The earth’s rocks are divided into three main types. Igneous rock is the original material that makes up the Earth, formed when magma rises to the surface and cools. The planet’s oldest rocks are all of the igneous type. Sedimentary rock is made up of particles of other rock that has been affected by contact with the atmosphere. Erosion caused by water, wind and ice breaks the rock down into tiny particle that are carried away and settle in rivers, lakes and other areas. Over time, the particles compress to form sedimentary rock. Metamorphic rock is formed by the natural effects of heat and pressure changing igneous and sedimentary rock.
The three main types, or classes, of rock are sedimentary, metamorphic, and igneous and the differences among them have to do with how they are formed.
Sedimentary
Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils. Examples of this rock type include conglomerate and limestone.
Metamorphic
Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbonlike layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface. Examples of this rock type include gneiss and marble.
Igneous
Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock. Examples of this rock type include basalt and obsidian.
Picture Credit : Google