Earth’s Internal and External Geological Processes

Posted on Dec 21, 2024 in Geology

The geosphere is an active system. As well as using external energy from the sun, it can generate energy internally and transmit it to the surrounding environment. It is also a system in dynamic equilibrium because, despite being in constant change (rise/denudation), the geological processes maintain internal and external balance, respectively. External geological processes are powered by solar energy and gravitational attraction.

Internal Processes

Internal processes are those caused by Earth’s internal energy. They have their origin in the interior of the Earth and are often manifested on the crust. The theory that provides a coherent and comprehensive explanation of these internal processes is the theory of plate tectonics. It proposes that the lithosphere is divided into a series of fragments, called lithospheric plates, that move horizontally with respect to one another, driven by mantle convection currents produced by the heat from the core.

Tectonic Phenomena Associated with Plate Boundaries

  • Constructive Borders: They are divergent edges of separation and are called oceanic ridges. They are underwater mountain ranges that limit a crack (rift) from which mantle material emerges.
  • Passive Borders: They are areas where plates slide past each other laterally. These edges do not generate or destroy oceanic lithosphere. They present fractures, called transform faults.
  • Destructive Borders: The boundary that separates the continental crust from the oceanic crust is an area whose characteristics contribute to the fracture itself, becoming an active margin. In it, subduction of the oceanic lithosphere, which has a greater density than the mainland, occurs beneath it.

The set of upward and downward movements of mantle material is called “convection currents” and would be caused by the planet’s internal heat and gravity.

External Processes

External processes take place through the combined action of two types of energy: solar heat and gravity.

Denudation covers all the processes (weathering, erosion, and transport) that cause the destruction of the relief.

Weathering originates materials of more or less large size and substances in solution from the bedrock. There are two types of weathering, physical and chemical, although they occur together:

  • Physical Weathering: Involves the fragmentation of rocks without chemical changes originating in minerals. The following processes are distinguished:
    • Decompression: As rocks are eroded superiorly, there is a release of weight which reduces the lithostatic pressure bearing on the rocks below, causing them to break into slabs or planes parallel to the topographic surface.
    • Gelifraction: When water introduced through cracks freezes, ice is formed. When experiencing an increase in volume, it exerts a broad effect on wedge cracks, splitting the rocks.
    • Thermoclastia: Successive expansion and contraction caused by heat and cold create tensions in the minerals that form rocks, which eventually disintegrate.
    • Haloclastia: As a result of crystal growth in areas near the sea with warm weather between the cracks of rocks, there is an increase in volume produced by breaking them.
    • Biological Weathering: The action of roots causes the opening of cracks and the movement of blocks.
  • Chemical Weathering: Water, oxygen, carbon dioxide, and other substances cause chemical reactions in the minerals of the rocks that contribute to their decomposition. Water is the medium in which chemical weathering occurs; it is the leading substance in solution and allows them to come into contact with the minerals in the rocks.
    • Dissolution: Water has great solvent power and can dissolve some mineral rocks, such as rock salt, gypsum, and limestone. Carbonation is a special case of dissolution, which involves the transformation of calcium carbonate into calcium bicarbonate by the action of carbon dioxide in the water.
    • Hydrolysis: The decomposition of aluminum silicate minerals by the action of hydrogen in water.
    • Oxidation: Oxygen dissolved in water causes the oxidation of certain minerals, such as those that are rich in iron.
    • Hydration: Some minerals and rocks have the ability to incorporate water into their crystal lattice, causing an increase in volume that may have implications for construction.
    • Biochemical Weathering: Some living organisms secrete acidic substances that allow their penetration into the rocks.

Denudation Dynamic Systems

The water cycle and air movement set in motion a number of agents that produce external geological processes of transport, erosion, and sedimentation.

Erosion is the dragging of materials and wear during transport. The act of modeling depends on the lithology, topography, climate, and vegetation.

