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Composition And Structure Of The Earth

  Since the Earth is approximately 6,370 km in radius and geologists have access only to the outer few kilometres, it is remarkable that they can claim to identify the structure and composition of the interior. Although natural processes have brought up material (including diamonds) from several hundred kilometres deep and scattered meteorites over the surface, the main means of knowing what is at the centre rest on an understanding of seismology. Fortunately, observation and inference yield a consistent picture, and one which implies how the present structure has come about. In the small amount of space available here, the easiest way of presenting this material is to describe what is known, then say how it is known and then conclude by outlining how it came about.

Geologists divide the solid Earth into three main layers. At the surface is the crust, ranging from 6 km thick in the oceans to 90 km in major mountain ranges, The continents are heterogeneous, as described under rock cycle, but the overall composition is andesite to granite (that is, mainly potassium and sodium aluminosilicates with some free silica). The ocean crust consists of basalt (mainly calcium/sodium aluminosilicates, with considerable pyroxene and olivine). Beneath the crust lies the mantle, reaching 2,900 km below the surface. Mantle rocks have been brought to the surface and are known to consist of peridotite, composed of the minerals olivine (85%) and pyroxene (15%). The form of the peridotite varies with depth, becoming more compact as pressure increases. Between 50 and 250 km is a zone where up to 5% of the rock is molten and the molten fraction has the composition of basalt. Below 2,900 km is the Earth\'s core, which is now believed to have two components: an inner core of radius 1,215 km, a solid ball made of a 20/80 nickel iron mix; an outer molten layer of 88% iron and 12% sulphur. The density of the core (10-13 tonnes per cubic metre) is very much higher than that of the mantle (3.3-5.4 tonnes per cubic metre).

The existence of the core was deduced from observations of earthquake vibrations, using knowledge of the effects of different states and densities on the speed and direction of travel. Earthquakes generate two kinds of vibration which travel through the Earth: P waves vibrate in the direction of travel, while S waves vibrate at right angles to it. P waves move faster and both travel faster in more dense material, except that S waves will not travel through a liquid. The absence of S waves from earthquakes on the opposite side of the world from the recorder is the clearest indication of the liquid core. There are other zones where P waves are not detected as a result of the effects of refraction when entering or leaving more dense layers. The central, solid core was inferred from the faster than expected arrival of P waves from earthquakes directly opposite the observatory and from others apparently reflected from the surface of the core. Confidence in these inferences about the interior was greatly strengthened when other researchers investigating the Earth\'s magnetic field showed that it could best be explained as the result of electrical currents flowing in a liquid metallic core.

The characteristics required to carry both seismic waves and electric currents in ways consistent with the observations also coincide with the indications from a study of meteorites. There are three kinds of meteorites, iron, stony/iron and stony, especially a group called ‘chondritic’ after the droplet-shaped particles in them. Chondritic meteorites consist of peridotite, nickel/iron, iron sulphide and olivine in the chondrites. This is also the composition of the solid components of other astronomical objects, including the Sun. It is now believed that the Earth formed by the accretion of particles similar to chondritic meteorites and was composed largely of iron (35%), oxygen (30%), silicon (15%) and magnesium (10%).

As gravity forced these particles together, the temperature rose so that the iron melted and settled toward the centre as a result of its greater density. Just as slag in a blast furnace rises to the surface, so the lighter metal silicates and oxides rose to form an upper layer—the mantle.

However, the early Earth had no crust, and an account of just how this was formed has been gained by insights from plate tectonics combined with known chemical differences. At hot spots in the upper mantle, convection currents formed, as they now do at mid-ocean ridges. Basaltic magma was lighter and more mobile than the peridotite and so rose to the surface, solidified and spread across the sea floor to form the ocean crust. At destructive plate margins this basalt was drawn below the surface, compressed and heated. Less dense and more mobile minerals would be left at the surface, as they are today in island arcs composed of andesite. Over hundreds of millions of years these island arcs would be progressively built up into continents, with associated erosion, deposition and mountain building. Beneath mountainous zones, melting occurred, with selective mobilization of some minerals and recrystallization of granite at depth. Subsequent erosion has revealed granite at the surface, including extensive areas like the Canadian shield.

This process, with selective mobilization of some elements and minerals, is consistent with observed differences between peridotite, basalt and granite. Olivine and magnesium are more plentiful in peridotite and so were left behind. Silica and aluminosilicates were progressively enriched, except that calcium is richer in basalt than granite. The result of these processes was to give the crust a different chemical composition from that of the mantle, with less magnesium and iron and more of the other metals, especially aluminium. Nevertheless, the crust is predominantly made of only a small number of elements: oxygen (47%), silicon (28%), aluminium (8%), iron (5%), calcium, sodium, potassium and magnesium—taking the total to 99%, so that the other 80 odd elements make up about 1% in total. In practice, these few elements are combined in complex and stable minerals, especially feldspars (aluminosilicates), which make them difficult to use as anything but stone. Fortunately other internal and surface geological processes have created local concentrations of many minerals which can be used as sources of useful elements. Biological processes have also contributed to these local concentrations, most notably in modifying the atmosphere. PS



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