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ФЕДЕРАЛЬНОЕ АГЕНТСТВО ПО ОБРАЗОВАНИЮ

ВОРОНЕЖСКИЙ ГОСУДАРСТВЕННЫЙ УНИВЕРСИТЕТ

И-139

Учебно-методическое пособие

по специальности 130301 (080100)

“Геологическая съемка, поиски и разведка

месторождений полезных ископаемых”

ГСЭ Ф.01 (английский язык)

Воронеж

2004

Утверждено научно-методическим советом геологического факультета (16.12.04, протокол №4)

Составители: Титова Л.Н.

Должикова К.Ю.

Учебно-методическое пособие подготовлено на кафедре английского языка факультета РГФ Воронежского государственного университета. Рекомендуется для студентов заочного отделения геологического факультета.

TEXT 1

1. Translate the following words and word combinations.

igneous rocks; to reach the surface; an intrusive igneous rock; to be too small for the unaided eye to see; as a result of erosion; rock-forming igneous minerals; at extremely high pressures; to grow in magmas; to be abundant in rocks of granitic composition; to occur in association with; cleavage; to be appropriate for minerals to start crystallizing; sodium- and potassium-rich feldspars; the overall composition of the rock; above destructive plate boundaries; to be move viscous than; to melt; molten; to reach progressively shallower depths; vulnerability to the process of erosion; to be granitic in composition; oceanic gabbro; to be neither plutonic nor volcanic; to be injected upwards; a dyke; coarse-grained rocks; at the time of their formation; to be injected in a horizontal sheet; much finer crystals; to provide evidence (that); a frozen mixture of the elements; to be of medium grain size; to undergo dramatic changes

2. Read and translate the text

Igneous intrusions

Rocks formed by cooling of a magma are called igneous rocks, from the Latin word for fire. Not all magma reaches the surface. Often it solidifies at depth, to form an intrusive igneous rock.

Volcanic rocks cool so quickly that usually the only crystals that grow within them are too small for the unaided eye to see. However, an intrusive rock cools more slowly, so there is time for the crystals of which the rock is formed to grow to lengths of millimetres or sentimetres before the rock is completely solidified. Crystals this big are easy to see when the body is eventually exposed on the surface as a result of erosion.

Minerals

Because the crust and mantle contain so much silicon and oxygen, all the common sorts of crystals that grow in magmas from these sources contain these elements. The crystalline structure of rock-forming igneous minerals is based on two- or three-dimensional arrays of silicon and oxygen held together by molecular bonds. These minerals, as well as the rocks that they form, are commonly referred to as silicates.

The most familiar of the silicate minerals is pure silica. This contains just silicon and oxygen, and has the formula SiO₂. In crystalline form this is the mineral quartz, except at extremely high pressures when the atoms are squeezed into more tightly-packed crystalline structure. All other silicate minerals contain various metallic elements in addition to silicon and oxygen. Quartz is a robust, hard-wearing mineral. It is able to grow in magmas whose SiO₂ content exceeds 52%. Quartz is abundant in rocks of granitic composition and is usually present, though scarce, in rocks of andesitic composition.

The potentially confusing use of silica meaning two quite different things is standard in geology. The silica (or SiO₂) content quoted for a rock is based on an overall chemical analysis of the elements making up the rock. This should not be taken to imply that all or any of the silicon and oxygen occurs in the form of quartz or any other pure silica mineral. On the other hand, when SiO₂ is given as the chemical formula of a mineral, it means that this mineral contains Si and O only, in the ratio 1 : 2. This specific ration is dictated by number of oxygen atoms each silicon atom is able to form a bond with. The three-dimensional structure of such bonds controls the properties of the crystal.

In magmas whose composition is basic (42 – 52% silica) or ultrabasic (45% silica), metallic elements are so abundant that no quartz crystals can grow, and the only silicate minerals to form contain a relatively high proportion of metals. The most extreme example is olivine, formula Mg₂SiO₄, in which there are twice as many metal atoms (in this case, magnesium) as silicon atoms. Actually, iron (Fe) atoms can fit in the same place as magnesium atoms, so olivine can have any composition ranging between Mg₂SiO₄ and Fe₂SiO₄, with Mg and Fe in any proportion.

Olivine most often occurs in association with pyroxene, another metal-rich silicate, with the formula (Ca,Mg,Fe)₂Si₂O₆. A rock composed essentially of just olivine and pyroxene is described as a peridotite, and these are the dominant minerals in mantle rocks.

In magmas of higher SiO₂ content, the proportions of magnesium and iron tend to be lower, whereas aluminium (Al), calcium (Ca), sodium (Na), and potassium (K) become more abundant. One result is the appearance of minerals known as feldspars.

The other important silicate minerals are mica and amphibole. These occur in granitic and andesitic rocks and depend on the presence of water in the magma.

Each of these minerals, and varieties within each type, can be distinguished by features such as colour, hardness, cleavage (regular planes of weakness in the crystal resulting from its atomic structure), and optical properties. With practice it is possible to recognize many of these when you find a coarse-grained igneous rock exposed in the field, without the aid of analytical instruments. Having identified each of the minerals making up rock, it is then possible to give the rock itself a name, and thence to deduce the conditions under which the rock is likely to have formed.