What is a Rock?

A rock is a naturally occurring solid aggregate of one or more minerals that forms the Earth’s crust.

• About 95% of the Earth’s crust consists of igneous and metamorphic rocks.
• Rocks are the parent material from which soils develop.

Important Facts

Particular	Description
Study of Rocks	Petrology
Study of Minerals	Mineralogy
Earth’s crust	Made of rocks
Soil originates from	Rocks (Parent Material)

Rock Cycle

1. Igneous Rocks (Primary Rocks)

Definition: Formed by the cooling and solidification of molten magma or lava. They are known as Primary Rocks because they were the first rocks formed on Earth.

Characteristics:

Characteristic	Description
Fossils	Absent
Layers	No layering
Hardness	Hard
Water holding	Low
Minerals	Rich in primary minerals

Types of Igneous Rocks:

• A. Intrusive (Plutonic): Cooling occurs inside the Earth. Cooling is slow. Large crystals are formed.
  Examples: Granite, Diorite, Gabbro
• B. Extrusive (Volcanic): Cooling occurs on Earth’s surface. Cooling is rapid. Fine crystals are formed.
  Examples: Basalt, Obsidian, Pumice

Important Examples:

Rock	Soil Formed
Basalt	Black Soil
Granite	Red Soil

2. Sedimentary Rocks (Secondary Rocks)

Definition: Formed by deposition and cementation of sediments.

Characteristics: Layered structure, Fossils present, Comparatively soft, Water permeable.

Formation Steps:

Examples:

Rock	Important Mineral
Sandstone	Quartz
Limestone	Calcium carbonate
Shale	Clay
Conglomerate	Pebbles

3. Metamorphic Rocks

Definition: Existing rocks changed due to Heat, Pressure, or Chemical activity.

Characteristics: Very hard, Compact, Crystalline, High density.

Examples (Parent Rock → Metamorphic Rock):

Parent Rock	Metamorphic Rock
Limestone	Marble
Granite	Gneiss
Sandstone	Quartzite
Shale	Slate

What is Weathering?

Weathering is the disintegration and decomposition of rocks into smaller particles at or near the Earth’s surface without transportation. It is the first step of soil formation.

Difference Between Weathering and Erosion

Weathering	Erosion
Rocks break at same place	Material transported
No transportation	Transportation occurs
Slow process	Faster process

A. Physical Weathering

No chemical change occurs; only rock size decreases.

Causes: Temperature, Wind, Water, Ice, Pressure.

Important Processes:

• Exfoliation: Outer layers peel off due to heating and cooling. Common in granite.
• Frost Action: Water freezes → Volume increases → Rock cracks. Occurs in cold regions.
• Abrasion: Wind or water rubs rock surface.
• Thermal Expansion: Repeated heating and cooling breaks rocks. Occurs in deserts.

B. Chemical Weathering

Minerals chemically change. Most common in humid climates.

Processes:

• Hydration: Mineral + Water → New mineral formed.
• Hydrolysis: Most important process. Water reacts with silicate minerals.
  Example: Feldspar → Clay minerals.
• Oxidation: Iron combines with oxygen. Produces reddish colour.
• Carbonation: CO₂ + Water → Carbonic acid → Limestone dissolves.
• Solution: Minerals dissolve directly in water. Example: Rock salt, Gypsum.

C. Biological Weathering

Caused by living organisms.

Examples: Plant roots, Earthworms, Bacteria, Fungi, Human activities.

Soil Definition & Terminology

Soil is the natural body developed from weathered rocks, containing minerals, organic matter, water, air and living organisms.

• Pedogenesis: Soil Formation.
• Pedology: Study of soil.
• Edaphology: Soil relation with plants.

Soil Forming Factors (Jenny 1941)

Factor	Meaning	Classification
CL	Climate	Active Factors (CO)
O	Organisms
R	Relief	Passive Factors (PRT)
P	Parent Material
T	Time

1. Climate: Most important factor. Controls Rainfall, Temperature, Weathering.
2. Parent Material: Determines Soil texture, Mineral composition, Fertility.
   Example: Basalt → Black Soil; Granite → Red Soil.
3. Organisms: Include Plants, Earthworms, Bacteria, Fungi, Human beings. Increase organic matter.
4. Relief: Controls Drainage, Soil depth, Erosion.
5. Time: Older soils are more developed.

Soil Forming Processes

• Addition: Organic matter, Dust, Fertilizers.
• Loss: Leaching, Erosion, Harvest removal.
• Transformation: Organic matter → Humus.
• Translocation: Movement of clay, Nutrients, Iron.

Horizon	Characteristics
O	Organic matter
A	Humus, roots, microorganisms
E	Leached layer
B	Clay accumulation
C	Weathered parent material
R	Hard rock

Eluviation vs Illuviation

Eluviation	Illuviation
Washing out	Deposition
A Horizon	B Horizon

Major Soil Forming Processes

Process	Region
Laterization	High rainfall
Podzolization	Cold humid
Calcification	Dry region
Salinization	Arid region
Gleization	Waterlogged

Father of Soil Science: Vasily V. Dokuchaev (Known as Father of Modern Soil Science).

Question	Answer
Primary rocks	Igneous
Secondary rocks	Sedimentary
Basalt forms	Black soil
Granite forms	Red soil
Weathering first step of	Soil formation
Study of rocks	Petrology
Study of soil	Pedology
Study of soil-plant relation	Edaphology
Father of Soil Science	Dokuchaev
Topsoil horizon	A
Bedrock	R
Eluviation	A Horizon
Illuviation	B Horizon
Soil formation equation	S = f(Cl, O, R, P, T)
Most important soil-forming factor	Climate

1. Igneous rocks are primary rocks.
2. Sedimentary rocks contain fossils.
3. Metamorphic rocks form due to heat and pressure.
4. Basalt forms black soil.
5. Granite forms red soil.
6. Weathering is the first step of soil formation.
7. Pedology is the study of soil.
8. Edaphology deals with soil-plant relationships.
9. Dokuchaev is the Father of Soil Science.
10. Climate is the most important soil-forming factor.
11. O horizon is rich in organic matter.
12. A horizon is topsoil.
13. B horizon is subsoil.
14. C horizon contains weathered parent material.
15. R horizon is bedrock.
16. Eluviation occurs in the A horizon.
17. Illuviation occurs in the B horizon.
18. Hydrolysis is the most important chemical weathering process.
19. Oxidation is common in iron-rich rocks.
20. Carbonation dissolves limestone.
21. Laterization occurs in humid tropical regions.
22. Calcification occurs in arid regions.
23. Salinization is common in dry climates.
24. Black soil is derived mainly from basaltic lava.
25. Soil develops from the weathering of rocks.