# Earth core

The earth's core is the metallic interior of the earth . Although the core with a diameter of 6942 km makes up only one sixth of the volume of the whole earth, it contributes around one third to its mass due to its high density . Evaluations of earthquake waves showed that the earth's core consists of a liquid outer core and a solid inner core . The earth's magnetic field is created in the outer core .

Shell structure of the earth's interior
Depth indications

## history

1904 beat Ernest Rutherford the radioactive decay before as a source of geothermal energy. As early as 1906, the British geologist Richard Dixon Oldham suspected on the basis of his evaluations of transit time differences in P waves and S waves , triggered by an earthquake , that the earth had a core, and estimated the radius of the core-mantle boundary to be 0.6 earth radii , ie at a depth of about 2500–2600 km. In 1914, the German geophysicist Beno Gutenberg calculated the depth of the core-mantle boundary to be 2900 km. The British mathematician and geophysicist Harold Jeffreys confirmed in his calculations in 1939 the limit at 2898 ± 3 km. Today it is assumed that the core-mantle boundary differs and is located on average at a depth of 2900 km.

The Danish seismologist Inge Lehmann found the boundary between the inner and outer core as a discontinuity in the speed of propagation as early as 1936 - the pattern of the P waves indicated a strong deflection at this interface.

## Structure and properties

The core has a volume of 17.548 · 10 10  km 3 and a mass of 1.9354 · 10 24  kg, i.e. 16.2% of the volume or 32.4% of the mass of the entire earth. It is not structured homogeneously. At the core-mantle boundary, the seismic P-waves of an earthquake slow down from 13.7 to 8.1 km / s and the speed of the S-waves is reduced from 7.3 km / s to 0. This leads to the conclusion admitted that the outer core must be liquid, since S waves cannot move in liquids. However, since the speed of the P waves in the inner core increases again, it is assumed that the inner core is solid.

depth Volume
distribution
Mass
distribution
Medium density
Earth crust Thickness: 5 to 70 km 00.8% 00.4% 02.8 g / cm 3
Mantle approx. 35-2900 km 83.0% 67.2% 04.5 g / cm 3
Earth core 2900-6371 km 16.2% 32.4% 11.0 g / cm 3

### Inner core

The inner core begins at a depth of 5150 km and extends to the center of the earth. Despite the very high temperatures in the inner core, which are around 6000 ± 500  K , this part of the earth's core consists primarily of solid metal. It is assumed that the metal alloy in the inner core is composed of 80% iron and 20% nickel , with the density of the core from the border to the outer earth core of about 12.2 g / cm 3 to the center being 12, 6 to 13.0 g / cm 3 increases. The enormous pressure of 330  GPa could explain that the iron-nickel alloy of the inner core is solid and not liquid as in the outer core.

### Outer core

The outer core begins at a depth of an average of 2900 km and ends at the border with the inner earth core at 5150 km. Its density increases with depth from 9.9 to 12.2 g / cm 3 . In addition to iron and nickel, about 10 to 15 weight percent lighter elements must be present, since the density is too low and the melting temperature too high for iron-nickel only. Depending on the temperature T at which the differentiation into core and shell could have taken place, silicon and oxygen are favored ( T high) or sulfur , carbon and hydrogen ( T less high). In this way, more precise knowledge of the composition of the outer core of the earth could help clarify the conditions for differentiation. Experimental methods to investigate the various possible mixtures at high temperatures and pressures for their density, sound velocity and distribution coefficient between the metallic phase and the cladding material are static compression with laser heating, shock wave experiments and quantum chemical calculations.

Although the material is as thin as water, the flow velocity is only in the order of magnitude of 1 mm / s, since the temperature difference between the core-shell boundary and the boundary to the inner core of almost 2000 K is almost entirely due to the pressure difference ( adiabatic Change of state ).

## Creation of the earth's magnetic field

The convection flow is driven both thermally and by concentration gradients. The thermal energy that is continuously given off to the earth's mantle comes partly from slow cooling, partly from crystallization heat at the inner core boundary, partly from compression - the growth of the inner core causes the entire core to shrink, which releases gravitational binding energy - and partly from radioactive decay heat . In addition, when solidifying at the inner core boundary, the light elements are enriched in the melt and are distributed upwards.

Under the influence of the Coriolis force , which is also responsible for the cyclones in the earth's atmosphere , the currents are forced on helical paths whose cylinder axes are aligned parallel to the earth's axis . Now magnetic fields cannot move freely in the electrically conductive liquid, but the field lines are carried along by the flow, wound up and stretched, which strengthens the fields. The direct cause of the magnetic fields are electrical eddy currents , which are caused by the slow drift of the field through the liquid. This self-amplification is saturated by ohmic losses increasing quadratically with the current strength .

Without the mechanical drive, the eddy currents and their magnetic field would subside within about 20,000 years. Simulations by the Institute for Geophysics at the Georg-August-Universität Göttingen have shown that the power required to operate the geodynamo is only 0.2 to 0.5 terawatts , far less than previously assumed. No decay heat in the earth's core is required to generate this power.

The earth's magnetic field existed over four billion years ago.

## Differential rotation of the inner core

In 1996 seismologists compared the fine structure of P-waves of seismic doublets. These are pairs of earthquakes of similar magnitude in almost the same place. The change in the fine structure depended on the time between the two quakes. The evaluation of 38 duplicates from 1967 to 1995 indicated a differential rotation : the inner core rotates a little faster than the jacket. Other such observations and evaluations confirmed this interpretation, but yielded contradicting values. Data up to 2007 could finally be interpreted in such a way that the relative angular velocity became larger and smaller over decades, with an average of about 0.4 ° per year. In the long term, it will be an irregular oscillation and an almost stable position of rest: The well-known east-west structure of the inner core - measured as a function of depth - indicates rotation rates compared to the mantle that are about six orders of magnitude slower, i.e. comparable to continental drift .

