The lithosphere is a colder, rigid, mechanically stronger solid outermost layer or shell of the Earth. In contrast, the asthenosphere is hotter, weaker, and ductile.
These two layers have many other differences that we will look at. However, before we examine these differences, let us briefly tell you what each is.
What is lithosphere?
The lithosphere is the outermost, rigid, solid part of the Earth. It lies beneath the atmosphere, water bodies or ice (glaciers), and above the asthenosphere. Look at it more like the outer shell of an egg.
The lithosphere has two layers, namely the crust and mantle lithosphere. These two layers vary in chemistry and composition.
What separates these two layers? A Mohorovičić discontinuity marked by an increase in seismic velocity. On the upper side of the Mohorovičić discontinuity, you have the crust, and on the lower side, the rigid part of the upper mantle. This lower layer is what we call the mantle lithosphere or lithospheric mantle.
We can further divide the lithosphere into two types: the oceanic and continental. As their name suggests, the oceanic lithosphere occurs in the ocean or seafloor and continental on the continents.
Lastly, the lithosphere is divided into seven large tectonic plates and eight smaller ones. These plates float and move on the asthenosphere. Their interactions cause earthquakes, deep oceanic trenches, volcanism, and mountain building.
What is asthenosphere?
The asthenosphere is a hotter, ductile, weak layer below the lithosphere. The lithosphere-asthenosphere boundary (LAB) separates it from the lithosphere above.
Unlike the Mohorovičić discontinuity, the LAB is a thermal boundary, not compositional. Rocks above LAB are rigid and stronger, and those beneath it are weak, ductile, and creep or flow under stress.
Usually, temperature and pressure influence the strength or stiffness of Earth’s material. The asthenosphere temperature and pressure are close to its melting temperature. It even has a small amount of melt between grain boundaries and perhaps a tiny water amount. This makes it ductile or able to flow.
Lastly, ductility and small melt and water allow the lithosphere to move above it.
Lithosphere vs. asthenosphere: How do they differ?
The lithosphere is colder, rigid, and mechanically stronger, while the asthenosphere is hotter, ductile, and weak. Also, these two layers have differences in viscosity, density, heat flows, location, composition, etc.
Let us discuss these differences in detail.
1. Mechanical strength
Their mechanical strength is the first and perhaps most important difference between these two layers. Some authors will call it flexural strength.
The lithosphere is at temperatures and pressures that make it mechanically strong or rigid. It will withstand stress, bend, or break. In contrast, the asthenosphere is mechanically weak. It is ductile and will flow or creep under stress like a viscous fluid. However, this flow happens slowly or on a geological time scale.
A good analogy is a bar of chocolate or wax. Chocolate or wax is stronger when cold and weaker when it is hot. This weakness is due to temperature differences.
Note that the difference in strength is not necessarily due to differences in composition. They occur due to the temperature and pressure regimes of these two layers.
2. Viscosity
The lithosphere is more viscous than the asthenosphere. According to one study, the viscosity contrast between these two is in 108 and 1010 order.
This huge difference influences some behaviors of these two layers. The more viscous one is rigid and doesn’t flow, while the less viscous one flows.
3. Temperature and geothermal gradient
The lithosphere is colder. Its temperature ranges from 100°C (212 °F) to 1300°C (2372°F), while the asthenosphere is hotter with temperatures of 1,400±100°C (2,552 ± 212°F). However, the crust surface is at 14°C (57°F).
The other difference related to temperature is the geothermal gradient. It simply means how temperature increases with depth.
The lithosphere has a higher geothermal gradient than the asthenosphere. It is 25 °C/km in continents compared to 0.6°C/km for the asthenosphere. This difference is due to how heat flows.
4. Movement
Both these layers move but in a different way. The asthenosphere, being ductile, flows by convection. A density difference from temperature composition will create a convection cell. This flow will force hotter, less denser rocks to move to cooler areas and vice versa.
On the other hand, the tectonic plates that make the lithosphere flow or slide above the asthenosphere as a single unit. Forces associated with subduction mainly influence this flow. Also, convection currents of the asthenosphere may contribute.
5. How heat flows
The other difference is how heat flows. In the lithosphere, heat flows by conduction. In contrast, heat flows by convection in the asthenosphere.
Heat conduction occurs between rocks in contact with a temperature difference. It doesn’t involve flow. It occurs on the lithosphere since it is rigid and doesn’t flow.
On the other hand, convection occurs in the asthenosphere since it can flow. Therefore, as materials in this layer move, they transfer heat.
6. Location
The lithosphere has two layers and lies above the asthenosphere. It has part of the upper mantle and crust, while the asthenosphere is a single layer. It lies below the lithosphere and is entirely in the mantle. However, it also has sections like the low-velocity and transition zones.
7. Composition
The lithosphere has mostly granitic rocks in the crust and peridotites on the rigid mantle part. In contrast, the asthenosphere has peridotites.
Granites are light-colored, coarse-grained plutonic rocks. These rocks are silica-rich and have mostly quartz and feldspar. Also, they are less dense.
On the other hand, peridotites are ultramafic rocks. Ultramafic rocks are low in silica and denser. These rocks have more than 90% iron and magnesium minerals.
Lastly, the crust may have sedimentary and metamorphic rocks besides granitic rocks.
8. State of matter
Both these layers are mostly solid. Usually, the lithosphere is 100% solid, while the asthenosphere has a tiny amount of melt, i.e., less than 1%.
9. Thickness
The lithosphere is 40 to 300 km (25-186 mi) thick, and the asthenosphere runs 40-280 km (25-174 mi) to 700 km (435 mi) above the surface. This makes the latter thicker than the former.
However, the thickness difference between these two is unimportant since they occur at different locations.
Also, it doesn’t influence the difference between these layers. Instead, temperature and pressure do.