The Earth's Structure

Our continents & the ocean floor are on the outer layer of the earth, the earth's crust. Compared to the whole radius of the earth (the median earth's radius measures 6.370 kilometres), the earth's crust is narrow. 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 in to the inside. Underneath Central Valley in New york, for example, the earth's crust is only twenty kilometres thick, while underneath the Himalayas it reaches the depth of 90 kilometres. In the upper crust, the seismic waves travel at different speeds. This indicates the existence of a great variety of rocks.

We know today, that the earth is surrounded by earth's crust consisting of a comparatively 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 parts, the outer & the inner core, & 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.


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


Between the crust & the upper mantle is a prominent boundary, named after its discoverer the Mohorovi discontinuity, or Moho for short. When analysing seismological knowledge, Mohorovi 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 contrast, the structure of the ocean crust is comparatively simple. It consists of smaller number of layers. First, there is a firm sediment measuring from a few hundred metres to up to kilometres. Underneath this sediment they discover a layer consisting of hard rock - chiefly basalt & a minor admixture of sedimentary rock. This layer measures on the average one,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. Beginning from the earth surface, they estimate that the temperature rises 30 degrees Celsius with each kilometre in to the earth's interior.

The earth's crust & 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 & the lithosphere are interactive. When material from the asthenosphere rises in the Mid-Oceanic Ridge, it becomes lithosphere. In the subductive zones (when plate slides underneath the neighbouring plate - proof-reader's note), the opposite occurs. In the midst of the earth's mantle the density increases & so does the speed of the seismic waves (P-waves). The lower part of the mantle consists chiefly 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.


The earth's mantle, situated underneath the earth's crust, represents about 82 percent of the earth's volume & 67 percent of its total mass. The seismic waves & volcanic debris provide knowledge concerning its composition. Based on its physical properties, they can divide the comparatively icy earth's mantle in to upper & lower layers. Analogous to what is happening in the earth's crust, here, , they 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 feasible, that the reason for the deviation of these waves is the convective flow in the inside of the earth.


They know comparatively small 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 & nickel, the outer core, which is 2300 kilometres thick, is molten iron & nickel. According to calculations, the density of the earth's core is times higher than the density of the earth's crust. Iron is the only substance they know that has this density, & it is therefore over likely that it forms most of the earth's core.