Sedimentary Rock Formation and Weathering

Chapter 6: Weathering, Soil, and Sedimentary Rocks

Overview of Processes Involved in Sedimentary Rock Formation

  • Formation of sedimentary rocks involves a complex series of steps:

    • Parent rock (pre-existing) to sedimentary rock transition.

    • Processes include:

    • Weathering (can be physical or chemical)

    • Erosion

    • Transport (as particles/solids or in solution)

    • Deposition (includes siliciclastic rocks and chemical rocks)

    • Lithification (which involves compaction and cementation)

Weathering

  • Definition: The physical breakdown (disintegration) and chemical alteration of Earth materials

  • Erosion: The removal of weathered material from its original location.

Types of Weathering

Mechanical Weathering

  • Definition: Mechanical weathering refers to the physical breaking of rocks into smaller pieces that retain the chemical composition of the parent material.

  • Processes include:

    • Frost wedging

    • Thermal expansion and contraction

    • Salt crystal growth

    • Pressure release

    • Organic activity

Chemical Weathering

  • Definition: rock materials are decomposed through chemical alteration

  • Chemical weathering is crucial because water is found in almost every environment, even arid climates. This process is typically slower compared to mechanical weathering.

  • Main processes include:

    • Hydrolysis

    • Hydration

    • Oxidation

    • Solution/Leaching

Mechanical Weathering Processes

Frost Wedging

  • Importance: Most significant physical weathering method in areas with recurring freezing and thawing.

  • Process:

    • Water enters large and microfractures in rocks.

    • When water freezes, it expands by approximately 9% in volume, exerting pressure that can crack rocks.

  • Scales of effect: Works at both small and large scales, creating angular blocks ranging in size from millimeters to kilometers.

Thermal Expansion & Contraction

  • Explanation:

    • Daily heating of rocks during sunny days followed by nighttime cooling results in mechanical breakdown.

    • Distinct rock-forming minerals expand differently when heated.

  • Notable temperature changes:

    • Some desert rocks can reach surface temperatures over 80°C.

    • Daily fluctuations can be as high as 40°C.

  • Significance: Though it can cause weathering, it is not a primary mechanical weathering factor.

Salt Crystal Growth

  • Process:

    • Evaporation of water that contains salt leads to salt crystals forming in pores and fractures.

    • Groundwater containing ions can precipitate materials to form salt.

  • Effect: Growth of these crystals can create significant stress, forcing cracks apart, and promoting granular disintegration.

  • Areas of impact: Most significant in semiarid environments and along coastlines.

Pressure Release

  • Explanation:

    • Rock masses originally buried deep within the earth's surface experience high confining pressures.

    • Upon erosion of overlying layers, these pressures decrease, prompting the buried rock to expand upward.

  • Resulting structures:

    • Development of fractures parallel to the surface resulting in sheet joints and dome exfoliation.

Organic Activity

  • Types include:

    1. Plant Root Wedging: Roots grow into cracks, physically wedging apart the rocks.

    2. Burrowing by Animals: Organisms such as worms ingest soil and rock fragments.

    3. Lichens: They expand and contract, plucking mineral grains and rock fragments from their surroundings.

Chemical Weathering Processes

Hydrolysis

  • Definition: A chemical reaction between silicate minerals and acids that leads to the breakdown of silicate minerals.

  • Example reaction:
    2KAlSi3O8 + 2H^+ + 9H2O ightarrow H4Al2Si2O9 + 4H4SiO_4 + 2K^+

  • Significance: The presence of $CO_2$, often from living plants, enhances this reaction, allowing for the transformation of unstable minerals into new, more stable forms, such as clays.

Hydration

  • Definition: The incorporation of water molecules into mineral structures to form new minerals.

  • Example reactions:

    • ext{Hematite (Fe}2O3) + ext{water}
      ightarrow ext{Goethite (FeOOH)}

    • ext{Anhydrite (CaSO}4) + ext{water} ightarrow ext{Gypsum (CaSO}4 ullet 2H_2O)

  • Discusses consequences of volume changes and physical disruption caused by these processes.

Oxidation

  • Definition: The chemical alteration of iron, manganese, and other minerals by oxygen dissolved in water.

  • Notable reactions include:

    • ext{Biotite and Pyroxene: } ext{Fe}^{2+}
      ightarrow ext{Fe}^{3+} + e^-

    • 2FeS2 + 3O{2}
      ightarrow ext{Hematite (Fe}2O3) + 4S

  • Significance: This process is associated with rusting and the transformation of cations necessary for maintaining mineral structure, thereby impacting the stability of existing minerals.

Solution/Leaching

  • Definition: The removal of soluble matter from bedrock due to water solutions, leading to the decomposition of rocks.

  • Note: Many minerals are not soluble in pure water but are in acidic or alkaline solutions. Essential examples include:

    • ext{Calcite, Dolomite, Gypsum, Halite}

  • Importance: Particularly prominent in wet climates and significant for the formation of caves via the dissolution of limestone.

Factors Influencing Weathering Rates

  • Chemical Weathering Factors:

    • Mineral stability: Higher crystallization temperatures typically lead to less chemical stability.

