Chapter 6

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CHAPTER

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ROCKS

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MINERALS

There are three main groups of rocks: Igneous; Sedimentary; and Metamorphic Rocks.

crystals and slow cooling in large crystals. Extremely sudden cooling will result in the formation of a natural glass which is noncrystalline. Large bodies of magma, trapped beneath the surface, cool very slowly because the surrounding rock conduct the heat slowly. Rapid cooling occurs in lava that loses heat rapidly to the atmosphere or to the overlying ocean water. The crystals large enough to be seen with naked eyes or with the help of a hand lens are called phaneritic textured crystals whereas those too small to be distinguished without the aid of a microscope are called aphanitic textured crystals of the igneous rocks. Where crystals in the rock are all within the same size range, the texture is described as equigranular. Where a few large crystals, called as phenocrysts, are embedded in a martix or groundmass of smaller crystals, the texture is porphyritic.

Igneous Rocks

Sedimentary Rocks

These are formed from lava hurled out of a volcano or from the cooling of hot magma below the crust. Granite is coarse-grained igneous rock that is formed by the slow cooling of magma. Basalt is fine-grained igneous rock, almost black, that is formed by quick cooling of lava. Igneous rocks are classified on the basis of chemical composition and texture. Chemical differentiation of magma gives rise to mafic and felsic types of igneous rocks. Texture relates to the sizes and patterns of the mineral crystals present in the rock. The size of mineral crystals in an igneous rock depends largely upon the rate of cooling of magma. As a general rule, rapid cooling results in small

These are made from sediments formed by the erosion and weathering of other rock types. Wind, water and snow erode rocks and carry the sediments to low lying areas. When deposited in the sea they are compressed and hardened to form layers of rocks. Sediment is fragmented mineral and organic matter derived directly or indirectly from pre-existing rocks and from life processes, transported and deposited by air, water or snow/ice. Sandstone is made from grains of sand, which have been naturally cemented together. Chalk is made up of millions of tiny calcium carbonate (lime) skeletons of microorganisms.

he earth’s solid layer, called lithosphere, is about 100 km in thickness. The word ‘lithosphere’ literally means ‘a sphere of rocks’. Down to a depth of 16 km from the earth’s surface, 95 per cent of the earth materials consist of rocks. The rocks are made of individual solid substances called minerals. Each mineral usually contains two or more elements, of which the whole earth is made of. ROCKS Petrologists (scientists who study rocks) define a rock as any natural mass of mineral matter that makes up the earth’s crust. All rocks are not hard. Clay, for example, is a type of rock that is soft. Chalk is still softer. Rock Types

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Throughout the geologic past, layers of sediment have accumulated to great thickness in certain favourable areas on the continents and on the ocean floor. As new layers are added, the underlying layers experience progressively deeper burial. Under increasing pressures imposed by the overlying load, water is excluded from the sediment. They become denser and strongly coherent. Ultimately, hard rock layers are produced, a process ter med lithification. In some instances, chemical changes also occur in the sediments following deposition. All processes of physical and chemical change affecting sediments during its conversion into solid rock are called diagenesis. The first level of classification of sedimentary rocks is into clastic and non-clastic divisions. The adjective “clastic” comes from the Greek word “klastos”, meaning “broken” and describes clastic sediments consisting of particles removed individually from a parent rock source. The naming of clastic rocks depends in part on the size of component mineral grains. A system of grading of mineral grains has been established by geologists, which is known as Wentworth Scale. The non-clastic sedimentary rocks are made of sediments of two basic types, chemical precipitates and organically derived sediments. Chemical precipitates are solid mineral matters precipitated from an aqueous solution in which the component ions have been transported. The rock salt and gypsum are its examples. The organically derived sediments consist of remains of plants or animals as well as mineral matters produced by the activities of plants and animals. Coal and limestone are very good examples of this kind. Important non-clastic sediments are the carbonate minerals, compounds of calcium or magnesium ion or both. Sulphate compounds are also minerals of the chemical sediments, anhydrite (calcium sulphate) and gypsum (hydrous calcium sulphate) are important minerals. Metamorphic Rocks These types of rocks are formed when a rock is subjected to heat and/or pressure (Fig. 6.1).

