Frequently Asked Questions About the Asthenosphere
The asthenosphere remains one of the least understood yet most important layers of our planet. Because it lies dozens to hundreds of kilometers beneath our feet, it generates many questions from students, educators, and curious minds. This FAQ addresses the most common questions about this dynamic layer, providing detailed answers based on current scientific understanding.
From its role in plate tectonics to its physical properties, the asthenosphere influences nearly every major geological process on Earth. Whether you're researching for an academic project or simply want to understand what lies beneath the surface, these answers will help clarify the complex nature of Earth's upper mantle. For more comprehensive information about Earth's internal structure, visit our main page.
What is the asthenosphere?
The asthenosphere is a semi-molten layer of the Earth's upper mantle located beneath the lithosphere, approximately 80-200 kilometers below the surface. This weak, plastic-like layer allows tectonic plates to move and slide over it. The name comes from Greek 'asthenos' meaning 'without strength,' referring to its relatively low resistance to deformation compared to the rigid lithosphere above. Despite being mostly solid, the asthenosphere contains about 1-2 percent partial melt, which dramatically reduces its viscosity. This layer was first proposed by American geologist Joseph Barrell in 1914 to explain how continents could rise and sink in response to loading by ice sheets during glacial periods. The asthenosphere's unique properties arise from the combination of high temperature (1,300-1,600°C) and pressure conditions that allow solid rock to flow like a very thick fluid over geological timescales of thousands to millions of years.
How deep is the asthenosphere?
The asthenosphere extends from about 80-100 kilometers depth to approximately 200-700 kilometers beneath the Earth's surface. The exact depth varies depending on location and whether it's under oceanic or continental crust. Beneath mid-ocean ridges, where new oceanic crust forms and temperatures are higher, the asthenosphere can rise to within 10-20 kilometers of the seafloor. Under old, cold oceanic lithosphere, the boundary sits around 80-100 kilometers deep. Continental regions show even greater variation: beneath young mountain belts like the Andes or Himalayas, the lithosphere-asthenosphere boundary may occur at 100-150 kilometers, while under ancient continental cratons like the Canadian Shield, the boundary can extend to 200-250 kilometers depth. The lower boundary of the asthenosphere is less well-defined, generally placed where the mantle transitions to higher viscosity material in the lower upper mantle, typically between 200 and 700 kilometers depth depending on regional temperature variations and mantle composition.
What is the difference between lithosphere and asthenosphere?
The lithosphere is the rigid outer shell of Earth including the crust and upper mantle, while the asthenosphere is the softer, partially molten layer beneath it. The lithosphere floats and moves on top of the flowing asthenosphere. The key distinction lies in their mechanical properties rather than chemical composition—both consist primarily of similar silicate minerals, but temperature differences create dramatically different behaviors. The lithosphere remains cool enough to behave as a brittle solid that fractures when stressed, which is why earthquakes occur within this layer. Its thickness ranges from about 5-10 kilometers at mid-ocean ridges to over 200 kilometers beneath old continental interiors. The asthenosphere, heated to temperatures approaching the melting point of its constituent minerals, deforms plastically through a process called solid-state creep. This allows it to flow slowly, with velocities typically measured in centimeters per year. The boundary between these layers, called the LAB (Lithosphere-Asthenosphere Boundary), is identified by a sharp drop in seismic wave velocities and an increase in electrical conductivity, both indicators of reduced rigidity and the presence of partial melt.
What is the asthenosphere made of?
The asthenosphere consists primarily of peridotite, an ultramafic rock composed mainly of the minerals olivine (about 60-70 percent) and pyroxene (20-30 percent), with smaller amounts of garnet, spinel, and other minerals. This composition is similar to the lithospheric mantle above it, but the asthenosphere's higher temperature causes a small fraction—roughly 1-2 percent—to exist in a molten state. This partial melt collects along grain boundaries between solid crystals, dramatically reducing the rock's strength and viscosity. The presence of volatiles like water and carbon dioxide, even in trace amounts of 50-200 parts per million, significantly lowers the melting temperature and enhances the rock's ability to flow. Seismic studies and laboratory experiments suggest that the asthenosphere may also contain regions enriched in basaltic components, particularly beneath mid-ocean ridges where melting is most extensive. The exact mineralogy varies with depth due to phase transitions—for example, olivine transforms to a denser crystal structure called wadsleyite at about 410 kilometers depth, marking the transition from upper to lower mantle.
What is the temperature of the asthenosphere?
The temperature in the asthenosphere ranges from approximately 1,300 degrees Celsius at its upper boundary to about 1,600 degrees Celsius at its base, though these values vary by location. These temperatures place the asthenosphere just below or at the solidus temperature—the point where rock begins to melt—for typical mantle peridotite. The temperature gradient in this region averages about 0.3-0.5 degrees Celsius per kilometer of depth, much lower than in the lithosphere above where heat is primarily conducted rather than convected. Beneath mid-ocean ridges, temperatures can exceed 1,400 degrees Celsius at relatively shallow depths of 50-60 kilometers, explaining the extensive melting that produces oceanic crust. Under continental cratons, where the lithosphere is thick and acts as an insulating blanket, the asthenosphere may be slightly cooler at equivalent depths. These high temperatures result from a combination of primordial heat left over from Earth's formation 4.5 billion years ago and ongoing heat production from radioactive decay of uranium, thorium, and potassium isotopes in the mantle. The temperature is high enough that the rock glows red-hot, though we cannot see it due to the overlying layers.
