Describe the characteristics and types of primary rocks.

Introduction

Primary or igneous rocks (Latin ignis, fire) are the first-formed rocks of the crust, crystallising directly from the cooling of magma or lava. Since all other rocks are ultimately derived from them, they are called parent rocks.

Characteristics

• Crystalline, hard, compact and generally massive in structure.
• Unstratified and devoid of fossils, having solidified from molten material.
• Texture depends on cooling rate: slow cooling at depth yields coarse crystals; rapid cooling at the surface yields fine-grained or glassy rocks.
• Closely associated with volcanic activity and rich in metallic minerals — iron, nickel, copper, chromite and platinum occur with igneous bodies.

Types by mode of occurrence

• Intrusive rocks solidify within the crust: plutonic rocks form at great depth with coarse grains (granite, gabbro), while hypabyssal rocks solidify at shallow depth in dykes, sills and laccoliths (dolerite, porphyry).
• Extrusive or volcanic rocks form when lava cools quickly on the surface — basalt of the Deccan Trap, and glassy or vesicular varieties such as obsidian and pumice.

Types by composition

• Acidic rocks (silica above about 65 per cent) such as granite are light-coloured and less dense.
• Basic rocks (silica roughly 45–55 per cent) such as basalt and gabbro are dark, denser and rich in ferro-magnesian minerals.

Conclusion

Igneous rocks build the shields, plateaus and volcanic landscapes of the earth and are the primary storehouse of its metallic mineral wealth.