Crust composition, Covers

Fig. 4.1 Schematic cross-section of continental crust showing geometry and global average composition of sedimentary cover.

Figure 7. (a) Idealized oxygen isotope profile of altered, sediment-covered oceanic crust based on Gregory and Taylor s (1981) study of the Ibra section of the Oman ophiolite and data for marine sediments taken from data sources cited in the text. The vertical black bar marks the range in 5 0 t ical of mantle peridotites the vertical white bar marks the range in 5 0 typical of fresh oceanic basalts, (b) VMues of typical of various major rock types in the Earth s cmst. Data sources are listed in the text. These ranges emphasize typical, representative values and purposefully exclude extreme examples of many rock types. The isotopic composition of most mantle peridotites (Fig. 8) and all NMORBs (Fig. 9) spans a range equal to the thickness of the vertical black hne. 

The Fabric of the Joints in the Human Body is a subject so much the more entertaining, as it must strike every one that considers it attentively with an Idea of fine Mechanical Composition. Wherever the Motion of one Bone upon another is requisite, there we find an excellent Apparatus for rendering that Motion safe and free We see, for Instance, the Extremity of one Bone molded into an orbicular Cavity, to receive the Head of another, in order to afford it an extensive Play. Both are covered with a smooth elastic Crust, to prevent mutual Abrasion connected with strong Ligaments, to prevent Dislocation and inclosed in a Bag that contains a proper Fluid Deposited there, for lubricating the Two contiguous Surfaces. So much in general. 

Today, the more external part of the crust or lithosphere constitutes the superficial covering of the earth. Two kinds of crust are easily distinguished by composition, thickness and consistency continental crust and oceanic crust. Continental crust has a thickness that, in mountain chains, may reach 40 kilometers. It is composed mainly of metamorphic rock and igneous blocks enriched with potassium, uranium, thorium and silicon. This forms the diffuse granitic bedrock of 45 % of the land surface of the earth. The oceanic crust has a more modest thickness, in the order of 5-6 kilometers, and is made up of basaltic blocks composed of silicates enriched with aluminium, iron and manganese. It is continuously renewed along mid-ocean ridges (cf Table 2.2). 

It is not certain that the average crustal relative abundances given by Ronov et al. are better than those of the NASC, which are found on all continents and in the sediments covering the igneous ocean floors. In any event, the value of average crustal abundances is limited mainly to placing limits on the bulk composition of the Earth and considering the extent to which various elements have been excluded from the core and crust. 

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