Why this matters now
Rock types are tested for their mode of formation and examples, and they connect to soils, minerals, fossils and landforms. The rock cycle is a favourite diagram-based concept.
Igneous rocks
Igneous rocks form by the cooling and solidification of magma/lava (“fire-formed”). Intrusive (plutonic) rocks cool slowly underground with large crystals (e.g. granite); extrusive (volcanic) rocks cool quickly at the surface with fine crystals (e.g. basalt, which forms the Deccan Traps). They are the primary rocks from which others derive.
Sedimentary rocks
Sedimentary rocks form by the deposition and compaction/cementation of sediments (“layered”). Examples: sandstone, limestone, shale. They often contain fossils and occur in strata, and host coal and petroleum.
Metamorphic rocks
Metamorphic rocks form when existing rocks are transformed by heat and pressure (“changed form”) without melting. Examples: limestone → marble; sandstone → quartzite; shale → slate; granite → gneiss.
The rock cycle
The three types are linked in a continuous rock cycle: igneous rocks weather and erode into sediments that form sedimentary rocks; burial and pressure turn these into metamorphic rocks; deep melting returns them to magma, which cools into new igneous rock — a never-ending recycling of crustal material.
UPSC angle
Match rock type → formation → example (igneous-cooling-granite/basalt; sedimentary-deposition-limestone; metamorphic-heat/pressure-marble). Know the rock-cycle transformations.
Frequently asked questions
What are the three types of rocks?
Igneous (from cooled magma/lava), sedimentary (from deposited sediments) and metamorphic (from transformed rocks).
What is the difference between intrusive and extrusive igneous rocks?
Intrusive (e.g. granite) cool slowly underground with large crystals; extrusive (e.g. basalt) cool quickly at the surface with fine crystals.
Which rocks contain fossils?
Sedimentary rocks, because they form by the gradual deposition of sediments in layers.
What is the rock cycle?
The continuous transformation of rocks among igneous, sedimentary and metamorphic types through weathering, deposition, heat/pressure and melting.