🌈Earth Systems Science Unit 5 – Weathering, Erosion & Soil Formation

What's This Unit All About?

• Explores the processes that shape and transform the Earth's surface over time
• Focuses on three main topics: weathering, erosion, and soil formation
• Examines the various factors that influence these processes, such as climate, rock type, and human activities
• Investigates the interplay between physical, chemical, and biological processes in shaping landscapes
• Highlights the importance of understanding these processes for managing natural resources and mitigating geohazards
• Provides a foundation for understanding the dynamic nature of the Earth's surface and its evolution over geologic time

Key Concepts and Definitions

• Weathering: the breakdown of rocks and minerals at or near the Earth's surface through physical, chemical, and biological processes
• Erosion: the removal and transport of weathered material by agents such as water, wind, ice, and gravity
• Soil formation: the process by which weathered rock and organic matter are transformed into soil through a combination of physical, chemical, and biological processes
• Mechanical weathering: the physical breakdown of rocks into smaller fragments without changing their chemical composition (frost wedging, exfoliation)
• Chemical weathering: the alteration of rock minerals through chemical reactions, often involving water and atmospheric gases (dissolution, oxidation)
• Biological weathering: the breakdown of rocks and minerals by living organisms, such as plants, fungi, and bacteria (root wedging, lichen acids)
• Sediment: loose, solid particles that are derived from the weathering of rocks and are transported by erosional agents

Types of Weathering

• Mechanical weathering
  • Frost wedging: the repeated freezing and thawing of water in rock cracks, causing the rock to split apart
  • Exfoliation: the peeling away of rock layers due to the release of pressure, often seen in granitic domes (Half Dome, Yosemite)
  • Thermal expansion and contraction: the repeated heating and cooling of rocks, causing them to crack and break apart (desert environments)
• Chemical weathering
  • Dissolution: the process by which minerals dissolve in water, often accelerated by acidic conditions (limestone caves)
  • Hydrolysis: the reaction between water and minerals, resulting in the formation of new minerals (feldspar to clay)
  • Oxidation: the reaction between oxygen and minerals, often resulting in the formation of iron oxides (rust-colored stains on rocks)
• Biological weathering
  • Root wedging: the growth of plant roots in rock cracks, causing the rock to split apart
  • Lichen acids: the secretion of organic acids by lichens, which can dissolve minerals and etch rock surfaces
  • Burrowing animals: the physical disturbance of soil and rock by animals, such as earthworms and rodents, which can expose fresh surfaces to weathering

Erosion: The Earth's Sculptor

• Erosion is the removal and transport of weathered material by agents such as water, wind, ice, and gravity
• Water erosion
  • Fluvial erosion: the erosion caused by flowing water in rivers and streams, which can carve valleys and canyons (Grand Canyon)
  • Coastal erosion: the erosion of shorelines by waves, tides, and currents, which can reshape coastlines and create features like sea cliffs and beaches
• Wind erosion
  • Aeolian erosion: the erosion caused by wind, which can transport fine sediments and create features like sand dunes and loess deposits (Sahara Desert)
  • Abrasion: the wearing away of rock surfaces by wind-blown particles, which can polish and shape rocks (ventifacts)
• Glacial erosion
  • Plucking: the removal of large rock fragments by glacial ice, which can create features like cirques and arêtes
  • Abrasion: the grinding and scouring of rock surfaces by debris-laden glacial ice, which can create features like U-shaped valleys and striations (Yosemite Valley)
• Mass wasting: the downslope movement of rock, soil, and debris under the influence of gravity, which can occur as slow creep or rapid landslides and rockfalls

Soil Formation Process

• Soil formation, or pedogenesis, is the process by which weathered rock and organic matter are transformed into soil
• Soil-forming factors: climate, parent material, topography, organisms, and time (CLORPT)
• Weathering of parent material: the breakdown of the underlying rock into smaller particles and minerals
• Addition of organic matter: the incorporation of dead plant and animal material into the soil, which provides nutrients and improves soil structure
• Leaching: the downward movement of dissolved minerals and clay particles through the soil profile, which can create distinct soil horizons
• Translocation: the movement of soil particles and minerals within the soil profile, often resulting in the formation of clay-rich subsoil horizons (Bt horizon)
• Soil profile development: the vertical arrangement of soil horizons, which reflects the combined effects of the soil-forming factors over time (O, A, E, B, C, R horizons)

Factors Influencing Weathering and Erosion

• Climate: temperature and precipitation patterns strongly influence the type and rate of weathering and erosion processes
  • Humid climates tend to promote chemical weathering and soil formation, while arid climates favor mechanical weathering and wind erosion
  • Freeze-thaw cycles in cold climates can accelerate mechanical weathering through frost wedging
• Rock type and structure: the mineral composition, grain size, and presence of fractures or bedding planes can affect the susceptibility of rocks to weathering and erosion
  • Rocks rich in feldspar and other easily weathered minerals are more prone to chemical weathering (granite)
  • Rocks with abundant fractures or bedding planes are more susceptible to mechanical weathering and mass wasting (shale, limestone)
• Topography: the slope, aspect, and elevation of the land surface can influence the intensity and direction of erosional processes
  • Steep slopes are more prone to mass wasting and rapid erosion, while gentle slopes may experience slower soil creep
  • South-facing slopes in the northern hemisphere receive more solar radiation, which can accelerate weathering and soil formation
• Vegetation cover: plants can protect the soil from erosion by intercepting raindrops, stabilizing soil with their roots, and adding organic matter to the soil
  • Deforestation and overgrazing can increase soil erosion rates by removing the protective vegetation cover
• Human activities: land use practices and infrastructure development can significantly alter weathering and erosion processes
  • Agriculture, mining, and construction can expose fresh rock surfaces to weathering and increase soil erosion rates
  • Dams and river channelization can disrupt sediment transport and alter downstream erosion and deposition patterns

Real-World Examples and Case Studies

• Grand Canyon, USA: a prime example of fluvial erosion, where the Colorado River has carved a deep canyon through layers of sedimentary rock over millions of years
  • The canyon walls display a variety of weathering features, such as exfoliation domes and desert varnish
• Loess Plateau, China: an extensive area of wind-deposited silt (loess) that has been shaped by both wind and water erosion
  • The region has experienced severe soil erosion due to deforestation and overgrazing, leading to the implementation of large-scale restoration projects
• Karst landscapes: areas underlain by soluble rocks, such as limestone and gypsum, which are prone to chemical weathering and the formation of distinctive landforms
  • The Guangxi region in China is famous for its towering karst peaks and extensive cave systems, formed by the dissolution of limestone
• Soil salinization in arid regions: the accumulation of salts in the soil due to high evaporation rates and poor drainage, which can lead to reduced soil fertility and crop yields
  • The Aral Sea basin in Central Asia has experienced severe soil salinization due to unsustainable irrigation practices, resulting in the shrinking of the Aral Sea and the formation of a salt desert

Why Should We Care?

• Understanding weathering, erosion, and soil formation processes is crucial for managing and conserving soil resources, which are essential for food production and ecosystem services
• Knowledge of these processes can help in predicting and mitigating geohazards, such as landslides, rockfalls, and coastal erosion, which pose risks to human lives and infrastructure
• Insights into the interplay between weathering, erosion, and climate can inform our understanding of past environmental conditions and help predict future landscape changes in response to climate change
• Recognizing the impacts of human activities on these processes is essential for developing sustainable land management practices and policies
• Studying weathering and erosion processes can also contribute to our understanding of the evolution of life on Earth, as these processes play a key role in the formation and modification of habitats and in the cycling of nutrients and minerals between the geosphere and biosphere