Plate Tectonics: Understanding Earth’s Dynamic Processes

Posted on Jan 5, 2025 in Geology

Pangaea: The Supercontinent

Alfred Wegener, a German meteorologist, proposed in 1912 that the continents were once joined together in a single landmass called Pangaea. He theorized that the continents gradually drifted apart over what he believed to be a flat ocean bottom.

Evidence Supporting Continental Drift

Wegener used several lines of evidence to support his theory:

  • Fossil Matches Across Oceans: Similar fossils are found near coastlines on different continents.
  • Rock Types and Structures Match: Geological formations and rock types align across oceans.
  • Ancient Glacial Features: Evidence of simultaneous glaciation is present on multiple continents.
  • Geographic Fit: The shapes of South America and Africa appear to fit together like puzzle pieces.

Initial Rejection of Continental Drift

Many scientists initially rejected Wegener’s theory because he couldn’t provide a satisfactory explanation for the mechanism driving continental movement. There was no known force strong enough to move entire continents.

Supporting Evidence and the Rise of Plate Tectonics

Later, two scientists provided crucial evidence supporting Wegener’s ideas:

  • Arthur Holmes proposed that thermal convection in the Earth’s mantle could drive continental movement.
  • Harry Hess’s discovery of Mid-Ocean Ridges and the theory of seafloor spreading provided further evidence. Deeper earthquakes were also found near trenches.

Types of Plate Boundaries

There are three main types of plate boundaries:

  • Convergent: Plates move towards each other, and lithosphere is consumed. There are three subtypes:

    • Oceanic-Oceanic
    • Oceanic-Continental
    • Continental-Continental
  • Divergent: Plates move apart, and new oceanic lithosphere forms due to volcanic activity, creating new rock and crust. Rocks are very young near divergent boundaries.
  • Transform: Plates slide past each other horizontally, and lithosphere is neither created nor destroyed.

Examples of Plate Boundaries

  • Divergent: The Mid-Atlantic Ridge, where the Eurasian and North American plates are separating (e.g., Iceland).
  • Convergent: The Mariana Trench, where the Philippine and Pacific plates are colliding (e.g., Mariana Islands).
  • Transform: The San Andreas Fault Zone, where the North American and Pacific plates are sliding past each other (e.g., California).

Types of Convergent Boundaries

  • Oceanic-Continental Convergence: An oceanic plate collides with a continental plate, and the denser oceanic plate subducts beneath the continental plate, forming trenches.
  • Oceanic-Oceanic Convergence: Two oceanic plates collide, forming deep-sea trenches.
  • Continental-Continental Convergence: Two continental plates collide, forming mountain ranges.

Continental Breakup

Continents break apart due to divergent plate boundaries. As plates move in opposite directions, the land between them stretches and thins. Volcanic activity, caused by the internal heat from rock movement, can also contribute to the breakup and the formation of mountains. This process is currently happening in the East African Rift Valley in Chile.

Rock Ages and the Mid-Atlantic Ridge

The rocks on the East Coast of North America are significantly older than those along the Mid-Atlantic Ridge. This is because the rocks of North America have existed for millions of years, while the rocks at the Mid-Atlantic Ridge are continuously being formed by seafloor spreading.

Convection Currents

A convection current is the movement of heat within a non-conductor material (like air, water, or rock). In the Earth’s mantle, rock beneath the plates moves in a circular pattern. As it gets closer to the core, it heats up and rises. It then spreads out, cools, and sinks back down, creating a continuous cycle.

Evidence of Plate Tectonics

Continental Evidence (Proposed by Wegener)

  • Fitting of coastlines
  • Matching geological structures and rocks on both sides of the Atlantic Ocean
  • Fossil evidence (e.g., Mesosaurus)
  • Climatic evidence (glacial remains in South America, South Africa, India, Antarctica, and Australia)

Oceanic Evidence (From the Mid-1960s Onward)

  • Sonar profiles of the Atlantic Ocean revealed a vast relief in the center.
  • Age of the ocean floor: The youngest rocks are found at the mid-ocean ridges, and rocks get progressively older towards the coasts.

Sources of Earth’s Internal Heat

  • Radioactive Decay: The breakdown of radioactive isotopes within the Earth, releasing heat.
  • Residual Heat: Leftover heat from the Earth’s formation when it was molten.

The Future of the Red Sea

The Red Sea will likely widen in the future as mountains continue to form along the Mid-Ocean Ridge, pushing the Arabian Peninsula eastward. This could eventually lead to the closure of the Persian Gulf and the formation of new mountain ranges.

Radiation: A form of energy that travels at the speed of light.

Orogen: An active mountain range where earthquakes and volcanoes are found. There are two main types:

  • Subductive orogen (e.g., Andes)
  • Collision orogen (e.g., Himalayas)

Volcanoes are found in subduction zones.

Formation of the Pyrenees

The Pyrenees were formed by the Alpine orogeny, a mountain-building event caused by the collision of the Iberian and Eurasian tectonic plates (continental-continental convergence).

Formation of the Cascades

The Cascades are a mountain range in western North America, near the Pacific Ocean.

Main Causes of Earthquakes

  1. Convergent Boundaries: Friction between plates causes reverse faults.
  2. Compression: Continental-continental convergence leads to intense earthquakes (e.g., Southern Spain, Almeria).
  3. Divergent Boundaries: Less intense earthquakes occur along normal faults.
  4. Transform Boundaries: Two plates sliding past each other.

Magma Formation and Volcanoes

  1. Increased Temperature: Friction from subducting plates.
  2. Decreased Pressure: Causes rocks to melt.

Benefits of Plate Tectonics for Humans

  • Fertile Soil: The physical breakdown and chemical weathering of volcanic rocks create fertile soils.
  • Ore Deposits: Rising magma can cool and solidify beneath volcanoes, forming crystalline rocks (like granite and plutonic rocks) rich in minerals. In Spain, many minerals are found in the south of Sierra Morena.

Hot Spots

A hot spot is an area in the mantle, close to the core, where rocks melt due to high pressure, generating magma. Examples include Yellowstone, the Azores, and the Hawaiian Islands. Hot spots can lead to the formation of seamounts and volcanic islands.