Rock Transformation: Understanding Earth's Weathering and Erosion Processes

Earth's Surface Processes: Weathering and Erosion

Whenever rocks are exposed at the Earth's surface, they are acted upon by external forces.

Understanding External Geological Forces

There are two distinct external processes that shape the Earth's surface:

• Weathering: The breakdown of rocks in situ (in their original place).
• Erosion: The further destruction, transport, and deposition of weathered rock material.

Types of Weathering

There are three main types of weathering.

Mechanical (Physical) Weathering

Mechanical weathering occurs when exposed rocks are broken up by physical forces without any change in their chemical composition.

Key Mechanisms of Mechanical Weathering

• Unloading: The expansion and fracturing of high-pressure rock when exposed to the surface.
• Frost Action (Freeze-Thaw): Water freezing and expanding in rock cracks.
• Root Wedging: Plant roots growing into and widening rock fractures.
• Temperature Changes: Expansion and contraction of rocks due to thermal fluctuations.

High Temperatures: Exfoliation (Onionization or Rock Peeling)

High temperatures can lead to exfoliation, also known as onionization or rock peeling. This process involves:

• During the day: The sun's rays heat the surface of the rock. The outer layer expands and pulls away from the cooler interior, causing cracks parallel to the surface to develop.
• During the night: The outer layer cools down rapidly without sun exposure. As it cools, the rock contracts, and cracks develop at right angles to the surface.

Eventually, the outer layer breaks up into large chunks of rock. These chunks then fall to the bottom of the hill under gravity, forming piles of rubble called scree.

Low Temperatures: Frost Shattering (Freeze-Thaw)

Low temperatures result in frost shattering, also known as freeze-thaw. This process occurs as follows:

1. Water fills a crack in a rock.
2. When temperatures drop below freezing, the water turns to ice and expands by about 9%, exerting pressure on the rock.
3. When the temperature rises, the ice melts and changes back to water.
4. Repeated cycles of freezing and thawing eventually cause the rock to shatter into several jagged pieces.

Other Mechanical Weathering Processes

• Gravity: Direct rockfall and collisions contribute to mechanical breakdown.
• Abrasion: The wearing away of rock surfaces by the friction of solid particles carried by wind, water, or ice.
• Unloading: The expansion and fracturing of deeply buried rocks when overlying material is removed, reducing pressure.
• Salt Crystallization: While often linked to chemical processes, the physical force of salt crystals growing in rock pores can cause mechanical breakdown. An example is the honeycomb weathering seen on pedestal rocks, such as at Yehliu, Taiwan.

Biological Weathering

Biological weathering involves the breakdown of rocks through the actions of living organisms.

Role of Plants

Seeds may fall into cracks in rocks where shelter and moisture help them grow into small plants or trees. As the plant grows, its roots develop and exert pressure, forcing the crack to widen. The rock eventually breaks up into smaller pieces.

Role of Animals

Burrowing animals such as rabbits, moles, and even earthworms can help to break down rocks by creating tunnels and exposing new surfaces to other weathering agents.