Comprehensive Study Notes – Plate Tectonics, Seafloor Spreading & Plate Boundaries

Introductory Context

  • Course focus: Marine geology → understanding the geological structure & evolution of the ocean basins.

  • Central paradigm: Plate-tectonics theory (accepted since the 1960s).

    • Analogy: Plate tectonics ≈ evolution for biology → a unifying framework that makes disparate observations coherent.

  • Classic geological puzzles solved by plate tectonics:

    • Why & how mountains form.

    • Marine fossils (e.g. mollusks) atop high mountains (Alps, Himalayas, Andes).

    • Localization of earthquakes & volcanoes (Pacific “Ring of Fire”).

    • Contrasts between continental rocks (granite, light-coloured, coarse-grained) vs. oceanic rocks (basalt, dark, fine-grained).

    • Tropical plant fossils in Antarctica (leaf “drip-tips”).

Fundamental Statement of the Theory

  • Earth’s outer shell (lithosphere) is broken into internally rigid plates that move relative to one another atop a ductile asthenosphere.

  • All major geological “action” (mountain building, seismicity, volcanism) concentrates at plate boundaries where plates:

    • Diverge,

    • Converge,

    • Shear (transform motion).

  • Driving force: convective heat transfer in the mantle; additional contributions from slab-pull & ridge-push.

1. Continental Drift (Alfred Wegener, 1912)

  • Wegener (meteorologist) synthesized four main lines of evidence:

    1. Puzzle-piece fit of continental margins (improves when shelves are matched).

    2. Fossil distributions that make sense only if the continents were once contiguous (e.g. Mesosaurus limited to S. America + Africa).

    3. Mountain-belt continuity: Appalachians–Caledonides–Scottish Highlands–Atlas Mountains align when Pangaea is reconstructed.

    4. Palaeoglacial striations radiate outward from a unified ice-sheet centre when Gondwana is assembled.

  • Limitation: lacked a viable mechanism → idea initially rejected (Wegener lost his university post).

2. Seafloor Spreading (Harry Hess, early 1960s)

  • Key insight: Oceans grow by creation of new basaltic crust at mid-ocean ridges (MORs) and consumption at trenches.

  • Mantle convection delivers magma upward at ridges; cooling lithosphere moves laterally & eventually descends (subducts).

Observational Evidence

  1. Bathymetry: Global ridge system encircles Earth; ridges mirror continental outlines.

  2. Magnetic “zebra stripes”:

    • Earth’s dipole field reverses irregularly (documented via dated lava flows).

    • Newly cooled basalt records ambient polarity → symmetrical, ridge-parallel bands of normal & reversed magnetization.

  3. Age pattern:

    • youngest\text{youngest} crust at ridge axis; age increases symmetrically with distance.

    • Global map: hottest colours = 0–10 Ma, coldest ≥ 180 Ma (none older due to continual recycling).

  4. Geophysical gradients:

    • Shallow earthquakes & high heat-flow at MORs.

    • Deep earthquakes, trenches & andesitic volcanism at subduction zones.

  5. Global magnetic field basics:

    • Generated by liquid Fe–Ni outer core; presently exits S-pole, enters N-pole.

    • Last major reversal (Brunhes/Matuyama) ≈ 0.78 Ma; field presently weakening → a future reversal plausible.

Quantifying Spreading Rate (Atlantic example)

  • Width (d)=5 000 km(d)=5\,000\,\text{km}; oldest Atlantic crust t=1×108 yrt=1\times10^{8}\,\text{yr}.

  • Full rate=dt=5×103 km1×108 yr=5×10−2 km/yr=5 cm/yr\text{Full rate}=\dfrac{d}{t}=\dfrac{5\times10^{3}\,\text{km}}{1\times10^{8}\,\text{yr}}=5\times10^{-2}\,\text{km/yr}=5\,\text{cm/yr}.

  • Half-rate per flank ≈2.5 cm/yr\approx2.5\,\text{cm/yr}.

  • Global range: ≈1–15 cm/yr\approx1 – 15\,\text{cm/yr} (fastest in east Pacific).

3. Plate Inventory

  • 9 major plates (Pacific, N. American, S. American, African, Eurasian, Indo-Australian, Antarctic, Nazca, Caribbean) + numerous microplates (Juan de Fuca, Cocos, Philippine, Scotia, etc.).

  • Some plates carry both continental & oceanic lithosphere (e.g. N. American plate: Canada + half Atlantic floor).

4. Plate Boundaries: Classification Matrix

Movement categories (3) × Crustal combinations (2) → 7 canonical types.

Relative motion

Ocean–Ocean (O–O)

Continent–Continent (C–C)

Ocean–Continent (O–C)

Divergent (constructive)

MOR (e.g. Atlantic Ridge)

Continental rift (East African)

—

Convergent (destructive)

Island arc + trench (Japan)

Collision orogen (Himalaya)

Andean-type margin (Chile, Cascades)

Transform (conservative)

Ridge-offset transforms (fracture zones)

San Andreas fault

—

5. Divergent Boundaries

Continental Rift → Ocean Basin Evolution Sequence

  1. Initial doming & fissuring (Ethiopian rift: fissures, incipient volcanism).

  2. Rift valley & linear lakes (Lake Tanganyika, Lake Nyasa).

  3. Narrow sea (Red Sea, Gulf of Suez/Aqaba, Gulf of California).

  4. Mature ocean with MOR (Atlantic).

  • Elevation of MORs: thermal buoyancy of hot, thin lithosphere.

