UPSC Mains 2022 GS1 Q4: Characteristics and Types of Primary Rocks

All rocks fall into three families based on how they form: igneous, sedimentary, and metamorphic. Open the answer by naming all three.

Igneous rock forms when magma or lava cools and solidifies. It is the starting point of the rock cycle, since all rock ultimately derives from cooled molten material.

Intrusive (plutonic) igneous rock cools slowly underground. Slow cooling lets large crystals grow, giving rocks like granite their coarse, visible texture.

Extrusive (volcanic) igneous rock cools quickly at the surface, as lava. Fast cooling leaves little time for crystals to grow, giving rocks like basalt a fine-grained texture.

Sedimentary rock forms from compacted, cemented sediment. Clastic sedimentary rock, like sandstone, forms from broken rock fragments deposited in layers.

Chemical or organic sedimentary rock, like limestone or coal, forms from precipitated minerals or accumulated organic matter instead of rock fragments.

Sedimentary rock typically shows visible layering, called stratification. It is also the only rock type that commonly preserves fossils, making it valuable for reading Earth’s history.

Metamorphic rock forms when heat and pressure transform an existing rock without melting it. The parent rock can be igneous, sedimentary, or even another metamorphic rock.

Foliated metamorphic rock, like schist and gneiss, develops a banded or layered texture under directed pressure. Slate, from shale, is a common foliated example.

Non-foliated metamorphic rock, like marble from limestone and quartzite from sandstone, recrystallizes without developing visible bands.

The rock cycle links all three: weathering turns any rock into sediment, heat and pressure turn any rock metamorphic, and enough heat melts any rock into new igneous rock.

Define each family with its formation process, give one intrusive/extrusive or clastic/chemical or foliated/non-foliated split per family where relevant, then close by tying them together through the rock cycle.