Maharashtra Board Class 6 Geography Chapter 7 Rocks and Rock Types PDF Download

Rocks And Rock Types

Do It Yourself!

Observe the pictures in figure 7.1 and answer the following questions.

What is the hill in 'A' made up of?

What is being done in 'B'?

What do we see in 'C'?

Is there any relationship between the three things shown above?

For what purpose do we use the things shown in A and C?

Show the rock specimens and your notes to the teacher. Discuss them.

Do It Yourself!

From the hills, river-beds or the land in your vicinity, collect rock specimens of different types, shapes, colours, etc. Observe the rock specimens and note down the following.

Place where the rock was found.

Colour of the rock.

Spots on the rock and their colour.

Weight of the rock (estimated; heavy / light).

Hardness of the rock (hard / brittle / medium).

Structure of the rock (Uniform grains / layers / hollowness).

Porosity of the rock (porous / non-porous).

Explanation

We have seen last year that the earth's crust is hard. It is made of rocks and soil.

Rocks are found on land surface and also below it. Rocks are a mixture of different minerals formed by natural processes in the lithosphere.

The properties of rocks depend on the constituent minerals and their proportion, as well as on the formation process. Silica, aluminium, magnesium and iron are the major rock-forming minerals. There are other minerals, too, in different rocks.

Types Of Rocks

There are three types of rocks according to their formation process:

Igneous Rocks / Primary Rocks

Sedimentary Rocks

Metamorphic Rocks

Teacher's Note

Rocks are everywhere around us. You can see rocks in hills, river beds, and even in old forts in India. Rocks are used to build houses and temples.

Exam Trick

Remember: Three rocks - Igneous (from fire), Sedimentary (from layers), and Metamorphic (from change). Think of how a lump of clay becomes a pot - that is like metamorphic rocks changing shape!

Points To Remember

Rocks are a mixture of minerals made by nature.

The three types of rocks are igneous, sedimentary, and metamorphic.

Rock properties depend on minerals and how they were formed.

Igneous Rocks

When the molten material, known as magma within the crust and lava on the surface, cools down, it solidifies giving rise to rocks. These are known as igneous rocks.

As these rocks are formed out of the material from the interior of the earth, these are also referred to as primary rocks. Mostly these rocks appear hard and homogeneous. These are heavy as well. We do not get fossils in these rocks.

The Maharashtra Plateau and the Sahyadris are formed out of igneous rocks. Basalt is a major example of igneous rock.

Pumice is an example of igneous rock. This is formed out of volcanic froth. It is a porous rock. As its density is quite low, it can float on water.

Do You Know?

The interior of the earth has very high temperature. As a result, the material in this part is in the molten state. Sometimes this material gets released through the fissures in the crust. This is called a volcano. Lava, gases, dust, ashes, etc. are thrown out during volcanic eruptions.

Do You Know?

Most of the hill forts in Maharashtra have lakes or elephant yards. Actually, these are old quarry sites. Rock was extracted from these sites and used for the construction of the fort and other buildings. In the pits and dugouts left behind after rock extraction, water was allowed to accumulate and these lakes, tanks etc. were created.

Use Your Brain Power!

What rock was used for building the forts in Maharashtra? Why?

Teacher's Note

Igneous rocks are very hard and strong. That is why old forts in Maharashtra were made with basalt rocks. If you visit a fort near your home, you can see how these rocks were cut and placed.

Exam Trick

Remember: Igneous = Fire rocks. Magma inside, Lava outside. Igneous rocks are hard and heavy like bricks used in buildings. No fossils in igneous rocks!

Points To Remember

Igneous rocks form when molten magma cools down.

These rocks are hard, heavy, and do not have fossils.

Basalt is the most common igneous rock in Maharashtra.

Pumice is a special igneous rock that can float on water.

Sedimentary Rocks

Due to continuous variations in the temperature, rocks develop cracks. Similarly, the water percolating through rocks dissolves the soluble minerals. This leads to the weathering of rocks. They get disintegrated or decomposed. That is, they get reduced to pieces. These rock particles get transported by rivers, glaciers, wind, etc. towards low lying areas and are deposited there. One after the other, layers of sediments get deposited in this way. The upper layers exert heavy pressure on the lower layers. This leads to compaction of material and development of sedimentary rocks.

One can easily see layers in the sedimentary rocks. While layers are getting deposited the remains of dead animals or plants get buried in these layers at times. Therefore, one may find fossils in sedimentary rocks. Sedimentary rocks are generally brittle and lightweight. Most of them are porous.

Sandstone, limestone, shale, corals, etc. are sedimentary rocks. One may find coal seams in sedimentary rocks.

Do You Know?

Near Jaipur city in Rajasthan, red sandstone is found. This is a type of sedimentary rock. This rock was used for the construction of the Red Fort at Delhi. As sandstone is relatively soft, it is easy to carve in it.

Teacher's Note

Sedimentary rocks form in layers over many years. You can see these layers in river banks and hills. The Red Fort in Delhi is built from sedimentary rock called red sandstone because it is soft and easy to carve.

Exam Trick

Remember: Sedimentary = Settle or settle down. Rivers carry rock pieces and they settle in layers. You can see clear layers in sedimentary rocks. Fossils are found here because dead animals got buried!

Points To Remember

Sedimentary rocks form from pieces of other rocks pressed together.

These rocks have clear layers that you can easily see.

Fossils of dead animals and plants are found in sedimentary rocks.

Sandstone and limestone are common sedimentary rocks.

Metamorphic Rocks

Volcanic activity and other earth movements constantly take place on the earth. While these are occurring, the igneous or sedimentary rocks in that region are subjected to tremendous pressure and heat. This leads to changes in the nature as well as the chemical composition of the original rocks. The crystals in the original rocks get re-crystallized. This means, the rocks get metamorphosed. The rocks formed through such a process are called metamorphic rocks. These rocks do not contain fossils. These rocks are heavy and hard.

Do You Know?

Fossils

The buried remains of dead animals or plants become subject to heavy pressure. As a result their impressions get marked in the rocks. At times the buried animals or plants get petrified. These are called fossils. The study of fossils provides information about the life in that period.

Do You Know?

Coal gets metamorphosed when it undergoes heavy pressure and intense heat. After coal gets metamorphosed into diamond, its price increases. We burn coal while we use diamonds as ornaments.

Do You Know?

The Taj Mahal at Agra is built in marble. Marble is a metamorphic rock. This rock was brought from mines in Makrana in Rajasthan.

One realizes while taking a boat ride in River Narmada through Bhedaghat gorge in Madhya Pradesh that the river has cut its gorge through marble rock. At sunrise, sunset and during full moon nights, these river banks present an enchanting scene.

Teacher's Note

Metamorphic rocks are made when other rocks are pushed deep inside the earth where it is very hot. The heat and pressure change the rocks completely. The Taj Mahal is made from marble, which is a metamorphic rock!

Exam Trick

Remember: Metamorphic = Meta (change) + morphic (form). These rocks change their form under heat and pressure. Coal becomes diamond! Limestone becomes marble! Think of it like putting clay in a very hot oven - it becomes hard and different.

Points To Remember

Metamorphic rocks form when heat and pressure change other rocks deep inside the earth.

These rocks are very hard and heavy and do not have fossils.

Diamond comes from coal and marble comes from limestone when metamorphosed.

The Taj Mahal is built from marble, a metamorphic rock.

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