The lithosphere is the outermost, rigid, solid layer of the Earth. It is like the shell of a coconut or egg.
Location-wise, this layer lies above the asthenosphere and below the atmosphere, ice, or water bodies. An asthenosphere is a weaker, ductile layer. By ductile, we mean it stretches or deforms under stress.
The lithosphere has two layers: the crust and the rigid or brittle upper part of the mantle. Geologists call this upper part of the mantle a lithospheric or lithosphere mantle.
Besides layers, the lithosphere is either oceanic or continental. Oceanic one occurs on the ocean or seafloor and continental on continents.
Also, the lithosphere is divided into seven major plates and eight smaller ones. We call them tectonic plates. They are made of a combination of oceanic and continental lithosphere. Also, they move over the asthenosphere.
These tectonic plates are dynamic, and their movement forms the basis of plate tectonic theory. They are dynamic because they constantly change their shape and size.
As these tectonic plates move, they may converge (move toward each other), diverge (move away from each other), or slide against each other. These movements cause earthquakes, volcanism, mountain-building, and deep ocean trenches.
Today, we will discuss the lithosphere, its two types, and its layers. We will also give you its thickness, composition, how it moves, etc.
Lithosphere thickness, types, and locations
The lithosphere thickness is 40-280 km (25-170 mi) or up to 300 km (186 mi). This thickness varies since a thermal or temperature boundary, not compositional, separates it from the asthenosphere below.
There are two types, namely oceanic and continental.
1. Continental
The continental lithosphere ranges from 40 to 280 km (25 to 170 mi) thick or up to 300 km (186 mi). Its average thickness is 200 km (124 mi), and its density is 2.7g/cm3.
The composition of this layer is mostly the less dense granites on the upper part and denser peridotites on the lower part. Granites are light-colored felsic silica-rich rocks with mainly quartz and feldspar and a small amount of mafic (iron and magnesium) minerals.
On the other hand, peridotites are dense, dark-colored ultramafic rocks. Ultramafic rocks have more than 90% mafic minerals.
You should also know that the oldest continental lithosphere occurs on cratons. These older continental lithosphere parts are thicker, less dense, and more stable. Their lower density is what makes these cratons stable.
Usually, the continental lithosphere is less dense than the oceanic. Therefore, it can’t subduct to great depths (more than 100 km or 64 mi) before it reappears. This may happen where two plates converge.
Therefore, it is not recycled at the subduction zone. This makes it a nearly permanent feature on Earth. Did you know that some cratons are over 3 billion years old?
2. Oceanic
The oceanic lithosphere is 50-140 km (31-87 mi) thick. Its average thickness is 100 km (62 mi), and its density is 2.9-3.0g/cm3.
However, on mid-ocean ridges (MOR), the younger lithosphere is no thicker than the crust. Mid-ocean ridges are divergent boundaries that create a new lithosphere, while destruction occurs at convergent boundaries, i.e., subduction zones.
The oceanic lithosphere thickness depends on age. The older it is, the thicker, with the oldest known being 140km (87 mi) thick. This happens because cooling converts part of the asthenosphere to the lithosphere. Such occurs far from MOR.
Compositionally, the oceanic mafic crust and ultramafic (peridotite mantle). These rocks are higher in magnesium and iron, making them denser.
Note: Some authors may include subducted lithosphere. It has a rigid, mechanically strong subducted zone. This zone is still attached to the continental plates above.
Lithosphere layers and their composition
The lithosphere has two layers. The outermost crust and the mantle lithosphere beneath the crust. These two layers are distinguished by their mineralogy and chemistry.
A discontinuity known as the Mohorovičić discontinuity separates the crust from the lithospheric mantle. It marks a change in composition and chemistry.
However, this discontinuity separates the crust from the asthenosphere on mid-ocean ridges. At these ridges, the lithospheric mantle has no thickness.
Let us talk a little more about each of these layers.
1. Crust
The crust is the outermost rocky shell of the Earth. Its surface has soil, rock outcrops, or sediments (occur in oceans and seas). The part with soil or subject to forming soil is known as pedosphere.
The crust’s surface is where plants grow, and animals live. We also build our houses, roads, railways, etc., on or near the crust’s surface.
Compared to other layers, the crust is a relatively thin layer of the Earth. It accounts for less than 1% of the Earth’s radius. Its temperature is between 100°C (212 °F) and 600 °C (1,112 °F). However, on the surface, the average temperature is 14°C (57°F)
The Earth’s crust is divided into 1) continental and 2) oceanic crust.
