Weathering: Definition and Causes of Physical Weathering

Definition of Weathering

Weathering refers to the natural process through which rocks and minerals are broken down into smaller particles due to various environmental factors. Unlike erosion, which involves the movement of weathered material, weathering involves the in-situ breakdown of rocks and minerals at or near the Earth’s surface. Weathering plays a crucial role in soil formation, landscape development, and the cycling of minerals in the environment.

Causes of Physical Weathering

Physical weathering, also known as mechanical weathering, involves the breakdown of rocks into smaller pieces without changing their chemical composition. The causes of physical weathering include:

1. Temperature Fluctuations:
   • Thermal Expansion and Contraction: Rocks expand when heated and contract when cooled. Repeated cycles of heating and cooling can cause rocks to crack and fracture. This process is particularly noticeable in desert environments where temperature variations between day and night are extreme. For example, the Death Valley in the USA experiences significant temperature fluctuations that contribute to physical weathering of its rocks.
   • Frost Wedging: Water that enters cracks in rocks freezes and expands when the temperature drops. This expansion exerts pressure on the rock, causing it to break apart. This process is common in mountainous regions with frequent freeze-thaw cycles, such as the Himalayas and Rocky Mountains.
2. Freeze-Thaw Cycles:
   • Frost Action: Water repeatedly freezing and thawing in rock crevices causes the rocks to split and break. This mechanism is significant in regions with cold climates and seasonal temperature variations. For instance, the Scottish Highlands experience substantial freeze-thaw action, which contributes to the physical weathering of its rocky landscapes.
   • Ice Wedges: In permafrost areas, ice wedges form as water seeps into cracks, freezes, and expands, leading to further rock fragmentation. This phenomenon is observed in the Alaskan Arctic and Canadian Arctic.
3. Biological Activity:
   • Root Expansion: Plant roots, particularly from trees and shrubs, can grow into cracks in rocks. As the roots expand, they exert pressure on the rocks, causing them to fracture and break apart. This type of weathering is observed in many forested regions, including the Amazon Rainforest.
   • Lichen and Moss Growth: Lichens and mosses secrete acids that can slowly break down the mineral structure of rocks. The physical growth of these organisms also contributes to rock fragmentation. Lichen weathering is prevalent on exposed rock surfaces in places like the Scandinavian Mountains.
4. Exfoliation:
   • Sheeting: This process involves the peeling off of outer layers of rock due to the release of pressure as overlying material is eroded away. Exfoliation is common in granite formations, such as those found in Yosemite National Park in the USA.
   • Unloading: Rocks that formed under high pressure can expand and fracture when the pressure is reduced, such as when overlying material is removed through erosion. This is seen in areas like the Granite Domes in California.
5. Salt Weathering:
   • Saline Action: In coastal areas or places with high evaporation rates, salt crystals form from evaporated seawater or saline groundwater. These salt crystals expand as they grow, exerting pressure on rock surfaces and causing them to disintegrate. An example of salt weathering is observed along the coastlines of the Mediterranean Sea.
6. Abrasion:
   • Wind and Water Erosion: Windblown sand and waterborne particles can impact rock surfaces, causing mechanical wear and tear. This type of abrasion is particularly evident in desert environments and riverbeds, such as the Arches National Park in Utah, USA.

Recent Examples and Observations

1. Himalayas:
   • The Himalayan region experiences significant physical weathering due to freeze-thaw cycles and frost action. The extreme altitude and cold temperatures contribute to the fracturing of rocks and the formation of scree slopes.
2. Death Valley, USA:
   • The extreme temperature fluctuations between day and night in Death Valley cause significant thermal expansion and contraction of rocks, leading to their fragmentation.
3. Scottish Highlands:
   • The Scottish Highlands experience intense frost wedging, with water freezing in rock crevices and causing the rocks to break apart.
4. Amazon Rainforest:
   • The physical weathering of rocks in the Amazon is influenced by root expansion and biological weathering from lichens and mosses.
5. Yosemite National Park, USA:
   • The exfoliation process is prominent in Yosemite’s granite formations, where sheet-like layers peel off due to pressure release.

Conclusion

Physical weathering is a key natural process that shapes the Earth’s surface by breaking down rocks into smaller fragments. Understanding the causes of physical weathering—such as temperature fluctuations, freeze-thaw cycles, biological activity, exfoliation, salt weathering, and abrasion—provides valuable insights into landscape formation and soil development. Recent examples from various geographic regions illustrate the diverse mechanisms through which physical weathering operates, highlighting its importance in environmental and geological studies.