Inner structure of the earth
The inner structure of the earth , which is examined primarily by geophysics , ideally consists of concentric spherical shells, the material of which each has a significantly different density . The spherical shell with the lowest density is furthest out and is known as the earth's crust . The spherical shell with the greatest density, actually a full sphere, lies in the center of the earth's body and is called the earth's core . The boundary surfaces of these shells are slightly flattened in the gravitational field .
The density stratification in the earth's body is accompanied by a chemical differentiation, i. That is, each spherical shell has a characteristic chemical composition. The core has a radius of about 3450 km and consists mainly of iron and nickel . This is followed by the 2900 km thick earth's mantle made of silicates and oxides with a high proportion of iron and magnesium . The outer shell of the earth's body is formed by the relatively thin (approx. 5–70 km) crust. This also consists mainly of silicates and oxides, but with a lower iron and magnesium content and an increased amount of aluminum and elements that are "insoluble" in the mantle rock (so-called incompatible elements ). With its structure of silicate crust and silicate mantle and an iron core, the earth is the prototype of the four terrestrial planets of the inner solar system .
In addition to the division of the spherical shells according to chemical aspects, another model is used that is based on the rheological properties of the material in the earth's body. This divides the earth's core into a crystalline inner core (radius: 1230 km) and a thin outer core (thickness: 2200 km). In addition, it does not differentiate between crust and mantle, but combines the crust with the rigid outermost part of the mantle to form the lithosphere , which is followed by a viscous mantle at the bottom.
Structure of the earth
Since the areas below a depth of a few thousand meters cannot be accessed directly by humans through drilling , the knowledge of the structure of the earth's interior is largely based on seismics , i.e. the recording and evaluation of mechanical waves that travel through the earth's body move, triggered z. B. by earthquakes or nuclear weapons tests. The resulting seismic profile of the earth's body is characterized by two striking discontinuity areas. These are considered to be the interfaces between the earth's crust and the earth's mantle or the earth's mantle and the core of the earth.
At the boundary between the core and the cladding , the density changes from 10 to 5 g / cm 3 due to the material . The differentiation into core and mantle material occurred within the first few million years after the formation of the dust in the primeval solar nebula through melting when protoplanets collided . A few ten million years later, a deep magma ocean emerged again during the collision between Proto-Earth and Theia , whose metallic cores united, see formation of the earth . The core was still completely liquid at the time. A few 100 million years later - it is unclear whether before or after the onset of deep mantle convection - iron and nickel began to crystallize in the center.
- Inner core of the earth: The solid inner core of the earth extends from the center of the earth to 5100 km below the earth's surface. The pressure here is up to 3.64 million bar and the temperature is assumed to be around 6000 K.
- Outer core of the earth: The outer core lies at a depth between around 2900 km and 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 amounts of sulfur or oxygen (see → Iron (I) oxide ). In interaction with the earth's rotation , the mobile iron melt is responsible for the earth's magnetic field due to its electrical conductivity .
- According to the PREM model , the earth's core with its 1.94 · 10 24 kg makes up about 32.5% of the earth's mass , but only 16.2% of its volume. This means that its mean density is over 10 g / cm 3 (compared to 5.52 g / cm 3 for the entire earth).
- D "layer : Above the core-mantle boundary is the so-called D" layer, which is viewed as a kind of transition zone between the earth's core and the earth's mantle. It has a strongly varying thickness from 200 to 300 kilometers and has a strong temperature gradient . From there mantle plumes rise .
- Lower mantle: The lower mantle consists of heavy silicates (mainly magnesium perovskite ) and a mixture of metal oxides such as periclase ( magnesium oxide ) and wustite ( iron (II) oxide ), which are collectively referred to as magnesiowustite . In the lower mantle, between 660 km and 2900 km depth, there is a temperature of around 2000 ° C.
- Transition zone: The area between 410 km and 660 km depth is considered to be the transition from the upper to the lower mantle, but is sometimes counted as part of the upper mantle. The boundaries are based on the depths of the mineral phase transitions of olivine , the main component of the upper mantle. Since the modified mineral structure with a change in the density and seismic velocity associated, these can discontinuities by seismological be detected and measured methods.
- Upper mantle: The upper mantle begins at a depth of 410 km and extends up to the earth's crust. It consists of peridotite , which is composed of olivine and pyroxene , and a garnet component. The uppermost area of the mantle includes the so-called lithosphere , which also includes the earth's crust , and the viscous asthenosphere below .
The earth's mantle makes up around two thirds of the earth's mass ; the mean density of its shells is between 3¼ and just under 5 g / cm 3 . The upper limit of the earth's mantle is called the Mohorovičić discontinuity (also called Moho for short ). It was detected as early as 1909 because of its striking density jump of around 0.5 g / cm 3 , which deflects strong earthquake waves or reflects them to the earth's surface.
