Internal Structure Of The Earth

Internal structure of the Earth:
1. Continental Crust
2. Lithosphere
3. Upper mantle (or asthenosphere )
4. Lower mantle (and Mesosphere )
5. Outer Core
6. Inner Core (or seed)
A. Moho
B. Gutenberg Discontinuity
C. Lehmann Discontinuity

The internal structure of the Earth is divided into several successive envelopes, the main ones being the crust , the mantle and core. This representation is very simplified since the envelopes can themselves be decomposed. To identify these layers, seismologists use seismic waves , and a law: As soon as the speed of a seismic wave changes suddenly and dramatically, there is change of environment, so layer. This method allowed, for example, determine the state of matter at depths that man can not reach (deep mantle, core).

These layers are bounded by discontinuities, such as the discontinuity of Mohorovic , that of Gutenberg , named after seismologist Beno Gutenberg, or that of Lehmann. To understand this constitution, we must go back to the formation of the Earth by accretion of meteorites , the different layers, so having put in place under the influence of various parameters, such as the density of its constituents.

Some historical milestones

From antiquity to the eighteenth ^century

Since ancient times, many people who have distinguished themselves in their attempts to explain the internal constitution of our globe. Some of these intellectuals have tried to stick to the vision of the land (topography, volcanoes, earthquakes), others also wanted to incorporate into their model for an explanation of the biblical texts (the flood). Then comes the period when the hypotheses are supported by experiments: this will be the era of geophysics. It is therefore in this gallery of portraits of mathematicians, philosophers, theologians and later naturalists, physicists and geologists. We will mention here only the best known.

For Aristotle ( ^fourth century BC. ), our planet is made of earth and rock surrounded by water and air. Next come a layer of fire and the stars. Copernicus until this vision is likely to continue, but in the middle of the seventeenth ^century a wealth of new ideas appear.

In 1644 , the Earth presented by Descartes in Principles of Philosophy "is an ancient sun has kept a core of solar-type but whose outer layers have evolved. Several successive layers from the center: rock, water, air and finally an outer crust balanced on the air. This crust has formed the broken reliefs and allowed water to pass from deep which formed seas and oceans.

At the same time, Athanasius Kircher also postulates that the Earth is a star but he has cooled below the crust molten material escapes from the center by some volcanoes. At the end of the ^seventeenth and during the ^eighteenth century , a lot of assumptions are made:

• Earth after an ancient comet: William Whiston (1667 - 1752)
• Earth was composed of a fluid mixture that was deposited by gravity over time: John Woodward (1665 - 1728) and Thomas Burnet (1635 - 1715)
• Hollow Earth several concentric shells and magnetized core separated by a vacuum: Edmund Halley (1656 - 1742)
• Earth completely hollow where the thin outer crust is in equilibrium between gravity and centrifugal force: Henri Gautier (1660 - 1737)

From the eighteenth ^to twentieth ^century

With the rise of geology , theories will have to stick to the observation and geophysical measurements. The limited influence of mountain masses on the local gravity tends to prove that the Earth is not hollow.

The slight flattening at the poles of the earth and the nature of certain igneous rocks are told to Georges de Buffon that the Earth was molten at its origin. Measuring the steady increase of temperature with depth in mines (1 C for 25 meters) induces Joseph Fourier and Pierre Cordier (1777 - 1861) to extrapolate and deduce that the center of our planet is melting at a temperature of several thousand degrees. The origin of this temperature is much debated: is the original heat on a globe during cooling or temperature rise due to internal chemical reactions or nuclear? Moreover, this heat would it not intense enough that all matter is internal gas beyond a certain depth?

To William Hopkins , the variation of the melting point of rocks based on the pressure is once again tipped the balance in favor of a solid core. The very low level of ground motion related to the tide (assessed by comparison with accurate measurement of ocean tides) argues, by (Lord Kelvin), a globe for the properties of an elastic solid and not fluid.

The analysis of the composition of terrestrial rocks and meteorites, and the extent of the global average density (5.5) affect several models where a thin light crust silicates covers a large metallic core denser. Finally, analysis of seismic data that will prove more accurate, will help establish the current model.

