The asthenosphere is one of the most special or unique layers of the Earth not known to other planets like Mars or Venus. It is what lubricates the Earth’s tectonic plates, which are responsible for earthquakes, volcanism, and mountain building.
The term asthenosphere was derived from an old Greek word asthenós (ἀσθενός), which means ‘weak’ or ‘without strength’ + the word ‘sphere’. It translates to a ‘weak sphere’ or a ‘sphere of weakness’. This term was suggested by Barrell (1914, p. 459) to refer to the mechanically weak layer beneath the lithosphere.
One unique thing about the asthenosphere is that it has solid rocks with only a tiny amount of melt (<1%). However, it is ductile and can flow or creep like a highly viscous fluid in geological time scales. This behavior allows it to convect and tectonic plates to move over it.
What is its thickness, depth, density, composition, and temperature? Why is it important? How does it differ from the lithosphere? You have many questions about the asthenosphere. We have the answers.
What is the asthenosphere?
The asthenosphere (weak sphere) is a hotter, ductile, mechanically weaker layer beneath the lithosphere. It starts at the lithosphere-asthenosphere boundary (LAB) and goes to a depth of about 700 km (431 mi) below the surface.
This layer starts at a zone with low P- and S- wave velocity, attenuated wave energy, and increased electrical conductivity. This zone is known as the low-velocity zone (LVZ). It occupies the top 50-150 km (31-93 mi) of the asthenosphere and occurs at depths of 80-300 km (50-186 mi) from the surface.
We mentioned the asthenosphere is weak and ductile. Why? It is so because temperature, pressure, and composition influence Earth’s material’s state of matter, strength, or stiffness. As the temperature nears the melting point, rocks become weaker, softer, and ductile.
The asthenosphere has temperature and pressure conditions that are near-melting point. It even has a small amount of incipient melting. This makes it comparatively weak, soft, and less strong (to lack flexural strength) than the lithosphere.
Therefore, when under a force, it slowly deforms, flows, or creeps like a moldable, plastic, ductile, or stretchable highly viscous fluid.
To understand the asthenosphere better, here are some of its behaviors:
1. It has a small amount of melt and perhaps water
The asthenosphere has a small amount of melt, usually less than 1%, and maybe a tiny amount of water.
This small melt occurs as a thin film along grain boundaries or as tiny nonconnected interstitial pockets. It cannot rise to form magma because it is trapped by surface tension.
A good analogy is squeezing all the water on a paper towel. It will not dry, i.e., it will remain damp and structurally weak. This happens because surface tension traps some water in towel fibers.
On the other hand, the water is probably from a small number of hydrous phases like amphibole, titanoclinohumite, mica, or other hydrates silicate in this mantle part. This water may be what lowers the melting point, causing a small amount of garnet lherzolite to melt partially.
The small amounts of melt and water slow seismic wave speeds. Also, it causes wave energy attenuation and high electric conductivity in the LVZ. One study notes that carbonate melt is most likely the cause of increased conductivity.
On the other hand, a decrease in water or the absence of a small amount of melt causes the LVZ termination.
Note: Some authors suggest that the amount of melt in the asthenosphere may be high if in interconnected tubes or at regions of high heat flow. Otherwise, it will not substantially slow seismic velocity and attenuate wave energy.
2. It allows the lithosphere to slide or move over it
Since it is soft, malleable, and has some melt or water, the asthenosphere offers lubrication, allowing the lithosphere plates to slide or move over it slowly. These lithosphere plates move by about 1-10 cm (0.25-2.5 inches) per year and form the basis of plate tectonics theory.
3. It is involved in isostatic adjustment
Since it is plastic and able to flow, the asthenosphere brings about isostatic equilibrium by moving upwards or downwards. By so doing, it balances gravitation from the lithosphere and its buoyance force.
For instance, if you load tons of glaciers on the lithosphere, being rigid will support the load, bend down, or break. When the lithosphere bends downward, it will force the asthenosphere to flow away, and when a rebound happens, it will flow back.
How quickly sinking or rebounding happens depends on the rate at which the asthenosphere flows. This rare is influenced by its resistance to flow or viscosity.
10 things to know about the asthenosphere
You have a basic understanding of the asthenosphere. Here are the ten things, including facts to know about the asthenosphere:
1. Lithosphere-asthenosphere boundary (LAB)
The lithosphere-asthenosphere boundary occurs at about 1300°C (2372°F) isotherm. It is a thermal, not a compositional boundary. However, it creates a mechanical boundary.
Therefore, it represents an insignificant change in chemical differences. If the asthenosphere rises and becomes cooler, it forms part of the lithosphere and vice versa.
Lastly, from a thermal perspective, the LAB represents a transition zone from convection in the asthenosphere to conduction heat transfer.