The asthenosphere belonging to the upper mantle (derived from the Greek asthenos "soft") extends, depending on the thickness of the lithosphere, from about 60–150 km to a depth of about 210 km. Due to partially melted rock material, it shows reduced seismic velocities and a viscoplastic rheology . With its fluidity, it is an important part of the concept of mantle convection: The lithospheric plates "float" on it, which are shifted against each other by the convection currents of the earth's mantle and thus lead to tectonic processes such as continental drift or earthquakes .
- Oceanic crust : The oceanic crust forms a comparatively thin layer around the earth's mantle with its thickness of 5 to 10 km. It consists of huge solid plates that are constantly in slow motion and float on the "fluid layer" (asthenosphere) of the upper mantle. At the spreading zones of the crustal plates, the mid-ocean ridges , alkaline magmas constantly penetrate and cool down. They solidify to basalt on and near the sea floor and to gabbro at greater depths of the crust . In this way - similar to an assembly line - new oceanic crust is produced. This is why the oceanic crust gets older and older as the distance from the ridge increases; this can be demonstrated over a large area due to its different magnetic polarity. Since it dips back into the mantle at subduction zones and sinks to the core-mantle boundary, it is nowhere older than 200 million years.
- Continental earth crust : It consists of individual plates, also known as continents, which are surrounded by oceanic crust. The continental crust also “floats” on the asthenosphere. Where it is thickest, it rises up as a high mountain massif ( isostasy ). In the detailed structure, the continental crust shows a division into a brittle upper crust and a ductile lower crust, which are caused by mineral transformations ( modification change ) and separated by the Conrad discontinuity .
The upper boundary of the earth's crust is either the bottom of the water or the interface between the atmosphere and dry land. This means that sediments in lakes and seas are added to the earth's crust.
The thickness of the continental crust is between 30 and 60 kilometers with a global mean around 35 km. It is mainly composed of crystalline rocks , the main components of which are quartz and feldspar . Chemically, the continental crust is made up of 47.2 percent by weight (62.9 atomic percent or 94.8 percent by volume) of oxygen, i.e. it forms a dense, rock-hard packing of oxygen, which is bound in the form of silicon dioxide (quartz), for example. In the earth's crust and on its surface, the rocks are subject to a constant transformation process, which is also known as the rock cycle . Today there are no more rocks that have remained unchanged since the first crust formation in the history of the earth . The oldest rocks ever found on earlier continental margins ( terrane ) have a protolith age of 4.03 billion years (see also The oldest rock ).
Exploration of the shell structure of the earth
Knowledge of the structure of the earth comes from various geophysical sources, geochemical or mineralogical analyzes of volcanic rocks, laboratory experiments on the stability of minerals and analogies to extraterrestrial celestial bodies.
Gravimetry and isostasis
The first indications of the earth's inner material resulted from its mean density of 5.5 g / cm 3 , which could be calculated using the law of gravitation by determining the earth's mass . Since rocks close to the surface have an average of 2.7 g / cm 3 , the interior of the earth must be at least 2 to 3 times denser (iron has about 8 g / cm 3 ).
Measurements of the vertical direction showed as early as the early 19th century that the interior of the earth has a lower density under high mountains . Precise gravity measurements ( gravimetry ) soon revealed that the solid earth's crust is thicker there than elsewhere, and that the earth's mantle below consists of heavier rocks. Large mountain massifs, like icebergs , dive deeper into the earth's interior the higher they are. This “swimming balance ” is called isostasis . By satellite geodesy can be similarly even deeper anomalies of the Earth's mantle locations.
The deepest well ever carried out took place in Russia on the Kola peninsula ( Kola well ) and led to a depth of 12.3 km. Here the uppermost layer of the continental crust could be explored, which at this point has a thickness of about 30 km. Another borehole, the so-called continental deep borehole (KTB), which has reached 9.1 km, was carried out near Windischeschenbach in the German Upper Palatinate . With a planned depth of 14 km, it would have been possible to explore the continental crust at the presumed seam where parts of the continents of Ur-Africa and Ur-Europe, drifting on the Earth's mantle, collided 300 million years ago (see also Armorica ).
Deep boreholes move in the upper to middle crustal area and can therefore only provide a small insight into the interior of the earth. If one were to reduce the earth to the size of an apple, our deepest drillings would correspond little more than drilling into the shell. Advancing to greater depths by drilling is currently beyond the technical possibilities: The high pressures (at a depth of 14 km approx. 400 MPa ) and temperatures (at a depth of 14 km approx. 300 ° C) require new solutions.
The greatest depth from which material penetrates to the surface of the earth and thereby creates the various forms of volcanism is found at the boundary layer between the outer core and the lower mantle, as can be observed, for example, in plumes . The material extracted from an eruption comes from different areas of the mantle and can be analyzed accordingly.
Further information on the properties of the mantle can be obtained by researching the mid-ocean ridges . The mantle, which lies directly below the plate boundary, rises to fill the space in the resulting gaps. Normally, the mantle rock melts due to the pressure relief and forms the new ocean crust on the sea floor after cooling . This approximately 8 km thick crust seals the access to the original mantle rock. An interesting exception is possibly the mid-ocean ridge between Greenland and Russia , the Gakkel Ridge , which is the slowest spreading ridge on earth at less than 1 cm per year. The earth's mantle rises very slowly here. Therefore, no melt and consequently no crust form either. The mantle rock could therefore be found directly on the sea floor.
