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Lithosphere

The term "lithosphere" comes from the Greek lithos (rock) and was introduced by Suess (1885). The term was later used by Barrell and ultimately defined by Isacks as a near surface layer of strength of earth. Even today it remains difficult to find a more precise definition than this. The lithosphere is the outer solid layer of the earth. As for the whole globe, the lithosphere can be divided according to its physical properties or according to its chemical (material) properties. Because there is an overlap between layers distinguished on the basis of different properties. Now, we differentiate lithosphere and asthenosphere on the basis of physical properties like density, temperature and seismic velocity, not only on the basis of its strength. The lithosphere is generally associated with high average densities, cooler temperatures and high average seismic velocities than the asthenosphere.

 

Definition

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The lithosphere is the outermost layer of Earth. Its surface is dimpled and pinched with deep ocean basins, where most of Earth's water collects and puckered with lofty mountain peaks. The lithosphere part of the crust ranges from 30 to 60 miles (50 to 100 km) in thickness and consists of two parts. Most of the lithosphere is solid rock, but the rock gets softer, almost plastic in its lower levels where it overlaps with the upper mantle. The upper part of the lithosphere is what we call the crust of Earth. The crust is divided into two distinct types, the ocean crust and the continental crust. The ocean crust is covered by the ocean. The continental crust is the part we live on. The relatively thin ocean crust ranges from 3 to 4.5 miles (5 to 7 km) in depth, while the thicker continental crust ranges from 22 to 43.5 miles (35 to 70 km). That makes the continental crust about seven times thicker than the oceanic crust. The lower part of the lithosphere overlaps part of the mantle beneath. This overlap zone is called the asthenosphere. The word means "weak". Asthenosphere rock is very hot and under great pressure from the crust above. The heat and pressure comes from the weight of the crust pressing down on it. the heat softens the rock and pressure causes it to flow very slowly. The schematic sketch of the lithosphere and related areas are illustrated below:


Plates

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The rheological stratification suggests that the lithosphere is a true stress guide and that the easiest motion to realize is horizontal gliding of the lithosphere on top of the asthenosphere. The motions in the asthenosphere are not identical to the motions of the rigid lithospheric plates and the whole system may be largely thought of as an exchange of matter primarily between lithosphere and asthenosphere. Certainly, the motions of the asthenosphere must be largely controlled by the peculiarities of the overlying lithosphere. As only the lithosphere reacts to stress as a brittle solid, all earthquakes must originate within the lithosphere. The global seismicity is consequently the expression of relative moveemnts between lithospheric plates. This leads to a global tectonic model in which the surface of the earth consists of rigid plates in relative displacement, gliding on top of a mechanically soft layer. 

The first global map of lithospheric plates appeared in published form in 1968 and there have since been many minor changes and revisions in the map. Boundaries have been relocated and new plates identified; differences have appeared in the naming of plates, as well. For a particular lithospheric plate to be identified and named, its boundaries should all be active. In other words, there must be good evidence of present or recent relative motion between the plate and all its contiguous plates. The global system of lithospheric plates consists of six great plates. These are listed in the table. Several lesser plates are also recognized, ranging from intermediate in size to comparatively small. Several sub plates are also recognized within the great plates.
Great Plates  Lesser Plates 
Pacific
American
Eurasian
Persian sub plate
African
Somalian sub plate   
Austral-Indian
Antartic
Nazca
Cocos
Philippine
Caribbean          
Arabian
Caroline
Bismark
Scotia



Composition

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From the point of view of plate tectonics, oceanic lithosphere is the relatively cold, strong, outer part of the Earth in the ocean basins. Far from mid-ocean ridges, this layer has a fairly uniform thickness of about 120 km. To understand how the Earth generates new oceanic lithosphere, it is helpful to know its composition and structure. Apart from the areas at the near mid ocean ridges, the seabed is covered in very fine mud. Drilling into this sediment layer shows that it can up to several hundred meters thick and that it is underlain by volcanic rocks of basaltic composition.

Continental lithosphere is a combination of two layers, in which each layer composed of materials that are rare in the Earth's upper part. Continental crust contains low temperature distillates which is produced during successive melting events. The underlying lithosphere is comparatively pure concentrate with high temperature, and contains highly refractory minerals. Materials with intermediate composition, which should have been far more abundant, is missing. 

Facts

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Some of the important facts related to the lithosphere are mentioned below. Please go through these facts and understand more about the lithosphere.
  • Lithosphere is the rigid, upper layer of Earth. It consists of the crust and top portion of the mantle.
  • Top of the mantle is rigid like the crust, but chemically it is different.
  • The lithosphere is divided into several parts, known as tectonic plates. The continents are situated on these plates.
  • Lithosphere has less thickness under the middle of the ocean.
  • The continent crust has almost 120 km thickness.

Importance

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The lithosphere is very much important because it act as a big reservoir of many resources. Almost all materials which is useful to human are supplied by lithosphere. The lithosphere serves as a source of different minerals. And it is a major source of fuels like petroleum, coal and natural gases. The combination of lithosphere, atmosphere and hydrosphere plays an important role in the existance of animals and plants in Earth. It provides the sufficient nutrients to plants. These plants are the major food source of human and the animals.

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