Describe the characteristics and types of primary rocks.

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Ghissu Editorial · Accepted Answer

Primary or igneous rocks form first from cooling magma or lava, and are crystalline, hard, mostly impermeable and free of strata and fossils. They are classed as plutonic like granite, hypabyssal like dolerite and volcanic like the basalt of the Deccan Traps.

**Introduction**

Igneous rocks are primary rocks because they formed first, from cooling magma or lava, and all other rocks are ultimately derived from them. India's examples range from Deccan basalt to Peninsular granite.

**Characteristics of Primary Rocks**

1. **Crystalline texture**: Interlocking crystals of feldspar, quartz and mica grow coarse when cooled slowly at depth and fine or glassy when cooled fast at the surface.
2. **No strata or fossils**: Having solidified from a melt, they lack the layering of sedimentary rocks and contain no fossils.
3. **Hard and impermeable**: They resist erosion and let water pass only along joints, where weathering forms tors and exfoliation domes.
4. **Mineral wealth**: They host ores of iron, copper and gold, and basalt weathers into the black regur soil of Maharashtra.

**Types of Primary Rocks**

1. **By place of cooling**: Intrusive rocks solidify underground, while extrusive or volcanic rocks cool on the surface.

SKETCH: tree | Types of Primary Rocks
- Igneous (Primary) Rocks
  - Plutonic: granite, gabbro
  - Hypabyssal: dolerite
  - Volcanic: basalt, pumice

2. **Plutonic and hypabyssal**: Granite and gabbro cool slowly at depth in batholiths, while dolerite cools at shallower depth in dykes and sills.
3. **Volcanic**: Basalt, obsidian and pumice cool quickly at the surface, as in the Deccan Traps, built by lava flows about 66 million years ago.
4. **By silica content**: Acidic granite is silica-rich, light and less dense; basic basalt and gabbro are iron- and magnesium-rich, dark and heavy.

**Conclusion**

As the parent of the rock cycle, primary rocks shape India's landforms, soils and mineral belts, from the fertile Deccan to the granites of the Peninsular shield.

Describe the characteristics and types of primary rocks.

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