Based on the mode of formation, rocks can be classified into three: 

igneous, sedimentary, and metamorphic.

a) Igneous rocks are formed by the molten rock material rising through the fissures in the crust and solidifying either on the surface of the Earth or within the crust itself. As all other rock types are formed from the igneous rocks, they are called primary rocks.

Example of Igneous rocks: Granite, Basalt, Obsidian, Scoria, Pumice, Diabase, Dacite, Diorite, Pegamatite, Rhyolite, Gabbro, Peridotite

b) Sedimentary Rocks: Rocks undergo weathering in course of time. The debris so formed will be deposited in the low regions as layers. These materials gradually get lithified and transformed into sedimentary rocks. As the sedimentary rocks are formed in layers, they are also known as stratified rocks.

Example of Sedimentary Rocks: Coal, Limestone, Sandstone, Shale, Gypsum, Conglomerate, Dolomite, Tuff, Breccia, Oil Shale, Coquina, Diatomite

c) Metamorphic rocks: When rocks undergo physical and chemical changes due to high temperature and pressure metamorphic rocks are formed.

Example of Metamorphic Rocks: Marble, Slate, Gneiss, Quartzite, Soapstone, Phyllite, Hornfels, Anthracite, Novaculite, Skarn, Schist, Mariposite

Types of Rocks Infographic by Outforia
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