Revision summary
Primary rocks are igneous rocks frozen from magma or lava. They are crystalline or glassy, usually hard, and lack fossils. Intrusive types include granite, gabbro, and dolerite. Extrusive types include basalt, rhyolite, and fragmental tuff. Texture follows cooling speed; mineralogy follows silica content.
Model answer
Copper italics in this answer — like this — are the key facts. Each one is unpacked in the Facts & figures rail.
Introduction
Primary rocks are igneous rocks formed by the cooling of magma or lava. They are the first crustal stock from which many sedimentary and metamorphic rocks are later made.
Body
Characteristics
- Igneous rocks are crystalline or glassy, generally hard, and they do not contain fossils because they freeze from melt, not from surface mud.
- Texture records cooling speed: coarse grains mean slow underground cooling; fine grains or glass mean rapid surface or underwater cooling.
- Chemistry ranges from silica-rich acidic rocks such as granite and rhyolite to silica-poor basic rocks such as gabbro and basalt.
- They may be vesicular where gas escapes, or porphyritic where large crystals sit in a finer groundmass.
- Joints, sills, dykes, and batholiths are structural habits, not separate chemistries.
Intrusive or plutonic types
- Granite is a coarse, quartz-feldspar rock of continental batholiths and forms much of the Peninsular basement.
- Gabbro is a coarse basic equivalent of basalt and appears in layered intrusions.
- Dolerite (diabase) fills dykes and sills with a medium grain, common in the Deccan and the Gondwana coalfields.
- Pegmatite veins carry very large crystals and sometimes mica or rare minerals.
Extrusive or volcanic types
- Basalt is the main plateau lava of the Deccan Trap, dark, fine-grained, and often columnar.
- Rhyolite and andesite are more silica-rich lavas of continental arcs; pumice and obsidian are glassy extremes.
- Tuff and volcanic breccia are fragmental ejecta welded or packed after eruption.
- Primary rocks therefore split first by place of cooling, then by silica and mineral content.
Flow diagram
Conclusion
- Primary rocks are igneous: hard, crystalline or glassy, and without fossils. Types are intrusive granite–gabbro–dolerite suites and extrusive basalt–rhyolite suites, sorted by cooling and chemistry.
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Are sedimentary rocks primary?
No. Primary here means first-formed igneous rock. Sedimentary rocks are secondary deposits.
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Is all basalt the same as granite chemically?
No. Basalt is basic and silica-poor; granite is acidic and quartz-rich.
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