9 hard and fast facts about the Earth's lithosphere
You live on it for your entire life, but how much do you actually know about the part of the Earth called the lithosphere? Join us as we dig down to see what exactly is going on in Earth's hard, crusty exterior.
What is the lithosphere?
The solid outer layer of the Earth is known as the lithosphere. The Earth's outermost layers, the crust and the brittle upper part of the mantle, are called the lithosphere. The asthenosphere, another component of the upper mantle, and the atmosphere form its boundaries.
It is presumed that similar structures exist on other planets, too (albeit with their own characteristics). Still, Earth is the only one we know about with any realistic level of detail. So, for that reason, the rest of this section will be dedicated to Earth's lithosphere alone.
The lithosphere's rocks are not dense, but they are nonetheless considered elastic. "Elastic" in the geological sense means that the material is able to deform and return to shape without shattering.
The asthenosphere is viscous, and geologists and rheologists (scientists who study the flow of matter) mark the difference in elasticity between the two layers of the upper mantle at the lithosphere-asthenosphere boundary (LAB). A solid material's capacity to stretch or deform under stress is measured by ductility. Compared to the asthenosphere, the lithosphere has far less flexibility.
Oceanic lithosphere and continental lithosphere are the two primary different forms of the lithosphere. The oceanic lithosphere is slightly denser than the continental lithosphere and is connected to the oceanic crust.
Tectonic activity is the most well-known characteristic of the Earth's lithosphere. The interaction of the enormous lithosphere slabs known as tectonic plates is referred to as tectonic activity.
The North American, Caribbean, South American, Scotia, Antarctic, Eurasian, Arabian, African, Indian, Philippine, Australian, Pacific, Juan de Fuca, Cocos, and Nazca tectonic plates are among those that make up the lithosphere.
The majority of tectonic activity occurs where these plates' boundaries meet, when they may collide, split apart, or move against one another. Thermal energy (heat) from the lithosphere's mantle component enables the movement of tectonic plates. The lithosphere's rocks become more elastic due to thermal energy.
Some of Earth's most dramatic geologic occurrences result from tectonic activity: deep ocean trenches, volcanoes, orogeny (mountain-building), earthquakes, and volcanoes.
The lithosphere can be shaped by tectonic activity: At rift valleys and ocean ridges, where tectonic plates are separating from one another, the oceanic and continental lithospheres are at their thinnest.
The Earth, as conceptualized as a system, comprises a series of so-called spheres, of which the lithosphere is but one. The other spheres are the cryosphere (Earth's frozen regions, including both ice and frozen soil), the hydrosphere (Earth's liquid water), the atmosphere (the air surrounding our planet), and the biosphere (which includes all life on the planet). These spheres affect various factors, including geography, biodiversity, and ocean salinity.
For instance, the pedosphere, formed of dirt and soil, is a lithosphere component. However, the lithosphere, atmosphere, cryosphere, hydrosphere, and biosphere all interact to form the pedosphere.
The tremendous movement of a glacier can potentially reduce enormous, hard boulders of the lithosphere to powder as part of the cryosphere. Rocks in the lithosphere may erode and weather due to the wind (atmosphere) or rain (hydrosphere), respectively. These eroded rocks combine with the organic elements of the biosphere, such as plant and animal remains, to form fertile soil or the pedosphere.
To affect temperature variations on Earth, the lithosphere also interacts with the atmosphere, hydrosphere, and cryosphere. For instance, temperatures on tall mountains are frequently much lower than on plains or slopes.
A chilly or even icy climate zone can be produced by the interaction of the lithosphere's mountain ranges with the decreased air pressure in the atmosphere and the hydrosphere's snowy precipitation. A region's climate zone affects the adaptations organisms in the region's biosphere need to make.
What are some hard facts about the lithosphere?
We've covered a lot of ground, so to speak, already, but if you want to know the main takeaways about the lithosphere, then check out these nine facts about it.
1. The Earth's oceanic lithosphere is constantly getting thicker
Believe it or not, parts of the Earth's oceanic lithosphere are gradually thickening. This is an extensive process that happens due to aging. As the lithosphere moves away from the Earth's mid-oceanic ridges, it tends to get thicker with distance.
The lithosphere eventually thickens due to prompt conductive cooling, which converts the hot asthenosphere into a lithospheric mantle. It becomes denser as it gets older.
While the thickness of the oceanic lithosphere may be less than that of our asthenosphere, for several million years, it is thought that it will surpass our asthenosphere once it crosses a certain threshold.
This is a key element of plate tectonics and is the primary reason oceanic crust tends to sink (usually) under continental crust at destructive plate boundaries.