The earth is shaken every day by earthquakes that are registered by measuring stations around the world. The seismic waves emanating from earthquakes traverse the entire body of the earth, with the seismic energy spreading in the various layers at different speeds. The propagation speeds depend on the elastic properties of the rock. The structure of the earth's interior can be examined from the transit times of seismic wave trains, the occurrence of reflected waves and other seismologically measurable effects such as damping or scattering .
In the year 1912 made Beno Gutenberg first time the border between the silicate mantle material and the nickel - iron - core at a depth of 2900 km from. Shortly before, the Croatian geophysicist Andrija Mohorovičić had discovered the interface between the earth's crust and mantle named after him . Both were possible because marked jumps in impedance - mainly caused by sudden changes in the propagation speed of earthquake waves, so-called "seismic discontinuities " - generate measurable reflected phases. Discontinuities can be chemical in nature. These are based on a change in the chemical composition of the earth's strata, resulting in changed elastic properties. In the transition zone , for example (MTZ, Eng .: mantle transition zone ) but there are also discontinuities associated without a change in chemical composition. These are based on phase transformations , with a mineral transforming itself into a differently structured and usually denser mineral of the same composition with an increase in pressure and / or temperature.
Meteorites, age of the earth
Our ideas about the substance of the earth's interior are based, in addition to the methods mentioned above, on analogy conclusions based on the composition of meteorites . Chondritic meteorites have hardly changed since the formation of the solar system. It is therefore believed that the overall chemical composition of the earth is similar to that of the chondrites, as these in turn are believed to resemble the planetesimals from which the earth was formed. The meteorites also include fragments of differentiated mother bodies: iron meteorites and the pallasites belonging to the stone-iron meteorites presumably come from the earth's core or the transition area between core and mantle of differentiated asteroids, while the achondrites come from their mantle or crust. The meteorites can therefore be used to investigate materials from the core and mantle area that are not accessible for direct investigations on Earth.
Meteorites play a huge role in dating the solar system and also the earth. Clair Cameron Patterson and Friedrich Georg Houtermans first concluded that the earth was 4.55 billion years old in the 1950s by means of uranium-lead dating on the iron meteorite Canyon Diablo . Dating methods based on other isotope systems ( e.g. 87 Rb- 87 Sr, 147 Sm- 143 Nd) have since confirmed this age. The oldest material found on earth are zirconium crystals in Western Australia with an age up to 4.4 billion years, which thus forms a lower limit of the earth's age.
- Cesare Emiliani : Planet Earth. Cosmology, Geology, and the Evolution of Live and Environment , ISBN 0-521-40949-7 , Cambridge University Press, Cambridge 1992.
- László Egyed : Solid Earth Physics , Akadémiai Kiadó, Budapest 1969, 370 pp.
- Walter Kertz : Introduction to Geophysics , Spektrum Akademischer Verlag 1970/1992, 232 pp.
- Karl Ledersteger : Astronomical and Physical Geodesy . In: Jordan / Eggert / Kneissl (ed.): Handbuch der Vermessungskunde, Volume V, Verlag JB Metzler, Stuttgart 1969, 871 pp.
- Harry Y. McSween, Jr .: Meteorites and Their Parent Planets. , ISBN 0-521-58303-9 , Cambridge University Press, Cambridge 1999.
- Frank Press , Raymond Siever: Understanding Earth , ISBN 0-7167-3504-0 , WH Freeman, New York 2000.
- David Graham Smith: The Cambridge Encyclopedia of Earth Sciences , ISBN 0-521-23900-1 , Cambridge University Press, Cambridge 1981.
- Harald Zepp : Ground plan general geography. Geomorphology , 3rd edition, ISBN 3-8252-2164-4 , Verlag Ferdinand Schöningh GmbH, Paderborn 2004.
- University of Tübingen : Structure of the Earth
- The Interior of the Earth (USGS )
- Simon Wellings: Oceanic crust - down to the core , All-Geo-Blogs, Metageologist, September 25, 2012 (English)
- Study Reconciles Long-Standing Contradiction of Deep-Earth Dynamics , PysOrg, August 25, 2005
References and footnotes
- Edward J. Tarbuck, Frederick K. Lutgens : General Geology . Ed .: Pearson Education Deutschland GmbH. 9th edition. Munich 2009, ISBN 978-3-8273-7335-9 , pp. 402–404 (English: Earth: An Introduction to Physical Geology . Translated by Tatjana D. Logan ).
- Note: The term earth surface , on the other hand, stands for the entire area of the earth's atmosphere, i.e. the surface of the terrain exposed in the air and the surface of the water.
- The planetary and meteorite system of the sun is characterized by a relatively low inclination ( ecliptic ) of the orbits of the celestial bodies under consideration to the equatorial plane of the sun. One can therefore speak of an “accretion disk” as it is usually characteristic of rotating systems.