Structure interns Earth
The internal structure of the Earth is divided into several successive envelopes, whose principal ones are the Earth's crust, the coat and core. This representation is very simplified since these envelopes can be broken up they-even. To locate these layers, the seismologists use the seismic waves, and a law: As soon as the speed of a seismic wave changes brutally and in an important way, it is that there is change of medium, therefore of layer. This method allowed, for example, to determine the state of the matter to depths which the man cannot reach. (deep coat - core)
These layers are delimited by discontinuities like the Discontinuité of Mohorovic, that of Gutenberg, named according to the seismologist Beno Gutenberg, or that of Lehmann. To include/understand this constitution, it is necessary to go back to the formation of the Earth, it was formed by accretion of Météorite S and during this formation, the various layers were installation because of the Density for example of its components.
Some historical stakes
Antiquity at the XVIIIe centurySince Antiquity many are those which were illustrated in their attempts at explanation of the internal constitution of our sphere. Some of these intellectuals sought to stick to the vision of the ground (relief, volcanos, earthquakes), others also wanted to incorporate in their model an explanation of the biblical texts (the flood). Will come then the period when the assumptions will be supported by experiments: it will be the era of geophysics. One thus finds in this gallery of portraits: mathematicians, philosophers, theologists then more tardily of the naturalists, physicists and geologists. We will retain here only most known.
For Aristote (fourth century BC), our planet consists of ground and rock surrounded by water then of air. Come then a layer from fire and the stars. Until Copernic this vision will evolve/move little, but in the middle of the XVIIe century an expansion of novel ideas appears.
In 1644, the Earth presented by Descartes in “Principles of philosophy” is an old sun which kept core of a solar type but whose external layers evolved/moved. Several layers follow one another starting from the center: rock, water, air then finally an external crust balances some on this air. This broken crust formed the reliefs and let pass water coming from the depths which formed seas and oceans.
At the same time, Athanasius Kircher also postulates him that the terrestrial sphere is a cooled star but which it contains under the crust a matter in fusion which escapes sometimes from the center by the volcanos.
At the end of XVIIe and during the 18th century, a great quantity of assumptions will be emitted:
XVIIIe at the XXe centuryWith the rise of the Geology, the theories will have to stick to the observation and geophysics measurements. The little of influence of the mountainous masses on local gravity thus tends to prove that the Earth is not hollow.
The light flatness of the sphere to the poles and the igneous nature of certain rocks make say to Georges de Buffon that the Earth was in fusion at its origin. The measurement of the regular increase in the temperature with the depth in the mines (1°C for 25 meters) encourages Joseph Fournier and Pierre Cordier (1777 - 1861) to extrapolate and deduce that the center of our planet is in fusion at a temperature of several thousands of degrees. The origin of this temperature will lengthily be discussed: remains original heat on a sphere in the course of cooling or rise in the temperature due to chemical reactions or nuclear internal? Moreover, wouldn't this heat be sufficiently intense so that all the matter interns is gas beyond certain depth?
For William Hopkins, the variation of the melting point of the rocks according to the pressure makes once again lean the balance in favor of a solid core. The very low level of the movements of the ground related to the tide (evaluated by comparison with the precise measurement of the oceanic tides) pleads, according to Lord Kelvin, for a sphere with the properties of an elastic solid and not of a fluid.
Analysis of the composition of the terrestrial and meteoritic rocks, as well as the measurement of the average density of the sphere (5,5) influential on several models where a fine light silicate crust recovers a bulky metal core denser. Lastly, the data analysis seismological which will prove increasingly precise, will make it possible to establish the current model.
Methods of investigation
Human explorationThe Speleology, activity with the multiple facets, hardly lends itself, even in its sporting component, with the establishment of records. A long time the dimension - 1 000 was only one dream which technology did not make it possible to concretize. It is in 1956 with the Gouffre Shepherd, in the Massif of Vercors (Isere), that this mythical depth was reached for the first time. In 2005, spectacular depth of - 2 000 meters was exceeded by speleologists with Krubera Cave (ex Voronja pit), in the the Western Caucasus (Abkhazie).
- 2000 meters C `is well, but what is there afterwards? What does one find low, below, increasingly deeper? What really hides behind the treasures of imagination of Jules Verne and his “ Voyage in the center of the Earth ” ? It is what we will try to discover here, we who we satisfy with effleurer the first hectometers of a sphere 12740 kilometers in diameter.
The variety of the grounds explored in the mines is much more important than the extents of sedimentary rocks traversed by the speleologists and the exploited grounds are much older. Moreover minors daily côtoient the phenomenon of rise in the temperature which as of the XVIIIe century will influence the assumptions of a sphere in the middle in fusion.
At all events, even the deepest mines of the world (~3 500m for the Western Deep Levels of South Africa in 2002) does nothing but scratch the Earth's crust and without the contribution of methods of indirect exploration, we would have remained completely ignoramuses of the deep contents of our sphere.
