Structure of the Earth

The Earth has a rough approximation of spherical shape (actual radius of the earth 6357-6378 miles), the interior of which is composed of several shells: In the center of the Earth's core is solid with a radius of about 1250 km, which is composed mainly of iron and nickel. This is followed by the liquid portion of the Earth's core includes (mainly iron) with a thickness of approximately 2200 km. In 2900 km thick layer of the so-called sheath of zähplastischem rock ( silicates and oxides ), and top of a relatively thin, hard crust. This is also made of silicates and oxides, but is enriched with elements that are not present in the rock mantle. With its shell-like structure of the Earth is also the prototype of the four terrestrial planets in the inner part of our solar system.

  • 3.1 Gravity and isostasy
  • 3.2 holes
  • 3.3 Volcanic activity
  • 3.4 Seismology
  • 3.5 meteorites, age of the earth

Emergence of the shell structure of the earth

Just as all the other planets of the solar system, the Earth was formed about 4.6 billion years ago from a rotating cloud of dust and gas, which slowly became denser by gravity and by aggregation of dust particles and larger bodies eventually formed kilometer-sized planetesimals. By collision between them proto- planets grew, until finally the planets were left who had withdrawn a large part of the free matter. The initially cold and homogeneous inside celestial bodies of the proto - Earth warmed to within about 100 million years ago by the energy released at the impacts of planetesimals, gravitational energy.

Due to the increase in mass, the Earth's core compacted. Radioactive decay processes accelerated its heating. After the earth had warmed up to about 2000 ° C - a temperature that melted in the iron and most of silicates - formed

The heavier droplets of molten metal migrated towards the center and gathered there to the iron core, whereby the lighter silicate melt was displaced from the center outward and became the mantle or the crust.

By long-standing differentiation thus arrived continuously lighter matter in the outer zones of the earth. The result was above the heavy iron core, a mantle rocks from medium density consisting of magnesium-iron silicates, and about an outer crust of lightweight material such as oxygen, silicon, aluminum, calcium, sodium and others. The light water whose origin is controversial to this day, got together to Urozeanen. The even lighter gases - among others from pores and volcanic fissures high rising - finally produced the Earth's atmosphere. The fact that the differentiation is still not closed, it can be seen for example on the gas emissions from volcanic eruptions, with huge amounts of gases escaping from the Earth's interior.

Structure of the Earth

The shell structure of the earth's interior is divided by two prominent seismic discontinuity surfaces. Separate the crust from the mantle and from this core.

Earth's core

  • Inner Earth's Core: The inner core of the Earth extends between 5100 km and 6371 km below the surface. It presumably consists of a fixed iron -nickel alloy. Here, the pressure is up to 3,640,000 bar and the temperature is at about 5000 ° C. Since a temperature measurement in the inner core is extremely difficult and characterized by uncertainty, the temperature may also be up to 8000 ° C at the core center, which would correspond to about the surface temperature of the sun (approx. 6000 ° C).
  • Outside Earth's core: The outer core lies at a depth of approximately 2900 km to 5100 km. At a temperature between 3000 ° C and about 5000 ° C, this part of the core is liquid. It consists of a nickel-iron melt ( " NiFe "), which may also contain small traces of sulfur or oxygen. In interaction with the Earth's rotation, the movable iron melt is responsible due to their electrical conductivity of the Earth's magnetic field.
  • After the PREM model makes the Earth's core with its 1.94 · 1024 kg approximately 32.5 percent of Earth's mass, but only 16.2 % of their volume. It follows that its average density is about 10 g/cm3 ( compared to 5.52 for the entire earth system ). The upper boundary of the Earth's core to the mantle back is core - mantle boundary or named after their discoverers, Emil Wiechert and Beno Gutenberg also Wiechert- Gutenberg discontinuity. Above the discontinuity is the so-called D "layer, which is considered as a kind of transition zone between the Earth's core and mantle. Has a greatly varying thickness of 200 to 300 kilometers and has a strong temperature gradient.


