Earth’s Surface Processes: Weathering, Erosion, Sedimentation, and Soil

Posted on Dec 2, 2024 in Geology

External Geodynamic Processes

External geodynamic processes occur on the Earth’s surface, driven by gravitational forces and solar energy. Important geological agents include rain and wind. These processes lead to weathering, erosion, transportation, and sedimentation.

Weathering

Weathering is the set of changes experienced by materials of the lithosphere in contact with the atmosphere, hydrosphere, or biosphere.

Physical Weathering

  • Fissuring: Rupture or disintegration of rocks due to tectonic stress.
  • Gelifraction: Breakage of rocks produced by ice.
  • Thermoclasty: Disintegration caused by temperature fluctuations.
  • Bioclasty: Breaking of rocks produced by living beings.

Chemical Weathering

Chemical weathering changes the composition of rocks through processes like:

  • Dissolution: Occurs due to water action.
  • Oxidation-Reduction: Involves the loss and gain of electrons.
  • Carbonation: Water containing carbon dioxide.
  • Hydrolysis: Rupture of the mineral structure by water.
  • Hydration: Changing a mineral’s phase by introducing water into its structure.

Erosion and Transportation

Erosion involves mobilizing materials weathered from rocks by water, ice, or air. Transportation is the movement of these dispersed materials from one place to another. Main transport agents include wind, rivers, sea waters, glaciers, and gravity. Forms of transportation include dissolution, flotation, traction (rolling or saltation), and suspension.

Sedimentation

Sedimentation is the settlement of materials in sedimentary basins, forming layers called strata. Forms of sedimentation include deposition of larger materials and decantation of dissolved materials.

Soils

Soils are materials formed from the alteration of the surface layer of the crust, a process called pedogenesis.

Factors Involved in Pedogenesis

  • Bedrock: Provides mineral components.
  • Time: Formation takes hundreds to thousands of years.
  • Climate: Influences weathering and vegetation types.
  • Living Organisms: Contribute organic matter and alter minerals.
  • Topography: Affects soil depth.
  • Human Action: Can rapidly remove soils.

Soil Composition

  • Inorganic Material: Derived from weathered bedrock.
  • Organic Matter: Living organisms and organic debris. Types include raw humus and young humus.

Good quality soil (loam) contains balanced proportions of components and pores between solid particles.

Soil Profile

  • A Horizon: “Cleaning” zone where salts are washed down.
  • B Horizon: “Precipitation” zone where salts accumulate.
  • C Horizon: Weathered bedrock.

Soil Types

  • Azonal Soils: Found in any climate and latitude, with little maturity.
  • Intrazonal Soils: More developed than azonal soils, including calcimorphic and siliceous soils.
  • Zonal Soils: Related to climate and distributed by latitude (e.g., tundra soils, black soils, gray-brown earth, red-Mediterranean soils, and soils of warm, humid climates).

Soil Conservation

Soil contamination can be minimized by avoiding excessive use of fertilizers and herbicides. Preventing soil loss is crucial.

Mineral Deposits of Sedimentary Origin

Mineral deposits are accumulations that can be economically profitable.

Fields of Alteration

Formed by water infiltration and weathering.

Mechanical Deposition

  • Eluvial: Formed by detrital materials.
  • Alluvial: Formed by river-transported weathered materials.

Chemical Deposition

Occurs through chemical precipitation due to variations in health, temperature, and element concentration.

Biochemical Deposition

Sedimentation caused by the metabolic action of organisms.

Diagenesis

The set of physical, chemical, and biological processes that transform sediments into sedimentary rocks. Diagenesis results in a sedimentary rock with three main constituents:

  • Skeleton: Framework of particles.
  • Matrix: Fine particles filling pores.
  • Cement: Material precipitating between grains.

Sedimentary Rocks

Characteristics:

  • Stratification: Overlapping horizontal layers (strata).
  • Structure: Geometric arrangement of components.
  • Texture: Internal order of rocks.
  • Composition: Related to formation conditions (oxidation, reduction, etc.).

Types of Structures

  • Internal Structures:
    • Massive parallel (high energy).
    • Ripple marks (water and wind flow).
    • Cross-stratification (slanting sheets).
    • Graded bedding (grain size variation).
  • Surface Structures:
    • Desiccation cracks (moist and dry conditions).
    • Current marks (erosion crevices).
    • Erosion scars.
  • Deformation Structures:
    • Tectonic origin.
    • Load structures (dense materials on soft ones).
    • Contorted structures or slumps (strata slippage).
  • Organizational Structures:
    • Reefs or stromatolites (calcareous skeletons).
    • Tracks, footprints, and bioturbation (organism activity).

Classification of Sedimentary Rocks

  • Detrital Sedimentary Rocks:
    • Conglomerate (pebbles).
    • Sandstone (sand).
    • Claystone (clay).
  • Non-Organogenic Detrital Sedimentary Rocks:
    • Carbon (plants).
    • Oil (hydrocarbons).
    • Other organic debris.
  • Chemical and Biochemical Sedimentary Rocks:
    • Phosphates (phosphorite).
    • Silica (quartz, silex).
    • Carbonates (limestone).
    • Ferruginous and aluminous (iron and aluminum, bauxite).
    • Evaporites (gypsum and sylvite).

Uses of Sedimentary Rocks

  • Industrial Use: Quartz arenites for glass.
  • Construction: Gravel.
  • Energy Production: Coal and oil.