Use the terms and fill in the gaps
A cross section (1) _________ the distinctive features. The magma (2)
_______ lies many miles below the Earth’s surface. It is a reservoir of gas-
rich (3) _ rock under (4) ___________ . This magma may (5) ________
outward, which results in the escaping of pressure through a (6) _____ in
the crust. As the (7) ______ rises, the pressure is reduced. Dissolved gases
are freed as (8) ______ _____ .This force of (9) _____ blasts opens and
forms a circular (10) ______ . This is an (11) _______ for ash, cinder and
(12) ________ . Explosions shape the volcano’s top as an (13) ___ cone
or (14) ________ .
READING COMPREHENSION
SCANNING - Igneous rocks (R.P. -5.5)
Read the following text and fulfill the after-reading exercise
These types of rock constitute approximately 75% of the continental crust and well over 90% of the oceanic crust. Some have been derived directly from the mantle, whereas others represent partly remelted oceanic or continental crustal materials.
Chemically speaking, igneous rocks exhibit limited compositional variations. Their principal constituent oxide is silica (SiO2), which ranges from about 40 to 75% by weight in the common rock types. Where the silica content is low, magnesium and iron oxides commonly are major components.
Occurrences of igneous rocks belong to two different categories: extrusive and intrusive. To the first group belong those that have reached the surface of the Earth in a largely molten condition. The second group of
igneous rocks consists of those that crystallized from magmas that solidified before reaching the surface of the Earth. Processes that formed them are thus hidden from view and must be deduced from the study of uplifted and deeply eroded parts of the crust.
Classification and chemical differentiation of igneous rocks
Igneous rocks are distinguished on contrasting chemical composition. Because the bulk-rock chemistry is reflected in the nature and proportions of the mineral present, we will utilize a simple classification based on the proportions of quarts, feldspars and dark ferromagnesian, minerals and on the composition of the plagioclase.
The most refractory minerals (i.e. those stable at the highest magmatic temperatures) form first and as the melt cools, crystallize, in some cases replacing earlier ones. This crystallization takes place discontinuously; formation of another group of minerals proceeds continuously, as in the feldspars.
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Bulk compositions reflect differences in mineral proportions among the various rock types. The solid and the liquid can be separated at this stage as well. The process is known as igneous differentiation and explains the contrasts in compositions of melts and solids produces during either partial fusion and / or partial freezing.
Correct the following statements where necessary (Igneous rocks)
1. Igneous rocks comprise the smallest proportion of the continental crust and the largest of the oceanic crust.
2. Most igneous rocks are formed from the crust.
3. Most igneous rocks are remelted oceanic and continental materials.
4. Igneous rocks have rather limited compositional variations.
5. The main constituent is oxide.
6. Silica has an average weight in the common rock types.
7. Magnesium and iron oxides are also major components.
8. There are two categories of igneous rocks-extrusive and intrusive.
9. Intrusive is those that have come to the surface of the Earth in a molten condition.
10. The second category is composed of those that are crystallized from magma that solidified before reaching the surface of the Earth.
11. The processes of the formation of igneous rocks can be studied in uplifted and deeply eroded parts of the crust.
3.1.2. You have one minute to read this part of the text -Classification and
chemical differentiation of igneous rocks
Complete the following text with the missing information. DON’T PEEP INTO THE TEXT!
Igneous rocks are distinguished on contrasting _____________ (1).
Because the bulk-rock chemistry is reflected in the nature and proportions of
the mineral present, we will utilize a simple _________ (2) based on the
proportions of _________ (3), feldspars and dark ferromagnesian, minerals
and on the composition of the plagioclase.
The most refractory minerals (i.e. those stable at the highest ______
________ (4) form first and as the melt cools, crystallize, in some cases
replacing earlier ones. This _________ (5) takes place discontinuously,
formation of another group of minerals proceeds continuously, as in the feldspars.
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Bulk compositions reflect differences in ___________ _______ (6)
among the various rock types. The ___________ (7) and the
______________ (8) can be separated at this stage as well. The process is
known as ___________ _______________ (9) and explains the contrasts
in compositions of melts and solids produce during either partial
__________ (10) and / or partial ________ (11).
3.2 INFORMATIVE READING – Igneous Rocks (R.P.-5.4) Read the following text and fulfill after-reading tasks.
Igneous rocks have crystallized from solidified magma. Igneous rocks can be classified in a number of ways and one of them is based on mode of occurrence. They occur either as intrusive (below the surface) bodies or as extrusive masses solidified at the Earth’s surface. The terms “intrusive” and “extrusive” refer to the place where rocks solidified.
The grain size of igneous rocks depends on their occurrence. The intrusive rocks generally cool more slowly than the extrusive rocks and crystallize to a large grain size. The coarser-grained intrusive rocks with grain size of more than 0.5 mm called plutonic or abyssal are referred to as intrusive igneous rocks because they are intruded into older pre-existing rocks. Extrusive or volcanic rocks have even finer grains, less than 0.05mm and are glassy.
Exposed igneous rocks are most numerous in mountain zones for two reasons. First, the mountain belts have been zones of major deformation. Second, uplifted in mountain belts have permitted plutonic masses to be formed.
The largest bodies of igneous rocks are called batholiths. Batholiths cooled very slowly. This slow cooling permitted large mineral grains to form. It is not surprising that batholiths are composed mainly of granitic rocks with large crystals called plutons. As is known, granites and diorites belong to the group of intrusive or plutonic rocks formed by solidification of igneous mass under the Earth’s crust. Granites sometimes form smaller masses called stocks, when the occurrence has an irregular shape but smaller dimensions than the batholiths.
Laccoliths and sills, which are very similar, are intruded between sedimentary rocks. Sills are thin and may be horizontal inclined or vertical. Laccoliths are thicker bodies and in some cases they form mountains.
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Dykes are also intrusive bodies. They range in thickness from a few inches to several thousand feet. Dykes are generally much longer than they are wide. Most dykes occupy cracks and have straight parallel walls. These bodies cool much more rapidly and are commonly fine-grained. For example, granite may occur in dykes that cut older rocks.
Pegmatites (quartz, orthoclase and mica) also belong to the group of plutonic or intrusive rocks. They occur in numerous veins, which usually cut through other plutonites, most often granite or adjacent rocks.
Extrusive igneous rocks have been formed from lava flow which come from fissures to the surface and form fields of volcanic rocks, such as rhyolite, andesite, basalt, as well as volcanic ashes and dust, tuff, etc. As a rule, these rocks of volcanic origin cool rapidly and are fine-grained. It is
interesting to note that basalt is the most abundant of all lava types. It is the principal rock type of the ocean floor.
(М.Я. Баракова, Р. И. Журавлева, Английский язык для горных инженеров, М. 2001)
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