Major drillingsThe objective of major drillings such that of the program KTB ( Kontinental Tiefbohrprogramm der Bundesrepublik ) which reached 9 800 meters under the Germany or that of 13 kilometers in the Peninsula of Cola (Russia), is better to know the Lithosphère and to reach the zone of transition between this one and the coat supérieur : the Moho.
If these drillings made it possible to confirm the structure and the composition of the crust, or to trace regional seismic profiles, they unfortunately did not make it possible to date reach the so much coveted subjacent layer. One thus could measure for example that the temperature of the rocks reaches approximately 300 °C with 10 kilometers of depth.
As the oceanic crust is thinner than the continental plates, several projects were born to try an opening on this level: MOHOLE then JOIDES in the USA, and programs international IPOD or ODP/DSDP. Alas, no ship still succeeded in drilling until the discontinuity of Mohorovicic.
The study of the meteoritesTo include/understand how the successive layers of the Earth gradually were different would be largely facilitated by the knowledge of the exact composition of the primitive material which gave him birth. The elements absolutely essential to the right formula are iron, nickel and the silicates. One finds these elements (and several others) in a type of meteorites called Chondrite S. They contain small spherical silicate zones solidified after fusion, the chondres, whose name is at the origin of the name of these meteorites.
Some of them as the Chondrite Allende contain a metal mixture of iron and iron oxide as well as a great quantity of carbon; others like the Chondrite of Indarch, metal iron and a magnesium silicate (MgSiO3): the enstatite, extremely frequent in the terrestrial coat. Other chondrites, more primitive, show iron completely oxidized, they are meteorites carbonaceous Ci: they are very close by their composition to the gas nebula which gave rise to the solar system there is approximately 4,57 billion years and with the Earth there is 4,45 billion years.
Among all these Chondrite S, only those containing 45% of enstatite present a chemical composition and isotopic in adequacy with the density and the current major nature of the Earth (several layers of light silicates and a core where migrated heavier metals). Obviously, these meteorites have a size well too low and are thus not differentiated: their elements remained distributed there in a relatively homogeneous way.
Indirect investigations (geophysics)
Seismic tomographyIt is the analysis of the recordings obtained thanks to the seismographs which will make it possible to completely renew the model of the Earth during the XXe century. The principle is relatively simple: following a seism one determines the position of his epicentre most precisely possible. Then one records the vibrations which are propagated through all the sphere. These undulatory phenomena are subjected to physical laws such as the reflection or the refraction. Moreover, they do not move all at the same speed according to the medium that they cross what makes it possible to evaluate the contents of the Earth by the attentive examination of the curves time/distance covered. The waves studied in the seismic Tomographie are the basic waves which traverse the terrestrial sphere in all the directions. The waves of surface, which cause the damage with human constructions, are propagated only in the crust and do not give any information on the deep layers.
Certain waves arrive quickly: they are the waves P (like Premières); others are delayed and recorded later: they are the waves S (like Secondes).
The speed of the two types of waves P and S varies according to the density of crossed material. The softer the crossed layer is, the more the waves are propagated slowly. Moreover, when a wave P arrives not perpendicularly on a zone of transition (interface coat-core for example) a small portion of its energy is converted in another form of wave (a fraction of P becomes S then). The interpretation of the seismographic statements is thus difficult because the layouts of many types of waves overlap there which should be disentangled and which one must explain the origin. To find itself there a little better, one indicated all these waves by different letters which one can then progressively combine of their evolution (see table below).
Thus a PP wave is a wave P which, after having undergone a reflection on the surface of the terrestrial sphere, remained in the coat before reappearing on the surface where it is detected. A wave PKP will be a wave P which arises on the surface after having crossed the external core liquidates (way = coat/core ext./coat). One can thus lengthen name as much as necessary. Let us take a rather complex example: a quasi vertical wave crossing the terrestrial sphere right through after having rebounded on the surface and to be last twice (with the outward journey and the return) by the core and seed will reappear on the affublée surface of the nice nickname, palindrome completely unpronouncable, of PKIKPPKIKP!
During the XXe century, several essential discoveries were made thanks to the seismic tomography.
In 1912, Beno Gutenberg (1889-1960) replaces the interface coat/core to 2900 km of depth thanks to the study of the waves P, giving its name to discontinuity between the lower coat and the external core, discontinuity known as of Gutenberg.
In same time, of 1923 with 1952, other geophysicists (Adams, Williamson, Bullen, Birch…) work on equations allowing to determine the variation of the density with the depth and the pressure which it generates.
