Types Of Rocks | Characteristics And Importance

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Table Of Contents

1. Meaning of rock
2. Types of rocks
3. Characteristics of rocks
4. Economic importance of rocks to man
5. Disadvantages of rocks to man.
6. Revision Questions.

Introduction

The earth’s crust consists of rocks. The Rock is any mineral material of the earth. It may be a combination of different mineral elements such as silica which contains silicon and oxygen.

All the Rock differ from one another in texture, structure, color, permeability, mode of occurrence and degree of resistance to denudation.

Types Of Rock

All rocks are classified into three major types, these include:

1. Indigenous rocks
2. Sedimentary rocks
3. Metamorphic rocks

1. Indigenous Rocks

Characteristics Of Indigenous Rocks

• Igneous rocks are crystalline in structure, i.e they contain crystals.
• They do not occur in layers, i.e they are non-stratified rocks.
• They do not contain fossils.
• They are usually hard and impervious.
• They are either acidic or basic.
• They vary in colour (light of dark)
• Crystal’s size vary from specks to glassy-mass.

Mode Of Formation: Igneous rocks are formed by the cooling and solidification of molten rocks called magma, ejected from beneath the earth’s crust. The magma which result from high temperature and pressure beneath the earth, forces itself towards the earth’s surface through cracks. As the magma moves towards the surface, it comes in contact with lower temperature, hence it cools and solidifies to form igneous rocks.

There are two types of igneous rocks

(a) Plutonic (instructive) igneous rocks: These are rocks formed when the molten magma cools and solidifies slowly before it gets to the surface of the earth to form large crystals. As a result of prolonged erosion, the plutonic igneous rocks will later be exposed to the surface. Examples of plutonic igneous rocks are granite, gabbro and diorite.

(b) Volcanic (extrusive) igneous rocks: This types of rocks are formed when the molten magma cools and solidifies rapidly on getting to the surface of the earth to form small crystals. Example of volcanic igneous rocks is basalt.

2. Sediment Rocks

Characteristics of sedimentary rocks

• Sedimentary rocks are stratified. They show distinct layers or strata.
• They may be coarse or fine grained depending on the nature of materials deposited.
• They are non crystalline.
• They are fossilized rocks. That is, they contain fossils of plants and animals.
• They are usually soft.
• They are not resistant to erosion.
• Examples are sandstone, coal, limestone.
• They are permeable.

Mode of formation: Sedimentary rocks are formed from sediments deposited either by water, by wind or by ice. These sediments are deposited in layer or strata one on top of the other, and after a long period of time, they become hardened by compression to form sedimentary rocks. Sedimentary rocks as stratified rocks and the plane between two strata is called bedding plane.

There are three types of sedimentary rocks based on their process of formation, these are:

(a). Mechanically formed sedimentary rocks

• They are derived from sediment of pre-existing rocks.
• The rocks debris are usually deposited by agent of erosion such as water, wind and glazier.
• The deposited materials are pressed together to become solid by the weight of overlying deposits.
• This process of solidification is called compaction or cementation.
• They may form conglometeries. Examples are sandstone, breccia, shale, clay and conglomerate.

(b). Organically formed sedimentary rocks: These are types of rocks formed from the remain of living organisms. When sedimentary rocks are formed from the remains of animals like corals or shellfish, such rocks are called calcareous rocks, e.g, limestone and chalk. But when sedimentary rocks are formed from the remains of vegetable matter such as swamps and Forest, they are called carbonaceous rocks, e.g. coal, peat, lignite and petroleum.

(c). Chemically formed sedimentary rocks

• These are rocks formed from precipitation.
• They are also formed from other rocks solutions or certain minerals from solution of salt.
• They are often referred to as evaporates.
• They are also formed from crystallization. Examples include potash, sodium chloride (common salt), nitrate, gypsum and dolomite.

Importance Of Sedimentary Rocks To Man

• Sedimentary rocks are source of materials for construction.
• They are source of minerals, e.g petroleum, coal and gold.
• They determine the quality of soil nutrients.
• They determine the availability and supply of underground water.
• They provide tourist attractions. example is Ogbunike cave in Anambra State.
• They are also source of raw materials, for example, limestone is used for cement.
• They source of food, e.g. rock salt.
• They are used as medicine, e.g, kaolin.
• They are used for scientific research, e.g. fossils.