  • Iceland: emergent segment of the North Atlantic ridge; exposes basaltic fissure vents, Thingvellir graben.

6. Convergent Boundaries

6.1 Continent–Continent

  • Example: India–Asia collision → Himalaya & Tibetan Plateau.

    • Marine sediments uplifted to >8 km; thickness doubled → plateau at ≈5 km\approx5\,\text{km}.

6.2 Ocean–Continent

  • Example: Nazca plate subducting beneath S. America → Andes + Peru-Chile trench.

    • Shallow-dip Benioff zone; arc-parallel seismicity; andesitic volcanism.

6.3 Ocean–Ocean

  • Example: Pacific ↘ beneath Philippine plate → Japan trench & island arc (curvilinear alignment, marginal Sea of Japan).

    • Older, colder slab subducts; steep Benioff zone; deep-focus quakes to ∼700 km\sim700\,\text{km}.

  • Deepest ocean spot: Mariana trench (≈11 km) along same Pacific boundary.

7. Transform (Shear) Boundaries

Oceanic Transforms & Fracture Zones

  • Offset ridge segments; active shear limited to the segment between opposing flow arrows.

  • Outside active section → inactive topographic scar (fracture zone) separating crust of different ages/densities.

Continental Transform: San Andreas System

  • Connects Gulf of California spreading centre with Juan de Fuca ridge.

  • Right-lateral slip ∼5 cm/yr\sim5\,\text{cm/yr}.

  • Hazard metrics: ≈62%\approx62\% chance of M≥6.7 quake in California within 30 yr; LA will move northward past SF in tens of Myr (if it survives!).

8. Hot Spots & Intraplate Volcanism

  • Stationary mantle plumes sourced near core–mantle boundary.

  • Plate motion across plume → linear age-progressive chains.

Pacific Example: Hawaiian–Emperor Chain

  • Active centre: Mauna Loa/Kīlauea (Big Island, rock $\le1\,\text{Ma}$).

  • Age increases NW to Midway & bends toward Aleutians (change in Pacific plate motion ≈ 30 Ma).

  • Volcanoes cool, subside → become guyots/seamounts.

Continental Example: Yellowstone Hot Spot Track

  • Initial flood-basalt outpouring (Columbia River Basalts) in Oregon/Washington.

  • Present geysers & caldera in NW Wyoming; earlier calderas form a NE-SW trail across Snake River Plain.

9. Paleogeography & The Wilson Cycle

  • Supercontinent cycles every ∼6×108 yr\sim6\times10^{8}\,\text{yr} (formation & breakup).

    1. Gondwana dominant (Carboniferous, 350 Ma).

    2. Pangaea assembled (Triassic 250 Ma) → Appalachian / Caledonian orogens.

    3. Breakup during Jurassic–Cretaceous, opening the Tethys Seaway → prolific petroleum source rocks in Middle East.

  • Net continental area slowly increases as accreted terranes weld onto cratons.

Florida’s Journey (animation summary)

  • 360 Ma: part of Gondwana near present Antarctic latitudes.

  • 200–150 Ma: embedded within central Pangaea (arid).

  • Post-Cretaceous: rifted away with North America to present 30° N latitude.

  • Result: uniquely travelled crustal fragment.

10. Quantitative & Conceptual Essentials

  • Rate equation: Rate=DistanceTime\text{Rate}=\dfrac{\text{Distance}}{\text{Time}} → used to infer spreading velocities.

  • Density/Buoyancy relationships:

    • Hot ⇒\Rightarrow less dense ⇒\Rightarrow rises (ridge crests).

    • Cold ⇒\Rightarrow more dense ⇒\Rightarrow subducts (slab pull).

  • Magnetic chronology: sequence of polarity chrons calibrated by radiometric dates on lavas.

  • Earthquake focal depths delineate slab geometry (Benioff-Wadati zones).

Ethical, Practical & Societal Implications

  • Natural-hazard assessment (earthquakes, tsunamis, volcanic eruptions) depends on accurate plate models.

  • Resource exploration (oil in passive-margin basins, metallic ores along arcs, geothermal in rifts/hot spots).

  • Climate modulation: mountain uplift (Himalayas) alters atmospheric circulation & CO₂ drawdown.

  • Satellite navigation & power-grid management may face risks during future geomagnetic reversals.

Self-Check / Review Prompts

  • Why are marine fossils found on Mount Everest?

  • Compute half-spreading rate for a 3,600 km basin aged 120 Ma.

  • List three differences between San Andreas & Peru-Chile trench.

  • Identify two present-day continental rifts and predict their geologic future.

  • Explain why the older of two converging ocean plates typically subducts.