  • Lower mantle: The transition between the core and the lower mantle is characterized by an abrupt decrease in density from 10 to 5 g/cm3. This is due to the change of iron to minerals: the lower shell is made of heavy silicates ( primarily magnesium perovskite ) and a mixture of metal oxides such as periclase ( magnesium oxide ), and wustite (iron (II ) oxide ), which are collectively referred to as magnesiowüstite. In the lower mantle, between 660 km and 2900 km depth, there is a temperature of about 2000 ° C. The thermal boundary layer ( D " layer) as a possible origin of plumes, which are Aufstrombereiche hot magma, viewed between the outer core and lower mantle.
  • Transition zone: The area between 410 km and 660 km depth is a transfer from the upper to the lower mantle, but is occasionally expected already for the upper mantle. The limits are based on the depths of the mineral phase transitions of olivine, the main constituent of the upper mantle. Since the modified mineral structure is accompanied by a change in the density and seismic velocity, these discontinuities can be detected and measured by seismic methods.
  • Upper mantle: the upper mantle begins at 410 km depth and extends up to the earth's crust. It consists of peridotite, composed of olivine and pyroxene, and a garnet component. The top part of the shell covers the so-called lithosphere, which includes the earth's crust on, and the underlying asthenosphere viscoplastic.

The mantle accounts for around two -thirds of the Earth's mass; the average density of its shells is between 3 ¼ and less than 5 g/cm3. The upper limit of the mantle is called Mohorovičić discontinuity ( Moho also abbreviated ). It has already been demonstrated in 1909 due to their striking density jump of about 0.5 g/cm3, are diffracted by the strong quake waves or reflected to the earth's surface.

Mantle convection

Which belongs to the upper mantle asthenosphere ( from the Greek asthenos " soft " ) ranges depending on the lithosphere thickness of about 60-150 km down to a depth of about 210 km. Due partially molten rock material, it has reduced seismic velocities and a viscous-plastic rheology. With its fluidity it is an important part of the concept of mantle convection: "swim" on her the lithospheric plates that are moved by convection currents in the mantle against each other and thus lead to tectonic processes such as continental drift and earthquakes.

Earth's crust

The earth's crust is the top layer of the lithosphere - which also includes the rigid lithospheric mantle of the upper mantle counts - and consists of two very different crustal types:

  • Oceanic crust: The oceanic crust forms with their thickness 5-10 km The relatively thin layer around the Earth's mantle. It consists of large rigid plates that are constantly in slow motion and on the " floating layer " ( asthenosphere ) of the upper mantle swim. At the spreading zones of crustal plates, mid-ocean ridges, constantly penetrate basic magmas up and cool down. They freeze at and near the seafloor basalt and in greater depth to crustal gabbro. So is - similar to an assembly line - new oceanic crust produced. Therefore, the oceanic crust with increasing distance from the back getting older, by their different magnetic polarity that is detectable over a large area. As she dives back into the mantle at subduction zones and drops down to the core - mantle boundary, it is nowhere more than 200 million years.
  • Continental crust: It consists of individual floes, which are also known as continents and are surrounded by oceanic crust. The continental crust " floats" on the asthenosphere. Where it is thickest, she stands out as a high mountain massif ( isostasy ). The detailed structure of the continental crust shows a dichotomy in a brittle upper crust and a ductile lower crust, which by Mineralumbildungen ( modification change) are related and separated by the Conrad discontinuity.

The upper boundary of the Earth's crust is either the base of the waters or the interface between the atmosphere and dry land. That is, sediments in lakes and seas will be added to the crust.

The thickness of the continental crust is between 30 and 60 km with a global average of 35 km. It consists mainly of crystalline rocks together, whose main ingredients include quartz and feldspar. Chemically, the continental crust to 47.2 weight percent ( 62.9 atomic percent and 94.8 percent by volume ) composed of oxygen, thus forming a dense rock hard packing of oxygen, however, is attached, for example, in the form of silicon dioxide ( quartz). In the earth's crust and on its surface the rocks are subjected to a constant process of transformation, which is referred to as a cycle of rocks. Today there are no more rocks, which have remained unchanged since the first crust formation in the Earth's history. The oldest rocks ever found on earlier continental margins ( terranes ) have a protolith age of 4.03 billion years ago ( see also The oldest rock ).

Exploration of the shell construction of the earth

Knowledge of the structure of the earth come from various geophysical sources, geochemical or mineralogical analyzes of volcanic rocks, laboratory experiments for the stability of minerals as well as analogies to Extraterrestrial Bodies.

Gravity and isostasy

The first indications of the inner material of the earth resulting from their average density of 5.5 g/cm3, which could be calculated by determining the mass of the Earth by gravity law. Since near-surface rocks have an average of 2.7 g/cm3, the Earth's interior must be at least 2 - to 3 - times denser be (iron has about 8 g/cm3).

Measurements of the perpendicular direction showed in the early 19th century that the Earth's interior under high mountains has a lower density. Through precise gravity measurements ( gravimetry) we soon realized that there the solid crust is thicker than elsewhere, and that the underneath mantle of heavier rocks there. Large massifs appear like icebergs, the deeper into the earth, the higher they are. This " floating balance " is called isostasy. Through satellite geodesy can be similarly locate even deeper anomalies of the mantle.


