5.7.1 Weathering of Rocks

What is Weathering?

It means the slow breaking down of large rocks into smaller pieces .

Example

• Look at the heaps of sand, soil, and stones often seen at the base of mountains and cliffs.
• These heaps are called sediments .
• They are formed by weathering.

Physical Weathering — Breaking Rocks Apart

This is the part of weathering where rocks physically break into smaller pieces.

No new substance is formed.

Causes of physical weathering

• Temperature changes
  • Rocks expand on hot days.
  • They contract on cold nights.
  • Over years, this stress cracks them.
• Growing tree roots
  • Tree roots push into the cracks of rocks.
  • As the roots grow thicker, they widen the cracks.
  • Slowly, the rock breaks apart.
• Freezing of water
  • Water seeps into the cracks of rocks.
  • When the water freezes, it expands.
  • The expanding ice splits the rock open.

Why this is a physical change

• The rock just breaks into smaller pieces.
• No new substance is formed.
• So this part of weathering is a physical change .

Chemical Weathering — Changing the Rock's Composition

This is the part of weathering where the rock chemically changes .

A new substance is formed inside the rock.

Example — basalt rock turning red

• The original rock is basalt .
• Basalt is normally black in colour.
• Basalt contains iron .
• Over a long time, the rock is exposed to water and moist air.
• The iron reacts with water and air.
• It forms iron oxide — the same red-brown substance as rust.
• This produces a red-coloured layer on the rock.

Why this is a chemical change

• A new substance (iron oxide) has been formed.
• So chemical weathering is a chemical change .

Weathering — Both Physical and Chemical Together

In nature, weathering is almost always a combination of both.

How both work together

• Physical part:
  • Temperature changes crack the rock.
  • Freezing water splits the rock.
  • Tree roots widen the cracks.
• Chemical part:
  • Water and air now reach freshly broken rock surfaces.
  • These surfaces react chemically.
  • New substances are formed.
• Over thousands of years, hard rock slowly becomes soil .
• This soil is the foundation of agriculture.