What is the lithosphere?
The lithosphere is made up of the Earth’s solid outer layer and it consists of the brittle upper portion of the mantle of the Earth and the crust, which is the Earth’s outermost layer. It is covered above by the atmosphere and below by the asthenosphere (another part of the upper mantle of the Earth).
Definition of lithosphere
The lithosphere is referred to as the rigid and outermost shell of Earth, and it is made up of the crust and a portion of the upper mantle of the Earth. On the basis of chemistry and mineralogy, we can distinguish the crust, upper and lower mantle of the Earth, in other words, it includes the crust and uppermost mantle, which make up our planet’s hard and rigid outer layer. A part of the lithosphere known as the pedosphere which is the uppermost part of the lithosphere chemically reacts with the atmosphere, hydrosphere, and biosphere during the soil-formation process. Hence, the pedosphere is the part of the lithosphere that has to do with the definition of the lithosphere in biology.
Lithosphere definition in geography
The term lithosphere is derived from the Greek words lithos, which means “rocky” and sphaira, which means “sphere.” Geographers use the term to refer to the layer of the Earth that extends from the surface to a depth of approximately 80 to 120 miles, depending on location, and the nature of the rocks which are relatively brittle and rigid. Along with the atmosphere, hydrosphere, and biosphere, it is one of the four major components of the Earth system.
Definition of Asthenosphere
The asthenosphere is a zone of the Earth’s mantle that lies beneath the lithosphere and is thought to be much hotter and more fluid than the lithosphere. The asthenosphere extends from about 100 kilometers (60 miles) to about 700 kilometers (450 miles) below the Earth’s surface.
What is asthenosphere?
The asthenosphere is the Earth’s layer beneath the lithosphere. It is a layer of solid rock in which extreme pressure and heat cause the rocks to glide like liquid. The asthenosphere’s rocks are not as dense as the lithosphere’s rocks and this low density allows for the lithosphere’s tectonic plates to move around on the Earth’s surface.
Lithosphere and Asthenosphere
The way rocks in the lithosphere respond to forces applied to them is something they all have in common because rocks tend to break under stress at the relatively low temperatures found near the Earth’s surface. Further down the Earth, as temperature and pressure rise, rocks are more likely to accommodate stress by changing shape, or by deforming, compressing, stretching, and bending, rather than breaking.
At a certain depth, the temperature will be high enough that rocks begin to behave like viscous fluids rather than brittle solids. That depth is known as the lithosphere’s bottom. Rocks below the lithosphere’s base are hot enough that they deform by flowing, despite remaining solid due to the high confining pressure produced by the weight of the rocks above. The asthenosphere is the layer on which the lithosphere rests.
As the rocks below move, the physical connection between the lithosphere and the asthenosphere causes a significant amount of pushing and pulling on the lithosphere. As a result, the lithosphere has fractured into a dozen large pieces known as lithospheric plates, or simply tectonic plates.
Although the lithosphere’s rocks are still considered elastic, they are not viscous but the asthenosphere is viscous, and the lithosphere-asthenosphere boundary (LAB) is where geologists and rheologists (scientists who study the flow of matter) distinguish the difference in ductility between the two upper mantle layers. The ability of a solid material to deform or stretch under stress is measured by ductility. The lithosphere has a much lower ductility than the asthenosphere.
What is the lithosphere made of?
The lithosphere is made up of rocks from 2 major layers of the Earth (crust and the mantle of the Earth). It contains the entire planet’s outer, thin shell, known as the crust, as well as the uppermost portion of the next-lower layer, known as the mantle. The lithosphere’s thickness varies; it is thickest below the continents and thinnest at mid-ocean ridges, which are raised areas of the seafloor where new seafloor crust is formed.
Composition of the lithosphere
- Physical composition
- Chemical composition
The physical properties of the lithosphere are composed of the crust as well as a portion of the upper mantle that is brittle and rigid.