2. The lithosphere was first conceived of just over 100 years ago
A. E. H. Love first suggested the idea of the lithosphere in his 1911 monograph "Some problems of Geodynamics." In several studies, Joseph Barrell expanded on the idea and coined the "lithosphere."
The idea was based on the observation of large gravity anomalies over continental crust. He deduced that there must be a strong, solid upper layer (which he named the lithosphere) over a weaker layer that may flow (which he called the asthenosphere).
Reginald Aldworth Daly developed these concepts in his groundbreaking book "Strength and Structure of the Earth." published in 1940. The majority of geologists and geophysicists quickly accept them. The theory of plate tectonics depends on these ideas of a strong lithosphere resting on a weak asthenosphere.
3. Earth's lithosphere can reach depths of up to 190 miles!
As we have previously discussed, the crust and the brittle upper mantle make up the lithosphere, the Earth's outermost layer. The Greek terms "lithos," which means stone, and "sphaira," which means globe or ball, are the source of the English word "lithosphere."
The lithosphere can extend as deep as 190 miles (300 kilometers), sandwiched between the atmosphere above and the asthenosphere below, according to The Geological Society.
Temperatures approach 2,400 degrees Fahrenheit (1,300 degrees Celsius) near the lithosphere-asthenosphere barrier, and rocks become viscous and move very slowly. Despite moving at a rate of one to two inches (2.5 to 5 centimeters) every year, the rocks remain solid due to the high pressure created by the miles of mantle and crust above.
The transition between the lithosphere and asthenosphere in the upper mantle is marked by a shift in ductility, which is a material's capacity to deform or stretch under stress.
4. The nature of Earth's lithosphere is revealed by studying earthquakes
The lithosphere is where we live, and it is here that we can observe the direct results of plate tectonics in the form of dramatic things like volcanoes and vast mountain ranges. But how can we be certain of what is occurring underneath the surface?
We can learn much about the Earth's interior via earthquakes and seismic waves, including where the lithosphere and asthenosphere are situated.
Primary (P) and secondary (S) waves propagate throughout the Earth's interior during an earthquake. Scientists may learn a lot about the composition, temperature, and pressure of the material that the waves have passed through thanks to specialized stations placed worldwide that can detect these waves and record their velocities.
While seismic waves slow down in liquids, they move more quickly through solid rocks and other dense materials. Seismic waves start to slow down at depths of around 60 to 90 miles (100 to 250 km), suggesting that they have entered the asthenosphere, a partially molten zone.
Rocks in the asthenosphere, a seismic low-velocity zone, partially melt as a result of either increased warmth or decreased pressure. According to the Geological Society, this partial melting occurs more frequently in hot regions and plate boundaries.
5. The lithosphere's composition is variable
Earth's two main rock layers make up the lithosphere (crust and the mantle of the Earth). It includes the uppermost part of the next-lower layer, known as the mantle, and the entire planet's thin outer shell, the crust. It is thickest below the continents, and weakest near mid-ocean ridges, which are raised seafloor regions where new seafloor crust is created.
Because of this, the Earth's lithosphere can vary in composition over the surface and with depth. Put simply, the lithosphere composition is determined by the following:
- Physical characteristics
- Chemical structure
The physical portions of the lithosphere are composed of the crust and a part of the upper mantle that is brittle and rigid.
Chemically speaking, things are a little more complex.
According to the type and composition of the rock strata at any given point, the lithosphere's chemical makeup can change a lot. For example, one crust component is soil, a mixture of worn rock minerals and organic matter. Elements found in soil include oxygen, silicon, aluminum, iron, calcium, salt, potassium, and magnesium. While the mantle is primarily formed of iron, it also contains magnesium.
6. Continents are usually thicker and less dense parts of the lithosphere
The mafic (silicate mineral or igneous rock rich in magnesium and iron) igneous rocks that make up the oceanic crust are far denser than the granite that makes up the majority of the continental crust, which is made up of a combination of igneous, metamorphic, and sedimentary rocks. Due to its thickness and low density, the continental crust rises higher on the mantle than the oceanic crust, which falls into the mantle to form basins. The planet's oceans are created when these basins are filled with water.
The crust is the upper 18.6 miles (30km) to 43.5 miles (70 km) of the normal continental lithosphere, which has a thickness of about 24.9 miles (40 km) to maybe 174 miles (280 km) or more. The chemical composition changes at the Moho (or Mohorovičić) discontinuity, which separates the crust from the upper mantle. The thicker and less dense mantle (mantle roots and craton — the stable interior portion of a continent generally composed of ancient crystalline rock) found in the oldest portions of the continental and mantle lithospheres contributes to stabilizing these two types of lithospheres.