Methods of investigation

Investigations direct

Human Exploration

The caving , multi-faceted activity, does not lend itself even in his sporting component, the establishment of records. Long document 1000 was only a dream that the technology did not realize. It was in 1956 at the Berger , in the Vercors ( Isere ), that this depth was reached mythical first. In 2005 , the depth of 2000 meters spectacular was exceeded by cavers to Krubera-Voronja (ex Voronja gulf) in the western Caucasus (Abkhazia).

Moreover, the variety of terrain explored in the mines is much larger than the expanse of sedimentary rocks covered by cavers and land operated are much older. The miners are in daily contact the phenomenon of elevated temperature from the eighteenth ^century will affect the assumptions of a globe in the heart melt. Anyway, even the deepest mines in the world (~ 3 500m for the Tau Tona of South Africa in 2002) only scratch the earth's crust.

What is hidden behind the real treasures of imagination of Jules Verne and his " Journey to the Center of the Earth "? Without the contribution of indirect methods of exploration, humans would have remained totally ignorant of the deep content of the world beyond the few first two or three kilometers.

The deep drilling

The goal of deep drilling, like that of the KTB (Kontinental Tiefbohrprogramm der Bundesrepublik), which reached 9800 meters in the Germany , or one of 13 km in the Kola Peninsula (Russia), is to better understand the lithosphere and achieve the transition zone between it and the upper mantle: the Moho.

If these wells have confirmed the structure and composition of the crust, or to draw regional seismic profiles, they have unfortunately not achieved to date the underlying layer coveted. It was thus possible to measure such as temperature rocks reaches about 300 C to 10 kilometers deep.

As the oceanic crust is thinner than the continental plates, several ocean drilling projects have emerged, then Moholi DSDP (1968-1983) in the U.S. and international programs like ODP (1985-2003) and IODP (2003 -2013). For now, no vessel has been able to drill up to the discontinuity Mohorovii.

The study of meteorites

Understanding how the layers of the Earth gradually differentiated would be greatly facilitated by knowing the exact composition of the primitive material that gave it birth. Elements absolutely essential to the formula are iron, nickel and silicates. We find these elements (and others) in a type of meteorites called chondrites. They contain small spherical zones of silicates solidified after melting the chondrules, whose name is the origin of the name of these meteorites.

Some of them, like chondrite Allende, contain a mixture of metallic iron and iron oxide, and a large amount of carbon while others, such as chondritis of Indarch, metallic iron and silicate magnesium (MgSiO _3), enstatite, extremely common in the mantle. Other chondrites, most primitive, show completely oxidized iron, it is the CI carbonaceous meteorites, and they are very close in composition of the gaseous nebula that gave birth to the solar system there are about 4.57 billion years, and to the Earth there are 4.45 billion years.

Among all these chondrites , only those containing 45% enstatite have a chemical and isotopic composition in line with the density and nature of existing deep Earth (several silicate layers and a core where light migrated heavier metals). These meteorites have a size too small to be differentiated: their components have remained relatively homogeneously distributed.

Investigations indirect (geophysical)

Seismic tomography

It is the analysis of records obtained through the seismographs that will completely redefine the model of the Earth in the twentieth ^century. The principle is relatively simple: after an earthquake is determined the position of its epicenter as accurately as possible. Then records the vibrations that spread across the globe. These wave phenomena are subject to physical laws such as reflection or refraction. Moreover, they do not move all at the same speed along the middle through which they can evaluate the content of the Earth by a careful examination of curves time / distance. Waves studied in the positron are seismic waves that traverse the bottom globe in all directions. Surface waves, which cause damage to human constructions, propagate only in the crust and give no information on the deeper layers.

Some waves arrive quickly: they are the P waves (such as First), while others are delayed and are recorded later: what are the S waves (as seconds).

	The P waves are vibrations that act in compression: the particles move in the direction of wave propagation, much like a spring. These compression waves propagate in solids, liquids and gases.
	The S waves are shear waves: particles move perpendicular to the propagation direction of the wave, like a swing on a rope. These shear waves propagate in solids but not in liquid or gaseous media.

