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Types and Characteristics of Rocks (UPSC Geography Notes)
Overview: Rocks are naturally occurring solid materials that form the Earth's crust. They are composed of minerals and vary in composition, texture, color, and hardness. The study of rocks, known as petrology, is essential to understand the Earth's crustal structure, plate movement, and the rock cycle — all important topics for UPSC Geography.
Table of Contents
- 1. What is a Rock?
- 2. Major Types of Rocks
- 3. Characteristics of Each Rock Type
- 4. The Rock Cycle
- 5. Quick Revision
- 6. UPSC Previous Year Question
1. What is a Rock?
A rock is an aggregate of one or more minerals. Rocks form the outer solid layer of the Earth and differ based on how they are formed, what they are made of, and their physical properties.
2. Major Types of Rocks
All rocks are classified into three main types based on their origin:
- 1. Igneous Rocks – formed by cooling and solidification of molten material.
- 2. Sedimentary Rocks – formed by deposition and compaction of sediments.
- 3. Metamorphic Rocks – formed by alteration of existing rocks under heat and pressure.
3. Characteristics of Each Rock Type
A. Igneous Rocks
- Formed when magma or lava cools and solidifies.
- Called primary rocks because they form the original crust.
- Contain interlocking crystals; usually hard and compact.
- Do not have layers or fossils.
Types of Igneous Rocks:
- Intrusive (Plutonic): Formed when magma cools slowly beneath the surface, producing large crystals. Example: Granite, Diorite, Gabbro.
- Extrusive (Volcanic): Formed when lava cools quickly on the surface, producing fine-grained rocks. Example: Basalt, Andesite.
B. Sedimentary Rocks
- Formed by accumulation, compaction, and cementation of sediments derived from pre-existing rocks.
- Occur in layers called strata.
- Contain fossils as they form under water or on land surface.
- Generally soft and porous compared to igneous rocks.
Types of Sedimentary Rocks:
- Clastic: Made from fragments of other rocks (sandstone, shale).
- Chemical: Formed by precipitation of minerals from solution (limestone, rock salt).
- Organic: Formed from remains of plants or animals (coal, chalk).
C. Metamorphic Rocks
- Formed when existing rocks are subjected to high temperature and pressure, causing physical or chemical changes.
- Usually harder and more compact than their original forms.
- Do not contain fossils due to heat and pressure.
Examples of Metamorphism:
| Original Rock | After Metamorphism |
|---|---|
| Shale | Slate |
| Limestone | Marble |
| Sandstone | Quartzite |
| Coal | Graphite |
4. The Rock Cycle
The rock cycle explains how the three main types of rocks are interrelated and can transform from one type to another over geological time.
Process Overview:
- Igneous rocks break down into sediments → form sedimentary rocks.
- Both igneous and sedimentary rocks can change under heat and pressure → become metamorphic rocks.
- Metamorphic rocks may melt → form magma → cool again to become igneous rocks.
This continuous transformation forms the rock cycle, showing the dynamic nature of the Earth’s crust.
5. Quick Revision
Sedimentary Rocks – Formed from sediments, layered, often contain fossils.
Metamorphic Rocks – Formed by heat and pressure, harder and denser.
Rock Cycle – Continuous process of transformation among rock types.
6. UPSC Previous Year Question
Q. Which one of the following pairs is correctly matched?
(a) Shale — Igneous Rock
(b) Basalt — Sedimentary Rock
(c) Limestone — Organic Sedimentary Rock
(d) Marble — Igneous Rock
Answer: (c) Limestone — Organic Sedimentary Rock
Related Keywords
Types of Rocks UPSC Notes, Igneous Sedimentary Metamorphic Rocks, Rock Cycle Geography, Physical Geography Notes, UPSC Prelims Geography, NCERT Geography Simplified.
Sources: NCERT Class XI – Physical Geography, Geological Survey of India, Britannica