The deepest hole ever conducted, found in Russia on the Kola Peninsula instead ( Kola hole) and led to a depth of 12 km. Here the top layer of the continental crust could be explored, which has a thickness of about 30 km at this point. Another hole, the so-called continental deep drilling ( KTB ), which has reached 9.1 kilometers, was carried out at Windisch Eschenbach in der Oberpfalz Germany. With a planned depth of 14 km, it would have been possible to explore the continental crust at the putative interface to the 300 million years ago collided parts of the drifting on the mantle Continents primal Africa and Ur - Europe ( see also Armorica ( Continent) ).

Deep holes move in the upper to middle crust area, and cover only a small glimpse into the earth. If one were to reduce the earth on apple size, so our deepest holes would not even correspond to the scoring of the shell. Push forward through holes to greater depths, currently exceeds the technical capabilities: The high pressures ( in 14 km depth about 400 MPa ) and temperatures ( in 14 km depth about 300 ° C) require new solutions.

Volcanic activity

The greatest depth, penetrates from the material to the surface and this produces the various forms of volcanism is found at the boundary between the outer core and the lower mantle, as is observed for example in Plumes. So this pumped for an eruption to day material comes from different areas of the mantle and can be analyzed accordingly.

Further evidence of the sheath properties can be won on the study of mid-ocean ridges. The jacket here, situated directly under the plate boundary rises to fill the space in the resulting gaps. Normally, the mantle rock melts down through the pressure relief and after cooling, forms the new oceanic crust to the ocean floor. This approximately 8 km thick crust sealed access to the original mantle rock. An interesting exception is possibly the mid-ocean ridges between Greenland and Russia, the Gakkel Ridge, which is the slowest spreading ridge on earth with less than 1 cm per year. The Earth's mantle rises to a very slowly. Therefore, no melt and, consequently, no crust is formed. The mantle rock could be thus be found directly on the seabed.


The earth is shaken daily by earthquakes that are recorded by monitoring stations around the world. Emanating from earthquake seismic waves traverse the entire earth system, wherein the seismic energy propagating in the different layers at different rates. The propagation speed depends on the elastic properties of the rock. From the travel times of seismic wave trains, the occurrence of reflected waves and other seismically measurable effects such as attenuation or scattering, the structure of Earth's interior can be examined.

In 1912, Beno Gutenberg was the first time identified the boundary between the silicate mantle material and the nickel-iron core at a depth of 2900 km. Shortly before the Croatian geophysicist Andrija Mohorovičić discovered named after him boundary between Earth's crust and mantle. Both were possible because striking impedance jumps - mainly caused by sudden changes in the propagation velocities of seismic waves, so-called " seismic discontinuities " - produce measurable reflected phases. Discontinuities may be chemical in nature. They are based on a change in the chemical composition of the layers of earth, with the result modified elastic properties. In the mantle transition zone, for example (MTZ, English:. Mantle transition zone ), there are also Diskontinuiäten that go without a change in chemical composition. These are based on phase transformations, wherein a mineral with the increase of pressure and / or temperature in a differently structured and IA dense mineral of the same composition transforms.

Meteorites, age of the earth

Our ideas about the material existence of the Earth's interior is based in addition to the above methods based on analogies with the composition of meteorites. Chondritic meteorites were hardly changed since the formation of the solar system. It is therefore assumed that the overall chemical composition of the soil to be similar to that of Chondrites as these probably again similar to planetesimals from which the soil is formed. Among the meteorites but are also fragments of differentiated parent bodies: iron meteorites and belonging to the stone -iron meteorite pallasite probably from the Earth's core or the transition region between the core and cladding of differentiated asteroids, while the achondrites originate from the mantle or crust. With the meteorite so materials can be examined from the core and cladding region, which are not accessible by the earth, for direct investigations.

Meteorite play a major role in the dating of the solar system and also the earth. Thus, it was concluded that the age of the Earth of 4.55 billion years ago, first in the 1950s by Clair Cameron Patterson, and Friedrich Georg Houtermans using uranium - lead dating to the iron meteorite Canyon Diablo. Dating methods on other isotope system (eg 87Rb - 87Sr, 147Sm - 143Nd ) have based since confirmed that age. The oldest material found on Earth are zircon crystals in Western Australia with an age of up to 4.4 billion years, thus forming a lower limit of Erdal Marketers.