Continental Drift and Plate Tectonics: A Deep Dive

Background to the Land of Continental Drift

During the nineteenth and twentieth centuries, the prevailing theory was contractionism. It proposed that the Earth, as it cools, contracts, resulting in large folds of the crust. This was later referred to as fixism.

The Wilson Cycle

The Wilson Cycle describes the cyclical process of ocean basin formation and destruction. It consists of the following stages:

1. Continental Fragmentation: Begins with the fragmentation of a continent by a hot spot. Magma from the mantle rises, causing a bulging of the lithosphere.
2. Formation of a Rift Valley: The lithosphere breaks, forming parallel faults. Central blocks sink by gravity, creating the Rift Valley.
3. Formation of a Young and Narrow Sea: Magma ascends from the mantle and solidifies at the bottom of the rift, creating new oceanic crust.
4. Extension of the Ocean Floor: The oceanic crust cools, becomes more rigid, and grows denser. Seas continue to open up into large oceans (like the Atlantic).
5. Rupture of the Mixed Plate and Subduction of the Oceanic Plate: The destructive limits stop being generated one or two blocks inland from the dorsal. This causes the rupture of the mixed layer, forming a continental and a denser oceanic plate. The oceanic plate will sink under the continental plate, penetrating and being destroyed in the mantle.
6. Continental Collision (Obduction): If the subducting oceanic plate is part of a mixed plate, convergence will culminate with a clash between two blocks with continental crust (obduction).

Plate Tectonics

The Earth's lithosphere is divided into rigid blocks (plates) that fit together. These plates can be classified as follows:

• Oceanic: Composed of oceanic crust (e.g., Pacific, Philippines).
• Mixed: Composed of both continental and oceanic crust (e.g., Eurasian, Indo-Australian).
• Continental: Composed of continental crust (e.g., Iranian).

Plate sizes vary:

• Large (100-150 km²): e.g., American, South American.
• Medium (20-60 km²): e.g., Eurasian, African.
• Small (10 km²): e.g., Nazca, Arabian.

Plate Boundaries

Divergent or Constructive Boundaries

These boundaries correspond to mid-ocean ridges and have the following characteristics:

• Rugged submarine ridges (1000-4000 km wide).
• Significant height (up to 3 km).
• Extensive length (up to 64,000 km).
• Formed by volcanic rocks.
• Intermittent volcanism.
• Moderate seismic activity.
• A rift valley (a deep valley where plates separate) is located in the center.
• The axis is interrupted by transform faults, forming sections of plates.

Convergent or Destructive Boundaries

These boundaries are characterized by subduction, where one plate moves beneath another. The deepest point is known as the Benioff zone, a plane that penetrates the interior of the other layer.

• Continental-Oceanic: An oceanic plate sinks beneath a continental plate because it is denser. The area where subduction occurs is covered by the ocean and creates a trench (e.g., the Andes). Characteristics of oceanic trenches include:
  • Deep depressions.
  • Maximum depth.
  • Elongated and narrow shape.
  • Variable width.
  • Sediments are deformed due to the pushing plates, forming an accretionary prism.
  • Significant volcanic and seismic activity.
  • Magmatism is generated due to increased temperature. Materials at a certain depth are subjected to high temperatures and pressures, fusing with the oceanic and continental lithosphere, creating volcanic arcs.
• Oceanic-Oceanic: One plate is subducted beneath the other, creating a trench and an insular island arc. Island arcs are strings of volcanic islands that form on the edge of the subducting crust (e.g., Japan).
• Continental-Continental: Both plates have the same density, so neither subducts completely. The subduction of oceanic lithosphere can initially cause the formation of a volcanic island arc by the fusion of the subducting rocks. The collision of the two continental blocks (obduction) forms an intracontinental mountain range (e.g., the Himalayas).

Transform or Conservative Boundaries

At these boundaries, lithosphere is neither created nor destroyed. Plates slide past each other laterally along a transform fault. This movement, in the same direction but opposite senses, creates friction in the lithosphere, causing frequent earthquakes. Seismicity is very significant at these boundaries.