Lesson Notes By Weeks and Term - Senior Secondary School 1

Rocks

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Date: 27th of February - 3rd of March 2023

Class: SSS 1

Term: 2nd Term

Week: 8

Subject: Agricultural Science

Topic: Rocks

Duration:45 minutes

Teaching Aid: A chart

Reference Books: Essential Agricultural Science for Senior Secondary Schools by O.A Iwena (pg 61-64)

Comprehensive Certificate Agricultural Science for Senior Secondary Schools by L.A et Al (pg69-73)

Behavioural Objectives: By the end of this topic, students should be able to

  1. Define rocks
  2. Describe the types of rocks with their characteristics

Presentation I

Step 1: Teacher revises last topic

Step 2: Teacher introduces and explains new topic

Step 3: Teacher writes note on the board

Step 4: Teacher evaluates students

Step 5: Teacher marks students' notes and give assignment

Previous knowledge: Students are familiar with the topic

Content

ROCK

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

Types of rocks

All rocks are classified into three major types:

Igneous, sedimentary and metamorphic rocks, based on their origin, mode of formation and appearance.

All these rock differ from one another in texture, structure, colour, permeability, mode of occurrence and the degree of resistance to denudation

IGNEOUS ROCKS

Characteristics

  1. Igneous rocks are glassy in appearance.
  2. Igneous rocks are crystalline in structure, that is, they contain crystals.
  3. They do not occur in layers that is, they are non-stratified rocks.
  4. They do not contain fossils.
  5. They are usually very hard and impervious.
  6. They are resistant to erosion and other elements of climate
  7. 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. As the magma moves towards the surface, it comes in contact with lower temperature; hence, it cools and solidifies to form igneous rocks,

Types of Igneous Rocks

There are two types of igneous rocks which are:

(a)        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.

(b) Volcanic (extrusive) igneous rocks:

These rocks are formed when the molten magma cools and solidifies rapidly on getting to the surface of the earth to form small crystals. An example of volcanic igneous rock is basalt.

SEDIMENTARY ROCKS

Characteristics of Sedimentary Rocks

  1. They occur in layers or strata or sheets.
  2. The rocks are coarse in texture
  • They do not exist in crystals, i.e, they are non-crystalline in structure.
  1. They contain fossils of plants and animals.
  2. They are not resistant to erosion.
  3. They are soft.
  • They react with weak acid e.g. dilute HCL

 

Mode of Formation

Sedimentary rocks are formed from sediments deposited either by water, by wind or by ice. These sediments are accumulated in layers 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 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:

(a) 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, breccias, shale, clay and conglomerate.

(b) 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, 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. Examples include, coal, peat, lignite, petroleum, etc.

(c) 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

  1. Some may occur in layers or strata.
  2. The rocks may be hard or soft.
  • They are non-crystalline in structure.
  1. They exist in different colours and texture.
  2. They may contain fossils.

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, gnesis, schist and graphite. For example, 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 and graphite is formed from coal.

ECONOMIC IMPORTANCE OF ROCKS IN AGRICULTURE

  1. Formation of soil: Soil is formed from the disintegration of rocks.
  2. Sources of plant nutrients: Plant nutrients like iron, calcium, potassium, etc are derived from rocks.
  3. Construction purpose: Some rocks like granite and sand stones are quarried and used for road and farm building construction
  4. Erosion control: Rocks are used to make a pile up across sloppy lands to prevent soil erosion.
  5. Obstacles to farm implements: Rocks are generally a source of hindrance to the free operations of farm implements.
  6. Domestic uses: Some rocks like granites are used for domestic purposes. An example is grinding stone.
  7. Sources of metals: Rocks are sources of metals which are derived from mines, e.g. gold silver, copper, aluminum etc
  8. As Ornamentals: Some beautiful rocks such as marble can be polished as ornamentals for decorating floors, walls of buildings, churches, etc.
  9. Sources of fuel: Sedimentary rocks like petroleum and coal are sources of fuel for domestic, commercial and industrial purposes.
  10. Sources of Food: Rock-salt, such as sodium chloride (table salt) from sedimentary rocks, provides minerals used in cooking our food.
  11. Sources of Minerals: Some rocks are sources of minerals such as gold, diamond, limestone, petroleum, etc. which can generate income to the nation.
  12. 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 glasses and house paints.

PROCESES OF SOIL FORMATION

The process of soil-formation is called weathering. Weathering is defined as the disintegration or breakdown of rocks into tiny pieces to form soil. In other words, weathering can also be defined as the breaking down of rock masses (rock minerals) into simpler forms through the agents of physical, chemical and biological processes.

(1) Physical process   (2) Chemical process  (3) Biological process

 

(1)       Physical Process: Agents of physical weathering are temperature, ice, wind, water and pressure.

  1. Temperature: The alternating heating and cooling of the rocks produce pressure within the rocks and cause them to break down into pieces.
  2. Wind: As a result of the grinding of rock surfaces by solid materials carried by wind water and moving ice (glacier), rocks break down to form soil.
  3. Ice: The conversion of water inside cracks in rocks into ice results in increase in volume. This increase in volumes results in more pressure being exerted on the rock walls which eventually break into smaller pieces.
  4. Water: Running water carries some fragments of rocks along its course and these rub against the surface of rocks in the river bed, thus breaking off small pieces of rocks.

(2)     Chemical Process: Agents of Chemical weathering include solution, carbonation, hydration, hydrolysis and oxidation.

  1. Solution: This occurs when water dissolves and soluble minerals present in the rock are carried from place while water flows.
  2. Carbonation: Atmospheric carbon dioxide mixes with rain water to form weak carbonic acid. This acid dissolves rocks, resulting in their breakdown.
  • Hydration: This is the attachment of water with rock minerals. This results in chemical alteration of the minerals, e.g the conversion of iron II rocks to hydrated rocks. Hematite rock is also changed to liminite.
  1. Hydrolysis: This is the reaction of water with rock minerals to produce a rock, entirely different from the original one. For example olivine rock is changed to serpentine.
  2. Oxidation: This is the reaction of rocks with oxygen from the atmosphere. This reaction eventually weakens and breaks down the rock to form soil.
  3. Biological Process: This involves the activities of plants and animals in the breaking down of rocks to form soil.
  4. It is caused by the action of animals like earthworms, termites and other soil organisms.
  5. Movements of these organisms cause small fragments of rocks to disintegrate.
  • Earthworms and termites burrow into the rocks and break off fragments of rocks.
  1. The roots of growing plants penetrate rocks through crevices, exerting pressures which split some rocks.
  2. The activities of man during farm operations such as ploughing and harrowing also break down rocks into tiny pieces.

Evaluation: describe the types of igneous rock

Assignment: present a diagram sowing the types of rocks



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