Rocks are natural aggregates of one or more minerals making up the Earth's crust, classified by their mode of formation into three families: igneous, sedimentary, and metamorphic.
- Igneous rocks form by the cooling and solidification of molten magma or lava; intrusive (plutonic) types like granite cool slowly underground with coarse crystals, while extrusive (volcanic) types like basalt cool fast on the surface with fine crystals. The Deccan plateau is basaltic.
- Sedimentary rocks form by the deposition, compaction, and cementation of sediments in layers (strata); examples are sandstone, shale, limestone, and coal. They alone may contain fossils.
- Metamorphic rocks form when existing igneous or sedimentary rocks are changed by heat and pressure without melting; limestone becomes marble, sandstone becomes quartzite, shale becomes slate, granite becomes gneiss, and coal becomes graphite or diamond.
- The igneous rocks are called primary or parent rocks because the others are ultimately derived from them; only sedimentary rocks normally preserve fossils.
- The continual conversion among the three families through weathering, melting, and metamorphism is the rock cycle.
The three families and their formation, intrusive granite versus extrusive basalt, the fossil-bearing nature of sedimentary rocks, and the metamorphic pairs (limestone to marble, coal to graphite) are recurring geomorphology facts.
Igneous (from magma or lava) versus sedimentary (from deposited layers) versus metamorphic (altered by heat or pressure); granite (intrusive, coarse) versus basalt (extrusive, fine); only sedimentary rocks contain fossils.
Three rock families: igneous (from magma), sedimentary (layered, fossil-bearing), and metamorphic (heat and pressure altered), linked by the rock cycle.