Plate Tectonics – Science 10 Lecture

Motivation

• Real-world geologic examples provided to spark interest and contextualize plate tectonics:
  • The Island of Iceland – sits astride the Mid-Atlantic Ridge, showcasing active rifting and volcanism.
  • The Chilean Earthquake – reference to powerful subduction-zone seismicity along the South American plate margin.
  • Mt. Everest – the world’s tallest peak, created by continental collision between the Indian and Eurasian plates.

  • San Andreas Fault – a well-known transform fault in California, illustrating the lateral sliding of the Pacific and North American plates, often leading to significant earthquake activity.

  • Divergent Boundaries – regions where tectonic plates move apart, leading to the formation of new oceanic crust and features such as mid-ocean ridges.

Plate Tectonics Overview

• Historical framing: drifting of continents from the supercontinent Pangaea explained by Continental Drift, now unified under modern Plate Tectonics.
  • Earth’s crust is divided into moving plates; their interactions account for the geographic arrangement of continents/oceans and for geologic hazards.

Alfred Wegener & the Birth of Continental Drift

• German meteorologist–geophysicist who published the idea in 1915.

• Core proposals:
  • All continents were once united in a single landmass: Pangaea.
  • About 250\,\text{million years} ago, Pangaea began fragmenting; the pieces drifted to current locations.
  • Shorelines (e.g., South America–Africa) appear to “fit” like a jigsaw puzzle—visual clue that motivated the hypothesis.

Evidentiary Pillars Supporting Continental Drift

• Four primary categories of evidence:
  • Jigsaw Fit of the Continents.
  • Fossil Evidence.
  • Geologic (Rock/Mountain) Correlation.
  • Paleoclimate (Climate) Evidence.

Jigsaw Fit

• Coastlines—especially South America vs. Africa—exhibit complementary shapes.
  • Implies they were once physically connected before the Atlantic opened.

Fossil Evidence

• Identical fossils on now-distant continents indicate former proximity:
  • Mesosaurus (fresh-water reptile) in both South America and Africa.
  • Glossopteris (seed fern) in South America, Africa, India, Australia, Antarctica.

Geologic Evidence

• Matching rock formations & structural trends across oceans:
  • Appalachian Mountains correlate with Greenland, Ireland, Great Britain, Norway ranges.
  • Shared rock types, ages, deformation patterns require past continuity.

Climate Evidence

• Glacial grooves & till recorded in now-tropical or temperate zones – e.g., India & Africa, signaling former high-latitude positioning.

• Coal beds (require lush, swampy, tropical settings) discovered in Antarctica – showing it was once near the equator.

Initial Rejection & Path to Acceptance

• Wegener could not identify a convincing physical mechanism for drift; peers dismissed the hypothesis.

• Mid-20th-century breakthroughs supplied mechanisms:
  • Seafloor-spreading (Harry Hess, 1962).
  • Mantle convection concept (Arthur Holmes 1927, elaborated through 1968 experiments/models).
  • Magnetic striping on ocean floor (Vine–Matthews 1963) proving symmetrical spreading about mid-ocean ridges.
  • Transform faults & hot-spot plumes (J. Tuzo Wilson) demonstrating plate segmentation and intraplate volcanism.

Chronology of Key Contributions

• 1596: Early recognition that continents may have moved (Abraham Ortelius).

• 1858: Antonio Snider-Peligrini—rock/fossil correlations.

• 1872: First mapping of Atlantic Mid-Ocean Ridge (HMS Challenger expedition).

• 1896: Discovery of radioactivity—showed Earth’s interior heat source.

• 1912: Wegener’s formal presentation of Continental Drift.

• 1953: Confirmation of continuous mid-ocean ridge spreading center.

• 1962: Hess’s seafloor-spreading model.

• 1963: Vine–Matthews–Morley explanation of magnetic striping.

• 1968: Unification of driving forces—mantle convection accepted; Plate Tectonics paradigm solidified.

