[Short Notes] NCERT class VII : Our Environment – Inside Our Earth – Chapter 2
The uppermost layer over the earth’s surface is called the crust. It is the thinnest of all the layers.The earth’s crust is made up of various types of rocks.
Any natural mass of mineral matter that makes up the earth’s crust is called a rock.
Types of Rocks-
- Igneous rocks-When the molten magma cools, it becomes solid. Rocks thus formed are called igneous rocks. They are also called primary rocks.
There are two types of igneous rocks:
Intrusive rocks-slow cooling of molten lava inside earth’s surface leading to rock formation, forms large grain structure, eg.Granite
Extrusive rocks– rapid cooling of molten lava on earth’s surface and turns to solid, fine grained structure eg.Basalt, found in deccan plateau
- Sedimentary rocks-Igneous rocks roll down, crack, and hit each other and are broken down into small fragments. These smaller particles are called sediments, when sediments are compressed and hardened sedimentary rocks are formed. eg. sandstone from sand
- Metamorphic rocks– turning of sedimentary & igneous rocks due to high temperature & pressure into a very strong rock.
Crust is about 35 km. on the continental masses and only 5 km. on the ocean floors.
The main mineral constituents of the continental mass are silica and alumina (sial).
The oceanic crust mainly consists of silica and magnesium; (sima).
Beneath the crust is the mantle which extends up to a depth of 2900 km. below the crust.
The innermost layer is the core with a radius of about 3500 km. It is mainly made up of nickel and iron(nifa).It has high temperature & pressure.
The radius of the earth is 6371 km.