ICSE Class 6 Biology Chapter 7 Rocks and Soil

Rocks And Soil

Syllabus

1. The three kinds of rocks - sedimentary, igneous and metamorphic - some examples of each kind - how they are made in nature. Rocks contain minerals - some common minerals and their uses (e.g. salt, talc, Plaster of Paris, chalk, lime, precious and semi-precious stones).

Ores are minerals from which metals can be extracted - the names of a few ores and the metals which can be obtained from them (e.g. Bauxite - aluminium, magnetite and haematite - iron, copper pyrites and malachite - copper) - Formulae of these ores are not required.

2. Erosion - wind, water and changes of temperature cause weathering of rocks (briefly) - this leads to the formation of soil.

3. Types of soils - sand, clay and loam - water retaining capacities of these - which one is suitable for plants. What plants take from the soil - minerals, moisture and nitrogen. How these are put back (i) by nature and (ii) by the farmer.

4. What are fossils? Fossil fuels - coal and petroleum - how they are formed (briefly) - burning fossil fuels gives us energy (as heat/light) - it uses up atmospheric oxygen and releases carbon dioxide into the air - why fossil fuels are considered non-renewable sources of energy.

Examining samples of different types of rocks, minerals, semi-precious stones (D).

Comparing the water retained by equal amounts of the three types of soils (D/E).

What happens if water in a plastic bottle is placed overnight in the freezer compartment of a refrigerator? Why should a glass bottle not be used? (D/E).

Finding out more about the extraction/uses of coal/petroleum. What alternatives are being used to reduce pollution caused by the burning of these? Discussion to follow.

Teacher's Note

Understanding rocks and soil helps us appreciate why certain regions are suitable for specific crops and why soil conservation is essential for agriculture and environmental health.

Introduction

Earth is a unique planet in the solar system which has air, water and a suitable surface temperature. Such a system has made existence of all forms of life possible on the earth.

From the surface inwards up to the centre, the earth shows three major regions:

1. Core: The central part

2. Mantle: The middle region

3. Crust: The uppermost layer

1. Core

The innermost region of the earth is called the core. It is mainly made up of iron and is the hottest region of the earth (about 5000-C temperature) which is solid in form. The outer portion of the core is made up of molten iron.

2. Mantle

This is the middle layer of the earth which is about 2900 km thick below the earth's crust. It is made up of molten and solid rocks and is much hotter than the earth's surface. The red-hot molten material is called magma. When this magma gets forced out of volcanoes, it is called lava. It is the slow motion of magma which causes disturbances on the earth, like earthquakes and volcanic eruptions. The uppermost part of the mantle together with the solid uppermost crust is called lithosphere (meaning rocky layer).

3. Crust

This is the uppermost layer, about 35 to 60 km in thickness which is made up of rocks. It supports all forms of life. About three-fourth of it is covered by oceans and the rest is divided into seven continents. Crust is made up of different materials, such as soil, minerals, water, air, etc., which supports life. Plants use these materials for their growth. We also get many useful substances from the earth's crust like coal, oil, natural gas, gems, precious stones, metals, etc.

Teacher's Note

The Earth's structure with its crust, mantle, and core explains why we have volcanic eruptions, earthquakes, and the formation of mountains - events that shape our world.

Rocks

Rocks are the supporting bases of the earth's crust. They are solid mineral deposits. These solid mineral deposits are known as rocks. These are made up of minerals in solid state, e.g., silicates, carbonates and oxides. The most abundant element is oxygen which is present in a combined state with silica in the form of silicate rocks.

There are metallic minerals, like aluminium, copper, lead, gold and silver as well as non-metallic minerals like quartz, gypsum (source of plaster of paris), mica, etc. Aluminium is the most common metal.

Types Of Rocks

Rocks are mainly of following three types -

(1) Igneous rocks

(2) Sedimentary rocks

(3) Metamorphic rocks

1. Igneous Rocks

Meaning "fire", referring to the solidification of the earth's internal molten magma. Millions of years ago, the parent rock material was in the form of molten state (magma) which was hot in liquid state and sticky. This liquid lava reached the surface of the earth during volcanic eruptions through the cracks and cooled down either on the surface of the crust or below it, thus forming igneous rocks. Due to wear and tear and also due to change in temperature and weather, the rocks got converted into soil.

Magma which cooled within the crust formed rocks like basalt, granite, and dolomite, etc. Magma bears many metallic ores (ores are minerals from which metals can be obtained).

2. Sedimentary Rocks

Sedimentary rocks are formed from the settling of material sediments derived from the pre-existing rocks and organic matter (living beings). These sediments are carried away mainly by water (river, lakes, etc.), wind and glaciers which later get deposited and form huge layered rocks, e.g., sandstone, limestone, claystone, etc. The constituents of the sedimentary rocks are mainly clay, sand, gravel and pebbles. Coal and oil are made up of the remains (fossils) of plants and animals. Sedimentary rocks also yield rich soil.

3. Metamorphic Rocks

These rocks are formed by alteration of the existing rocks by heat, pressure, etc. These are altered forms of igneous and sedimentary rocks under very high pressure and heat, making the rocks hard and compact. Marble and slate are good examples of metamorphic rocks.

Teacher's Note

The three types of rocks tell the story of our planet's dynamic processes - from volcanic activity to sediment transport to mountain building - all happening over geological timescales.

Uses Of Rocks

1. Most of the houses are built from stones and minerals extracted from rocks.

2. Many sculptures and pieces of art are made of rocks and minerals.

3. Rocks and minerals are used in art and craft.

4. Jewellery is made from metals and precious and semi-precious stones. These materials are obtained from rocks. Some precious stones are diamond ("Heera"), emerald ("Panna"), ruby ("Manik") and Sapphire ("Neelam"). At the same time, stones like turquoise ("Feroza"), topaz ("Pukhraj"), aquamarine ("Beruji") are not so costly. These are semi-precious stones. They are also used in making jewellery.

5. Rocks are a big sources of minerals such as aluminium, copper, gold, gypsum, antimony, asbestos, bauxite, cobalt, plaster of paris, etc. All these minerals are of great importance in our lives.

Activity 1

Request your parents to take you to any jeweller's shop/artificial jewellery shop. Ask the shopkeeper to show you various stones used for making jewellery. Note their names and try to find out from which type of rocks they are originated.

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