The mechanical coupling of the inner core to the inner area of ​​the liquid outer core is responsible for the fluctuations and is of a magnetic nature, while that to the earth's mantle is gravitational.

## exploration

A direct investigation of the earth's core is currently not possible. The Kola borehole , which is the deepest hole ever drilled, was 12.3 km deep, which corresponds to only 1518 and thus around 0.2% of the distance to the center of the earth. However, there is the possibility of gaining knowledge about the earth's core through indirect evidence:

• Statistical mechanics : Statistical mechanics allows conclusions to be drawn from the microscopic properties of the particles on the macroscopic behavior of the material in question. The conditions of the earth's core, such as extreme pressure and temperature, cannot be created in the laboratory or can only be created with great difficulty for experimentation. Statistical mechanics provides theoretical clues for the material properties under such conditions.
• The earth's magnetic field indicates that there must be electrically conductive material in the state of a fluid inside the earth . Theories about a geodynamo that generates the earth's magnetic field usually contain assumptions about the properties of the earth's core. Fluctuations in the earth's magnetic field and measurements with very low-frequency radio waves also indicate that the mantle has a low, depth-dependent electrical conductivity .
• Rocks in the earth's crust and mantle have densities between 2.5 and 4 g / cm³. For the entire earth body, however, there is a density of about 5.5 g / cm³. It follows from this that there must be areas with a much higher density in the interior of the earth.
• Iron meteorites were created from the metallic cores of differentiated asteroids , i.e. those that were made up of an iron-rich core and a mantle of rock, similar to the earth. According to today's ideas, these were shattered by collisions after their creation.
• Longitudinal compression or compression waves ( also called P-phases after English pressure , "phase" in the meaning of a period of time in the seismogram) emanating from a seismic source (for example earthquakes or explosions) pass the boundary to the earth's core (core Shell boundary) and are broken there on entry and exit (PKP phase, K for core). The earth's core acts like a lens for the PKP phases, leading to a focal circle approx. 145 ° from the epicenter . Since the earth's core deflects all direct P-phases between a distance of 100 ° to 145 ° through this effect, the ring-shaped so-called umbra forms here. In this umbra you can measure other nuclear phases, for example the PKiKP phase, which is reflected at the inner core of the earth. The reflections observed at the outer and inner core boundary show that the impedance there changes abruptly, at a distance shorter than one wavelength. The PKIKP seismic phase running through the inner core, distinguished by refraction from the PKP phase running past the inner core, led to its discovery in 1936 by the Danish seismologist Inge Lehmann .
• Since liquids have no shear resistance, shear waves can not propagate in the outer core. At the core-cladding boundary, shear waves are partially reflected back into the cladding (SS) and partially converted into compression waves in the core (e.g. SKS). Conversely, at the boundary between the outer and inner core of the earth, compression waves are also converted into shear waves, which then propagate more slowly in the inner core than the compression waves. When such shear waves (e.g. PKJKP) were observed in the inner core (from the pattern of location-dependent arrival times), this reinforced the suspicion that the inner core of the earth was solid.
• Super rotation : earthquake waves from different earthquakes from the same place of origin, which run through the earth's core, are deflected more and more differently in the earth's core with increasing time interval (different arrival point on the opposite side of the earth). The differences in deflection are most likely due to inhomogeneities in the inner solid core which change their location as a result of a slightly faster rotation of the core. These analyzes show that the inner core of the earth rotates 0.3 to 0.5 degrees per year faster than the earth's mantle and crust. With that it makes an additional turn within approx. 900 years. The energy for this is presumably provided by electromagnetic forces of the geodynamos in the outer core of the earth.

## Age and unsolved problems

A liquid core of the earth probably existed shortly after the earth was degassed and differentiated 4.45 billion years ago. There are several model calculations with different starting patterns for cooling and thus the formation of the solid inner core; the newer models assume an age of around one billion years (± 0.5), while older models estimate two to four billion years. The extent to which radioactive decay processes and their thermal energy play a role for the earth's core can only be roughly estimated due to the uncertain proportion of decaying nuclides in the earth's core. From a geochemical point of view, it seems possible that a low content (5 ppm) of potassium exists in the earth's core. The extent to which convection processes occur in the inner core of the earth is also unclear, but according to current estimates it is unlikely. Such estimates are also highly dependent on the assumed age of the solid inner core and its exact composition.

## literature

• Heinrich Bahlburg, Christoph Breitkreuz: Fundamentals of geology . Elsevier, 2004, ISBN 3-8274-1394-X .
• Edward J. Tarbuck, Frederick K. Lutgens : General Geology . German edition edited and supplemented by Bernd Lammerer. 9th, updated edition. Pearson studies, Munich [a. a.] 2009, ISBN 978-3-8273-7335-9 (English: Earth: An Introduction to Physical Geology . Translated by Tatjana D. Logan ).
• Martin Okrusch, Siegfried Matthes: Mineralogy: An introduction to special mineralogy, petrology and deposit science . Part III. Springer, 2009, ISBN 978-3-540-78200-1 , Chapter 27. Structure of the Earth's interior .
• Hidenori Terasaki, et al. : Deep earth - physics and chemistry of the lower mantle and core . John Wiley & Sons , New York 2016, ISBN 978-1-118-99247-0 .

Wiktionary: Earth core  - explanations of meanings, word origins, synonyms, translations

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