The Earth’s interior is structured by layers defined by their physical properties, which are governed by temperature and pressure. The mantle, located between the crust and the core, contains two distinct mechanical zones: the Asthenosphere and the Mesosphere. Although chemically similar, these regions exhibit vastly different physical behaviors, such as strength and flow characteristics. Understanding these differences is central to comprehending processes like plate tectonics and mantle convection.
Locational Boundaries and Depth
The Asthenosphere is situated immediately beneath the rigid Lithosphere, which comprises the Earth’s crust and the uppermost part of the mantle. This mechanically weak layer generally begins at a depth of approximately 80 to 100 kilometers below the surface. It extends downward through the upper mantle, with its lower boundary often considered to be around 410 kilometers, though some models place its base closer to 350 kilometers.
The Mesosphere, often referred to as the lower mantle, is significantly larger and deeper than the Asthenosphere. It begins below the mantle’s transition zone, a region of rapidly changing mineral structure that ends at the 660-kilometer discontinuity. This boundary marks a major transition in the Earth’s interior structure, separating the upper mantle from the lower mantle. The Mesosphere then continues to the core-mantle boundary, ending at a depth of about 2,900 kilometers.
Physical State and Material Behavior
The most significant difference between the two layers lies in their rheology. The Asthenosphere, whose name is derived from the Greek word for “without strength,” is mechanically weak and ductile. Although it is predominantly solid, the material is close to its melting point, which gives it a soft, plastic-like consistency. This characteristic allows it to flow slowly over geological timescales, enabling the movement of the rigid tectonic plates that sit above it.
This layer is often described as a low-velocity zone because seismic waves slow down as they pass through it, a phenomenon linked to its reduced shear strength. The Asthenosphere’s low viscosity is a direct result of its temperature being near the rock’s solidus. This partial thermal softening allows for the convection currents that drive plate motion.
The Mesosphere, in sharp contrast, is far more rigid and immobile, despite being subjected to higher temperatures. The immense pressure at this depth dramatically increases its strength and shear resistance. Its material behaves like a stiff solid, exhibiting significantly less plasticity than the Asthenosphere. It is so strong that earthquakes do not originate within it, as the rock is prevented from fracturing.
Movement does occur in the Mesosphere, but it is much slower and more constrained than the flow in the Asthenosphere. This region acts as a high-strength, high-viscosity solid that transmits seismic waves more quickly than the layer above it. Its greater rigidity means it makes up the vast majority of the mantle’s volume.
Driving Forces: Temperature, Pressure, and Density
The distinct physical properties of the Asthenosphere and Mesosphere are controlled by the competing effects of temperature and pressure gradients with depth. In the Asthenosphere, the temperature closely approaches the melting point of the rock, weakening the material. This proximity to the solidus is the primary reason for its low strength and ductile behavior. The presence of trace amounts of melt further contributes to this mechanical weakness.
In the Mesosphere, although temperatures are much higher than in the Asthenosphere, the rate of pressure increase is greater than the rate of temperature increase. This extreme lithostatic pressure effectively raises the melting point of the rock, suppressing any tendency toward melting and forcing the material into a highly compressed, solid state. The resulting material is considerably denser and more resistant to deformation.
The boundary at 660 kilometers is also associated with mineral phase transitions, which contribute to the Mesosphere’s rigidity. At this depth, the silicate minerals change their crystal structure to denser forms, such as the transition from ringwoodite to bridgmanite and periclase. These denser mineral phases are less compressible and structurally more rigid, contributing to the Mesosphere’s overall stiffness and marking the transition from the upper to the lower mantle.