Lecture 4

Plate Tectonics: Overview

Key Definitions

• Tectonics: Movements of Earth’s lithosphere (crust + upper mantle).

• Plate Tectonics: Movement of discrete segments of Earth’s lithosphere in relation to one another.

• Continental Drift: Theory that continents move horizontally over the Earth's surface.

Major Features of Earth's Surface

• Observations of major features:

  • Look for seafloor characteristics and formation methods.

  • Investigate disparity between eastern and western continental sides.

  • Understand the formation of underwater mountain ranges in oceans.

  • Explore explanations for high elevations in areas like the Himalayas.

  • Analyze the relationships between submarine trenches and geological activity (earthquakes/volcanoes).

  • Comprehend how geographical features like the Red Sea originated.

History of Plate Tectonics Theory

• Initial skepticism and later acceptance of plate tectonics in geoscience.

• Importance of scientific method in the development of the theory.

Learning Outcomes

After this lecture, one should be able to:

• Describe evidence supporting the idea that continents were once joined.

• Define what a tectonic plate is.

• Distinguish between the theory of plate tectonics and the concept of continental drift.

• Explain the theory of plate tectonics and the driving forces behind it.

• Describe observations confirming the theory of plate tectonics.

Evidence for Continental Drift

• Continental Drift Hypothesis (1915): Proposed by Alfred Wegener, asserting that Pangaea existed around 180 million years ago.

• Key Evidence:

  • Geometrical fit of continental edges (e.g., South America and Africa).

  • Distribution of similar fossils across disparate continents (e.g., Glossopteris, Mesosaurus, Lystrosaurus).

  • Geological similarities in rock formations in South America and Africa.

  • Connectivity of mountain chains across continents.

  • Analysis of historical climate indicators (e.g., glacial deposits).

Fossil Evidence

• Unique fossils found on separate continents show previous land connections.

• If continental drift didn't occur:

  • Species evolved separately, contradicting Darwin’s evolutionary theory.

  • Alternative migration theories (e.g., aquatic movement) lack support.

Geological Similarities

• Identical rock sequences in Brazil and South Africa suggest simultaneous formation under equivalent conditions.

• This anomaly indicates prior proximity of these landmasses.

Glacial Evidence

• Glacial marks suggest prior land links among continents, with orientations indicating interconnectedness.

• Glacial deposits found in now arid Southern regions imply significant climatic shifts after continental separation.

Paleoclimate Indicators

• Evidence of tropical plant fossils in cold regions (e.g., Antarctica) and glacial formations in tropical climates indicates significant climate changes.

• Such indicators bolster the case for prior continental unity.

Shift from Continental Drift to Plate Tectonics

• Skepticism surrounding Wegener’s hypothesis due to a lack of a mechanism for continental movement.

• Rejection of the continental drift hypothesis led to its classification as a failed theory, while laying groundwork for plate tectonics.

• Renewed research in the 1950s provided substantial geological and oceanic evidence, revitalizing interest in continental displacement:

  • Recognition of ocean floor topography (mid-ocean ridges).

  • Studies on seismic activity patterns concentrated on oceanic terrains.

  • Documentation of magnetic field reversals and their long-term impacts.

  • Insights gained from drilling expeditions demonstrating patterns of ocean sedimentation.

Development of Plate Tectonics Theory

• Chronological contributions to the theory's formation:

  • Recognition of plate movements (1596).

  • Correlation between rock types and fossils across continents (1858).

  • Mapping of the Atlantic Mid-Ocean Ridge (1872).

  • Discovery of radioactive heat in Earth’s interior (1896).

  • Conceptualization of convective mantle drives (1927).

  • Mid-ocean ridge recognition (1953) and subsequent naming of oceanic spreading mechanisms (Harry Hess, 1962).

  • Establishment of tectonic plate-driven exhaustion of lithosphere at oceanic trenches, necessitating a balanced recycling process.

Tectonic Plates and Their Characteristics

• The lithosphere comprises approximately 15 major tectonic plates.

• Characteristics:

  • Plates fit together like interlocking puzzle pieces.

  • They consist of more than just continents, terminating where they meet other plates.

  • Located atop the upper mantle (asthenosphere).

Types of Plate Boundaries

• There are three key types of plate boundaries that define interaction characteristics (details will follow in future lectures).

  • Behavioral properties at boundaries are essential to understanding plate tectonics and are integral to geological studies.

Mid-Ocean Ridges and Sea Floor Spreading

• Hypothesized as zones for new oceanic crust formation.

• Observations confirm the following properties:

  • Mid-ocean ridges serve as underwater mountain belts and spreading centers.

  • Seismic waves propagate more slowly over ridge zones due to heat and potential partial melting.

  • Volcanoes beneath ridges corroborate crustal generation at these sites.

Age of Ocean Floors

• Young rocks near mid-ocean ridges age progressively moving away, substantiating sea floor spreading

• Average sediment thickness indicates general vitality and age of ocean basins (averaging 260 million years minimum).

  • Observations detail sediment deposition rates and continental crust transformations.

Paleomagnetism and Plate Movement

• Paleomagnetic studies reveal historical magnetic orientation correlations with age.

• Research suggests historical polarity reversals create magnetic stripes amidst mid-ocean ridges, a vital component for understanding plate movement.

Hot Spots and Their Geological Implications

• Hotspot volcanic activity indicative of tectonic progression:

  • Formation of island chains (e.g., Hawaii) due to hot mantle plumes.

  • As tectonic plates traverse over these plumes, aligned volcanic formation accounts for directional plate motion tracking.

Evidence Gathering and Contemporary Techniques

• Current methodologies including GPS installations track tectonic shifts accurately over time, highlighting relative plate displacements.

• Continued investigations elucidate movement rates across tectonic plate boundaries, contributing essential data toward understanding dynamic geological interactions.

Conclusion and Summary of Plate Tectonics

• The unifying theory of plate tectonics integrates frameworks supported by paleomagnetic data, geological observations, and fossil correlation.

• Ongoing discoveries continue to refine knowledge and understanding of Earth's surface transformations over geologic time, firmly establishing the dynamic nature of our planet's lithosphere.