The speed of the two types of P and S waves varies with the density of material traversed. Crossing over the layer is soft, the waves propagate more slowly. Moreover, when P wave arrives perpendicular to a non-transition zone (mantle-core interface for example) a small portion of its energy is converted into another waveform (a fraction of P becomes S). The interpretation of seismic surveys is difficult because it overlaps the alignment of many types of waves must be disentangled and that must explain the origin. To navigate a little better, we have designated all these waves by different letters that can then be combined as and when they evolve (see table below).

Thus a wave PP is a P wave which, after undergoing reflection at the earth's surface, remained in the mantle before reappearing at the surface where it is detected. PKP wave is a P wave that emerges at the surface after passing through the liquid outer core (mantle path = / kernel ext. / Coat). We can extend the term as necessary. Consider a fairly complex example: a wave almost vertically through the earth from side to side after bouncing on the surface and be passed twice (outbound and return) by the kernel and the seed will reappear on the surface of the decked nice nickname, palindrome completely unpronounceable to PKIKPPKIKP!

During the twentieth ^century, several important discoveries were made using seismic tomography.

In 1909 , Andrija Mohorovii detected in Croatia interface crust / mantle now called by his friends, and in honor of its discoverer, Moho.

In 1912 , Beno Gutenberg (1889-1960) puts the interface Coat / nucleus to 2900 km depth through the study of P waves, giving its name to the discontinuity between the lower mantle and outer core, called the Gutenberg discontinuity.

In 1926 , Harold Jeffreys (1891-1989) establishes the fluidity of the metal core.

In 1936 , Inge Lehmann (1888-1993) discovers the seed (or core): metal part inside the nucleus. Its strength will be determined later in the following decades.

At the same time, 1923 to 1952 , other geophysicists ( Adams , Williamson , Bullen , Birch ...) are working on equations to determine the density variation with depth and the pressure it creates.

Know the essential structure of our globe must necessarily be accompanied by a study of its internal dynamics to better understand its evolution, seismic upheavals, changes in the magnetic field, etc..

The study of magnetism

The terrestrial magnetism is a very complex phenomenon to interpret. The Earth behaves as a sort of dynamo self-sustaining which generates a strong magnetic field (the one that deflects the needle of the compass, and protects us from some cosmic disturbances such as solar winds that could disrupt electronic devices). This field is variable in time and has even reversed hundreds of times since the beginning. Interpret this dynamic is inseparable from understanding the composition of Earth's internal structures and their movements.

Attempts at numerical modeling and laboratory experiments are under consideration. If they have not yet created a dynamo in a sphere, they have shown that convection columns appear at certain temperatures depending on the viscosity of the liquid and the speed of rotation. These movements are consistent with the assumptions of creating the Earth's electromagnetic field as we know it.

Current model

Structure Details

Detailed structure.

(1) continental crust mostly solid granite, topped in places by sedimentary rock. It is thicker than oceanic crust (30 km to 100 km under the mountain). The rind or crust is about 1.5% of the volume land. It was formerly called SIAL (silicon + aluminum).

(2) oceanic crust solid mainly composed of basaltic rocks. Relatively thin (about 5 km). It is also known as ADAM (silicon + magnesium).

(3) subduction zone where one plate sinks up to several hundreds of kilometers into the mantle.

(4) upper mantle , which is less viscous (more "ductile") that the lower mantle because the physical limitations that make them prevail in the liquid part. It consists mainly of rocks such as peridotite (its minerals are olivine , pyroxene , garnet ). Upon contact between the crust and upper mantle, can sometimes identify an area called LVZ (see No. 11).

(5) rash on areas of volcanism active. Two types of volcanoes are represented here, the deepest of them is called "hot spot". It would be of volcanoes whose magma come from the depths of the mantle near the boundary with the liquid core. These volcanoes are therefore not related to plate tectonics, and so do not follow the movements of the crust, they are virtually immobile in the earth's surface, and form the archipelago of islands like Tahiti.

(6) Lower mantle properties of an elastic solid. The mantle is not liquid as one might think looking at the lava flows of volcanic eruptions some but it is less "rigid" than the other layers. The mantle represents 84% of the land.

