A rock can be defined as any mineral material of the earth. The earth’s crust consists of rocks. A rock may be a combination of different mineral elements such as silica which contains silicon and oxygen.
Types of rocks
All rocks our classified into three major types: igneous, sedimentary and metamorphic rocks, based on their origin, mode of formation and appearance.
All these rocks differ from one another in texture, structure, color and permeability, mode of occurrence and the degree of resistance of denudation.
Igneous rocks
Characteristics of Igneous rocks
- Igneous rocks are glassly in appearance.
- They do not occur in layers, that is they are non-stratified rocks.
- They are crystalline in structure; that is, they contain crystals.
- They do not contain fossils.
- They are usually very hard and impervious.
- They are resistant to erosion and other elements of climate.
- They are light and dark in colour.
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 results from high temperature and pressure beneath the earth, forces itself towards the earth’s surface through cracks. Has the magma moves towards the surface, it comes in contact with lower temperature; hence, it’s cools and solidifies to form igneous rocks.
Types of igneous rocks
There are two types of igneous rocks which are:
Plutonic (or instrusive) 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.
Volcanic (or exstrusive) igneous rocks
These rocks are formed when the molten magma cools and modifies rapidly on getting to the surface of the earth to found small crystals. An example of volcanic igneous rocks is besalt.
Sedimentary rocks
Characteristics of sedimentary rocks
- They occur in layers, strata or sheets.
- The rocks are coarse in texture.
- They do not exist in crystals. i.e. they are non-crystalline in structure.
- They are soft.
- They react with weak acid, e.g. dilute Hcl.
- They are not resistant to erosion.
- They contain fossils of plants and animals.
Mode of formation
sedimentary rocks are formed from sediments deposited either by water, by wind or by ice. These sediments accumulated in layers or strata, one on top or the other and after a long period of time, they become hardened by compression to form sedimentary rocks sedimentary rocks are stratified rocks and the plane between two strata is called bedding plane.
Types of sedimentary rocks
There are three types of sedimentary rocks based on their process of formation.These are
Mechanically formed sedimentary rocks
These are formed from sediments of other rocks that have accumulated and cemented together over a long period of time. Examples include: sandstone, breccia, shale, clay and conglomerate.
Organically formed sedimentary rocks
These are rocks formed from the remains of living organisms. When sedimentary rocks are formed from the remains of animals like corals or fish shells, search rocks are called calcareous rocks, examples are 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. Examples include coal, peat, lignite, petroleum, etc.
Chemically formed sedimentary rocks
These rocks are precipitated chemically from rock solutions. Examples include potash, sodium chloride (common salt),nitrate, gypsum and dolomite.
Metamorphic rocks
Characteristics of metamorphic rocks
- Some may occur in layers or strata.
- The rocks may be hard and massive.
- They are non-crystalline in structure.
- They may contain fossils
- They exist in different colours and texture.
Mode of formation
Metamorphic rocks are changed rocks. The 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 changed by such forces. Examples of metamorphic rocks include slate, marble, quartzite, gneiss, schist and graphite. For example, slate is from clay; marble is formed from limestone; quartzite is formed from sandstone; gneiss is formed from granite; schist is formed from shale and graphite is formed from coal.
Economic importance of rocks in agriculture
Formation of soil
Soil is formed from the disintegration of rocks.
Sources of plant nutrients
Plant nutrients like iron, calcium, potassium, etc are derived from rocks.
Construction purposes
Some rocks like granite and sand stones are quarried and used for the road and farm building construction.
Erosion control
Rocks are used to make a pile up across sloppy lands to prevent soil erosion.
Obstacles to farm implements
Rocks are generally a source of hindrance to the food operations of farm implements.
Sharpening of farm tools
Some rocks are used for sharpening farm tools like knives and cutlass.
Breaking hard objects
Some rocks are used for breaking hard objects like kernels and seeds.
Useful as grits
Some rocks are used as grits in livestock feed formulation.
Useful in fertiliser blending
Some rocks are useful in blending of fertilisers in agriculture.
Useful as liming materials
Rocks generally contain calcium and magnesium which are useful as liming materials.
Useful as feed additive
Some rocks are very useful as feed addictive, e.g. salt lick.
Sources of plant nutrients
Rocks are good sources of plant nutrients in the soil. For examples, micra releases potassium in the soil.
Other economic importance of rocks to man
Domestic uses
Some rocks like granites I used for domestic purposes. An example is grinding stones.
Sources of food
Rock-salt, such as sudden chloride (table salt) from sedimentary rocks, provides minerals used in cooking food.
Sources of fuel
Sedimentary rocks like petroleum and coal are sources of fuel for domestic, commercial and industrial purposes.
Construction purposes
Rocks such as granite and limestone are used for construction, e.g. roads, buildings and bridges.
As centre of tourism
Rocks are also used as centre of tourism, e.g. Olumo rock in Ogun State.
For research purposes
Some rocks are used by scientists for research purposes.
Sources of metals
Rocks are sources of metals which are derived from mines, e.g. gold, silver, copper and aluminum.
As ornamentals
Some beautiful rocks such as marble can be polished as ornamentals for the correcting floors, walls of building, churches, etc.
Sources of minerals
Some rocks are sources of minerals such as gold, diamond, limestone and petroleum which can generates income to the nation.
Raw materials for industries
Some sedimentary rocks are used as raw materials for industries. For example, limestone is used for making cement. Dolomite and marble are used to manufacture glass and house paint.