The chemical composition of the lithosphere varies according to the layers. For example, soil, a mixture of weathered rock materials and organic matter makes up the crust and some of the elements discovered in the soil include oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. While magnesium and iron make up the mantle and the core is mostly made of iron.
Lithospheric plates are broken regions of the Earth’s crust and upper mantle that move across a deeper plasticine mantle. The Earth’s crust is divided into 13 major lithospheric plates but there are approximately 20 total lithospheric plates. Each lithospheric plate is made up of a layer of the oceanic crust or continental crust that is superficial to the mantle’s outer layer and is generally thought to be about 60 mi (96 km) thick. Each cross-section of the lithospheric plate may contain only continental or oceanic crust. These layers make the lithospheric plates move on top of the asthenosphere (the outer plastically deforming region of the Earth’s mantle).
The term ‘plate‘ is misleading, given that Earth is an oblate sphere but lithospheric plates are curved and fractured into curved sections similar to peeled orange sections. Analyzing lithospheric plate movements and dynamics necessitates more sophisticated mathematics that takes into account plate curvature which helps to clear the term plate.
There are three types of boundaries between lithospheric plates in geology. The first is when the lithospheric plates move apart at divergent boundaries, resulting in the formation of crust. The second occurs when the lithospheric plates move together in collision zones at convergent boundaries, where the crust is either destroyed by subduction or uplifted to form mountain chains. Finally, transform faults are formed as a result of lateral movements between the lithospheric plates at the sites of plate slippage.
There are specific geophysical forces that are characteristic of plate dynamics at each of the unique lithospheric plate boundaries. For instance, shearing forces exist between the lithospheric plates at transform boundaries while tensional forces dominate plate interaction at divergent boundaries, and the compression of lithospheric plate material predominates at subduction zones.
Plate tectonic dynamics, driven by deeper thermal processes, i.e. stress, and cause elastic strain on lithospheric materials results in rock fractures in the lithosphere which release energy in the form of seismic waves (an earthquake).
Types of lithospheres
- Continental crust
- Oceanic lithosphere
The continental crust is composed of a variety of igneous, metamorphic, and sedimentary rocks and the average composition is granite, which is much less dense than the oceanic crust’s mafic igneous rocks. The continental crust rises higher on the mantle than the oceanic crust, which sinks into the mantle to form basins, due to its thickness and low density. These basins, when filled with water, form the planet’s oceans.
The thickness of the continental lithosphere ranges from about 40 km to perhaps 280 km; the upper 30 to 50 km of the typical continental lithosphere is the crust. The change in chemical composition occurs at the Moho discontinuity which distinguishes the crust from the upper mantle. In the oldest parts of the continental lithosphere (underpin cratons), and the mantle lithosphere, there is a thicker and less dense mantle (roots of cratons) which aids in the stabilization of the continental and mantle lithosphere.
Because of its low density, the continental lithosphere that reaches a subduction zone can only subduct for about 100 km (62 mi) before resurfacing. As a result, the continental lithosphere is not recycled in subduction zones in the same way that the oceanic lithosphere is. This feature makes the continental lithosphere, to be a nearly permanent feature of the Earth.
The oceanic lithosphere or crust is composed of magma that erupts on the seafloor to form basalt lava flows or cools deeper to form the intrusive igneous rock gabbro. The seafloor is covered in sediments, which are primarily muds and the shells of small sea creatures. The oceanic crust and can be found in ocean basins.
The thickness of the oceanic lithosphere is typically 50–140 km but the layer beneath the mid-ocean ridges is not thicker than the crust. Furthermore, the oceanic lithosphere is primarily composed of the mafic crust and ultramafic mantle (peridotite) and it is denser than the continental lithosphere, in which the mantle connects with felsic rocks in the crust. It continues to thicken as it ages and moves away from the mid-ocean ridge. This thickening occurs as a result of conductive cooling, which converts the hot asthenosphere to the lithospheric mantle.
