AsthenosphereEdit
The asthenosphere is a zone within the Earth's upper mantle that lies beneath the rigid lithosphere. It is defined less by a sharp boundary than by its mechanical behavior: rocks here deform in a ductile, or plastic, fashion under high temperature and pressure, allowing the overlying tectonic plates to glide over it. This layer is part of the Earth's mantle and is closely linked to the operation of plate tectonics in shaping the planet's surface. In many regions, the asthenosphere extends from roughly 100 kilometers (about 60 miles) below the surface to depths approaching 700 kilometers (around 440 miles), with its base transitioning into the more sluggish, more rigid portion of the mantle. The rocks are broadly consistent with peridotite in composition, but their physical state is governed as much by temperature, pressure, and water content as by mineralogy.
Because the asthenosphere is relatively weak, it functions as a lubricating layer that permits the lithospheric plates to move, bend, and interact. The concept rests on rheology—the study of how materials deform—where the asthenosphere behaves in a viscous, time-dependent way, contrasting with the brittle behavior typical of the lithosphere at shallow depths. The presence of this weak layer helps explain how large-scale motion of continents and oceanic plates occurs without requiring every fault to fracture. The term is tied to geophysical observations such as low seismic shear velocities and increased ductility, which scientists infer from what is seen in the seismology record and from models of mantle convection within the Earth’s interior. The asthenosphere is not a uniform slab but a broad, dynamic region whose properties can vary with tectonic setting and depth.
Characteristics
- Depth and thickness: The asthenosphere spans a broad depth range, typically from about 100 km down to the base of the mantle, with regional variation. Its thickness and the exact depth to its base are influenced by factors such as tectonic setting (oceanic vs continental) and thermal structure.
- Temperature and rheology: Elevated temperatures bring mantle rocks closer to their solidus, enabling ductile flow over geological timescales. Water-bearing minerals and grain-size reduction can further decrease viscosity, contributing to the layer's weak character.
- Composition: While chemically similar to the mantle rocks that make up the lithosphere—primarily peridotite—the asthenosphere's behavior is governed by temperature, pressure, and water content more than by composition alone.
- Seismic properties: The asthenosphere exhibits reduced shear-wave speeds relative to the surrounding mantle, creating what is known as a low-velocity zone in seismological images. Seismic anisotropy in this region records distortions and preferred flow directions, providing clues about mantle convection patterns.
- Role in plate motion: By decoupling the lithospheric plates from the deeper mantle, the asthenosphere facilitates the horizontal movement of plates and the occurrence of subduction, rifting, and continental drift. See the plate tectonics framework for how these processes fit together.
Formation and evolution
The asthenosphere emerges from the thermal and pressure conditions of the upper mantle. Its existence reflects the balance between cooling and heating within the mantle and the presence of minerals capable of deforming plastically under application of long-term stress. The boundary between the lithosphere and asthenosphere is commonly described as a rheological boundary rather than a fixed physical one, with the lithosphere becoming progressively weaker and more ductile into the underlying asthenosphere. The exact depth range and degree of partial melt, if any, are subjects of ongoing research, with regional differences tied to tectonic history and current geodynamic regime.
Evidence and interpretation
Geophysical methods, especially seismology, provide the strongest constraints on the nature of the asthenosphere. Tomographic imaging reveals regions of reduced seismic shear velocities that correspond to the LVZ (low-velocity zone) associated with the asthenosphere. Geodesic measurements and studies of surface deformation help quantify how easily the lithosphere can slide over the mantle. In addition, studies of mantle minerals and experimental petrology inform how peridotite responds to high temperature and pressure, including the potential roles of partial melting and water in lowering viscosity. The interpretation of these data continues to evolve, with some models emphasizing thermal effects, others highlighting modest amounts of melt, and still others exploring how grain size, texture, and water content influence rheology.
Controversies and debates
Degree of partial melt: A long-standing topic is whether the asthenosphere contains appreciable partial melt or whether its apparent weakness can be explained by temperature and grain-size effects alone. Proponents of a melt-rich interpretation point to certain geophysical signals and petrological constraints, while others argue that solid-state mechanisms, such as grain-boundary sliding and temperature-dependent viscosity, can account for much of the observed behavior without widespread melting. Both positions cite seismological and lab-based evidence, and the debate centers on the relative contribution of melt versus purely thermomechanical factors.
Sharp vs. gradual boundary: Some researchers favor a relatively sharp rheological boundary between the lithosphere and asthenosphere, while others describe a gradual transition in properties with depth. The real Earth may exhibit regional variations in how quickly mechanical properties change, reflecting differences in temperature gradients, water content, and tectonic history.
Regional variability: The depth and strength of the asthenosphere appear to vary between oceanic and continental regions, and between subduction zones and areas of passive margin. This regional heterogeneity has implications for how mantle convection operates on a global scale and for how plates interact at different edges of the world.
Whole-mantle versus layered convection: There is a broader debate about whether mantle convection is best described as a layered, "stagnant" model with a distinct boundary between upper and lower mantle, or as a whole-mantle system with more integrated dynamics. The behavior of the asthenosphere is central to these models, because its properties influence whether surface plates couple more or less strongly to deeper mantle flow.