2. Location and thickness
The asthenosphere runs from approximately 40-280 km (25-174 mi) to 700 km (435 mi) from the Earth’s surface. On the oceans, it starts at 50-140 km (31-87 mi) below Earth’s surface; on the continents, it is at 40-280 km (41-174 mi).
This layer is entirely part of the upper mantle. It starts from the LAB boundary and ends above the lower mantle. From this, we can deduce that the asthenosphere thickness ranges approximately from 420- 660 km (270-410 mi). However, this will vary.
To quickly jog your mind, the upper mantle is part of the mantle, which is one of the four layers of the Earth. The other layers are the crust, outer, and inner core.
Lastly, the asthenosphere is divided into the low-velocity zone (LVZ), transition zone, and Repetti discontinuity sections. These sections are as follows.
i). Low-velocity zone (LVZ)
The low-velocity zone is the uppermost part of the asthenosphere and is approximately 50-150 km (31-93 mi) thick. It runs from an 80 to 300 km (50-186 mi) depth, where seismic wave velocity drops significantly. Below, its seismic velocity increases as it melts, and water disappears.
Towards the lower boundary of the LVZ, at depths of 220 km (140 mi), there is a 220-km discontinuity or Lehmann discontinuity. This discontinuity occurs on continental crust, not seen in oceanic. Thus, it is not global. Some authors suggest it marks the end of the pliable asthenosphere.
ii). Transition zone
The transition zone lies at a depth of 410 and 660 km (255 and 410 mi) from the surface. It has an average thickness of about 250 km (155 mi) and a sharp step-like increase of seismic waves marks it.
The transition zone is sometimes known as the mesosphere. However, some authors consider the mesosphere to include the lower mantle, going up to 2,900 km (1,802 mi) deep.
Also, this term may mean the third 35 km thick layer in the atmosphere above the stratosphere and below the thermosphere.
iii). Repetti discontinuity
Repetti discontinuity starts at a depth of 660 km to 700 km (410-435 mi) and has an average thickness of 40 km (12 mi). It separates the upper and lower mantle.
Note: There is no consensus on the extent of the asthenosphere since all the mantle beneath the lithosphere can flow plastically. Some may equate it to a low-velocity zone or from the lithosphere to the transition zone on the upper mantle. Others consider it up to the upper mantle, lower boundary, or the entire mantle.
3. What is the asthenosphere temperature?
The temperature of the asthenosphere is approximately 1,400±100°C (2,552 ± 212°F).
How did we arrive at this number? It is simple. The thermal boundary between the lithosphere and asthenosphere is assumed by convention to be the 1,300 °C (2,372°F) isotherm. Some authors may place it at 1280°C (2336°F) isotherm.
This thermal boundary is cool enough to make peridotites above behave rigidly and remain strong. Beneath it, peridotites become hot enough, weaken, and behave plastically. Also, a small amount of the rocks will melt but not generate substantial magma volume.
The convective currents in the asthenosphere cause a low geothermal gradient of 0.6°C (33°F)/km compared to 25°C (77°F)/km near the Earth’s surface. Considering the average thickness of the asthenosphere, the temperature range of 1,300-1,500°C (2,372-2,732°F) of the asthenosphere is practical.
4. What is the density of the asthenosphere?
The density of the asthenosphere is about 3.3 g/cm3 on the upper part. It increases the transition of 3.5 to 3.7g/cm3 and the transaction zone.
This change occurs as pressure results from peridotites forming packed atomic structure minerals of the same composition.
5. What is its composition?
The asthenosphere has peridotites, especially lherzolite, composed mainly of olivine and small amounts of pyroxenes. Pyroxenes present are orthopyroxene (enstatite) and clinopyroxenes (diopside). Also, it has a smaller amount of minerals like chromite and spinel.
However, as pressure increases, olivine changes to b-spinel and pyroxenes to garnets at 390-410 km (242-255 mi) depth. At 520 km (323 mi) depth, b-spinel forms g-spinel.
5. Viscosity
The viscosity of the asthenosphere is in the order of 1019 Pa.s. One study put this viscosity at 3 × 1018 Pa.s for a thinner asthenosphere of 140 km (89 mi) and 4 × 1019 Pa.s for a thick one, about 380 km (236 mi). Some authors put it in the order of 1020–1021 Pa.s.
Since it accommodates the rising and sinking of the lithosphere by a ductile flow, measuring the isostatic rate of adjustment can enable us to calculate the asthenosphere’s viscosity.
6. How does the asthenosphere move?
It moves by convectional currents. Since this layer can flow or creep, a density difference from variation in temperature or composition can make the asthenosphere flow or create a slow convection cell.
