ICSE Class 6 Geography Chapter 6 Rocks

Chapter Six

Rocks

Syllabus

Rocks: Rocks - difference between minerals and rocks, types of rocks: igneous, sedimentary, metamorphic; their characteristics and formation; rock cycle.

Rocks And Minerals

Rocks consist of mineral elements. Though scientists have identified more than 2000 mineral elements, yet only a few of these are important as constituents of rocks. Distinction must be drawn between mineral elements found in the earth's crust and the earth as a whole.

The most abundant mineral elements of the earth, as a whole, are iron, silica, magnesium, nickel, sulphur and calcium. The most abundant mineral elements of the crust are silicon, aluminium, iron, magnesium, potassium and sodium. The earth's crust contains more of silicon and aluminium whereas the earth as a whole contains more of iron, silica and magnesium.

A rock can be defined as an aggregate of minerals that forms more or less a definite unit of the earth's crust. The term rock refers not only to any hard solid matter like granite, but also to soft loose particles like sand, silt and clay derived from earth. The earth's crust is made up of various types of rocks that differ from one another in texture, structure, colour, permeability, mode of occurrence and degree of resistance to denudation.

Teacher's Note

Rocks form the foundation of mountains and cliffs we see around us. The rocks in your garden or local construction site are examples of the three main types discussed here.

Types Of Rocks

Rocks are classified in several ways but important classification is done on the basis of their origin. Three main types of rocks based on origin are (i) Igneous; (ii) Sedimentary, and (iii) Metamorphic.

Igneous Rocks

Formation: The word 'Igneous' is derived from the Latin word 'ignis' meaning fire. These rocks were formed due to cooling, solidification and crystallisation of hot molten material of the earth known as magma. Found at great depths in the interior of the earth. The chemical composition of these rocks varies from alumino-silicate minerals like quartz and feldspar to granite and rhyolite collectively known as Sialic rocks. Since the igneous rocks were the first to be formed, they are called primary rocks. They form the basis of formation of other types of rocks.

Characteristics Of Igneous Rocks

1. They are hard and compact.

2. They are formed by solidification of molten magma.

3. They are granular and crystalline.

4. They are either fine grained, smooth and compact or may have large crystals with coarse texture.

5. The size of their crystals depends upon the rate of cooling of the molten material.

6. These rocks are less affected by chemical weathering because water does not percolate in these rocks.

7. Most of them consist of silicates.

8. They do not have layers. They are generally weathered by mechanical weathering.

9. Igneous rocks are associated with volcanic activity and are found mostly in volcanic regions.

Classification Based On Origin

Based on their origin, the igneous rocks are classified into two types - (i) Extrusive Igneous Rocks and (ii) Intrusive Igneous Rocks.

(i) Extrusive Igneous Rocks: They are formed by the cooling of molten magma on the earth's surface. The magma which is brought to the surface through fissures or volcanic eruptions, solidifies to form rocks. Hence, such rocks are smooth, crystalline and fine grained. They are also called as volcanic rocks. Basalt is a common extrusive igneous rock and forms lava flows, lava sheets and lava plateaus. Some kinds of basalt solidify to form long polygonal columns. For example, the Giant's Causeway in Northern Ireland.

(ii) Intrusive Igneous Rocks: When the magma solidifies within the earth's crust it cools slowly forming coarse textured, hard rocks with large crystals, e.g. granite, gabbro, dolerite. When the magma solidifies below the earth's surface it forms different types of intrusive rocks like batholiths, laccoliths, sills, dykes, necks, etc. (Fig. 6.2).

(a) Batholiths: They are deep-seated intrusion of igneous rocks. They are usually dome-shaped with no definite floor and they form cores of mountain ranges (Bathos meaning 'depth'). They may be exposed to earth's surface through continued erosion. They are chiefly composed of granite. Some of the important examples are Ranchi Batholiths.

(b) Laccoliths: The magma does not reach the crust of the earth. Magma becomes solid just below the crust. Different types of intrusions of magma form differently shaped landforms. The upper surface is dome-shaped but the bottom is flat (from Greek lakkos meaning cistern).

(c) Sills: Sometimes magma flows between layers of rocks horizontally. It then hardens there. This layer of intrusive rock is called Sill.

(d) Dykes: The magma when forced upwards fills vertical cracks or fissures in existing rocks and it then hardens there to form dykes.

(e) Necks: Sometimes the passage of an extinct volcano is filled with magma. It then solidifies there and is known as volcanic neck or plug.

