Rocks are naturally occurring solid materials composed of minerals. They are classified into three main types based on their formation process:
- Igneous Rocks (Formed from magma/lava)
- Sedimentary Rocks (Formed from deposited sediments)
- Metamorphic Rocks (Formed from heat and pressure)
1. Igneous Rocks (Primary Rocks)
- Formed by the cooling and solidification of magma/lava.
- Found in volcanic and tectonic regions.
- Do not contain fossils.
Types of Igneous Rocks
✅ Intrusive (Plutonic) Igneous Rocks – Formed inside the Earth’s crust from slow cooling magma (e.g., Granite, Diorite).
✅ Extrusive (Volcanic) Igneous Rocks – Formed on the surface by fast-cooling lava (e.g., Basalt, Pumice, Obsidian).
Examples
- Granite (Used in construction)
- Basalt (Forms ocean floor)
- Obsidian (Glassy volcanic rock)
2. Sedimentary Rocks (Secondary Rocks)
- Formed by the accumulation and compression of sediments over time.
- Often contain fossils.
- Found in riverbeds, lakes, and ocean floors.
Types of Sedimentary Rocks
✅ Clastic Sedimentary Rocks – Formed from weathered rock fragments (e.g., Sandstone, Shale).
✅ Chemical Sedimentary Rocks – Formed from mineral precipitation (e.g., Limestone, Gypsum).
✅ Organic Sedimentary Rocks – Formed from organic remains (e.g., Coal, Chalk).
Examples
- Sandstone (Used in buildings)
- Limestone (Used in cement production)
- Coal (Used as fuel)
3. Metamorphic Rocks (Changed Rocks)
- Formed when igneous or sedimentary rocks undergo heat, pressure, or chemical changes.
- Harder and more compact than original rocks.
- Found in mountainous regions and tectonic zones.
Types of Metamorphic Rocks
✅ Foliated Metamorphic Rocks – Have layered or banded structure (e.g., Slate, Schist).
✅ Non-Foliated Metamorphic Rocks – Have no distinct layers (e.g., Marble, Quartzite).
Examples
- Marble (Used in sculptures)
- Slate (Used in roofing)
- Quartzite (Used in construction)
Comparison Table of Rock Types
| Rock Type | Formation | Examples | Features |
|---|---|---|---|
| Igneous | Cooling of magma/lava | Granite, Basalt, Obsidian | Hard, No fossils |
| Sedimentary | Deposition of sediments | Sandstone, Limestone, Coal | Fossils present |
| Metamorphic | Heat & Pressure | Marble, Slate, Quartzite | Layered or compact |
MCQs on Classification of Rocks
1. Which type of rock is formed by cooling and solidification of magma?
a) Sedimentary
b) Igneous
c) Metamorphic
d) Fossilized
Answer: b) Igneous
2. Which of the following is a sedimentary rock?
a) Granite
b) Marble
c) Sandstone
d) Basalt
Answer: c) Sandstone
3. What type of rock is formed due to heat and pressure?
a) Igneous
b) Sedimentary
c) Metamorphic
d) Fossilized
Answer: c) Metamorphic
4. Which igneous rock is found on the ocean floor?
a) Granite
b) Basalt
c) Limestone
d) Quartzite
Answer: b) Basalt
5. Coal is an example of which type of rock?
a) Igneous
b) Metamorphic
c) Sedimentary
d) Fossilized
Answer: c) Sedimentary
6. What is the parent rock of Marble?
a) Granite
b) Limestone
c) Basalt
d) Sandstone
Answer: b) Limestone
7. What is the main characteristic of metamorphic rocks?
a) Presence of fossils
b) Formed by cooling of magma
c) Formed by heat and pressure
d) Found only in oceans
Answer: c) Formed by heat and pressure
Short Answer Questions
1. How are igneous rocks formed?
Igneous rocks form by the cooling and solidification of magma or lava.
2. What are the three types of sedimentary rocks?
The three types are:
- Clastic (formed from fragments, e.g., Sandstone)
- Chemical (formed by mineral precipitation, e.g., Limestone)
- Organic (formed from remains of living organisms, e.g., Coal)
3. What is the difference between intrusive and extrusive igneous rocks?
- Intrusive (Plutonic) Igneous Rocks: Cool inside the Earth’s crust, forming large crystals (e.g., Granite).
- Extrusive (Volcanic) Igneous Rocks: Cool on the surface, forming small crystals (e.g., Basalt).
4. What are metamorphic rocks?
Metamorphic rocks are formed when igneous or sedimentary rocks undergo heat and pressure.
5. Why do sedimentary rocks contain fossils?
Sedimentary rocks form in layers over time, preserving remains of plants and animals.
Conclusion
The classification of rocks is based on their formation process. Understanding these types helps in geology, resource exploration, and environmental studies.
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