Earth present crust, chemical composition

Various algorithms are also in use to estimate the contribution of mineral dust (MD). With MD is meant all fugitive windblown and mechanically resuspended dust with a composition comparable to the earth s crust. Since chemical analyses of PM samples measure elements directly, the approach here is to sum over those elements known to be present in the earth s crust Al, Si, C03, Ca, Fe, K, Mn, Ti and P [10]. Weights were first recalculated to correct for their oxidised form (e.g. Si is usually present as SiC>2). MD is a parameter difficult to estimate. The use of other algorithms in the estimation of MD results in different values, e.g. the one formulated by Denier van der Gon et al. [11]. Also, local anthropogenic sources may contribute (e.g. metallurgical industry). 

Earth s crust ranges in thickness from 10 to 50 km and contains at least trace amounts of 88 chemical elements. It can be subdivided into two distinctly different regimes the oceanic crust that underlies the oceanic basins and the continental crust. The two differ in composition — the oceanic crust being richer in iron, magnesium, and calcium, the continental crust being richer in silicon, aluminum, and alkali elements. The 88 natural elements are all present in both crusts, though in somewhat different concentrations. Nevertheless, only 12 elements, and the same 12 elements in each case, are... 

The crust, hydrosphere and atmosphere formed mainly by release of materials from within the upper mantle of the early Earth. Today, ocean crust forms at midocean ridges, accompanied by the release of gases and small amounts of water. Similar processes probably accounted for crustal production on the early Earth, forming a shell of rock less than 0.0001% of the volume of the whole planet (Fig. 1.2). The composition of this shell, which makes up the continents and ocean crust, has evolved over time, essentially distilling elements from the mantle by partial melting at about 100 km depth. The average chemical composition of the present crust (Fig. 1.3) shows that oxygen is the most abundant element, combined in various ways with silicon, aluminium (Al) and other elements to form silicate minerals. 

The most complete and reliable data about the chemical composition of the Venusian surface comes from three Soviet missions, the Venera 13, Venera 14, and Vega 2 probes. These spacecraft actually reached the planet s surface and conducted studies of elements and compounds present on the planet s surface. In atypical experiment, one of the lander s tools would drill a hole into the planet s surface about 1.2 inches (3 cm) deep and extract a sample about 1 cm3 in volume. The chart on page 110 summarizes data obtained from these three missions and gives the composition of Earth s continental crust for purposes of comparison. Notice that the major differences in crustal composition between the two planets appears to be in the relative abundance of Si02 (45.6 percent on Venus compared with 60.2 percent on Earth) and of MgO (about 11.5 percent on Venus compared with 3.1 percent on Earth). Otherwise, the two planets do indeed appear to be almost "sister planets," at least with regard to the composition of their outer crusts. 

The planet Earth was thus formed. Heat was created as the coalescence (of planetesimals) proceeded due to gravity, and heat also came from radioactivity of several radioactive elements such as aluminum-26. So the newly formed body was heated and the core was melted. As the material becomes liquid (as a result of melting), the materials contained in the liquid separate out according to their densities. The more dense material would sink closer to the bottom (core). Thus, the present layer structure of the Earth formed. The innermost core is a dense solid of about 1,200 km radius, whose density is about 12.6 g per cubic centimeter (12.6 x 10 kg/m ). It is made of mostly iron metal and a small amount of nickel. By the way, the density of iron metal is only 7.8 x 10 kg/m under the ordinary pressure. The next layer is the outer core (up to 3,500 km from the center of the Earth), which is liquid and has a density of 9.5-12x10 kg/m. The chemical composition seems to be about the same as that of the inner core. There is an abrupt change in density in the next layer, mantle. The width of mantle is about 2,900 km (3,500-6,380 km from the center). Its density ranges from 4 to 5.5 x 10 kg/m. The mantle is made of mostly magnesium-iron silicates (silicon oxides). The outermost layer is the thin crust of about 35 5 km on the land portion, and about 6 km under the ocean portion. 

Minerals are the most abundant type of solid matter on the crust of the earth they are homogeneous materials that have a definite composition and an orderly internal structure. Minerals make up most of the bulk of rocks, the comminuted particles of sediments, and the greater part of most soils. Over 3000 minerals have been identified, and new ones are discovered each year. Only a few hundred, however, are common most of the others, such as, for example, the precious stones, are difficult to find (Ernst 1969). Table 3 lists common minerals and mineraloids. Many schemes have been devised for classifying the minerals. In the scheme presented in Table 4, minerals are arranged in classes according to their increasing compositional chemical complexity. 

It is clear that the Earth s mantle has at least two Os-isotopic reservoirs - a plume-related isotopically enriched reservoir and a chondritic upper mantle reservoir. Both have long histories (Fig. 3.32). The variations in composition within the upper mantle reservoir reflect Re-depletion and enrichment related to melt extraction. The isotopically enriched plume reservoir represents chemically isolated, rhenium-enriched, recycled oceanic lithosphere. There is some evidence to suggest that this enriched reservoir may have been in existence since the early Archaean (Walker Nisbet, 2002) and was the source of some Archaean komatiites and the 3.81 Ga Itsaq Gneiss chromitites. If this is true, then basaltic crust was being created and recycled even before 4.0 Ga. Estimates of the present size of this high Re/Os basaltic reservoir vary from 5% to >10% of the whole mantle (Bennett et al., 2002 Walker et al., 2002). 

It has already been shown that the composition of the Earth s continental crust and the Earth s mantle have evolved chemically over time (see Section 4.3 and Chapter 3, Section 3.2.3). Hence, as the continental crust has grown, so has its composition changed, as is apparent from the differences in the REE content and Rb/Sr ratio of granitoids and the Th/La ratio of sediments (Section 4.3.2). These chemical differences could indicate that the mechanism of crust formation has also changed with time. Further support for this hypothesis comes from Plank s 2005 study of crustal Th/La ratios, discussed above. Plank argued that the present-day high Th/La ratio (0.28-0.31) of the continental crust is the product of internal crustal fractionation. However, Archaean continental crust has a much lower Th/La ratio (0.18) than modern continental crust, and does not require intracrustal differentiation, and so may have formed in a different manner. 

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