FUNDAMENTALS OF PHYSICAL GEOGRAPHY

The heat and/or pressure alter the characteristics of the original rock by forming new minerals. Clay when subjected to heat and/ or pressure gets metamorphosed to slate. Similarly, limestones give rise to marble. The metamorphic rocks can be broadly grouped into two major classes: cataclastic rocks and recrystallised rocks. Cataclastic rocks are formed by mechanical disruption (breaking and crushing) of the original minerals without appreciable chemical change. The process is described as dynamic metamorphism. The recrystallised rocks are formed by the recrystallisation of the original minerals. Minerals with different chemical formulae and crystal lattices are produced due to recrystallisation. The recrystallised rocks are further divided into two subclasses: contact and regional metamorphic rocks. The contact metamorphic rocks are formed by recrystallisation under high temperature caused by the intruding magma. The rocks are not subjected to bending and breaking but new mineral/s, emanating from magma are added to metamorphosed rocks. The regional metamorphic rocks undergo recrystallisation during the process of being deformed by shearing, often under conditions of high pressure or high temperature or both. As metamorphism continues a large percentage of the minerals assume plate-like shape and are assembled in parallel orientation in the rock, a structure known as foliation. Strongly developed foliation is known as schists. In schists, crystals of certain minerals grow in size. These large crystals are referred to as porphyroblasts. Another structure found in metamorphic rocks is lineation. In this, mineral grains are drawn out into long, thin, pencil-like objects, all in parallel alignment. In another for m of structure, known as banding, minerals of different varieties or groups are segregated into alternate layers. These layers are usually of light or dark shades, making the banding conspicuous. Gneiss and schist have well marked banded structure.

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ROCKS AND MINERALS

Fig.6.1 : Rock Cycle

MINERALS

Crystal Structure

The mineral is a naturally occurring homogeneous solid that is an inorganic substance, having an orderly atomic structure and a definite chemical composition. The basic source of all minerals is the hot magma in the interior of the earth. When magma cools, crystals of minerals appear. These first crystals may sink in the magma so that the composition of the magma changes with depth. Thus, a sequence of minerals is formed in the rocks as the magma cools. Besides, certain minerals such as coal, petroleum and natural gas are organic substances found in solid, liquid and gaseous forms respectively.

Mineral crystals fall into six crystal systems (Fig.6.2), each of which is defined in terms of its crystallographic axes. (i) Isometric System : Three axes of crystal are of equal length and lie at right angle to each other, e.g. Halite mineral. (ii) Tetragonal System : Three axes make right angles with each other. The two horizontal axes, a and b, are of equal length but the third, c axis, is of different length, e.g Zircon mineral. (iii) Hexagonal System : Four axes are present. Three of the axes are horizontal and of equal length, intersecting in angles of 600 and 1200. The fourth axes is perpendicular to other axes and is of different length, e.g. Beryl mineral. (iv) Orthorhombic System : Three axes intersect at right angles to each other, but they are of unequal lengths, e.g. Staurolite mineral.

Physical Properties of Minerals Minerals can be identified by their hardness, colour, the way they reflect light (lustre), the way they break (cleavage and fracture), and their density.

FUNDAMENTALS OF PHYSICAL GEOGRAPHY

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(v) Monoclinic System : The three axes are of unequal length. Two axes intersect at right angles whereas the third axis intersects obliquely forming an acute angle, e.g. Gypsum mineral. (vi) Triclinic System : The three axes are of unequal length and intersect at oblique angles, e.g. Albite mineral. Cleavage and Fracture The smooth planar surfaces of weakness along which a mineral has pronounced tendency to split is known as cleavage planes. It bears a close relationship to internal atomic structure and external crystal form of mineral. Minerals

lacking cleavage break along various types of fracture surfaces. For example, the curved fracture surfaces of a glass constitute conchoidal fracture, that is seen in mineral quartz. Specific Gravity Each mineral has a certain specific gravity which is the ratio of its density to the density of water at 40 Celsius. Specific gravity of minerals is a property of great importance because it determines the density of a given rock and rock density in turn determines the gross layered structure of the earth.