Usually, heat, such as the loss of heat from radioactive decay within the interior of Earth, drives convection currents more like what you see in lava lamps. For instance, warmer, less dense parts of the asthenosphere will rise while denser, less warm parts sink to take the space left.
Don’t forget that heat flows by convection currents in the asthenosphere, not conduction like in the lithosphere, which is how heat is carried closer to the Earth’s surface. Also, these currents contribute to the movement of tectonic plates to a lesser extent.
Note: There is no consensus on whether convection occurs in individual layers or the entire upper mantle, mesosphere, or mantle. Some models support convection in independent layers; others believe it appears in the whole mantle.
7. What is its state of matter?
The asthenosphere is essentially solid, i.e., solid rocks with a small amount of melt (usually less than 1%). The prevailing temperature and pressure regime make it ductile or flow.
Although in the geophysical realm, the asthenosphere is a fluid in geological time scale, it is solid, not like a molten sea of magma.
However, regions beneath volcanic provinces may have magma blobs that accumulate and rise.
8. Delamination or decoupling may occur
Sometimes, a denser lithospheric may detach, delaminate, or decouple from the upper crust and sink into a less dense, plastic asthenosphere, including at subduction zones.
This will push the less dense asthenosphere upwards, causing a crustal uplift. Also, it may melt the overlaying lithosphere and raise the geothermal gradient.
Usually, the asthenosphere is denser than the lithosphere, and ordinary lithosphere wouldn’t sink. However, delamination will only occur in the cold, older, or metamorphosized lithospheric mantle that is denser.
9. Why is the asthenosphere important?
Understanding the asthenosphere’s chemical and physical properties or behavior will unlock our understanding of plate tectonics.
Plate tectonics is what causes earthquakes, volcanism, and mountain building. These processes occur at plate boundaries, i.e., where these plates converge, diverge, or slide past each other.
Secondly, it is a magma-generating zone, and its upwelling at midocean ridges creates a new lithosphere. Even direct cooling and stiffening of this layer form the lithosphere.
Third, the heat associated with its upwelling and the ocean spreading center drive ocean metamorphism.
Lastly, this layer participates in isostatic adjustment.
10. How does the asthenosphere differ from the lithosphere?
The asthenosphere is a hotter, ductile, and mechanically weaker layer, while the lithosphere is colder, rigid, and mechanically stronger. Also, the asthenosphere is less viscous, and heat transfer occurs by convection, while the lithosphere is more viscous, and heat transfer occurs mainly by conduction.
Note that these differences aren’t influenced by thickness but by viscosity, which is influenced by temperature and pressure.
Frequently Asked Questions (FAQs)
Asthenosphere flow occurs mainly by dislocation and diffusion creep. The hot mantle in this layer will flow mostly by dislocation and colder by diffusion.
Yes, but only to a small extent. Forces that influence the movement are associated with subduction, i.e., balance between slab pull and drag force, gravitational ridge push force, transform resistance force, and subduction resistance force.
No. Although Venus has gravity, topography, and mantle thickness of about 100-1,000 km (62-621 mi) that correlates to Earth’s, it doesn’t have weak asthenosphere or plate tectonics. Therefore, its lithosphere and mantle are strongly coupled. One reason is the absence of water.
No. Martian lithosphere doesn’t have asthenosphere or plate tectonics despite having comparable size, mass, and density to Earth and a less rigid lithosphere with a surface temperature of 450°C.(842°F). A reason is the absence of water that makes its asthenosphere stiff.
References
- Grotzinger, J. P., & Jordan, T. H. (2014). Understanding earth (7th ed.). W.H. Freeman and Company.
- Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.
- Gupta, H. K. (2021). Encyclopedia of solid earth geophysics (2nd ed.). Springer.
- Levin, H. L., & King, D. T. (2016). The Earth through time, 11th edition (11th ed.). Wiley.
- Scarselli, N., Adam, J., & Chiarella, D. (2020). Regional Geology and tectonics (2nd ed., Vol. 1). Elsevier.
- Condie, K. C. (2016). Earth as an evolving planetary system (3rd ed.). Academic Press.
- Barrell, J. (1914). The Strength of the Earth’s Crust. The Journal of Geology, 22(7), 655–683. http://www.jstor.org/stable/30060774
- Klein, C., & Philpotts, A. R. (2017). Earth materials: Introduction to mineralogy and petrology (2nd ed.). Cambridge University Press.
- Martin, R. E. (2018). Earth’s evolving systems: The history of planet earth (2nd ed.). Jones & Bartlett Learning.
- Kusky, T. M., & Cullen, K. E. (2005). Encyclopedia of earth and space science. Facts on File.
- Frisch, W., Blakey, R. C., Blakey, R. C., & Meschede, M. (2011). Plate tectonics continental drift and Mountain Building. Springer.