How does the asthenosphere affect plate tectonics?
The asthenosphere serves as the essential lubricating layer that enables plate tectonics by allowing rigid lithospheric plates to move horizontally across Earth's surface. Its low viscosity—approximately 100 to 1,000 times less viscous than the lithosphere—means it can flow in response to stress over timescales of thousands to millions of years. Mantle convection within the asthenosphere creates circulation patterns where hot material rises beneath mid-ocean ridges, spreads laterally, cools, and eventually sinks at subduction zones. These convection currents exert shear stress on the base of tectonic plates, contributing to their motion. However, research since the 1990s has shown that the primary driving forces come from the plates themselves: slab pull (dense oceanic lithosphere sinking into the mantle) accounts for about 70 percent of plate driving force, while ridge push contributes 10-20 percent. The asthenosphere also plays a crucial role in volcanic activity—decompression melting occurs when hot asthenospheric material rises toward the surface, producing magma that feeds volcanoes at mid-ocean ridges, hotspots like Hawaii, and continental rift zones. Without the asthenosphere's unique properties, Earth would likely be a tectonically dead planet like Mars, with a single rigid shell and no plate movements.
Can you see the asthenosphere on an Earth layers diagram?
Yes, the asthenosphere appears on comprehensive Earth layers diagrams, typically shown as a distinct zone within the upper mantle, positioned between the lithosphere above and the deeper mantle below. However, not all simplified diagrams include it because Earth's layers can be classified in two different ways: by chemical composition (crust, mantle, core) or by mechanical properties (lithosphere, asthenosphere, mesosphere, outer core, inner core). Diagrams emphasizing mechanical behavior will clearly show the asthenosphere as a separate layer, usually colored differently from the lithosphere and labeled with its approximate depth range of 80-700 kilometers. More detailed scientific diagrams often indicate the asthenosphere with wavy lines or arrows to represent its flowing, ductile nature, contrasting with the rigid, straight-edged appearance of the lithosphere. Educational resources from institutions like the Smithsonian National Museum of Natural History or USGS typically include the asthenosphere in their Earth structure diagrams. When viewing these diagrams, look for labels indicating the lithosphere-asthenosphere boundary (LAB) or the low-velocity zone (LVZ), both of which mark the top of the asthenosphere. Some advanced diagrams also show variations in asthenosphere depth under different tectonic settings, such as thinner beneath ocean ridges and thicker under continents.
What is the density of the asthenosphere?
The density of the asthenosphere ranges from approximately 3.4 grams per cubic centimeter at its upper boundary to about 4.4 grams per cubic centimeter at greater depths, with an average of around 3.6-3.8 g/cm³ throughout most of the layer. This makes it denser than the overlying lithosphere, which has densities of 2.7-3.0 g/cm³ for continental crust and 2.9-3.3 g/cm³ for oceanic lithosphere and lithospheric mantle. The density increase with depth results from two factors: compression due to increasing pressure, which forces atoms closer together, and changes in mineral structure. Despite being slightly less rigid due to partial melting, the asthenosphere remains denser because pressure effects dominate over the small amount of melt present. This density structure is crucial for understanding mantle dynamics—denser material tends to sink while less dense material rises, driving convection. Interestingly, even though the asthenosphere is denser than the lithosphere, the lithosphere doesn't simply sink through it under normal conditions because both layers are already in a state of gravitational equilibrium, with the lithosphere's greater rigidity allowing it to maintain its position. Only when oceanic lithosphere becomes old, cold, and sufficiently dense—typically after about 180-200 million years—does it become negatively buoyant enough to sink into the asthenosphere at subduction zones, initiating the recycling process that drives plate tectonics.
Asthenosphere Properties by Geographic Location
| Location Type | Depth to Top (km) | Depth to Bottom (km) | Temperature Range (°C) | Dominant Process |
|---|---|---|---|---|
| Mid-Ocean Ridge | 10-20 | 100-150 | 1,350-1,500 | Decompression melting, crustal formation |
| Young Oceanic Lithosphere | 60-80 | 200-250 | 1,300-1,450 | Passive mantle flow |
| Old Oceanic Lithosphere | 80-100 | 300-400 | 1,280-1,400 | Plate cooling, eventual subduction |
| Continental Rift Zone | 50-70 | 150-200 | 1,350-1,500 | Lithospheric extension, volcanism |
| Stable Continental Craton | 150-250 | 400-700 | 1,250-1,350 | Minimal activity, ancient lithosphere |
| Subduction Zone | 80-120 | 600-700 | 1,200-1,600 | Slab dehydration, arc volcanism |
External Resources
- Encyclopedia Britannica provides additional historical context on the discovery and naming of the asthenosphere by Joseph Barrell.
- National Geographic Education offers interactive diagrams showing how the asthenosphere facilitates plate tectonic movement.
- Incorporated Research Institutions for Seismology explains how seismic waves are used to map the asthenosphere and other Earth layers.