Weathering and Erosion - Comprehensive Study Notes

Weathering and Erosion

  • Overview of the process flow in the Earth surface system
    • Tectonic processes uplift the land surface above sea level. Once rocks are exposed to air and water, they undergo chemical and physical weathering, breaking down to produce sediment.
    • Convection in the atmosphere generates wind, rain, and snow; flowing water, ice, and air erode and transport sediment to sites of deposition.
    • Leaching by downward-percolating rainwater, along with the addition of organic material, produces soil.
    • Coastal deposition and coastal erosion are part of the sediment transport and deposition cycle.
  • Examples illustrated in the material (from field/landscape images):
    • New boulders tumble from a cliff in New Mexico.
    • Silt collects along a stream in Indiana.
    • Soil forms on bedrock of chalk in southern England.
    • Waves move sand on a beach in Brazil.
    • Erosion carves the coastal cliffs of Ireland.

The Rock Cycle

  • Core idea: The rock cycle links weathering, erosion, deposition, and the formation of igneous, sedimentary, and metamorphic rocks through time.
  • Key sequence (as shown in the figure):
    • Erosion, transportation, and deposition produce sediment.
    • Heating and remelting produce magma and igneous rock.
    • Burial and/or heating transform rocks into metamorphic rock.
    • Melting in the mantle provides new material to the crust.
    • Subduction returns crustal material to the mantle.
    • Erosion, transportation, and deposition continue to recycle materials, feeding back into sedimentary rock formation.
  • Short-form cycle described:
    • Erosion, transportation, and deposition → Sedimentary rock
    • Heating and melting → Igneous rock
    • Burial/heating → Metamorphic rock
    • Burial, heating, and remetamorphism → metamorphic cycle continues

Sediments, Soils, and Bedrock

  • Definitions:
    • Soil: the uppermost veneer of the Earth that directly interacts with rain and organic matter.
    • Sediment: naturally occurring material broken down by weathering and erosion, transported and deposited.
    • Bedrock: the foundation on which sediment rests; bedrock is attached to the Earth’s crust.

Ancient Soils: Paleosols

  • Paleosols are ancient soil horizons preserved in the rock record.
  • They are difficult to identify in the rock record and often resemble mudstones; in some cases, paleosols may appear similar to sandstones.

Weathering

  • Key factors controlling weathering:
    • Surface area: weathering acts faster on rocks with higher surface area because more surface is exposed to air and water.
    • Chemical and physical processes must interact with rock surfaces to affect them.
    • Weathering acts faster on rocks with high surface area due to greater contact with weathering agents.

Weathering: Physical vs Chemical

  • Physical Weathering (mechanical):
    • Breaks consolidated rocks into clasts (sediment grains of any size).
    • Does not alter chemical/mineralogical composition of the rock.
  • Chemical Weathering:
    • Involves chemical reactions that alter or destroy minerals when rock comes into contact with water and/or air.

Weathering: Spatial Occurrence

  • Weathering takes place on land (subaerial) and also in the ocean (submarine).
  • Usually associated with interactions at plate boundaries.
  • Subaerial: on exposed land surfaces.
  • Submarine: occurs beneath the ocean surface.

Physical Weathering: Mechanisms and Concepts

  • Stress-release weathering:
    • Geothermal gradient: pressure and temperature increase with depth; erosion of overburden exposes deeper rocks, reducing pressure and cooling them.
    • Result: rocks crack and break as they are exposed to lower pressure and cooler temperatures.
  • Joints: natural cracks in rocks; provide planes of weakness for weathering fluids to penetrate.
  • Reduced pressure and cooling change rock shapes, making brittle rocks break and form joints.

Physical Weathering: Common Processes

  • Freeze-Thaw (Frost) Wedging:
    • Water expands when frozen; water trapped in joints can freeze and expand, enlarging the original joint.
  • Salt Wedging:
    • Salt dissolved in water precipitates in joints and grows into larger crystals, forcing rocks apart.
  • Root Wedging:
    • Plant roots grow into bedrock joints; as roots grow they apply pressure, contributing to rock breakup.

