There are 3 types of Rocks- Igneous Rocks, Sedimentary Rocks and Metamorphic Rocks .
Now we would learn how these rocks are formed. Let us tell you a secret ,if you know the meaning of Igneous,Sediments & Metaphor then, understanding formation of rocks becomes very easy.
Let's first learn about Igneous rocks. To understand Igneous rocks ,let's first know meaning of Igneous .Igneous means resulting from or produced from fire. To understand it further let's first understand volcano. Inside earth core ,there are lot of molted metals & minerals. These get melted due to very hot temperature inside the earth. The melted material is called magma. It puts lot of pressure on earth's surface. Sometimes earth's surface breaks & this magma comes out.When it comes out, it is called lava.Some of magma can't come out & it settles inside the earth .Whatever magma cools inside under the surface of earth ,it changes into Granite. It is a kind of Igneous rock.The lava ,which comes out & it gets in touch with air, it cools down faster. This cooled down down lava forms Basalt rocks. Basalt rocks are also igneous rocks. So there are two main types of Igneous rocks, one which are formed when magma comes out ,which are Basalt & others when magma cools down under surface of earth ,these are granite rocks.
Now let's move to 2nd type of Rocks .These are called sedimentary rocks.The word sedimentary is made from sediments, which means Material that settles to the bottom of a liquid (pause) .Take few stones & a glass of water. Put these stones in glass. See these settle down.This settled down material is called sediments. It is a perfect example how sedimentary rocks are formed.
Let's understand more. Sedimentary rocks are formed by the layers of sediments.Sediments are small parts of rocks, dead animals or plants, which have been eroded by strong winds, strong flow of water in rivers & glaciers.Most of these particles ultimately reach the sea.These get deposited on the sea bed.As more & more of these particles settle on each other ,they press each other & compression happens.The sediments at bottom change into sedimentary rocks.This process takes thousands of years.
Sedimentary rocks have very clear different layers,these are known as beds or bedding planes Limestone & chalks are two kinds of sedimentary rocks.Type of these rocks depend upon sediments which get compressed in formation of these rocks. Like Limestone is formed by compression of skeletons & shells of marine animals in the sea.
Sandstone is formed by compression of sand.You know coal is also a sedimentary rock. It is formed by compression of dead trees, plants which are also called fossils.
Now let's learn about 3rd type of rocks.These are called Metamorphic rocks. Let's again focus on names. Metamorphic is derived from word morph, which means to be transformed, to change. Yes you understood right, metamorphic rocks are changed forms of igneous & sedimentary rocks.
Metamorphic rocks are formed when other types of rocks are changed due to lot of heat or pressure .See in this picture, lot of heat from molted magma & lot of pressure from upper layers is causing few of the sedimentary rocks to change
Marble & slate are two types of metamorphic rocks. Marble is formed when limestone ,which is a sedimentary rock ,comes under lot of pressure from above. Similarly Slate is formed when clay is changed by heat & pressure.