Here is more about their thickness, temperature, and composition:
- Continental crust: Continental crust occurs on continents. It has a thickness of 30–70 km (19-43 mi) and a 2.6–2.7 g/cm3 density. Its composition is mostly rich quartz and felsic-rich, especially granitic rocks lower in density. However, it also has sedimentary and metamorphic rocks.
- Oceanic crust: It occurs on the ocean and seafloor. The ocean crust is approximately 6-12 km (4-7 mi) thick, and its density is 2.8–3.0 g/cm3. It has mainly mafic rocks like basalt, gabbro, or diabase. Since mafic rocks are higher in magnesium and iron, they are denser.
2. Lithosphere mantle
The lithospheric mantle is the uppermost rigid solid part of the upper mantle. It lies between the Mohorovičić discontinuity and the lithosphere-asthenosphere boundary (LAB).
The LAB is a thermal boundary with an isotherm of 1300°C (2372°F). Above this temperature, rocks are mechanically strong. Such rocks behave elastically over geological time scales (thousands of years or more) under stress. Prolonged stress makes them break.
On the other hand, beneath the LAB, you have a ductile asthenosphere layer. This layer is mechanically weaker. Also, it deforms under stress like a thick, viscous fluid.
What about its thickness and temperature? The lithospheric mantle is about 100-200 km (62-124 mi) thick. Its temperature varies from 600 °C (1,112 °F) to 1300°C (2372°F) at the LAB boundary, and its density is about 3.22–3.29g/cm3 in situ.
Compositionally, the mantle lithosphere has mainly peridotites. These dense ultramafic mantle rocks make it denser.
Lastly, like the crust, the lithospheric mantle has an oceanic section and a subcontinental part.
The lithosphere is not entirely rigid
Contrary to what many assume, the lithosphere isn’t entirely rigid or mechanically strong. Put differently, it doesn’t behave like a stiff or brittle surface rock across its entire depth.
Instead, it progressively becomes weaker and easily deformed with depth. At the boundary with the asthenosphere, rocks are near their melting point. Below this boundary, rocks become easily deformed, making them ductile.
How does the lithosphere flow?
The lithosphere is a cooler, rigid solid layer that floats in the asthenosphere. When under force, it will withstand, bend, or break. However, it is highly viscous and doesn’t flow. It makes the tectonic plates.
On the other hand, the asthenosphere is ductile and even has a tiny amount of melt. This allows or even lubricates the lithospheric plates to move or over it.
Usually, the lithospheric tectonic plates move 1-10 centimeters a year. Thus, we cannot perceive the movement.
What forces influence this movement? The forces that influence the movement are associated with subduction. These forces include the balance between the slab pull and drag force, gravitational ridge push force, transform resistance force, and subduction resistance force.
However, the heat‐driven convectional flow of the asthenosphere may contribute, but only to a small extent.
Some studies suggest the lithosphere is coupled with some asthenosphere up to 250 km (155 mi) and moves together. This thick entity makes the tectosphere. However, this movement is only known on continental crust, not oceanic. Therefore, the lithosphere moves on top of the asthenosphere in the oceanic crust.
Usually, the coupled asthenosphere has subtle chemical differences. These differences make it relatively buoyant relative to the surrounding asthenosphere.
Lastly, whether the coupled lithosphere and part of the asthenosphere that flows can exert enough drag force is debatable.
Entomology and historical background
The word lithosphere comes from the Ancient Greek word ‘lithos’, which means rocky, and ‘sphaíra’, which means sphere.
Augustus Edward Hough Love, a British mathematician, developed the concept of the lithosphere in 1911. However, the word lithosphere was introduced by an American geologist named John Barrell. He is also the one who also came up with the name asthenosphere.
Later, in the 1940s, Reginald Aldworth Daly, a Canadian geologist, expanded it. Now, it is a widely accepted concept.
Frequently Asked Questions (FAQs)
The lithosphere is important because it forms a surface where plants and animals live and thrive. The land we farm, where animals graze, and where we build houses is on this layer. Also, we mine minerals and fossil fuels inside this layer.
The seven major tectonic plates comprise the larger part of the lithosphere and eight minor. These plates are the Antarctic, African, North American, Pacific, South American, and Eurasian. Minor plates include Indian, Nazca, and Juan de Fuca.
References
- Grotzinger, J. P., & Jordan, T. H. (2014). Understanding earth (7th ed.). W.H. Freeman and Company.
- Tewari, H. C., Prasad, R. B., & Kumar, P. (2018). Structure and tectonics of the Indian continental crust and its adjoining region: Deep Seismic Studies. Elsevier.
- Xia, B., Thybo, H., & Artemieva, I. M. (2020). Lithosphere mantle density of the North China Craton. Journal of Geophysical Research: Solid Earth, 125(9). https://doi.org/10.1029/2020jb020296