asthenosphere is the mechanically weak, upper part of the mantle, sitting beneath the rigid lithosphere, which forms the tectonic plates.
The upper boundary, the Lithosphere-Asthenosphere Boundary (LAB), typically begins 80 to 200 kilometers below the surface. This transition zone is defined by a change in mechanical behavior, occurring where the rock temperature reaches approximately 1,300 degrees Celsius. This heat causes the mantle rock, primarily peridotite, to become ductile.
The asthenosphere extends downward to about 700 kilometers, marking the base of the upper mantle. Beneath this lies the mesosphere, or lower mantle, which is more rigid due to increased pressure. Seismologists identify the asthenosphere as the “low-velocity zone” (LVZ) because seismic waves travel more slowly through its less rigid material.
The mechanical distinction is crucial: the lithosphere is rigid and brittle, while the asthenosphere is soft and capable of flow. This structural arrangement provides the necessary conditions for the processes that shape the Earth’s surface.
Understanding Plasticity and Partial Melting
The defining characteristic of the asthenosphere is its plasticity, allowing solid material to deform and flow slowly under stress. This behavior is described as viscoelasticity: it acts like an elastic solid over short periods, but flows like a viscous fluid over geological time scales. The rock is not a molten liquid, but a hot, semi-solid extremely close to its melting point.
The rock’s ability to flow is due to high temperature and specific pressure conditions. The temperature gradient causes the rock to approach its solidus, resulting in a small percentage of partial melting, typically between 0.1% and 5%.
This small fraction of liquid melt, along with volatiles like water and carbon dioxide, significantly reduces the rock’s strength and viscosity. The material is weak enough to allow mineral grains to slide past each other over time. This low-viscosity zone yields readily to persistent stresses, enabling the movement of the overlying lithospheric plates.
How the Asthenosphere Drives Plate Movement
The plasticity of the asthenosphere facilitates plate tectonics, the process responsible for the movement of continents, earthquakes, and volcanism. The flowing nature of this layer acts like a lubricant beneath the rigid lithospheric plates.
The driving force for plate movement is mantle convection, the slow circulation of material due to heat transfer from the Earth’s core. Hot material rises, cools, and then sinks in a continuous cycle. The asthenosphere channels this convective flow, and the movement of this viscous material drags and pushes the overlying tectonic plates.
The weak layer also enables isostasy, the vertical movement of the lithosphere to maintain gravitational equilibrium. When a large load, such as an ice sheet or mountain range, is placed on the lithosphere, the asthenosphere’s flow allows the crust to sink slowly. Conversely, when the load is removed, the lithosphere slowly rebounds, demonstrating how the asthenosphere maintains the Earth’s surface balance.