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.

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🔹 Types of Rocks

There are three major types:

1. Igneous Rocks (Primary rocks)
2. Sedimentary Rocks (Secondary rocks)
3. Metamorphic Rocks (Transformed rocks)

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🌋 1. Igneous Rocks (Primary Rocks)

🔸 Definition

Formed by the cooling and solidification of molten magma or lava.

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🔸 Types of Igneous Rocks

1. Intrusive (Plutonic Rocks)

• Formed inside the Earth
• Cooling is slow → large crystals

👉 Examples:

• Granite
• Diorite

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2. Extrusive (Volcanic Rocks)

• Formed on the Earth’s surface
• Cooling is fast → small crystals

👉 Examples:

• Basalt
• Obsidian

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🔸 Characteristics

• Hard and compact
• No layers (non-stratified)
• No fossils
• Rich in minerals

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🔸 Importance

• Source of metals (iron, nickel)
• Used in construction
• Forms the base of all other rocks

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🌊 2. Sedimentary Rocks (Secondary Rocks)

🔸 Definition

Formed by the deposition, compaction, and cementation of sediments over time.

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🔸 Formation Process

Weathering → Erosion → Deposition → Compaction → Cementation

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🔸 Types of Sedimentary Rocks

1. Clastic (Detrital)

• Formed from broken rock fragments

👉 Examples:

• Sandstone
• Shale

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2. Chemical

• Formed by evaporation of water

👉 Examples:

• Rock Salt
• Gypsum

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3. Organic

• Formed from remains of plants and animals

👉 Examples:

• Coal
• Limestone

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🔸 Characteristics

• Layered (stratified)
• Contain fossils
• Relatively soft
• Porous

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🔸 Importance

• Storehouse of fossils
• Source of coal, petroleum, natural gas
• Important for agriculture (soil formation)

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🔥 3. Metamorphic Rocks (Transformed Rocks)

🔸 Definition

Formed when existing rocks (igneous or sedimentary) are changed by heat, pressure, or chemical processes.

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🔸 Types of Metamorphism

1. Contact Metamorphism

• Caused by heat (near magma)

2. Regional Metamorphism

• Caused by pressure and temperature over large areas

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🔸 Examples (with parent rocks)

Parent Rock	Metamorphic Rock
Granite	Gneiss
Limestone	Marble
Shale	Slate

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🔸 Characteristics

• Hard and compact
• May show layering (foliation)
• No fossils
• Crystalline structure

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🔸 Importance

• Used in buildings and sculptures (marble)
• Source of minerals
• Economically valuable

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🔄 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)

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🔸 Key Point

👉 The rock cycle shows that no rock is permanent.

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📊 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

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🧠 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

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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.