Geological Processes Shaping Earth’s Surface

Posted on Jun 22, 2025 in Geology

External Geodynamic Processes

External geodynamic processes occur on land or close to it. External geological agents include: rain, rivers, wind, sea, and ice. These agents produce weathering, erosion, transportation, and sedimentation. Moreover, they redistribute land, shaping the relief, and transform sediments into sedimentary rocks.

Weathering: Transformation of Earth’s Surface

Weathering is the set of changes experienced by lithospheric materials in contact with the atmosphere, hydrosphere, or biosphere. These changes can be physical or chemical.

Two Types of Weathering:

  • Physical Weathering: Rock Disintegration

    Physical weathering involves cracking, breakage, or disintegration of rocks due to compressive stresses, relaxation efforts, temperature variations, or the action of living organisms. Tectonic stresses act comprehensively, producing folds or fractures.

    • Gelifraction: The breakage of rocks produced by ice.
    • Thermoclasty: The disaggregation of rocks subjected to constant temperature fluctuations.
    • Bioclasty: The breaking of rocks produced by living organisms.
  • Chemical Weathering: Mineral Alterations

    The key players producing these alterations are water, oxygen, and carbon dioxide. There are five types:

    • Solution: Occurs through the action of water, which dissolves minerals like rock salt and halite.
    • Oxidation-Reduction: Oxidation is the loss of electrons, and reduction is the gain of electrons. This occurs when oxygen or hydrogen bind to minerals or rocks containing metallic elements.
    • Carbonation: Occurs when water containing CO2 reacts with compounds like limestone, transforming insoluble limestone into soluble calcium bicarbonate.
    • Hydrolysis: Rupture of the mineral structure due to the action of OH- and H+ ions from water.
    • Hydration: Involves the modification of a mineral phase by introducing water into its structure.

Erosion: Mobilization of Earth Materials

Erosion is the mobilization of materials (produced as a result of rock weathering) by water, ice, or air. Erosion depends on the geological characteristics of the rock type and the existing relief. Erosion processes include homogeneous abrasion, wear, corrosion, and etching.

Transport: Movement of Sediments

Transport is the movement of dispersed materials from one place to another. It depends on both the geological agents and the materials themselves. Materials undergo wear during transport: they may become rounded, rolled, polished, or striated, and accumulate in selected sedimentary basins based on their densities and sizes.

The Main Transport Agents Are:

  • Wind: Can carry materials in contact with the ground surface by slip, rolling, saltation, or suspension.
  • Rivers: Transport materials either in contact with the riverbed (bedload) or suspended within the water body.
  • Sea: Materials are transported by waves, currents, or tides, similar to river waters.
  • Glacial Ice: Materials on the ground surface are covered by ice and dragged along the bed, at the front, and at the margins of the glacier tongue.
  • Gravity: Transports materials by slip when reliefs are sloped. If the slip is slow, it is called flow; if it proceeds by small upward and downward movements of surface particles, it is called creep.

Main Forms of Transport:

  • Solution: Materials, such as ions, travel dissolved within the water.
  • Flotation: Materials are carried on the surface of the transport agent.
  • Rolling, Traction, or Saltation: Materials travel in contact with the bed, turning, sliding, or moving through small elevations and falls.
  • Suspension: Particles are carried within the transport medium.

Sedimentation: Formation of Sedimentary Layers

Sedimentation is the settlement of materials in sedimentary basins. During sedimentation, materials accumulate in layers called strata.

Sedimentation Methods:

  • Deposition: Materials travel larger distances by rolling or saltation drag.
  • Decantation: Materials in suspension settle when gravitational forces exceed the forces that tend to keep particles within the transport medium.
  • Precipitation: Occurs when the concentration of dissolved materials in solution exceeds their solubility limit.