Every day, you get to see a lot of rocks around you – on the mountains, around the riverbeds, in the parks and even on the streets. Have you ever wondered what are these rocks made of? Rocks come in many different colours, shapes, and sizes and no two rocks are exactly alike.
How are rocks classified?
Geologists have classified rocks into three basic groups on the basis of their formation in nature. Igneous, Sedimentary and Metamorphic.
1. Igneous rocks
Igneous rocks are also known as ‘fire rocks’. Igneous rocks start their life in molten form much below the surface of the earth. Whenever there is a volcanic eruption, they are shot out of a volcano as lava, and are cooled back into rocks as they hit the ground.
Types of Igneous rocks
There are two types of igneous rocks found in nature,
- Intrusive igneous rocks that solidify beneath the earth’s surface.
- Extrusive igneous rocks that solidify on the surface of the earth.
2. Sedimentary rocks
Sedimentary rocks are formed when bits of rock and material such as shells and sand get packed together. Together, all these particles are known as sediments. Very slowly, over a period of time, these sediments harden and turn into a rock.
Nature – Sedimentary rocks are generally very fragile and may break apart or crumble easily. Most of the rocks that you see on the ground like sand, pebbles, or stones are sedimentary rocks. Some examples of sedimentary rocks are limestone, sandstone and shale.
3. Metamorphic rocks
Metamorphic rocks are made out of the first two types of rocks, deep within the earth’s crust. We call these metamorphic or ‘changed’ rocks. Sometimes sedimentary and igneous rocks are subjected to pressures and heat so intense that they are completely changed. They become metamorphic rocks!
Nature – Metamorphic rocks often have ribbon-like layers and may have shiny crystals. Examples of this rock type include marble and gneiss.
What is the rock cycle?
The process of the formation of all three types of rock is called the Rock cycle.