The earth’s crust has various types of rocks made of minerals, with silicates being the most abundant in the crustal layer. Rocks are grouped into three major types, depending on their origin.
The rocks formed by gradual cooling and solidification of molten rock, called magma, which gets erupted from volcanoes. As the earth got cooled to a solid crust, the original crust consisted of igneous rocks. Igneous rocks, are therefore, known as primary rocks also. Ex. Basalt, Granite.
These rocks are made up of sediments deposited usually on the floors of sea and lakes. Sediments may consist of gravel, sand, silt or clay. The loose sediments get compacted into rocks by the pressure of overlaying sediments and the presence of cementing materials like lime. Sedimentary rocks are also known as stratified rocks as they occur as layers or strata one above the other. They are the most widespread types of rocks on the Earth. Ex. Limestone, Sandstone. Petroleum and natural gas are also found in some types of sedimentary rocks.
Rocks are formed by modification of pre-existing igneous or sedimentary rocks, when they are subjected to extreme heat or pressure, or both. The minerals in them get transformed e.g. limestone gets converted to marble, sandstone becomes quartzite.