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Translations of Encyclopedia about Geology

 

The earth's structure

A great part of the earth's interior is inaccessible. If we want to get an image of its structure, we must use indirect research methods. One of the most important methods is measuring the seismic waves, which are deflected in places where the rock composition changes. The seismographers register and analyse the speed and the depth of the echo. We also obtain information by studying the thermic, magnetic, and gravity fields of the earth. The first geological map of Great Britain was compiled in 1835 by the geologist Sir Henry de la Bech (1796-1855).

We know today, that the earth is surrounded by earth's crust consisting of a relatively light rock. Underneath is the earth's mantle. On the outside, the mantle is formed by a solid rock, which turns liquid as it descends closer to the earth's core. The core consists of two parts, the outer and the inner core, and its material is mostly iron. The temperature of the inner core is probably similar to the temperature that exists on the surface of the sun.

Our continents and the ocean floor are on the outer layer of the earth, the earth's crust. Compared to the entire radius of the earth (the median earth's radius measures 6.370 kilometres), the earth's crust is very thin. Underneath the oceans, the thickness of the earth's crust measures between 8 to 15 kilometres. Underneath the continents it reaches 30 to 100 kilometres into the interior. Underneath Central Valley in California, for example, the earth's crust is only 20 kilometres thick, while underneath the Himalayas it reaches the depth of 90 kilometres. In the upper crust, the seismic waves travel at very different speeds. This indicates the existence of a great variety of rocks.

Between the crust and the upper mantle is a prominent boundary, named after its discoverer the Mohorovič discontinuity, or Moho for short. When analysing seismological data, Mohorovič discovered that in that particular place the travelling speed of seismic waves noticeably changes, which also means that there is a different material in the earth's core.

In comparison with the ocean crust, the structure of the continental crust is much more varied and stronger. It contains rocks that are up to 3,8 million years old. The effects of wind erosion, deformation, rising and lowering created many diverse layers. The surface is often formed by sediments and volcanic rocks, which have a very low density. Underneath we find a folded metamorphic sedimentary layer, linked partially to a granitic layer. The lower portion of the crust contains crystallised and metamorphic rock layers extending all the way to the earth's mantle.

In contrast, the structure of the ocean crust is relatively simple. It consists of smaller number of layers. First, there is a firm sediment measuring from a few hundred metres to up to three kilometres. Underneath this sediment we find a layer consisting of hard rock mainly basalt and a minor admixture of sedimentary rock. This layer measures on the average 1,5 kilometre. It is followed by a five-kilometre thick layer of basalt or gabbro, which is penetrated by giant cones of magma streaming up from the earth's mantle. The temperature in the earth's interior rises in direct relation to the depth. Starting from the earth surface, we estimate that the temperature rises 30 degrees Celsius with each kilometre into the earth's interior.

The earth's mantle, situated underneath the earth's crust, represents approximately 82 percent of the earth's volume and 67 percent of its total mass. The seismic waves and volcanic debris provide information concerning its composition. Based on its physical properties, we can divide the relatively cool earth's mantle into upper and lower layers. Analogous to what is happening in the earth's crust, here, too, we register changes in the seismic waves. These changes occur in a specific area, in the depth of 100 to 200 kilometres. This region is called the Gutenberg Zone. It is possible, that the cause of the deviation of these waves is the convective flow in the interior of the earth.

The earth's crust and the upper part of the earth's mantle, up to the Gutenberg Zone, form the lithosphere. The asthenosphere, which is less firm, lies underneath. The asthenosphere and the lithosphere are interactive. When material from the asthenosphere rises in the Mid-Oceanic Ridge, it becomes lithosphere. In the subductive zones (when one plate slides underneath the neighbouring plate proof-reader's note), the opposite occurs. In the middle of the earth's mantle the density increases and so does the speed of the seismic waves (P-waves). The lower part of the mantle consists mainly of silicon. Having a density of 9,4 gram per square centimetre it is connected to the earth's core. This region is called Wiechert-Gutenberg discontinuity.

We know relatively very little about the earth's core. Seismic waves penetrate the core only partially or are deflected by it. While its inner part, with a diameter of 2400 kilometres, consists of solid iron and nickel, the outer core, which is 2300 kilometres thick, is molten iron and nickel. According to calculations, the density of the earth's core is four times higher than the density of the earth's crust. Iron is the only substance we know that has this density, and it is therefore more than likely that it forms most of the earth's core.

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