Plate Tectonics: Earth’s Dynamic Geological Processes

Posted on Aug 24, 2025 in Geology

Plate tectonics is currently the theory that best explains the dynamics of Earth’s geological processes.

Theory of Plate Tectonics

This theory proposes the dynamics of the Earth based on several key factors:

  • Ocean floor
  • Lithospheric plates
  • Plate boundaries
  • Lithospheric dynamics

Ocean Floor Dynamics

Oceans cover 71% of the planet’s surface. Beneath the waves lies an underwater relief known as the oceanic ridge, where sediments are scarce.

Characteristics of the Oceanic Lithosphere

  1. Youthful Age: Oceanic lithosphere is relatively young, typically no older than 200 million years.
  2. Ridge Formation: Ridges are composed of recent basaltic rocks and are characterized by transform faults.
  3. Age Progression: The oceanic lithosphere becomes progressively older as it moves away from the ridge.
  4. Sediment Accumulation: Sediment layers are thicker in areas further from the ridge.

Dynamics of the Ocean Floor

Oceanic lithosphere is continuously created in some areas and destroyed in others, ensuring the Earth’s size remains constant.

  • Creation of Oceanic Lithosphere:

    The oceanic lithosphere is formed at mid-ocean ridges, spreading outwards to both sides.

    • Mid-ocean ridges represent the birthplaces of new oceanic lithosphere.
  • Destruction of Oceanic Lithosphere:

    Oceanic lithosphere is consumed in subduction zones, descending into the mantle.

    • Subduction zones balance the creation of oceanic lithosphere at ridges.

Lithospheric Plates

The lithosphere, Earth’s outermost rigid layer, is divided into several plates:

  • Large Plates: Eurasian, African, Indo-Australian, Pacific, North American, South American, and Antarctic.
  • Medium-Sized Plates: Nazca, Caribbean, Cocos, Arabian, and Philippine.

Plate Boundaries

(Content for Plate Boundaries is not provided in the original document.)

Lithospheric Dynamics

Lithospheric plates move over the ductile materials of the upper mantle. The primary drivers of this movement are thermal energy and gravity:

  • Thermal Energy: Generates convection currents within the mantle.
  • Gravitational Energy: Activates plate movement at ridges (ridge push) and in subduction zones (slab pull).

Lithospheric Movements and Geological Cycles

The movements of the lithosphere can be explained by various geological models.

The Wilson Cycle

The Wilson Cycle describes a sequence of plate tectonic stages, categorized into tensional processes (ocean formation) and compressional processes (orogen formation). The main stages include:

  1. Initial Pre-Drift Extension: Characterized by the thinning of continental lithosphere.
  2. Rift-to-Drift Phase: Involves the opening of an oceanic basin and the formation of a rift zone.
  3. Seafloor Spreading: The distance between continental margins increases.
  4. Basin Widening: A new ocean develops and expands.
  5. Subduction of Oceanic Lithosphere: The basin begins to close due to a subduction zone.
  6. Continent-Continent Collision: The ocean disappears through an obduction process, leading to continental collision.

Opening of an Ocean Basin

Pangea was the supercontinent that began to divide through the opening of new ocean basins. The stages of ocean opening are:

  1. Formation of a Thermal Dome:
    • Generation of a thermal plume (rising magma) in the upper mantle.
    • The overlying lithosphere rises and arches.
  2. Formation of a Continental Rift:
    • Tensional stress causes faults (fractures), and the central area collapses.
    • A valley or rift forms, bounded by elevated ridges.
  3. Creation of Oceanic Lithosphere:
    • Magma erupts and solidifies, forming new crust.
    • The valley widens, and the continental blocks separate.
    • A narrow sea begins to form.
  4. Development of an Ocean:
    • If the extensional process continues, a growing ocean basin is created.

Orogenesis: Mountain Building

Orogenesis is the process of mountain range (or orogen) formation, typically involving folding and faulting. It is driven by different processes:

  • Accretionary wedge formation
  • Magmatism
  • Overlapping of two continental crusts

Orogens can be classified as accretionary or collisional.

Accretionary Orogenesis

Accretionary orogens form in subduction zones where oceanic lithosphere descends beneath continental lithosphere. The stages include:

  1. Formation of an Accretionary Wedge: Occurs in the subduction zone.
  2. Magmatism and Metamorphism: Involves the generation of magma, leading to magmatic and metamorphic rock formation.

The Andes Mountains are a prime example of accretionary orogens.

Collisional Orogenesis

Collisional orogens result from subduction zones where one continental lithosphere collides with another. The stages are:

  1. Oceanic Subduction: The oceanic basin disappears as one plate subducts.
  2. Closure of the Ocean Basin and Continental Collision: This occurs through an obduction process, leading to the direct collision of continental landmasses.