Weathering Processes and Effects

Weathering Overview

Definition: Weathering is the process that breaks down and decays rocks through physical or chemical methods.
Difference from Erosion: In weathering, the material doesn't move; it just breaks apart.
Term for Broken Down Particles: The particles created by weathering are called regolith ("lith" means stone).

Physical/Mechanical Weathering

Definition: This type of weathering happens due to stress on rocks.

Freeze/Thaw Action

Description: This is the most common type of weathering, especially in places where water can collect in rock cracks.
Conditions:

• High altitudes experience frequent temperature changes.
• Water collects in rock cracks and freezes at night when it gets below 0°C.
  Process:

1. When water freezes, it expands by 9%, causing pressure on the rocks (about 100 kg/cm²).
2. During the day, the ice melts, allowing water to seep deeper into the cracks.
3. Repeating this cycle weakens the rocks and causes them to break.

• Rocks that break off are called scree.
  Where It’s Common: Found in areas like West Ireland, the Alps, or the Arctic.

Exfoliation (Onion Weathering)

Description: This involves layers of rock peeling off, usually in deserts.
Key Term: Diurnal Temperature Range - the difference in temperature between day and night.
Mechanism:

• In deserts, daytime temperatures can exceed 40°C, and nighttime temperatures can drop below freezing.
• The outer rock layers heat up faster than the inside layers, leading to cracks.
• Larger cracks can result in the outer layers peeling away.
• In granite, quartz expands three times more than feldspar, breaking down the rock.
• Erosion from above allows the rocks to crack and create dome-like formations.
  Resulting Features: Tors are the remaining rock pieces after weathering, seen in places like Three Rock Mountain, Co. Dublin.

Chemical Weathering

Effect: This change alters the minerals in the rocks.
Key Ingredient: Water plays a big role in chemical weathering.

Carbonation

Process:

• Rainwater absorbs CO₂ to create a weak acid called carbonic acid (H₂CO₃).
• This acid reacts with limestone (calcium carbonate, CaCO₃).
• The reaction:
  $CO₂ + H₂O = H₂CO₃$
  $H₂CO₃ + CaCO₃ = Ca(HCO₃)₂$
• The soluble product is washed away, creating caves and other features.
  Examples of Features: Things like karst landscapes, limestone pavements, swallow holes, and stalactites (found in The Burren, Co. Clare).

Hydrolysis

Description: This is the most common type of chemical weathering and is important for forming soil.
Result: It turns minerals like feldspar into clay soil.

• Weathering doubles in speed with every 10°C rise in temperature.

Oxidation

Process: This occurs when oxygen reacts with minerals, breaking them down, especially in sandstone.
Visual Indicator: Weathered rocks often show a reddish color like rust.

Biological Weathering

Mechanisms:

• Plant roots and burrowing animals help break down rocks.
• Human activities like burning fossil fuels lead to acid rain, increasing weathering rates.
• Deforestation exposes rocks, making them more vulnerable to weathering.
  Distinctive Irish Landscape: The granite in the Wicklow Mountains.
  Composition: Granite is mainly made up of feldspar, mica, and quartz.
  Effects of Weathering:
• These rocks were once protected by layers of shale and sandstone, but weathering has made them vulnerable.
• The result is rounded hills and formations known as tors, which show flat tops and horizontal layers.
  Key Exam Point: Understand hydrolysis and freeze/thaw action when discussing unique landscapes.