A rock is a natural solid aggregate of minerals forming the Earth’s crust. Rocks are classified based on their origin, composition, and formation process.
🔹 Types of Rocks
There are three major types:
- Igneous Rocks (Primary rocks)
- Sedimentary Rocks (Secondary rocks)
- Metamorphic Rocks (Transformed rocks)
🌋 1. Igneous Rocks (Primary Rocks)
🔸 Definition
Formed by the cooling and solidification of molten magma or lava.
🔸 Types of Igneous Rocks
1. Intrusive (Plutonic Rocks)
- Formed inside the Earth
- Cooling is slow → large crystals
👉 Examples:
- Granite
- Diorite
2. Extrusive (Volcanic Rocks)
- Formed on the Earth’s surface
- Cooling is fast → small crystals
👉 Examples:
- Basalt
- Obsidian
🔸 Characteristics
- Hard and compact
- No layers (non-stratified)
- No fossils
- Rich in minerals
🔸 Importance
- Source of metals (iron, nickel)
- Used in construction
- Forms the base of all other rocks
🌊 2. Sedimentary Rocks (Secondary Rocks)
🔸 Definition
Formed by the deposition, compaction, and cementation of sediments over time.
🔸 Formation Process
Weathering → Erosion → Deposition → Compaction → Cementation
🔸 Types of Sedimentary Rocks
1. Clastic (Detrital)
- Formed from broken rock fragments
👉 Examples:
- Sandstone
- Shale
2. Chemical
- Formed by evaporation of water
👉 Examples:
- Rock Salt
- Gypsum
3. Organic
- Formed from remains of plants and animals
👉 Examples:
- Coal
- Limestone
🔸 Characteristics
- Layered (stratified)
- Contain fossils
- Relatively soft
- Porous
🔸 Importance
- Storehouse of fossils
- Source of coal, petroleum, natural gas
- Important for agriculture (soil formation)
🔥 3. Metamorphic Rocks (Transformed Rocks)
🔸 Definition
Formed when existing rocks (igneous or sedimentary) are changed by heat, pressure, or chemical processes.
🔸 Types of Metamorphism
1. Contact Metamorphism
- Caused by heat (near magma)
2. Regional Metamorphism
- Caused by pressure and temperature over large areas
🔸 Examples (with parent rocks)
| Parent Rock | Metamorphic Rock |
|---|---|
| Granite | Gneiss |
| Limestone | Marble |
| Shale | Slate |
🔸 Characteristics
- Hard and compact
- May show layering (foliation)
- No fossils
- Crystalline structure
🔸 Importance
- Used in buildings and sculptures (marble)
- Source of minerals
- Economically valuable
🔄 Rock Cycle (Very Important)
🔸 Concept
Rocks continuously change from one type to another through the rock cycle.
👉 Process:
- Magma → Igneous rock
- Igneous → Sedimentary (weathering & deposition)
- Sedimentary → Metamorphic (heat & pressure)
- Metamorphic → Magma (melting)
🔸 Key Point
👉 The rock cycle shows that no rock is permanent.
📊 Comparison Table
| Feature | Igneous | Sedimentary | Metamorphic |
|---|---|---|---|
| Formation | Cooling of magma | Deposition of sediments | Heat & pressure |
| Structure | Non-layered | Layered | Foliated/non-foliated |
| Fossils | Absent | Present | Absent |
| Hardness | Hard | Soft | Very hard |
🧠 Important Exam Points
- Igneous rocks are called primary rocks
- Sedimentary rocks contain fossils
- Metamorphic rocks form due to heat & pressure
- Basalt forms oceanic crust
- Granite forms continental crust
Rocks are the fundamental building blocks of the Earth’s crust. Their formation and transformation through the rock cycle play a crucial role in shaping landforms, supporting life, and providing valuable resources.