As the oceanic lithosphere ages, it thickens and becomes denser. In reality, it is a thermal boundary layer for mantle convection. It can also be observed that the oceanic lithosphere is less dense than the asthenosphere, however, after tens of millions of years, it continues to become denser than the asthenosphere.
The oceanic crust is lighter than the asthenosphere based on chemical differences but the thermal contraction of the mantle lithosphere causes it to be denser than the asthenosphere. The gravitational instability of the mature oceanic lithosphere, affects the subduction zones, causing the oceanic lithosphere to sink beneath the overriding lithosphere.
On a whole, at mid-ocean ridges, new oceanic lithosphere is constantly produced and recycled back to the mantle at subduction zones resulting in it being much younger than the continental lithosphere. The oldest oceanic lithosphere is approximately one hundred and seventy (170) million years old.
- The term lithosphere comes from the Greek words litho, which means “rocky,” and sphaira, which means “sphere.”
- In 1911, mathematician A.E.H. Love was the first to describe the concept of the earth’s structure consisting of an outer layer in his monograph titled “Some problems of Geodynamics.“
- Love’s theory was expanded upon by a geologist named Joseph Barrell, who discovered that the continental crust had both an upper solid crust and a semi-molten underlying layer. The semi-molten layer was dubbed the asthenosphere, and the upper solid crust was dubbed the lithosphere. He based his theories on gravity anomalies that existed over the earth’s continental crust.
- The thickness of the oceanic lithosphere ranges from 5 to 10 miles.
- The continental lithosphere is about 22 miles thick, but it can get up to 37 miles thick under certain mountain ranges.
- The continental lithosphere has been around for billions of years, whereas the oceanic lithosphere is much younger and is constantly being formed from mantle material at mid-ocean ridges.
- The African Plate, Antarctic Plate, Eurasian Plate, Indo-Australian Plate, North American Plate, South American Plate, and Pacific Plate are the lithosphere’s main plates.
- The Arabian Plate, the Caribbean Plate, the Cocos Plate, the Indian Plate, the Juan de Fuca Plate, the Nazca Plate, the Philippine Sea Plate, and the Scotia Plate are all smaller but significant plates.
- When a continental plate collides with an oceanic plate, the oceanic plate sinks beneath the continental plate.
- Natural resources such as coal, various fuels, and metals can be extracted from the lithosphere. It is also beneficial to plants because it provides them with the minerals they require for growth.
- The continental lithosphere is made up of felsic rock, which is an igneous rock. This rock is abundant in the elements necessary for the formation of quartz and feldspar.
- Mafic crust and ultramafic mantle make up the oceanic lithosphere. The mafic crust is composed of iron and magnesium-rich silicate minerals. The ultramafic crust is composed of Peridotite, a mineral composed of olivine and pyroxene that contains less than 45 percent silica.
- Because earthquakes occur when tectonic plates shift or collide, the lithosphere is the location where all earthquakes occur on Earth.
- 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 terranes.
- The movement along California’s San Andreas Fault line has resulted in the formation of 17 terranes in the San Francisco Bay region.
- It is made up of tectonic plates, which are the planet’s continents.
- It is thought that the continents began as a single solid landmass known as Pangaea, which then split into several tectonic plates, resulting in the formation of the individual continents.
- The lithosphere is divided into two parts: continental and oceanic lithosphere.
- While the top layer of the lithosphere is generally the same temperature as the surface, it rises by 35°C for every 100 meters below the surface, reaching a maximum temperature of 1280°C where it ends at the asthenosphere.
The Interaction of the Lithosphere with Other Spheres
The cool, brittle lithosphere is one of five great “spheres” that shape the Earth’s environment. The biosphere (Earth’s living things); the cryosphere (Earth’s frozen regions, including both ice and frozen soil); the hydrosphere (Earth’s liquid water); and the atmosphere (the air surrounding our planet) are the other spheres. These spheres interact to influence a wide range of factors, including ocean salinity, biodiversity, and landscape.
