Readng comprehension

3.1 DETAILED READING: Occurrence and classification of metamorphic rocks

Read the following text and fulfill the after-reading tasks (R.P – 7.1)

Recrystallized or deformed preexisting rock units constitute approximately 15 % of the continental crust. Metamorphic rocks are products of pressure-temperature conditions intermediate between those of igneous and sedimentary environments. Thus, on the one hand, metamorphic processes merge with diagenesis, a sedimentary phenomenon; on the other hand, metamorphism leads to partial fusion of deeply buried continental and oceanic crust and in the generation of granite and andesitic magmas.

Two principal metamorphic processes may be distinguished: mechanical deformation and chemical recrystallized. The former process includes grinding, crushing and ductile deformation of an initial rock, phenomena that reflect readjustment of the material. Recrystallization takes place because preexisting mineral assemblages are destabilized by changes in the temperature, pressure or chemical milieu. Nearly all-metamorphic rocks show a combined influence of both mechanical deformation and chemical reaction; they differ principally in the degree of development of these effects. Metamorphic rocks exhibit contrasts reflecting variations in bulk composition as well, due to differences in original rock chemistry or to alteration.

Mechanical deformation is called cataclasis and metamorphic rocks in which reduction to small particle size is conspicuous are known as cataclastic rocks. In contrast to cataclastic rocks, which are produced by dominantly mechanical deformation, contact metamorphic rocks are formed by a significant increase in temperature in the absence of differential stress. Contact metamorphic rocks are localized as concentric zones surrounding igneous bodies emplaced at upper levels of the crust.

The most common varieties of metamorphic rocks develop on a regional scale in response to both deformation and mineral reaction. Such widespread rocks are known as regional metamorphic rocks and are typified by orientated mineral fabrics. Foliation and lineation are widespread features reflecting differential motions of the rock sections undergoing recrystallization.

(Barret E., Hunt A. And Milner B.” Earth and Atmosphere”, 1993, Longman)

3.1.1. State whether the following sentences are T (TRUE) or F (FALSE), according to the information from the text

1. Metamorphic rocks compose about 25% of the continental crust.

2. Metamorphic rocks are a combination of sedimentary and igneous rocks altered under pressure and heat conditions.

3. There are two main metamorphic processes.

4. Mechanical deformation includes grinding and crushing of the country rock.

5. Pre-existing mineral assemblages are chemically recrystallized by changes in the temperature, pressure or chemical milieu.

6. All metamorphic rocks show a combination of both mechanical deformation and chemical reaction.

7. Metamorphic rocks show contrasts because of the differences in original rock chemistry or change.

8. Mechanical deformation forms the so-called cataclastic rocks.

9. Contact metamorphic rocks are formed by a great increase in heat without differential stress.

10. Contact metamorphic rocks are found in zones surrounding sedimentary rocks on the surface of the crust.

11. Regional metamorphic rocks are a result of both deformation and mineral reaction.

12. Foliation and lineation are characteristic features of various rock motions in the process of recrystallization.