NCERT Geography class 7 chapter 2

The earth is about 460 crore years old and its surface is continuously undergoing changes since then. The study of the interior is done by geologists and the study is called geology. On the basis of density and other physical and chemical properties of rocks, there are 3 main layers- crust; mantle; and core. The crust is the solid outermost layer in which life exists; also known as lithosphere; it is further divided into two parts- continental crust (SiAl) and oceanic crust (SiMa); average density is about 3g per cubic cm. The mantle lies between crust and core; magma is found in this layer; approximately 2900km thick; density ranges between 3.5g to 5.5 g per cubic cm. The core is the innermost layer; also known as barysphere; made of heavy metals (NiFe); average density between 5 to 13g per cubic cm. Rocks and minerals are found in the earth’s crust. On the basis of mode of formation, rocks are of three types- Igneous, Sedimentary, and Metamorphic. Igneous is the primary rock formed due to the cooling and solidification of hot molten magma; can be intrusive (cool below the surface of the earth) or extrusive (cool on the surface of the earth). Sedimentary rocks are formed due to the deposition of sediments brought by the agents of deposition i.e. rivers, wind, or glaciers; most widespread( 75% of total land area); contains fossils (remains of plants and animals). Metamorphic rocks are formed due to the changes in physical and chemical properties of the other two types of rocks because of excessive heat and pressure; the act of metamorphism takes hundreds of years; very hard; do not have empty spaces in them. The rocks actually never die. They change from one form to another, this process is known as the rock cycle. Please refer to my vide for further details on the topic- https://youtu.be/j9URhZbRi6I