Lecture 2.3 Weathering of rocks

Introduction to Weathering

• Weathering is the process that breaks down bedrock into smaller particles called sediment.

• All materials, including concrete, are subject to weathering impacts.

• Weathering works to disintegrate rocks, regardless of type or strength.

Categories of Weathering

Mechanical Weathering

• Involves physical processes that break down rocks without changing their chemical composition.

• Main agents include:

  • Pressure expansion

  • Frost wedging: ice expands by 9% when it freezes, causing cracks in rocks to widen.

  • Root wedging: plant roots grow into cracks, pushing them apart.

  • Salt expansion: salts precipitate from evaporating water, occupy rock cracks and expand, leading to formation of Tafony (cracked rock).

Chemical Weathering

• Involves chemical reactions that alter the mineral composition of rocks.

• Key processes include:

  • Carbonic acid formation: carbon dioxide in water forms a weak acid that dissolves minerals (especially limestone, rock salt).

  • Oxidation: loss of electrons from minerals (e.g., rusting of iron).

  • Hydrolysis: water reacts with minerals to form new minerals and dissolve existing ones, such as feldspar transforming to clay.

Mechanisms of Erosion

• Erosion is primarily a mechanical process that removes sediment and soil from weathered areas.

• Major driving forces of erosion:

  • Water

  • Wind

  • Gravity

  • Ice (glaciers)

• Erosion resistance affects geological features; less resistant materials erode away, leaving resilient formations like the cliffs of the Grand Canyon.

Unique Geological Features

• Features like hoodoos are formed from rocks of varying erosion resistance, creating aesthetically unique landscapes in places like Bryce Canyon National Park.

• The old man in the mountain in New Hampshire illustrates damage due to ice wedging and erosion.

Impact of Surface Area in Mechanical Weathering

• Smaller rock pieces have a greater surface area exposed to weathering compared to larger chunks.

• Example: a cubic rock measuring 2x2 units has 24 square units of surface area but can be broken into smaller cubes that collectively reach 96 square units of exposure.

Case Study: Half Dome

• Half Dome in Yosemite National Park showcases onion-skin weathering due to pressure release after uplift and exposure.

• Forms recognizable patterns and flaking of granite.

Mechanical vs. Chemical Weathering

• Mechanical weathering physically breaks rocks into pieces, increasing surface area for chemical reactions.

• Chemical weathering dominates in warm, humid environments due to the presence of water and higher temperatures, accelerating mineral degradation.

Summary of Chemical Processes

• Rainwater, which is slightly acidic, dissolves certain minerals, leading to features like sinkholes and caves via karst processes.

• Oxidation is crucial in weathering, as it alters minerals through electron exchange, often changing the appearance of rocks.

Conclusion

• Understanding the interplay of mechanical and chemical weathering is essential for recognizing geological features and processes in nature.