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ТЕКСТ В The Earth's Crust

Most mineral resources are derived from the Earth's crust. The crust is composed of minerals that are crystalline solids with specific and rather simple composition. Minerals in the Earth's crust are concentrated into specific groups which are called rocks. Two distinctly different types of crust are recognized: oceanic and continental.

Since it is difficult to investigate the floor of the ocean, the composition of the oceanic crust is not known completely. Scientists say that it is relatively constant in composition. The oceanic floor consists largely of minerals rich in calcium, magnesium, iron and silicon, and it is formed by the cooling of lavas extruded on the sea floor to form a type of rock called basalt. It is subjected to the same forces of erosion and weathering.

The continental crust contains less iron and magnesium than the oceanic crust, but relatively more silicon, aluminium, sodium and potassium. The continental crust is more complicated and has a more variable thickness and a less well defined structure.

A systematic examination of all known rock types shows that two principal types predominate: 1) Igneous rocks which are formed by the cooling and crystallization of liquids from deep in the crust called magma; 2) Sedimentary rocks which are formed by

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sedimentation and gradual cementation of sediments by the action of water, ice, wind and organisms. They are layered or stratified. Most of the sediments are deposited in the sea along the continents.

As sediments grow larger and are buried deeper, increasing pres­sure and rising temperature produce physical and chemical changes in them. The resulting metamorphic rocks generally show whether they originated from sedimentary or igneous rocks. This process is slow — hundreds of millions of years are necessary. As weathering and erosion occur, some substances are dissolved and removed in solution while others are transported as suspended particles.

Continental crust contains extremely varied types of rock. It is quite possible to say that the rock-forming processes which we can observe today, have been active for at least 3,500 million years.

The oceanic crust, by contrast with the continental crust, shows little variation in composition. It leads to the idea that the rocks of the sea floor might not contain as many valuable mineral resources as do the rocks of the continental crust. The solution of the problem will be one of the main problems of oceanographic research in future.

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