Rocks: A rock is a natural substance made up of solid crystals of different minerals. Rocks are formed when different minerals join together and make a solid lump. Every rock has a unique structure. There are certain rocks that are made up of only one mineral. For example, limestone rock contains only calcite mineral, while granite is made up of quartz and feldspar. The common minerals present in a rock are: silicates, carbonates and sulphides. There are three basic types of rocks: igneous, sedimentary and metamorphic.
Igneous Rocks: Igneous rocks are made up by the solidification of the Earth’s volcanic material, molten magma. The word igneous is derived from the Latin word Ignis which means ‘fire born’. These rocks are formed when the magma cools down either under the Earth’s surface or lithosphere (intrusive rocks) or above the Earth’s surface (extrusive igneous rocks). Inside the Earth, these rocks are formed when the magma is squeezed between the layers of the Earth followed by slow cool-down and solidification. These types of rocks have grainy structure with large crystals. Above the surface, these rocks are formed when the molten magma flows on the surface through a vent and spreads and cools rapidly. These types of rocks have smooth texture with fine crystals. The four types of igneous rocks are: granite, obsidian, basalt and pumice.
The four types of igneous rocks are: granite, obsidian, basalt and pumice. Almost 95% of the Earth’s crust is made up of igneous rocks.
