Rocks and Rock Types Class 6 Geography Notes Maharashtra State Board
From the hills, riverbeds, or the land in your vicinity, collect rock specimens of different types, shapes, colours, etc. Observe the rock specimens and note down the following.
A 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).
We have seen last year that the earth’s crust is hard. It is made of rocks and soil. Rocks are found on the 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.
The interior of the earth has a 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.
Types of Rocks
There are three types of rocks according to their formation process:
- Igneous Rocks/Primary Rocks
- Sedimentary Rocks
- Metamorphic Rocks
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.
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.
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 the compaction of material and the 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.
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.
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 in Delhi. As sandstone is relatively soft, it is easy to carve in it.
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. Study the table given below and try to understand the metamorphosis.
Coal gets metamorphosed when it undergoes heavy pressure and intense heat. After coal gets metamorphosed into diamonds, its price increases. We burn coal while we use diamonds as ornaments.
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.
The figure shows the distribution of major rocks in Maharashtra. List the districts in Maharashtra where rocks other than basalt are found. The basalt rock formed out of volcanic eruption has spread over a vast region of our State. Granite rock is found in the eastern parts and in South Konkan. Laterite is found in South Konkan. Because of the thick and extensive layers of basalt, Maharashtra does not have large reserves of mineral wealth. Therefore, mining activity is concentrated in South Konkan and in eastern Maharashtra.
Comprehensive Maharashtra State Board Class 6 Geography Notes Rocks and Rock Types can help students make connections between concepts.