Theory of Plate Tectonics

Overview of the Theory of Plate Tectonics

The Theory of Plate Tectonics is a fundamental concept in Physical Geography that explains the forces at work along the boundaries of the Earth's crustal plates. It describes a dynamic Earth where the outer shell is broken into distinct segments that move relative to one another. The modern theory is considered a unified model, composed of two predecessor theories developed in the 20th century:

• The Theory of Continental Drift: Proposed by Alfred Wegener in 1912.

• Sea Floor Spreading Theory: Proposed by Harry H. Hess in 1960.

Academic Assessment and Marking Criteria (2019 Marking Scheme)

For university-level or state examination preparation (based on the 2019 3C Plate Tectonics module), the following criteria are essential for a comprehensive discussion on the theory of plate tectonics:

• Total Marks: The question is typically valued at 30 marks.

• Structure: Evaluation consists of 15 Significant Relevant Points (SRPs).

• Theorist/Theory Credit: One SRP is awarded for naming a specific theorist (e.g., Wegener or Hess) or their specific theory.

• Geographical Location: One SRP is awarded for providing a relevant geographical location used within the discussion.

• Diagrams:     * One SRP is awarded for a relevant labelled diagram. Note: A diagram without labelling receives 0 marks.     * Up to two additional SRPs (2 x SRPs) can be earned for additional relevant information provided on a labelled diagram, provided this information is not already present in the written account.

• Context: The question is not tied specifically to the geography of Ireland.

1. The Theory of Continental Drift (1912)

Alfred Wegener, a German meteorologist, proposed the Theory of Continental Drift in 1912. He hypothesized that the Earth's continents were not stationary but were once part of a single, massive landmass.

• Pangaea: Wegener proposed that around $200 \text{ million years}$ ago, all continents were joined together in an original supercontinent called Pangaea.

• Breakup of Pangaea: He believed Pangaea eventually split into two smaller supercontinents:     * Gondwanaland: The southern landmass.     * Laurasia: The northern landmass.

• Drift: Over millions of years, these landmasses drifted into their current positions across the Earth's surface.

Evidence for Continental Drift

Wegener was the first scientist to support his theory with empirical evidence, though he lacked a mechanism for the movement.

• Matching Coastlines and Rock Types:     * The shapes of coastlines, specifically the east coast of South America and the west coast of Africa, fit together like pieces of a jigsaw puzzle.     * Field studies confirmed that these matching coastlines also share the exact same rock types and the same geological age.

• The Distribution of Identical Fossils:     * Fossils of the Mesosaurus, an ancient freshwater reptile, were discovered in both Brazil and South Africa.     * Since the Mesosaurus was a freshwater creature and could not have swum across the vast, salty Atlantic Ocean, its presence on both continents suggests they were once joined.

• Matching Mountain Ranges:     * The Appalachian mountain range on the east coast of the United States and the Caledonian mountain range (running through Northern Ireland, Scotland, and Scandinavia) form a continuous, unbroken line when the continents are reconstructed.     * These ranges also share a similar rock composition, specifically granite.

Historical Note: Wegener could not explain the physical forces powerful enough to move entire continents. He died in 1930 during an expedition to Greenland while attempting to find definitive proof for his theory.

2. Sea-Floor Spreading Theory (1960)

In 1960, Harry H. Hess, an American geologist, proposed the Sea-Floor Spreading Theory. This theory provided the missing physical mechanism that Wegener’s theory lacked.

• Mechanism of Development: The theory proposes that the ocean floors are being pulled apart along massive cracks centered on mid-oceanic ridges.

• Conveyor Belt Motion: New oceanic crust is continuously created at these ridges and spreads away from them in a motion similar to a conveyor belt.

• Magma and Lava: Hess believed that magma rose from the Earth's interior through these cracks. Once it reached the surface, it became lava, spread across the ocean floor, cooled, and solidified into new crust.

• Conservation of Crust: This process does not mean the Earth is expanding. Instead, the crust is being destroyed at subduction zones at the same rate it is being created at mid-oceanic ridges.

Proofs of Sea-Floor Spreading

• The Age of the Sea Floor:     * Rocks are youngest along the axis of the mid-oceanic ridges.     * Rock age increases progressively and the depth increases the further one moves away from the ridge.

• Specific Examples:     * The ocean floor near the Mid-Atlantic Ridge is approximately $10 \text{ million years}$ old.     * The ocean floor closer to the continental margins of North America and Europe is approximately $180 \text{ million years}$ old.

• Volcanic Islands: Younger volcanic islands, such as Iceland, are found directly near these ridges, further proving that geological activity is concentrated there.

3. The Unified Theory of Plate Tectonics (1968)

In 1968, researchers Vine and Matthews combined the Theory of Continental Drift and the Sea-Floor Spreading Theory to establish the comprehensive Theory of Plate Tectonics.

• Structure of the Lithosphere: The theory states that the Earth's crust (lithosphere) is broken into eight major plates and several minor plates.

• Driving Force: Convection Currents: The movement of these plates is caused by convection currents, which are the circular movements of magma within the mantle. These currents act as a thermal engine for plate movement.

• Rate of Movement: Plates move at an average speed of approximately $3\text{ cm/year}$.

• Plate Composition: Plates can be classified as either Continental or Oceanic.

• Tectonic Activity: Scientists observed that major tectonic events, such as earthquakes and volcanic eruptions, occur primarily at plate boundaries (fault lines).

Major Tectonic Plates Identified

• Pacific Plate

• North American Plate

• South American Plate

• Nazca Plate

• Antarctic Plate

• African Plate

• Arabian Plate

• Eurasian Plate

• Indo-Australian Plate

• Philippines Plate

Types of Plate Margins

• Destructive Margin: Where plates move toward each other and one is destroyed (subduction).

• Collision Margin: Where two continental plates collide, often forming mountain ranges.

• Constructive Margin: Where plates move apart, allowing new crust to form (e.g., Mid-Atlantic Ridge).

• Conservative Margin: Where plates slide past each other without creating or destroying crust.

Summary of Theoretical Progression