Fig.6.2 : Mineral Crystals — Various Forms

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ROCKS AND MINERALS

Hardness The degree to which a mineral surface resists being scratched is known as its mineral hardness. Hardness is geologically important because it determines how easily a mineral is worn away by the abrasive action of streams, waves, wind and glaciers in the processes of erosion and transportation. Ten standard minerals constitute the Mohs Scale of hardness ranging from the softest to the hardest (Appendix II). Lustre The appearance of a mineral surface under reflected light is referred to as its mineral lustre. It is described by several descriptive adjectives, such as metallic (metal like), adamantine (diamond like), vitreous (glass like), resinous (oil like), pearly or silky (pearl or silk like). Colour Certain minerals possess a distinctive mineral colour that facilitates recognition. The impurities present in the mineral provides shades of colour to a mineral.

resources. The most basic group, essential resources, comprises soil and water. Energy resources can be divided into the fossil fuels (crude oil, natural gas, coal, oil shale and tar sand) and the nuclear fuels (including uranium, thorium and geothermal power). Metal resources range from structural metals such as iron, aluminum and titanium to ornamental and industrial metals such as gold, platinum and gallium. Industrial minerals include more than 30 minerals, such as salt, asbestos and sand (Appendix III). Mineral deposits have two geological characteristics that make them a real challenge to modern civilisation. First, almost all of them are non-renewable resources. The geological processes that form them are much slower than the rate at which we exploit them. There is no likelihood of our ability to grow mineral deposits at a rate equal to our consumption. Second, mineral deposits have a place value. We cannot decide where to extract them; nature made that decision for us when the deposits were formed. Facts About Minerals •

Streak When a mineral is rubbed across the white porceline plate, known as streak plate, it may leave a streak of mineral powder of distinctive colour. The colour of powder of mineral may sometimes be different than the colour of the mineral. Besides physical properties, minerals are also identified on the basis of their optical properties. These optical properties are evaluated by means of a microscope using polarised light rays and are of great value in mineral identification.

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Economic Importance of Minerals Mineral resources can be divided into four main groups: essential resources, energy resources, metal resources, and industrial

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There are at least 2,000 minerals that have been named and identified. However, most rocks are made up of not more than 12 different classes of minerals. The most common element in the earth’s crust is oxygen. The second most common element is silicon. Mineral Quartz is silicon dioxide and is very common mineral. The largest diamond ever found was the Cullinan diamond, discovered in a mine in South Africa in 1905. It was 3,106 carats, which means it weighed more than 600 g. Diamond and Graphite are both forms of carbon but their atoms are arranged differently. Diamond is the hardest mineral whereas Graphite is soft, black and feels greasy. Under the sea, minerals dissolved in water crystallise around the vents of faults or fissures.

FUNDAMENTALS OF PHYSICAL GEOGRAPHY

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EXERCISES

Review Questions 1. Answer the following questions: (i) Define a rock. (ii) Name the types of rocks. (iii) Which are the two main types of igneous rocks? (iv) What is meant by ‘texture’ of the rocks? (v) What is lithification? (vi) Name the scale that grades the size of mineral grains. 2. Distinguish between: (i) Rocks and minerals; (ii) Chemical precipitates and organically derived sediments; (iii) Cataclastic rocks and recrystallised rocks; (iv) Foliation and lineation. 3. Give a word for the following: (i) Few large crystals embedded in a matrix of smaller crystals. (ii) The processes of physical and chemical changes affecting sediments during their conversion into solid rocks. (iii) Structure of a metamorphic rock in which mineral grains are drawn out into long, thin pencil like objects, all in parallel alignment. (iv) The smooth planar surfaces of weakness along which a mineral has pronounced tendency to split. 4. Classify the following rocks into igneous, sedimentary and metamorphic: (i) Granite; (ii) Slate; (iii) Marble; (iv) Limestone; (v) Clay; (vi) Basalt; (vii) Sandstones; (viii) Coal; (ix) Chalk; (x) Gypsum; (xi) Gneiss; and (xii) Schist. 5. Write short notes on: (i) Sedimentary rocks; (ii) Types of metamorphism; (iii) Economic importance of minerals. 6. Describe the formation of igneous rocks giving suitable examples of various types. 7. Discuss physical properties of minerals. Finding Out Collect rock samples of different kinds and write their main features as seen through naked eyes.