Exogenic Processes: Weathering Overview

Exogenic Processes: Weathering

• Weathering is the breakdown of rocks at the Earth's surface, driven by rainwater, temperature changes, and biological activity.
• Importantly, it does not involve the removal of rock material.
• A recap question: what natural agents or processes break down rocks into smaller pieces?

Main Categories of Weathering Processes

• Weathering is categorized as physical and chemical.
• Weathering commonly occurs in situ (in place).

Physical Weathering Processes

• Frost wedging: Water seeps into cracks, freezes, expands, and forces the rock apart.
• Salt crystal growth: Dissolved salts in water evaporate, crystallize, and exert pressure on surrounding rock, causing it to break apart.
• Abrasion: Rocks are worn away by the constant collision of loose particles.
• Biological activity: Plants and animals (including humans) contribute to mechanical weathering.

Major Processes of Chemical Weathering

• Dissolution: Molecules break down into ions – a classic example is the dissolving of salt.
• Oxidation: A reaction occurs between minerals and dissolved oxygen in water.
• Hydrolysis: Minerals react with water, changing their composition.

Factors Affecting Weathering

• Climate:
• Rock type: Weathering rates are roughly inversely related to the order of mineral crystallization in the Bowen's Reaction Series.Cold and dry climates result in slow chemical weathering, while high temperatures and rainfall contribute to rapid chemical weathering.
• The minerals that crystallize first are typically less resistant to weathering.
• Rock structure: Cracks and openings in rocks allow agents of weathering (water, plant roots, etc.) to enter more easily, speeding up the process.
• Time: Longer exposure to weathering agents leads to a higher rate of weathering. The ro ck becomes progressively weaker over time.