The continental lithosphere can only descend for a distance of roughly 62 miles (100 km) before resurfacing due to its low density. As a result, unlike the oceanic lithosphere, the continental lithosphere is not regenerated in subduction zones. Because of this characteristic, the continental lithosphere is a relatively constant feature of the planet.
7. And it is thinner in the oceans
Magma that erupts on the ocean floor to produce basalt lava flows or cools at depth to produce the intrusive igneous rock gabbro that makes up the oceanic lithosphere or crust. Sediments, mainly composed of mud and the shells of small sea animals, cover the seafloor. Ocean basins contain the oceanic crust and all these other materials.
The oceanic lithosphere normally ranges in thickness from 31 miles (50 km) to 75 miles (120 km); however, the layer below the mid-ocean ridges is not thicker than the crust. Additionally, the oceanic lithosphere is denser than the continental lithosphere, in which the mantle links with felsic rocks in the crust and is predominantly made up of the mafic crust and ultramafic mantle (peridotite).
It continues to thicken as it ages and moves away from the mid-ocean ridge. Conducted cooling, which changes the warm asthenosphere into the lithospheric mantle, causes this thickness.
The oceanic lithosphere thickens and gets denser with time. The thermal boundary layer for mantle convection is what it actually is. The oceanic lithosphere is similarly seen to be less dense than the asthenosphere, yet throughout millions of years, it has gradually increased in density.
Due to chemical differences, the oceanic crust is lighter than the asthenosphere, yet the mantle lithosphere is denser than the asthenosphere due to thermal contraction. The oceanic lithosphere sinks beneath the overriding lithosphere due to the gravitational instability of the mature oceanic lithosphere, which impacts the subduction zones.
Overall, the oceanic lithosphere at mid-ocean ridges is much younger than the continental lithosphere because it is continually created and recycled back to the mantle at subduction zones. A little over 170 million years have passed since the oldest oceanic lithosphere was formed.
8. Sometimes, bits of ocean crust gets piled onto the land
Sometimes, thanks to the constant ebb and flow of plate tectonics, a little piece of the ocean crust can become preserved on the continents. When a continental plate and an oceanic plate collide, the continental plate scrapes off the top layers of the oceanic plate, which are known as ophiolites.
One famous example is the movement along California’s San Andreas Fault line has resulted in the formation of 17 ophiolites in the San Francisco Bay region. Ophiolites can be found all over the planet, with other notable examples including the Lizard in Cornwall, England, parts of Cyprus, Marmaris, Turkey, and many others.
Understandably, the geology of these structures is markedly different from other rock strata in an area. These rock sections are also often very rich in valuable resources like ore minerals of chromite, copper, iron-nickel laterite, titanomagnetite, and platinum group elements (PGE).
9. Sometimes, we can also get little bits of the deep Earth in the lithosphere
By looking at components called mantle xenoliths ("alien/foreign rocks") that have been raised in volcanic pipes like kimberlite, lamproite, and others, geoscientists can directly investigate the characteristics of the subcontinental mantle.
In geology, the term "xenolith" is almost exclusively used to describe inclusions in igneous rock entrained during magma ascent, emplacement, and eruption.
A magma chamber's borders, the walls of an erupting lava conduit or explosive diatreme (a long vertical pipe or plug of magma), or the base of a body of flowing lava on the Earth's surface are all possible places where xenoliths can be captured and transported to the surface.
An igneous body may contain one or more xenocrysts, which are foreign crystals. Diamonds and quartz crystals found in kimberlite diatremes are two examples of xenocrysts. Even in geographically constrained places, xenoliths can vary from place to place.
For instance, the rhyolite-dominated lava of the Niijima volcano in Japan contains two distinct forms of gabbroic xenoliths that originated under various pressure and temperature circumstances.
Numerous techniques, such as examinations of the abundances of the isotopes of osmium and rhenium, have been used to learn more about the histories of these xenoliths.
These studies have demonstrated that, despite the mantle flow associated with plate tectonics, the mantle lithospheres beneath some cratons have survived for longer than 3 billion years. That is quite amazing.
And that, lithosphere-lovers, is your lot for today.
The lithosphere is an integral part of the planet on which we live. It is, in effect, the only part of the planet's solid bits that we are all intimately familiar with. We have seen that it can vary widely and is in constant flux thanks to the never-ending process of plate tectonics and mantle convection.
We know that other rocky planets probably have lithosphere too, but how similar they are to our own planet is yet to be understood fully.