Modern Plate Tectonics Theory (Essence)

• Lithosphere: rigid outer shell (~100\,\text{km} thick) broken into \approx 7 major & many minor plates.

• Plates float atop the asthenosphere (partially molten, ductile upper mantle).

• Convection currents transport heat; rising & sinking mantle material drags plates.

• Interactions at plate boundaries drive:
  • Earthquakes.
  • Volcanism.
  • Mountain-building (orogeny).
  • Seafloor generation & destruction.

Global Distributions Explained by Plate Boundaries

Earthquakes

• Concentrated along plate edges—not random.

• Key belts:
  • “Ring of Fire” encircling the Pacific—dominant convergent & transform margins.
  • Mid-Atlantic Ridge—divergent boundary quakes.
  • Himalayas—continental-continental convergence.

• Mechanism: stress accumulation from relative motion released as seismic energy.

Volcanoes

• Also plate-boundary-focused.
  • Ring of Fire hosts \approx 75\% of Earth’s active subaerial volcanoes.
  • Mid-Atlantic Ridge produces fissure eruptions (e.g., Iceland).
  • Continental rifts (East African Rift) and subduction zones (Aleutians, Andes) dominate.
  • Hot-spot tracks (Hawaiian-Emperor chain) illustrate plate motion over fixed plumes (intraplate setting).

Mountain Ranges

• Form via crustal shortening, thickening, uplift at convergent margins.
  • Alpine-Himalayan belt ("Alpide") from India–Eurasia and Africa–Eurasia collisions.
  • Andes: oceanic-continental subduction.
  • Cordilleras encircling Pacific reflect compressive tectonics.

Science Trivia Break

• Bees possess two large compound eyes + three ocelli (simple eyes) – total 5 eyes.

• Can detect ultraviolet light; flowers display UV nectar guides invisible to humans, enhancing pollination efficiency.

• Illustrates adaptation via specialized sensory perception; example of how unseen wavelengths influence ecological interactions.

Review Checklist (Previous Lesson)

• Revisit:
  • Continental Drift Theory.
  • Supporting evidence categories.
  • Transition to Plate Tectonics.
  • Spatial patterns of volcanoes, mountains, earthquakes.

Classroom Activity 1: Distribution of Volcanoes & Earthquakes (Pair-Share)

Phase I – Data Collection

• Form pairs (Student A & B).
  • Student A: list 10 recent volcanic events.
  • Student B: list 10 recent earthquake events.

• For each entry record:
  • Date (use DD\text{/}MM\text{/}YYYY).
  • Country & geologic origin (e.g., subduction zone, divergent rift).
  • Brief damage summary (casualties, infrastructure, economic loss).

• Output on separate intermediate paper; follow provided tabular format.

Phase II – Mapping Exercise

• Materials:
  • 3 pieces short-size clear plastic cover.
  • 3 permanent marker colors: red, blue, black.
  • Two distinct pen shades for plotting: one shade earthquakes, another volcanoes.

• Instructions:
  • Overlay plastic cover on world map; mark an “x” at each listed location.
  • Use distinct color/pen for quake vs. volcano marks.

• Submission deadline: \text{July }1,2025.

Conceptual & Real-World Connections

• Iceland serves as a microcosm: ridge push, volcanism, seismicity all visible at surface.

• Chilean subduction zone exemplifies how oceanic-continental convergence yields both megathrust quakes and stratovolcano arcs.

• Everest dramatizes continent-continent collision, illustrating vertical crustal motion when subduction is impeded.

• Activities reinforce the empirical link between plate boundaries and hazard geography—useful for disaster-risk reduction planning and engineering.

Ethical & Practical Implications

• Understanding plate dynamics aids hazard preparedness, informs building codes, guides land-use planning.

• Empowers societies to mitigate loss of life/property through evidence-based policy.

• Scientific humility: Wegener’s initial rejection shows that paradigm shifts require open-mindedness and robust evidence.