(7) Panache hottest material, starting from the boundary with the core, partly melts when arriving near the Earth's surface and produces the hot spot volcanism.

(8) Outer core liquid mainly composed of iron-nickel alloy (approximately 80% - 15%) plus a few lighter elements. Its viscosity is close to that of water, its average temperature is 4000 C and density 10. This huge quantity of molten metal is certainly agitated (by thermal convection and chemical (separation, demixing of the phases ) but also due to various movements of rotation and precession of the Earth). Flow of liquid iron can lead to electric currents (by Seebeck effect ) that give rise to magnetic fields which reinforce the current, creating a dynamo effect , by interviewing each other. The liquid core is causing the Earth's magnetic field.

(9) Inner core solid (also called "seed") essentially metal (an alloy of mostly iron and nickel) formed by gradual crystallization of the outer core. The pressure, which is 3.5 million bars (350 GPa), maintains a solid despite temperatures exceeding 5000 C and a density of about 13.
The inner core is still a mysterious place, and several questions remain:

• Recent studies suggest that the inner core is not stationary relative to the rest of the Earth: It could thus present a differential rotation , that is to say he would not turn at exactly the same speed as the rest of the planet's angular velocity of rotation would be greater by 0.3 to 0.5 degrees per year ), but according to other data, the observed motion could correspond in fact to a swing of the core around a mean position, with a sum of movements would be zero in the long term. Indeed, lateral differences in temperature at the base of the mantle appear to create a "footprint" detectable on the seed, affecting the rate of crystallization of iron. Outside, the existence of this impression is it seems possible that if the effects of these temperature differences always practice the same places the seed for several hundred million years, which would therefore incompatible with a permanent rotation, but could be a simple oscillation .
• There is also a doubt that the inner core is really strong, because in some ways it behaves like a liquid, while other data confirm that it is solid. Swedish and Russian researchers have demonstrated that under the conditions prevailing at the center of our planet, the alloy that makes up the inner core does not look like metals which are known to the surface, but would rather mechanical properties comparable to those of sand , which would explain the ambiguous results regarding her condition .
• Finally, recent studies show that the seed itself seems divided into two parts, one internal and one external then. The inner part, called the kernel because of its shape, is more pure iron than the outside, and is characterized by a crystal structure anisotropic ^, .

The inner and outer rings represent 15% of the land.

(10) mantle convection cells where the material is moving slow. The mantle is the seat of convection currents which transfer the bulk of the heat energy from Earth's core to the surface. These currents cause the drift of continents, but their precise characteristics (speed, magnitude, location) are still poorly understood.

(11) Lithosphere : it consists of the crust (tectonic plates) and a portion of the upper mantle. The lower limit of the lithosphere lies at a depth of between 100 and 200 km, the limit where the peridotites are approaching their melting point.
Sometimes found at the base of the lithosphere (some of them include geologists) an area called LVZ (for "Low Velocity Zone) where there is a decrease in velocity and attenuation of seismic waves P and S. This phenomenon is due to partial melting of peridotites which leads to greater fluency. The LVZ is generally not present in the roots of the mountains of the continental crust.

(12) asthenosphere : the zone below the lithosphere

(13) Gutenberg Discontinuity : mantle transition zone / core.

(14) Moho : transition zone crust / mantle (it is included in the lithosphere).

Features

Respective dimensions of different layers and approximate temperatures prevailing there.

Internal Heat

On the figure cons, the temperatures are in degrees Celsius as a guide. Can not be measured directly but only inferred, they are approximate (we sink more and more margin for error is large). Much of the internal heat of the Earth (87%) is produced by the radioactivity of rocks by natural decay of uranium from thorium and potassium.

Variable Radius

The Earth is not perfectly spherical and the actual radius equatorial is greater than twenty miles radius polar.

Indeed surprising that follows: the Mississippi River , whose source is located near the Great Lakes and empties into the Gulf of Mexico at a level (distance from the center of the globe) higher than its source. If the altitude was estimated from the center of the Earth, so water would flow from the lowest to the highest point. In reality, the sea level is always taken as reference altitudes, thinking in terms of mechanical energy is valid.

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