A. Definition:
- Primary rocks = Also known as igneous rocks, they are the first formed rocks of the Earth’s crust.
- Formed by cooling and solidification of magma/lava (molten material from the Earth’s mantle).
- Basis for formation of sedimentary and metamorphic rocks.
B. Characteristics:
- Crystalline in nature – composed of interlocking crystals.
- Non-layered and hard – no stratification.
- Contain no fossils – due to high formation temperatures.
- Form the foundation – of continents and ocean floors.
- Often rich in minerals like quartz, feldspar, mica.
C. Types of Primary (Igneous) Rocks:
1. Based on Formation Location:
- Intrusive/Plutonic rocks – Formed below Earth’s surface.
- Slow cooling → large crystals.
- Examples: Granite, Diorite.
- Extrusive/Volcanic rocks – Formed on the surface.
- Rapid cooling → fine-grained texture.
- Examples: Basalt, Rhyolite.
- Hypabyssal rocks – Formed near the surface (intermediate depth).
- Examples: Dolerite.
2. Based on Chemical Composition:
- Acidic rocks – High silica (>65%), light-colored.
- Example: Granite.
- Basic rocks – Low silica (<55%), dark-colored, denser.
- Example: Basalt.
Conclusion:
Primary rocks are the oldest and most fundamental rocks, forming the core of the lithosphere. Their classification helps understand Earth’s geological history.
