The Lithosphere , Also called rock sphere, is the most superficial layer that forms the Composition of the earth's crust And has an average thickness of 100 kilometers.
Below the lithosphere, in the upper mantle is a soft and plastic layer, known as asthenosphere ("weak sphere"). The upper layer of the asthenosphere, whose temperature and pressure conditions allow part of the layer to be of molten rock, is the one that separates the lithosphere from other layers.
The lithosphere is separated from the asthenosphere by the layer of molten rock and consequently the first moves independently of the second.
The lithosphere is a brittle solid similar to rocks on the surface. The rocks of the lithosphere progressively heat up and become more ductile as they are found at greater depth. In contrast, the upper asthenosphere is soft because it is in a melting point with the lithosphere.
In general, the eight most abundant elements that are part of the lithosphere are called geochemical elements and are:
- Oxygen (49.50%)
- Silicon (27.72%)
- Aluminum (8.13%)
- Iron (5.0%)
- Calcium (3.63%)
- Sodium (2.83%)
- Magnesium (2.09%)
- Potassium (2.59%)
Here are some of the main features that define the lithosphere as a terrestrial layer:
Characteristics of the lithosphere
1- Rigid components
The whole of the elements that make up the lithosphere are rigid and their components can be inorganic, undissolved, produced by the decomposition and the weathering of the superficial rocks. According to the rigidity of the lithosphere and its components this is divided into:
- Thermal lithosphere (conduction of heat predominates over convection of heat).
- Seismic lithosphere (a reduction in the propagation velocity of the S waves and a high attenuation of the P waves).
- Elastic lithosphere (layer that moves according to the Movement of tectonic plates ).
In general, rocks of the lithosphere account for 95% of known minerals. In its superior it is superiorly limited by the atmosphere and the hydrosphere. Both also influence the processes that transform the surface of the earth.
2- Predominance of sedimentary rocks
The lithosphere is composed of sedimentary and igneous rocks. The upper part of the lithosphere in 95% is composed of magmatic or igneous formations, although often presents sedimentary rocks. In the continents, the lithosphere is composed mainly of granite rocks superimposed by a solid layer.
Sedimentary rocks are formed by accumulation of sediments, transported by water, ice or wind. These rocks are subjected to diagenesis, ie physical and chemical processes that encourage the materials to solidify.
These types of rocks form on the banks of rivers, in ravines, valleys, seas and at the mouths of rivers. The igneous rocks have magmatic origin, that is to say they are formed when the magma cools.
There are two types of igneous rocks: plutonic or intrusive and volcanic or extrusive. The intrusive rocks are formed inside the surface of the earth by the magma that solidifies, while the extrusive ones are formed by the magma outside the earth. They are usually the result of a rash.
According to their texture, the ingrown rocks are classified as: vitreous, aphanic or fine-grained, faneritic or coarse-grained, porphyritic, pegmatitic and pyroclastic.
And according to their chemical composition, they are classified in: dark or ferromagnesic and clear. The latter are rich in iron, magnesium and low in silica.
For its part, sedimentary rocks are classified according to their origin in: detrital rocks, organogenic rocks, chemical rocks and marls. And according to its composition in: terrígenas, carbonáticas, silíceas, organic, ferro-alumínicas and phospháticas.
3- Soil of organic and inorganic substances
The constituent parts of the lithosphere are soil minerals, organic substances and living organisms, water, gases. The organisms after decomposition become part of the humus (fertile part of the soil).
In this sense, the upper layer of the lithosphere, the soil, depends greatly on the cycle of atoms involving plants, animals and microorganisms.
The inorganic part of the soil changes under the influence of living matter, water and gas. The crushing of rocks happens not only by physical erosion, but also by the decomposition of living organisms.
The physical wear of the rock is a result of the vital activity of plants and microorganisms. For example, vegetation, especially climbing plants, is attached to the rock by tearing off pieces of it.
Subsequently, these pieces are wrapped in other plants that penetrate them. And in this line, the carbonic acid formed during the process of respiration and wilting of the plants also affects the upper layer of the lithosphere.
4- Division of plates
The lithosphere is divided into lithospheric plates. According to the theory of plate tectonics, lithospheric plates are limited to areas of seismic, volcanic and tectonic activity, that is to say plate boundaries, which are classified as: divergent, convergent and changing limits.
From geometrical considerations, it is evident that at the same point only three plates can converge. A point at which four or more plates converge, is unstable and decomposes rapidly over time. In turn, there are two fundamentally different types of earth crust: the continental crust and the oceanic crust.
Some of the lithospheric plates are composed entirely of the oceanic crust. For example the Pacific plate, which is the largest plate in the world. While others are composed of a block of continental crust and oceanic crust.
These fuse and constantly change their shape and can be divided as a result of rifting and join to form a single plate, as a result of collisions.
Lithospheric plates can also sink into the mantle of the planet, reaching the depth of the outer core of the earth. Due to the constant movement of the plates, their limits change with time and the size of some is unknown. In turn, the speed of movement of the plates has also changed over time.
Consistent with the above, the horizontal movement velocity of the lithospheric plates currently ranges from 1 to 6 centimeters per year.
However, the speed of movement to different directions may be different. For example, the speed of the Atlantic plate in the northern part is 2.3 centimeters a year while in the southern part it is 4 centimeters a year.
Generally, the plates separate faster near the Eastern Pacific ridge on Easter Island where it is determined that its velocity is 18 centimeters a year. Conversely, the plates move more slowly in the Gulf of Aden and the Red Sea whose speed is 1-1.5 centimeters a year.
The major plates are: North American, African, South American, Pacific, Eurasian, Australian and Antarctic. The Pacific plate covers a significant portion of the Pacific Ocean basin and is the largest in the world. Most large plates include a whole continent or an entire ocean. For example, the South American plate contains the entire subcontinent.
This fact constitutes an important antithesis to the continental drift hypothesis of Alfred Wegener, who proposed that continents moved across the ocean floor, not with it.
5- Plate movement
On the other hand, Wegener considered that none of the plates is defined completely by the margins of a continent. Although it has now been shown that this part of his hypothesis is incorrect.
Another idea of Alfred Wegener's theory is that the plates move coherently in relation to all other plates. As plates move, the distance between two points on the same plate is constant, while the distance between points on different plates changes gradually.
That is to say that the distance between two cities in South America does not change, however much the plates move since the cities are located in the same plate. On the other hand, the distance between Rio de Janeiro and London changes gradually.
References
- Bird, J. M. and B. Isacks. Plate Tectonics. American Geophysical Union, Washington, D.C., 1972.Motz, L. The Rediscovery of the Earth. National Council of Science and Technology, 1982.
- Wilson, J. T."Continental drift and plate tectonics". Selections from Scientific American, 2nd edition. Blume, Madrid, 1976.
- Tarbuck, E.J. And Lutgens, F.K. Earth Sciences: An Introduction to Physical Geology. 8ª ed. Madrid: Pearson Prentice Hall, 2005.
- Monroe, J.S.; Wicander, R.; And Pozo Rodriguez, M. Geology: Dynamics and evolution of the Earth. Cambridge: Cambridge University Press.