Modeling Processes

  • Flux: When the clay materials of the slopes are saturated with water, they have properties of a viscous fluid and slide down the slopes, forming mudflows.
  • Landslides: During the rainy season, water infiltration increases the weight of some layers of rock, reducing the internal friction coefficient, which causes layers to slip upon lower ones.
  • Creep: A slow gravity decrease is the result of the rising effect of expansion (heat), shrinkage (cold), and gravity. It affects the outer layer.
  • Solifluction: A movement resulting from the combination of flow and creep.
  • Detachments: Individual materials (rock fragments of smaller or larger size) drop and not en masse, as in previous processes.

Formation of Rocks and Geochemical Cycling

Magmatic or Igneous Processes

These processes include those resulting from the solidification of magma or the crystallization of minerals from melts at high temperatures. Magmatic rocks may be volcanic, whether resulting from the consolidation of lava at the surface by rapid cooling (these rocks have a very low degree of crystallization), plutonic (which is abundant in crystals because they result from the consolidation of magma at depth, slowly), or philonian (which are consolidated into veins causing cracks and fractures).

Sedimentary Processes

Processes that result in sedimentary rocks, so called because they come from deposited material from the weathering and erosion of the rocks of the Earth’s surface due to the weather and geological agents operating externally, driven by solar energy and gravity.

Metamorphic Processes

This group includes all those processes involving the transformation of existing rock (usually sedimentary) without significant changes in the chemical composition and the effects of pressure and temperature within the lithosphere. The latter, in conditions of extreme temperature and pressure, can be fused (anatexis), and magmatic rocks rise.

Risks Arising from Internal Processes

Active volcanism is associated with interactions at plate boundaries. Volcanic hazards generate large economic losses, although the loss of life is relatively low.

  • Risks caused by the emission of solids: The danger comes from the impact of pyroclastics.
  • Risks arising from the emission of liquids: The moving lava flows from the volcano crater to the base.
  • Risks associated with the emission of gaseous products: Hot clouds of gas and fine solids are ejected in explosive eruptions at speeds of hundreds of km/h.
  • Mudflows or lahars: When snow melts on volcanoes, mudflows occur that can produce devastating effects.
  • Volcanic Subsidence: Debris avalanches are occurring as collapsed volcanic cones whose danger increases with the slope of the volcano.
  • Magmatic-phreatic eruption: As magma flows through an aquifer or seawater enters the caldera of a volcano, the internal pressure increases due to the sudden formation of steam, and the eruption multiplies in violence.
  • Tsunamis: Giant waves are produced by volcanic eruptions and earthquakes or major disasters in coastal areas.

Earthquakes are paroxysmal (violent) movements that occur on the surface of the Earth’s crust due to the production of seismic waves within the crust due to tectonic processes (plate boundaries, faults) or volcanic activity. Risks from:

  • Collapse of buildings, which produce the highest percentage of deaths and economic damage, destruction of public buildings, roads and railways, bridges, dams, nuclear power plants, etc.
  • Fires caused by broken electric cables and gas pipes.
  • Mudslides and falling slopes.
  • Tsunamis, alterations of aquifers and river channels.

Salt Diapirism

Salt masses are found sandwiched between layers, and because of their lower density, they tend to rise, causing instability in the field that carries some risks: damage to buildings and roads and sinking by dissolving the salt.

Risks from External Processes

Landslides

Water infiltration on slopes can cause landslides or mass movements of soil. These movements, in many cases, are fast and may cause hazards. All these risks are called gravitational risks.

Avalanches

Avalanches are movements of snow on slopes. Sometimes they carry with them rocks and trees, which helps increase their danger. The effect is triggered by the snow, by vibrations caused by noise, earthquakes, explosions, and rockfalls located above.

Vertical Movements, Subsidence, and Collapse

The difference between subsidence and collapse is that subsidence movements are slow, and instead, collapses are relatively rapid.

Expansive Soils

Expansive soils are soil or sedimentary rock with plenty of clay or plaster, which increase in volume by absorbing water. The risk is in the process of ground swelling and deflation, which produce increases in volume and subsequent reduction with cracks that cause breakage of pipes, unstable buildings, slope deterioration, and deformation of pavements.