Continental Drift & Pangaea

Introduction to Pangaea

• Pangaea = Greek for “all land”; refers to a hypothesised super-continent that once contained all of Earth’s dry land in a single, continuous mass.
• Estimated formation & breakup timeline:
  • Formed during late Paleozoic (
    $ext{~300–335 Myr ago}$).
  • Began rifting apart in early Mesozoic, roughly $200\ \text{Myr ago}$.
• Present distribution of continents & oceans reflects the fragmentation, drift, and subsequent interactions of the Pangaean pieces.

Alfred Wegener & the Continental-Drift Hypothesis

• German meteorologist/geophysicist.
• Published “Die Entstehung der Kontinente und Ozeane” (1915) introducing continental drift.
• Core propositions:
  • Continents are not fixed; they plow slowly through oceanic crust.
  • All were once united as Pangaea and later drifted to current positions.
• Contemporary reaction:
  • Widely dismissed (no convincing mechanism; physics of the era predicted impossible drag forces).
  • Re-evaluation began in the 1950s with seafloor-spreading data and paleomagnetism.
• Personal tragedy: died in Greenland (1930) while collecting climate data.

Four Classic Lines of Evidence for Pangaea

1. Geometric / “Jigsaw” Fit

• West African coastline fits snugly against the east coast of South America; additional matches include:
  • North America vs. NW Africa / Iberia.
  • Greenland vs. Europe.
• Quantitative reconstructions (Bullard fit, 1965) achieve <$100\ \text{km}$ average misfit when using the 1000-fathom (≈$1830\ \text{m}$) contour rather than modern shorelines.

2. Fossil Correlation

• identical, non-swimming terrestrial or freshwater organisms found on now-separated continents—impossible to disperse across open oceans.
  • Mesosaurus (fresh-water reptile) – eastern South America & western Africa only.
  • Cynognathus (≈$3\ \text{m}$ Triassic land reptile) – South America & Africa.
  • Lystrosaurus (Triassic land reptile) – Africa, India, Antarctica.
  • Glossopteris (seed fern) – all southern continents (Gondwanan distribution).
• Significance: demonstrates physical connection allowing shared terrestrial habitats.

3. Rock & Mountain Correlation

• Mountain belts that match in lithology, structure, metamorphic grade, and radiometric age.
  • Appalachians (eastern N. America) correlate with
  • Caledonides (Scotland, Scandinavia, Greenland)
  • Anti-Atlas (NW Africa).
  • Collision & folding dated to $\sim300\ \text{Myr}$ (Alleghanian/Variscan orogenies) — contemporaneous with Pangaea assembly.
• When continents are re-joined, these belts form continuous linear chains.

4. Paleoclimate Indicators

• Glacial striations & tillites of late Paleozoic age found in present-day tropical latitudes (S. America, S. Africa, India, Australia, Antarctica) show flow directions pointing away from a central ice cap—coherent only if continents once clustered around the South Pole.
• Bituminous coal beds (fossilised tropical swamp peat) discovered in temperate/polar regions (e.g., Pennsylvanian coals in N. America & Europe; Permian coals in Antarctica) imply those landmasses resided nearer the equator during formation.
• Additional climate proxies: evaporites, reef limestones, and redbeds, all plotting into latitudinal belts consistent with drift reconstructions.

Supplementary / Modern Evidence (post-1950s)

• Seafloor spreading (Mid-Atlantic Ridge magnetic anomalies).
• Paleomagnetic apparent polar-wander paths that converge when continents are fit together.
• Age progression of oceanic crust: <200\ \text{Myr} everywhere, confirming continual renewal at ridges.

Terminology & Key Fossils

• Supercontinent cycle – repeated assembly/dispersal every $\approx500\ \text{Myr}$.
• Gondwana – southern portion of Pangaea; included present S. America, Africa, India, Australia, Antarctica.
• Laurasia – northern lands (N. America, Europe, Asia) after initial Pangaean rifting.
• Fossils (memorise species ↔ continents):
  • Mesosaurus: S. America, Africa (freshwater).
  • Lystrosaurus: Africa, India, Antarctica (land).
  • Cynognathus: S. America, Africa (land).
  • Glossopteris: all Gondwanan.

Chronological Milestones

• $1915$ – Wegener publishes hypothesis.
• $1930$ – Wegener dies; theory stagnant.
• $1950–63$ – paleomagnetism, Vine-Matthews-Morley, Hess’ seafloor-spreading—continental drift becomes plate-tectonics.
• $1965$ – Bullard et al. quantitative fit & plate-tectonic framework.

Ethical, Philosophical, & Practical Implications

• Illustrates paradigm shift: evidence can accumulate decades before acceptance; mechanism often required for consensus.
• Demonstrates interdisciplinary nature of Earth science (geology + biology + climatology + geophysics).
• Informs resource exploration:
  • Hydrocarbon basins, coal seams, and mineral belts predicted by re-assembling ancient plates.
• Climate-change studies: past continental positions modulate ocean circulation & CO$_2$ drawdown.

Quick Numerical / Statistical References

• Scale bars often used in paleogeographic maps: $1000\text{ mi} = 1600\text{ km}$.
• Mesosaurus range: limited to fluvial environments within $\sim200\ \text{km}$ of coastlines.
• Mountain-belt age ≈ $300\ \text{Myr}$; coal-bearing strata ≈ $300\text{–}320\ \text{Myr}$ (Carboniferous).

Study Tips & Connections

• Reconstruct continents on a blank world map; draw fossil localities & glacial striae—visual reinforcement.
• Link to previous lectures on plate tectonics, mantle convection, Wilson cycle.
• Practice explaining each evidence line and why it refutes the “land-bridge” alternative.
• Watch recommended video (YouTube link from slide) to visualise Pangaea breakup and present drift rates (≈$2–15\ \text{cm yr}^{-1}$).

Possible Exam Prompts

• Describe two fossil and two rock-type correlations that support continental drift.
• Explain why glacial deposits in present-day tropics argue for continental mobility rather than global ice coverage.
• Outline reasons Wegener’s theory was rejected, and describe the data that eventually validated it.