Rocks and Minerals – Types & Formation
Delhi Police & SSC Exams Preparation
1. Introduction
The Earth's crust is made up of rocks, and every rock is composed of minerals. Understanding rock types and their formation helps us study the Earth's history, resources, and internal processes like volcanism and erosion.
Key Concepts:
- Rock: Mixture of minerals
- Mineral: Naturally occurring inorganic substance
- Importance: Study Earth's history and resources
- Processes: Volcanism, erosion, and formation
Significance:
Fundamental for understanding Earth's composition and geological processes
Exam Trick: "Rock = mixture of minerals, Mineral = naturally occurring inorganic substance"
2. What Are Minerals?
Characteristics:
- Nature: Naturally occurring inorganic substances
- Composition: Definite chemical composition
- Examples: Quartz, Mica, Feldspar, Calcite
- Uses: Glass, jewelry, fertilizers, industries
Role:
Basic building blocks of rocks and essential for various industries
Exam Trick: "Minerals = Nature's Building Blocks of Rocks"
3. Types of Rocks
| Type | Formation | Example | Key Feature |
|---|---|---|---|
| Igneous Rocks | Cooling & solidification of magma/lava | Basalt, Granite | "Mother rocks" — all others form from them |
| Sedimentary Rocks | Deposition of sediments by water, wind, or ice | Sandstone, Limestone | Have layers and may contain fossils |
| Metamorphic Rocks | Alteration of existing rocks under heat & pressure | Marble, Slate, Quartzite | Harder and more compact |
4. Igneous Rocks – The Primary Rocks
Formation & Types:
- Formation: Cooling of molten magma or lava
- Intrusive Igneous: Form inside Earth (slow cooling → large crystals)
- Extrusive Igneous: Form on surface (fast cooling → small crystals)
- Examples: Granite (intrusive), Basalt (extrusive)
India Fact:
Deccan Plateau = made of Basalt (extrusive igneous rock)
Exam Trick: "IGNEOUS = Fire-Born Rocks (from lava/magma)"
5. Sedimentary Rocks – The Layered Rocks
Formation Process:
- Process: Weathering → erosion → deposition → compaction → cementation
- Fossils: Often contain fossils of plants & animals
- Examples: Limestone (from shells), Sandstone (from sand), Shale (from clay)
- Features: Layered structure, may contain fossils
Characteristics:
Formed by accumulation and compaction of sediments over time
Exam Trick: "Sedimentary = Settle Down Rocks (formed by settling of sediments)"
6. Metamorphic Rocks – The Changed Rocks
Formation:
- Process: Existing rocks change under high pressure and temperature
- Changes: Structure, hardness, and color transformation
- Features: Harder and more compact than original rocks
- No Melting: Transformation occurs without melting
Rock Transformations:
| Original Rock | Metamorphic Rock |
|---|---|
| Limestone | Marble |
| Shale | Slate |
| Sandstone | Quartzite |
| Coal | Graphite |
Exam Trick: "Metamorphosis = Change → Metamorphic = Changed Rocks"
7. The Rock Cycle
Cycle Process:
Magma
↓ (Cooling)
Igneous Rock
↓ (Weathering, Erosion)
Sediments
↓ (Compaction)
Sedimentary Rock
↓ (Heat & Pressure)
Metamorphic Rock
↓ (Melting)
Magma again!
Significance:
Shows that all three rock types are interconnected — one can transform into another through geological processes
8. Quick Comparison Table
| Feature | Igneous | Sedimentary | Metamorphic |
|---|---|---|---|
| Formation | Cooling of magma/lava | Deposition of sediments | Heat & pressure |
| Fossils | Absent | Common | Rare |
| Texture | Crystalline | Layered | Hard, compact |
| Example | Basalt, Granite | Sandstone, Limestone | Marble, Slate |
| Found in India | Deccan Plateau | Vindhya Range | Himalayas |
PYQs (Delhi Police & SSC)
Which of the following is an example of an igneous rock?
Options: (A) Limestone (B) Basalt (C) Sandstone (D) Marble
Category: Rock Types
Show Answer
B. Basalt
Which rock is known as the "primary rock"?
Options: (A) Sedimentary (B) Metamorphic (C) Igneous (D) None of these
Category: Rock Classification
Show Answer
C. Igneous
Which of the following rocks is formed by the deposition of sediments?
Options: (A) Granite (B) Sandstone (C) Basalt (D) Gabbro
Category: Formation Process
Show Answer
B. Sandstone
Marble is formed by the metamorphism of which rock?
Options: (A) Limestone (B) Sandstone (C) Shale (D) Coal
Category: Rock Transformation
Show Answer
A. Limestone
Which rock is likely to contain fossils?
Options: (A) Sedimentary (B) Igneous (C) Metamorphic (D) Volcanic
Category: Fossil Preservation
Show Answer
A. Sedimentary
Which of the following pairs is correctly matched?
Options: (A) Coal → Marble (B) Limestone → Granite (C) Limestone → Marble (D) Sandstone → Shale
Category: Rock Transformations
Show Answer
C. Limestone → Marble
The Deccan Plateau is made up of which type of rock?
Options: (A) Sedimentary (B) Igneous (C) Metamorphic (D) Fossiliferous
Category: Indian Geography
Show Answer
B. Igneous
Final Exam Tips
Igneous → Fire rocks → No fossils
Sedimentary → Layered → Fossils present
Metamorphic → Changed → Hardest type
Remember conversions: Limestone → Marble, Sandstone →
Quartzite
Deccan Plateau = Basalt = Igneous
Master Geology for Delhi Police Exam!
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