Igneous Rocks

 

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 larger 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 0.05 mm 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, uplifts 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 they may be horizontal, inclined or vertical. Laccoliths are thicker bodies and in some cases they form mountains.

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 flows 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.

Igneous rocks are rich in minerals that are important economi­cally or have great scientific value. Igneous rocks and their veins are rich in iron, gold, zinc, nickel and other ferrous metals.

Пояснения к тексту:

· abyssal – а абиссальный, глубинный; hypabbissal – a гипабиссальный

· adjacent – а смежный, примыкающий

· body – n тело, вещество; solid (liquid, gaseous) bodies - твердые (жидкие, газообраз­ные) вещества; породная мас­са; массив; месторождение; пласты

· cool – v охлаждать(ся); осты­вать; прохладный

· dyke – n дайка

· extrusion – n вытесне­ние; выталкивание; ant intru­sion – n вторжение; геол. интрузия (внедрение в породу извер­женной массы)

· fine – а тонкий, мелкий; мел­козернистый; высококачественный; тонкий; прекрасный, ясный (о погоде); изящный; fine-graded (fine-grained) - мелкозернистый, тонкозернистый; fines – п pl мелочь; мелкий уголь

· fragmentary – а обло­мочный, кластический

· inclined – а наклонный

· mica – n слюда

· permit – v позволять, раз­решать; syn allow, let; make possible

· rock – n горная порода; igneous ~ изверженная порода; sedimentary ~ осадочная порода

· shallow – a мелкий; поверх­ностный; ant deep - глубокий

· sill – n силь, пластовая интру­зия

· stock – n шток, небольшой ба­толит

· vein – n жила, прожилок, пропласток

 

 

2. Укажите, какие предложения соответствуют содержанию текста.Подтвердите свои ответы фактами из текста.

 

1. Igneous rocks have been formed by sedimentation.

2. Intrusive rocks have been formed by the cooling of rocks of the Earth's crust.

3. Extrusive rocks have been formed the same way.

4. The grain size of igneous rocks depends on mode of occur­rence.

5. Exposed igneous rocks are numerous in mountain zones.

6. Granites and diorites belong to the group of extrusive rocks.

7. As a rule, granite may occur in dykes.

8. Pegmatites do not belong to the group of plutonic or intrusive rocks.

 

3. Письменно ответьте на следующие вопросы по тексту:

1) Have igneous rocks crystallized from magma or have they been formed by sedimentation?

2) Which types of igneous rocks do you know?

3) What does the grain size of igneous rocks depend on?

4) Can you give an example of intrusive or plutonic rocks?

5) Are diorites intrusive or extrusive formations?

6) What do you know about batholiths?

7) Do pegmatites belong to the group of plutonic or volcanic rocks?

8) How do pegmatites occur?

9) What minerals are igneous rocks rich in?

4. Найдите в правой колонке русские эквиваленты следующих слов и словосочетаний и выпишите их:

1. adjacent layers	а) способ залегания
2. abyssal rocks	б) крупнозернистый
3. dimensions of crystals	в) зоны крупных нарушений
4. valuable minerals	г) абиссальные (глубинные) породы
5. shape and size of grains	д) смежные пласты (слои)
6. mode of occurrence	е) размеры кристаллов
7. coarse-grained	ж) взбросы
8. uplifts	з) форма и размер зерен
9. zones of major deformation	и) ценные минералы

5. Соедините главные предложения с придаточными:

1. Abyssal rocks belong to the group of intrusive rocks	а) thatare economically important.
2. Uplifts in mountain belts have permitted erosion to the depths	б) whichusually cut through plutonites.
3. Granites and diorites occur as batholiths	в) becausethey are intruded into pre-existing rocks.
4. Pegmatites (quartz, orthoclase and mica) occur in numerous veins	г) at whichplutonic masses are formed.
5. Extrusive igneous rocks have been formed as lavas	д) wherethe changes in temperature are great.
6. Igneous rocks are rich in minerals	е) whichcome from fissures to the surface of the Earth's crust.
7. Physical weathering occurs in the deserts and in high mountains	ж) whichare large irregular masses.

6. Закончите следующие предложения подходящими по смыслу словами из текста:

1. Batholiths are composed of....

2. Laccoliths are very similar to sills but sills....

3. Granites belong to the group of....

4. Pegmatites are also....

5. Dykes are intrusive bodies which....

6. Extrusive rocks have been formed from....

7. As for the origin of intrusive rocks, they....

8. The grain size of igneous rocks may be different. For ex­ample....

7. Письменно переведите следующие предложения на английский язык:

 

1. Изверженные породы образовались от затвердевшей магмы.

2. Размер зерна изверженных горных пород зависит от их залегания.

3. Самые большие массивы изверженных горных пород называются батолитами.

4. Пегматиты принадлежат к группе вулканических или интрузивных горных пород.

 

Вариант № 3

 

1. Прочитайте и письменно переведите следующий текст: