GLY 101: Earth Revealed - Sediments: Weathering and Erosion

GLY 101: Earth Revealed - Sediments: Weathering and Erosion

Course Overview

  • Instructor: Dr. Emma Morris

  • Semester: Fall 2025

  • Primary Reading: Marshak, Essentials of Geology, Interlude B

Introduction to Sediments

  • Definition of Sediment: Sediments are made up of loose fragments of rocks or minerals, shells, shell fragments, or mineral crystals that precipitate from water.

  • Types of Sediments:

    • Loose fragments of

    • Minerals

    • Shells & shell fragments

    • Mineral crystals precipitating out of water.

Understanding Geological Terms

  • Bedrock: Solid rock underneath soil and other unconsolidated materials.

  • Soil: A mixture of organic matter, minerals, gases, liquids, and organisms that together support life.

  • Unconsolidated Sediment: Loose, uncemented material that can be easily transported by natural forces.

Weathering vs. Erosion

  • Weathering:

    • Definition: The breakdown and alteration of bedrock by physical and chemical processes, resulting in a regolith--a layer of loose material that's available for transport.

    • Processes:

    • Physical/Mechanical Weathering

    • Chemical Weathering

  • Erosion:

    • Definition: Physical movement of rocks, minerals, and soils, primarily driven by gravity, water, wind, and ice.

    • Result: Movement of solid products of weathering away from their original site.

Types of Weathering

Physical Weathering

  • Definition: Mechanical disintegration of physical rocks and minerals without changing the chemical composition.

Main Processes of Physical Weathering:

  1. Temperature Changes (Thermal Stress):

    • Rocks experience expansion and contraction due to temperature fluctuations from day to night.

    • Process involves exfoliation, where the outer layer of rocks flakes off in thin sheets.

    • Ex: Spheroidal weathering in granite observed in La Serena, Chile.

    • Unloading can also lead to exfoliation, wherein the removal of overlying materials induces pressure changes resulting in rock expansion.

  2. Ice/Water Freeze-Thaw Cycles:

    • Frost wedging: Water freezes in rock cracks, expands, and forms wedges, causing rock breakdown.

    • Swelling of clays due to absorbed water also assists in the breakdown of surrounding harder rocks.

  3. Growth of Salt Minerals (Haloclasty):

    • Occurs primarily in coastal areas with constant sea spray forcing rocks to interact with salts, leading to honeycomb weathering, where pits are formed in the process.

  4. Biological Activity:

    • Plant roots growing within rocks widen existing cracks, ultimately breaking rock pieces apart.

    • Tunneling animals (e.g., moles, prairie dogs) facilitate rock and soil fragmentation.

Chemical Weathering

  • Definition: Process involving chemical reactions that alter or destroy minerals, typically facilitated by water.

  • Key Reactions:

    • Dissolution

    • Oxidation

    • Reduction

    • Hydration

    • Hydrolysis

    • Biochemical processes.

Types of Chemical Weathering Processes:

  1. Dissolution:

    • Characterized by the action of slightly acidic solutions leading to pit formation in rocks, contributing to features such as karst topography, exemplified by the limestone pavement of the Burren, Ireland.

    • Impact of Acid Rain: Dissolution also affects both natural landscapes and human-made structures over time.

  2. Hydrolysis:

    • The dissociation of water into H⁺ and OH⁻ ions, typically involving natural acids.

    • Example Reaction: Orthoclase K-spar converts to Kaolinite clay.

    • Hydration: Involves minerals absorbing water under humid conditions, enhancing weathering rates.

  3. Oxidation:

    • Oxygen combines with minerals, typically from dissolved O₂ in soil water or the atmosphere.

    • Leads to the formation of hydrated iron oxides, changing minerals' properties, such as iron-bearing minerals turning to red-orange rocks (e.g., pyroxene to limonite).

  4. Reduction:

    • Occurs in water-saturated environments with low oxygen, allowing iron atoms to gain electrons (Fe³⁺ to Fe²⁺), resulting in soluble ferrous iron which could color soils grey, blue, or green.

  5. Biochemical Weathering:

    • Lichens secrete chelating agents that increase chemical weathering on rock surfaces.

    • Plants and bacteria contribute to the process through the release of CO₂ that acidifies water.

    • Organisms like barnacles and limpets secrete organic acids that weather coastal rocks.

Interplay of Physical and Chemical Weathering

  • Relationship: Physical weathering increases the surface area of rocks, thereby enhancing the effectiveness of chemical weathering.

  • Effect on Rock Composition:

    • Intact rock reduces surface area.

    • Increased cracks and fragmentation through weathering lead to greater surface area, facilitating faster chemical reactions.

    • Example: Rock decomposition, resulting in clay formation and sediment movement.

Differential Weathering

  • Definition: Uneven weathering of rock materials due to varied resistance properties of rocks.

  • Examples:

    • Edges and corners of rocks weather differently, leading to diverse landform shapes.

    • Key Features:

      • Differentiation occurs between weaker shale and stronger sandstone.

      • Results in characteristic landforms like "buttes" and "mesas," where hard rock acts as a cap over softer materials, effectively protecting them from erosion.

    • Visual Examples:

      • Hoodoos and spires seen in Bryce Canyon and Zion National Parks reflect regional geological patterns.

Conclusion

  • The study of sediments and weathering provides insights into geological processes that shape the Earth's landscape, manifesting through the continuous cycle of sedimentation and erosion driven by physical and chemical forces of nature.