  • Physical and Chemical Weathering Factors:

    • Parent material: Variations exist based on rock type (e.g., ultramafic, mafic, felsic, sandstone, dolomite, limestone, salt).

    • Climate: Chemical weathering rates increase in warm, wet environments; while physical weathering prevails in arid, cold conditions.

    • Atmospheric composition: Influenced by volcanic activity and human-induced phenomena (e.g., acid rain).

    • Fractures and particle size: Increase surface area for more effective weathering responses.

Soil Formation

  • Definition: Soil is a mixture consisting of weathered rock material, water, air, and organic matter, which includes sand, silt, clay, and humus—decomposed organic carbon-rich material.

Controls of Soil Formation

  • Parent Material:

    • Residual soils (derived from underlying bedrock).

    • Transported soils (formed from materials carried from elsewhere).

  • Time and Climate:

    • Time: Varies based on geological and climatic conditions—critical for soil development.

    • Climate: The most significant factor. Influenced by temperature and precipitation levels.

  • Biological Factors:

    • Organisms contribute to soil properties and provide organic matter. Especially noteworthy are microbes like fungi and bacteria.

  • Slope:

    • Steep slopes typically lead to less developed soils; orientation impacts soil character significantly.

Soil Profile Structures

  • O Horizon: Organic matter layer.

  • A Horizon: Topsoil—intense biological activity.

  • B Horizon: Subsoil—zone of accumulation.

  • C Horizon: Composed of partially altered parent rock with minimal organic matter.

Major Soil Types

  • Pedalfers: Found in humid climates (e.g., much of Canada).

  • Pedocals: Typically seen in arid climates.

  • Laterites: Found in tropical climates, characterized by reddish color high in iron and aluminum.

Soil Degradation

  • Definition: Decrease in soil productivity or loss of soil integrity.

  • Types recognized include:

    • Erosion

    • Chemical degradation (due to overuse, poor fertilization practices, pollution, salinization)

    • Physical deterioration (compaction from machinery or livestock)

Control of Soil Erosion

  • Strategies to minimize erosion include:

    • Limit bare soil exposure.

    • Reduce overgrazing.

    • Implement alternating strips of grass or similar plants in row cropping.

    • Practice crop rotation.

Weathering and Natural Resources

  • Residual concentrations can include minerals like bauxite, iron, manganese, tin, gold, and kaolinite.

  • Gossans are formed through the alteration of iron and sulfur-bearing minerals, often relating to underlying ore deposits.

Weathering, Sediment, and Sedimentary Rocks

  • Definition: Sediment generation arises from mechanical and chemical weathering, producing raw materials for soil and sedimentary rocks. Sediment can be categorized into:

    • Detrital (based on particle size):

    • Gravel: > 2 mm in diameter

    • Sand: 1/16 mm to 2 mm

    • Silt: 1/256 mm to 1/16 mm

    • Clay: < 1/256 mm

    • Chemical sediments: May originate from biological activities.

Sediment Transport and Deposition

  • Methods of Transport: Ice, wind, water.

  • Processes include:

    • Abrasion and rounding of particles.

    • Sorting processes, which lead to an increase in sediment maturity.

Deposition Environments

  • Types of Environments:

    • Continental

    • Transitional

    • Marine

Lithification: Converting Sediment into Sedimentary Rock

  • Processes include:

    • Compaction: Reduction of pore space and volume in sediments.

    • Cementation: Involves the addition of cement, leading to increased mechanical strength. Common cementing agents include:

    • Calcite

    • Silica

    • Iron oxide

Types of Sedimentary Rocks

  • Based on source material origins:

    • Detrital Sedimentary Rocks: Involves transported sediments as solid particles (includes rocks like conglomerate, sandstone, siltstone, shale).

    • Chemical Sedimentary Rocks: Composed of precipitated materials once in solution as a result of biological or inorganic processes.

Reading the Story in Sedimentary Rocks

  • Geologists infer deposition environments through analyses of:

    • Rocks

    • Fossils

    • Sedimentary structures

  • Principle of Uniformitarianism: States that natural laws and processes are consistent over time, allowing current observations to interpret past events—"The present is the key to the past."

Sedimentary Structures

  • Types include:

    • Strata or beds

    • Graded bedding

    • Cross-bedding

    • Ripple marks

    • Mudcracks

Fossils in Sedimentary Rocks

  • Significance: Fossils reveal adaptations of organisms, providing insights into ancient environments.

  • Considerations: Fossil structures and preservation can affect the fossil record, with insoluble materials like calcite being more stable.

Important Resources in Sedimentary Rocks

  • Hydrocarbons (coal, petroleum, natural gas) originate from marine organism remains.

  • Upward migration through porous rock formations leads to trapping in geological structures.

  • Uranium sources, especially carnotite, are concentrated in sedimentary rocks, with North America relying heavily on these for nuclear reactor fuel.

  • Banded Iron Formation: Characterized by layers of chert and iron oxide; formed 2.0 to 2.5 billion years ago and is significant for iron mining today.