Intrusive igneous rocks are also called plutonic rocks because of slow cooling of magma at great depth which leads to the formation of crystals of large size.

When magma cools at intermediate depth and forms minor rocks like sills and dykes it is called Hypobyssal rocks.

Classification On The Basis Of Chemical Composition

Igneous rocks are classified into (i) Acid Igneous rocks and (ii) Basic Igneous rocks on the basis of their chemical composition.

(i) Acid Igneous Rocks: These rocks have a silica content between 65 to 85 per cent. They generally lack in iron and magnesium. Granite is acid igneous rock. It is made up of large crystals of quartz, feldspar and mica. It is commonly used as a building material.

(ii) Basic Igneous Rocks: They have higher percentage of oxides of denser elements and silica content varies between 40 to 60 percent. It is heavy and dark in colour. Basalt, dolerite and gabbro are typical examples of this group. Igneous rocks are usually compact and of interest to builders and sculptors. They may also be associated with ores of metals. Important igneous rocks are gabbro, basalt, granite, dolerite, feldspar, pumice, obsidian, permadite and soria.

Teacher's Note

Granite countertops in kitchens and basalt used in road construction are common examples of igneous rocks we use in daily life.

Sedimentary Rocks

Formation: When igneous rocks are exposed on the surface of the earth, they are broken down by weathering and carried away by forces of gradation like rivers, glaciers and ocean waves. They are then deposited as sediments. Any exposed rock, due to the various agents of erosion may weather and disintegrate. The sediments are deposited over long periods in seas, lakes, streams, etc. Subsequent layers are added and due to their own weight from above, the sediments get cemented together to form sedimentary rocks. They are secondary rocks because they are formed by the solidification of sediments of original igneous, sedimentary or metamorphic parent rocks.

Characteristics Of Sedimentary Rocks

1. Sedimentary rocks are formed from materials derived from other rocks. They may also have plant and animal remains. These rocks may thus contain fossils.

2. They may be formed through different processes and are also classified on the basis of their formation. The process that turns loose sediments into rocks is called lithification.

3. They are most widespread on the surface of the earth and constitute about 75 per cent of the surface area of the globe.

4. Sedimentary rocks are generally not crystalline. They are soft (as compared to igneous rocks) and layered as they are formed by deposition of sediments. The different types of sediments are consolidated and compacted by different types of cementing elements like calcium, silica, etc.

5. Sedimentary rocks make extensive landforms. They do not make massive rock forms like batholiths and laccoliths.

Processes Involved

Lithification means turning the loose sediments into hard rock (lith). Transported sediments are deposited layer upon layer. This builds up pressure and temperature. Three processes are involved in turning the loose materials into hard rocks - evaporation, compaction and cementation.

(i) Evaporation: In this case, water from loose materials is evaporated as in the case of rock-salt. The accumulation of rock salt mainly takes place through evaporation before being compacted and cemented.

(ii) Compaction: In this process, sediments after piling up are gradually squeezed by the weight of overlying layers and hardness of the underlying layers. Sand is compacted in this way to sandstone.

(iii) Cementation: It is the bonding together of compacted sediments by natural materials such as calcium compounds, silica and iron, all of which have the properties of bonding on the loose materials.

Classification On The Basis Of Formation

(i) Mechanically formed rocks: These contain fragments from breaking up of other rocks. These sediments are also called Clastic sediments. The breaking of older rocks takes place through denudation by agents like running water, moving glaciers and wind. Mechanically formed rocks include shale, sandstone, clay, conglomerates and loess. In this process, the sediments are gradually squeezed by the weight of overlying sediments and the lower layers harden to form rocks. Since such rocks are formed in layers, they are known as Stratified Rocks. For example, sand becomes sandstone.

(ii) Chemically formed rocks: These are formed by the direct precipitation of mineral matter from solution. The accumulation takes place in lakes and lagoons. They are compacted through evaporation. For example, gypsum and rock salt and potash. Rock salt is found in Dead Sea, Aral Sea as well as in Sambhar Lake in Rajasthan.

(iii) Organically formed rocks: These rocks contain remains of dead plants and animals. Limestone (or calcareous rocks) is formed by skeletons, shells and animal remains. It contains lime proportions of time.

Teacher's Note

Sandstone used in building construction and limestone found in chalk deposits are sedimentary rocks formed over millions of years from compressed sediments.

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