Asthenosphere

Lore & Background

The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. It is composed of peridotite, a rock containing mostly the minerals olivine and pyroxene. The lithosphere-asthenosphere boundary is conventionally taken at the 1,300 °C isotherm. Closer to the surface at lower temperatures, the mantle behaves rigidly; deeper below the surface at higher temperatures, the mantle moves in a ductile fashion. The asthenosphere is where the mantle rock most closely approaches its melting point, and a small amount of melt is likely present in this layer.

Seismic waves pass relatively slowly through the asthenosphere compared to the overlying lithospheric mantle, giving it the name low-velocity zone (LVZ), although the two are not strictly the same; the lower boundary of the LVZ lies at a depth of 180 to 220 km, whereas the base of the asthenosphere lies at about 700 km. The LVZ also has high seismic attenuation and significant anisotropy. The discovery of the LVZ alerted seismologists to the existence of the asthenosphere. The decrease in seismic wave velocity from the lithosphere to the asthenosphere could be caused by the presence of a very small percentage of melt, though since the asthenosphere transmits S waves, it cannot be fully melted.

The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about. Due to the temperature and pressure conditions, rock becomes ductile, moving at rates of deformation measured in cm/yr over lineal distances eventually measuring thousands of kilometers. In this way, it flows like a convection current, radiating heat outward from the Earth's interior. The rigid lithosphere is thought to 'float' or move about on the slowly flowing asthenosphere, enabling isostatic equilibrium and allowing the movement of tectonic plates.

Reader's Guide

The asthenosphere is significant as the mechanically weak layer that underlies the lithosphere and facilitates plate tectonic movement and isostatic adjustments. Its ductile behavior, resulting from partial melting and other mechanisms such as grain boundary sliding and strain-rate weakening, allows the rigid lithospheric plates to move atop it. The asthenosphere is the primary source of magma on Earth, generated by decompression melting as it wells upward, producing mid-ocean ridge basalt and magmas in subduction zones and continental rifts. Its discovery was prompted by the identification of the low-velocity zone in seismic studies, which revealed its physical properties. The lithosphere-asthenosphere boundary is relatively sharp and coincides with the onset of partial melting or changes in composition or anisotropy. The lower boundary of the asthenosphere is less well-defined but approximately coincides with the 670 km discontinuity, linked to a transition in mantle mineralogy. Numerical models suggest that strain-rate weakening contributes to the particularly weak asthenosphere below the Pacific plate. Overall, the asthenosphere is central to understanding Earth's internal dynamics, heat transfer, and magma generation.

Did You Know?

• The asthenosphere is almost solid, with less than 0.1% of the rock melted, yet this slight melting contributes to its mechanical weakness.
• Seismic waves pass relatively slowly through the asthenosphere, which is why it is called the low-velocity zone (LVZ).
• The lithosphere-asthenosphere boundary is conventionally taken at the 1,300 °C isotherm.
• Decompression melting of asthenospheric rock is the most important source of magma on Earth, producing mid-ocean ridge basalt.