The pedosphere, for example, is a component of the lithosphere composed of soil and dirt, and the interaction of the lithosphere, atmosphere, cryosphere, hydrosphere, and biosphere results in the formation of the pedosphere. The powerful movement of a glacier (cryosphere) can reduce massive, hard rocks to powder. Weathering and erosion from wind (atmosphere) or rain (hydrosphere) can wear down rocks in the lithosphere. The pedosphere is formed when organic components of the biosphere, such as plants and animals remain, combine with eroded rocks to form fertile soil.
To influence temperature differences on Earth, the lithosphere interacts with the atmosphere, hydrosphere, and cryosphere. Tall mountains, for example, typically have much lower temperatures than valleys or hills. The lithosphere’s mountain range interacts with the atmosphere’s lower air pressure and the hydrosphere’s snowy precipitation to create a cool or even icy climate zone. The climate zone of a region, in turn, influences the adaptations required by organisms in the region’s biosphere.
Mantle of the Earth and the Earth’s core
The Mantle of the Earth
The Earth’s mantle’s most important characteristics are that it is made of solid rock and is extremely hot. Seismic waves, heat flow, and meteorites have all provided evidence that the mantle is composed of rock, and its properties of ultramafic rock peridotite (peridotite is a mineral that is rarely found on the Earth’s surface), which is composed of iron- and magnesium-rich silicate minerals also attest to the fact that it is made of solid rock. . The mantle is known to be extremely hot because of the heat that radiates from it as well as its physical properties.
Within the Earth, heat moves in two ways; conduction and convection. Conduction is described as heat transfer that occurs as a result of rapid atom collisions, which can only occur if the material is solid. Heat moves from warmer to cooler areas until they are all at the same temperature. The mantle is hot primarily as a result of heat conducted from the core.
Convection on the other hand is the process by which a material with the ability to move and flow develops convection currents. Convection in the mantle is identical to convection in a pot of water on the stove. This is because as material near the core heats up, convection currents form within the Earth’s mantle, and as the core heats the base layer of the mantle material, particles accelerate, causing their density to decrease and rise.
The convection current is initiated by the rising material which means that the warm material spreads horizontally when it reaches the surface and when it is no longer in contact with the core, the material cools off. It eventually cools off and becomes dense enough to sink back into the mantle and the material travels horizontally back to the bottom of the mantle where it is heated up by the core. The mantle convection cell is complete when it reaches the location where warm mantle material rises.
The Earth’s core
A dense metallic core lies at the planet’s center and for a variety of reasons, scientists believe the core is made of metal and it has been observed that the density of Earth’s surface layers is much lower than the overall density of the planet, as calculated by the rotation of the planet. For instance, if the surface layers of the Earth are less dense than average, then the interior of the Earth must be denser. According to calculations, the core is approximately 85 percent iron metal, with nickel-metal accounting for the majority of the remaining 15 percent.
Metallic meteorites are also thought to represent the core because the planet would not have a magnetic field if its core was not made of metal. This conclusion is based on the fact that iron and other metals are magnetic, and because seismic waves stop at the inner core, scientists know that the outer core is liquid and the inner core is solid. Convection in the liquid outer core causes a strong magnetic field while heat from the even hotter inner core causes convection currents in the outer core. The inner core’s breakdown of radioactive elements generates the heat that prevents the outer core from solidifying.
Where is the lithosphere located?
It lies beneath the atmosphere and above the asthenosphere.
What is the composition of the lithosphere?
The physical composition of the lithosphere consists of the crust and the upper mantle. While the chemical composition consists of elements such as iron, magnesium, oxygen, silicon, aluminum, potassium, calcium, and sodium.
How is the lithosphere formed?
It is formed at the mid-ocean ridges where hot magma upwells and cools to form plates as it moves away from the spreading center.