3. Metamorphic Rocks

Characteristics Of metamorphic Rocks

• Some may occur in layers or strata.
• The rocks may be hard or soft.
• They are not crystallized in structure.
• They exist in different colors and texture.
• They may contain fossils.

Mode Of Formation: Metamorphic rocks are changed rocks. This types of rocks are formed from pre-existing igneous or sedimentary rocks which have been changed as a result of great heat and pressure. The original character and appearance may be greatly altered or change by some forces. Examples of metamorphic rocks are slate, marble, gneiss, schist and graphite.

Slate is formed from clay, marble is formed from limestone, quartzite is formed from sandstone, gneiss is formed from granite, schist is formed from shale, while graphite is formed from coal.

Importance Of Metamorphic Rocks

• Marble for floor tiles.
• Graphite for lead pencils
• Gneiss for construction purposes.
• Slate, marble are minerals/raw materials for Industries.
• Kaolin for ceramic products.
• Foreign exchange earnings, e.g, from graphite and marble.
• Emeralds for jewelries.

Difference Between Sedimentary And Metamorphic Rocks

• Resistance: Metamorphic rocks are generally more resistant than most sedimentary rocks.
• Lustres: Metamorphic rocks are crystalline while sedimentary rocks are non-crystalline.
• Permeability: Sedimentary rocks are more permeable while metamorphic rocks are more impervious.
• Mineralogy: Metamorphic rocks contain non-ferrous minerals while sedimentary rocks contain hydrocarbons as minerals.
• Relief: Sedimentary rocks generally sustain lower relief than metamorphic rocks.
• Fossils: The fossils in metamorphic rocks have been destroyed by heat and pressure while sedimentary rocks have beautiful fossils.
• Jointing: Sedimentary rocks are more jointed than metamorphic rocks.
• Mode of formation: Metamorphic rocks are formed out of existing rocks, whereas sedimentary rocks are products of weathering. Metamorphic rocks are formed due to heat and intense pressure while sedimentary rocks are formed through the deposit of withered materials, remains of organic matters, and evaporation of chemically-charged water.

Economic Importance Of Rocks To Man

1. Sources of minerals: Some rocks are sources of minerals such as gold, diamond, limestone and petroleum which can be exported to provide foreign exchange for a country. Petroleum, coal, limestone are derived from sedimentary rocks while gold, diamond and tin are the right from igneous and metamorphic rocks.
2. Sources of fuel: Sedimentary rocks like petroleum and coal are sources of fuel for domestic and industrial uses.
3. Construction purposes: Some rocks like granite and Sand stone are quarried and used for road, bridge, and building construction.
4. Tourist centers: Huge rock masses on mountains serve as tourist centers.
5. As ornament: Some beautiful rocks such as marble can be polished as ornament for decorating floors, walls of buildings, churches and tombstones.
6. Sources of food nutrients: Rock salts such as sodium chloride (table salt) from sedimentary rocks provide minerals used in cooking.
7. Formation of soil: Soils are formed from the disintegration of rocks.
8. Sources of metals: Rocks are sources of metals which are derived from mines, e.g, gold, silver, iron, aluminum and copper.
9. Domestic uses: Some rocks like granite are used for domestic purposes, e.g, grinding stones.
10. Raw materials for Industries: Some sedimentary rocks are used as raw materials in Industries, e.g, limestone is used for making cement, dolomite and marble are used for manufacturing of glasses and house paints.

Disadvantages Of Rocks To Man

1. Rocks form a barrier Between two Towns or villages etc. Rocks hinders communication.
2. They hinder the free flow of movement.
3. The occupy space.
4. They usually serve as hide out for criminals.
5. There are sometimes used as objects of worship by some religious group.
6. They make habitation of people difficult.

Revision Questions

1. (a) Name 3 different types of rocks (b) discuss the characteristics and mode of formation of one of the rocks name in (a) above.
2. (a) Describe four characteristics of igneous types of rocks (b) using specific examples, state four ways in which metamorphic rocks are useful to man.
3. (a) What are sedimentary rocks? (b) Name the three classes of sedimentary rocks according to their mode of formation.
4. Give an account of the economic importance of rocks.