From now on, the essence of the structure of our sphere is posed. Remain to improve dynamic comprehension of it interns for better including/understanding its evolution, its sudden starts, the variations of the magnetic field, etc
The study of magnetismThe terrestrial magnetism is a phenomenon very interesting and extremely complex to interpret. The Earth behaves as a kind of self-sustained Dynamo which generates an important magnetic field (that which deviates the needle of the compass and protects us from certain cosmic disturbances). This field is variable in time and it was even reversed hundreds of times since the origin. To interpret this dynamics is indissociable comprehension of the composition of the internal structures of the terrestrial sphere and their movements.
Attempts at numerical modeling and experiments in laboratory are being studied. If they did not make it possible yet to create a $dynamo effect in a sphere, they showed that columns of convection appear at certain temperatures according to the viscosity of the liquid and number of revolutions. These movements are compatible with the assumptions of creation of the terrestrial electromagnetic field such as we know it.
Detailed structure(1) continental Crust solid primarily granitic surmounted by place of sedimentary rocks. It is thicker than the oceanic crust (of 30 km to 100 km under the mountainous solid masses). The crust or Earth's crust accounts for approximately 1,5% of terrestrial volume. It was in the past called SIAL (silicon + aluminum).
(2) oceanic Crust solid primarily made up of basaltic rocks. Relatively fine (approximately 5 km). It is also called also SIMA (silicon + magnesium).
(3) Zone of subduction where a plate is inserted sometimes until several hundred kilometers in the coat.
(4) higher Coat which is more viscous than the lower coat because the physical constraints which reign there return it liquid partly. It is made primarily of rocks such as the peridotite (its minerals are: olivine, pyroxene, garnet). With the contact between the crust and the higher coat one can sometimes detect a zone called LVZ. (see n°11).
(5) Eruptions on zones of active volcanicity. Two types of volcanicities are represented here, deepest of both is known as “of hot spot”. They would be volcanos whose magma would come the depths of the coat close to the limit with the liquid core. These volcanos would thus not be related to the tectonic plates and, thus not following the movements of the Earth's crust, they would be thus almost motionless on the surface of the sphere, and would form the archipelagoes of islands like that of Tahiti.
(6) Coat lower than the properties of an elastic solid. The coat is not liquid as one could believe it by looking at the lava flows of certain volcanic eruptions but it is less " dur" that other layers. The coat accounts for 84% of terrestrial volume.
(7) Plume of hotter matter which, on the basis of the limit with the core, melts partially while arriving close to the surface of the Earth and produces the volcanicity of hot spot.
(8) external Core liquid primarily composed of iron (approximately 80%) and nickel plus some lighter elements. Its viscosity is close to that of water, its average temperature reaches 4000 °C and its density 10. This enormous quantity of fused metal is certainly agitated (by convection, but also following the various rotation movements and of precession of the terrestrial sphere). Liquid iron flows can generate there electric currents which give rise to magnetic fields which reinforce the currents thus creating a $dynamo effect by discussing the ones the others. The liquid core is thus at the origin of the Terrestrial magnetic field.
(9) Noyau interns solid (or granulates) primarily metal made up by progressive crystallization of the external core. The pressure maintains it in a solid state in spite of an higher temperature with 5000 °C and a density from approximately 13. Internal and external core account for 15% of terrestrial volume.
(10) Cells of convection of the coat where the matter is moving slow. The coat is the seat of currents of convection which transfer the major part of calorific energy from the core of the Earth towards surface. These currents cause the continental drift but their precise characteristics (speed, amplitude, localization) are still badly known.
(11) Lithosphere: it is consisted of the crust (tectonic plates) and part of the higher coat. The lower limit of the lithosphere is with a depth ranging between 100 and 200 kilometers, with the limit where the peridotites approach their melting point. Sometimes one finds at the base of the lithosphere (certain geologists there include) a zone called LVZ (for “Low Velocity Zone”) where a reduction speed is noted and a marked attenuation of the seismic waves P and S. This phenomenon is due to the partial Fusion peridotites which involves a greater fluidity. The LVZ is generally not present under the roots of the mountainous solid masses of the continental crust.
(12) Asthenosphere: it is the lower zone of the higher coat (in lower part of the lithosphere)
(13) Discontinuity of Gutenberg: zone of transition coat/core.
(14) Discontinuité of Mohorovicic: zone of transition crust/coat (it is thus included in the lithosphere).
On this figure, the temperatures are given in degrees Celsius as an indication. Not being able to be measured directly but only deduced, they are approximate (the more one is inserted and the larger the margin of error is). Moreover, the terrestrial sphere is not perfectly spherical and the real ray equatorial is higher of a score of kilometers than the polar ray . This made that the the Mississippi, which has its source close to the Big lakes, flows at a “altitude” (that of its mouth in the Gulf of Mexico) larger than “the altitude” of its source, if altitude is measured compared to the center of the ground. Water “is descended” while running out if one reasons in terms of mechanical potential energy, the sea level being taken as reference of altitudes.
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