Chemical Weathering: Key Processes

  • Dissolution:
    • The process of dissolving material in water.
    • Water molecules are polar and can hold ions in solution; dissolution causes grain surfaces to become pitted as ions are carried away.
    • Example: limestone dissolution by meteoric water creates dissolution troughs.
    • Representation: CO2 + H2O
      ightarrow H2CO3
      ightarrow H^+ + HCO_3^- (CO2 dissolved in water reduces pH and enhances dissolution)
  • Congruent Dissolution (Simple solution):
    • A mineral dissolves into solution without precipitation of another mineral.
    • Typically involves meteoric water and readily affects calcium carbonate and evaporites.
    • Bonds are destroyed, releasing ions into solution.
    • CO2 dissolved in meteoric water increases dissolution (see above).
  • Incongruent Dissolution (Hydrolysis):
    • Breakdown of silicates by acids with release of cations but not complete dissolution of the original mineral.
    • Other minerals (typically clays) form from the released metallic cations.
  • Oxidation:
    • Reactions involving loss of electrons; common for iron (Fe) and manganese (Mn) minerals.
    • Example: ext{Fe}^{2+}
      ightarrow ext{Fe}^{3+} + e^- ext{ (in the presence of O}_2 ext{)}
  • Hydration/Dehydration:
    • Addition/removal of water to the crystal structure of a mineral, producing a new mineral and volume change.
    • Volume change causes physical disruption of surrounding rocks.
    • Examples: hematite to goethite transformation.

Weathering Products

  • At weathering sites, secondary minerals develop.
  • The dominance of particulate residues depends on the relative stability of minerals present.
  • Soluble constituents are transported by water to final basins.

Weathering: Practical Examples and Progression

  • Feldspar Weathering (as an example of chemical weathering):
    • Intact rock containing feldspar; chemical weathering weakens rock and increases surface area, speeding up the weathering process.
    • Result: rock breaks into loose grains; feldspar converts to clay; quartz remains as quartz grains; subsequent rounding of quartz grains as they are transported.
  • Visualization: Quartz persists while feldspar weathers to clay; over time, clays are removed from the surface in moving water.

Weathering Rates

  • Weathering rates are difficult to quantify, especially in ancient geologic systems.
  • Climate effects:
    • Physical weathering is more active in cold (freeze-thaw) and arid (salt wedging) climates.
    • Chemical weathering is greater in humid, hot climates.
  • Slope effects:
    • Weathering is more effective on gentle slopes where water is retained in the soil.
  • Silicate rocks: weathering partly predictable based on mineral stability; broader references exist (e.g., Geology Cafe).

Differential Weathering

  • Different rock types weather at different rates due to mineralogy and rock strength.
  • Example: Weak shale vs. strong sandstone show differential weathering patterns.

Submarine Weathering

  • Halmyrolosis: alteration of sediments through reactions with seawater.
    • Submarine weathering of basalts around ocean ridges results in leaching and hydration of basalts and changes seawater composition due to ion exchange.
    • Low-temperature alteration occurs as seawater infiltrates fractures in the ocean crust (up to 2–5 km).
    • Olivine weathers quickly to clays, which also become hydrated.
    • Hydrothermal vents release hot waters containing suspended fine-grained minerals which precipitate minerals including pyrite and chalcopyrite.
  • Scale of process: The entire ocean circulates through ocean-floor hydrothermal systems over millions to tens of millions of years.

Submarine Life and Weathering Context

  • Crysomallon squamiferum (scally-foot gastropod): a snail that lives around black smokers and is armored with iron sulfides, illustrating biological interactions in submarine environments.

Erosion, Transport, and Sediment Deposition (Glaciers and Rivers)

  • Glaciers erode and transport rocks, contributing to erosion and transport processes.
  • Transported sediments accumulate in the ocean.
  • Rivers collect and transport sediments, linking erosion to global sedimentary cycles.