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Plate Tectonics — Field Guide

Alfred Wegener, a German meteorologist, noticed that the coastlines of South America and Africa fit like puzzle pieces. Matching fossils, glacial deposits...

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Alfred Wegener, a German meteorologist, noticed that the coastlines of South America and Africa fit like puzzle pieces. Matching fossils, glacial deposits, and rock formations on opposite Atlantic shores convinced him: continents move . Key sections include: PLATE TECTONICS; Wegener, 1912; Magnetic stripes, 1960s; The Grand Synthesis, 1967-68; The Plates; Boundaries; Subduction; Collision; Hotspots; Earthquakes.

Key sections

  • 01PLATE TECTONICS
  • 02Wegener, 1912
  • 03Magnetic stripes, 1960s
  • 04The Grand Synthesis, 1967-68
  • 05The Plates
  • 06Boundaries
  • 07Subduction
  • 08Collision
  • 09Hotspots
  • 10Earthquakes
  • 11Past Supercontinents
  • 12Pangaea Ultima
  • 13Closing

Topics covered

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Slide outline
  1. 01PLATE TECTONICS
  2. 02Wegener, 1912
  3. 03Magnetic stripes, 1960s
  4. 04The Grand Synthesis, 1967-68
  5. 05The Plates
  6. 06Boundaries
  7. 07Subduction
  8. 08Collision
  9. 09Hotspots
  10. 10Earthquakes
  11. 11Past Supercontinents
  12. 12Pangaea Ultima
  13. 13Closing
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Slide 01

PLATE TECTONICS

  • A Field Guide · Slide One of Thirteen
  • Continents adrift on stone
  • Earth's lithosphere is fractured into rigid plates that creep, collide, and tear apart over geologic time.
Slide 02

Wegener, 1912

  • 02 · The Heretic
  • Continental drift — and the laughter that followed
  • Alfred Wegener, a German meteorologist, noticed that the coastlines of South America and Africa fit like puzzle pieces. Matching fossils, glacial deposits, and rock formations on opposite Atlantic shores convinced him: continents move.
  • "The Earth's crust ... must be conceived as floating on a viscous substratum." — Wegener, Die Entstehung der Kontinente
  • His mechanism was wrong (he proposed continents plowing through ocean crust) and geophysicists derided him. He died in 1930 on a Greenland ice expedition, his theory still rejected.
  • Fig. 1 — Matching fossils and coastlines (red dots: Mesosaurus, Glossopteris)
Slide 03

Magnetic stripes, 1960s

  • 03 · The Smoking Gun
  • The ocean floor records its own history
  • Sonar surveys mapped a colossal mountain chain running down the middle of every ocean basin — the mid-ocean ridges. Then magnetometers pulled behind ships found something stranger: parallel stripes of normal and reversed magnetism, mirrored on either side of the ridge crest.
  • Lava erupts at the ridge, freezes magnetic field of the day into the rock
  • New crust spreads outward — older crust pushed away
  • Stripes match Earth's known polarity-reversal record
  • Vine, Matthews & Morley (1963): the ocean floor is a tape recorder. Spreading rates ~ 2-15 cm/yr.
  • Fig. 2 — Symmetric magnetic stripes flanking a spreading ridge
Slide 04

The Grand Synthesis, 1967-68

  • 04 · The Revolution
  • From outlaw idea to textbook truth in a decade
  • By the late 1960s, several lines of evidence converged into one unifying theory:
  • 1965 — Tuzo Wilson identifies transform faults; proposes "plates"
  • 1967 — Jason Morgan and Dan McKenzie independently formalize plate motions on a sphere
  • 1968 — Le Pichon publishes the first global plate model with six plates
  • 1968 — Isacks, Oliver & Sykes correlate earthquake distributions with plate boundaries
  • Plate tectonics is to geology what evolution is to biology: a single framework that organizes nearly every observation. It went from heresy to orthodoxy in roughly five years.
  • Wilson
  • Morgan
  • McKenzie
  • Le Pichon
  • Synthesis
Slide 05

The Plates

  • 05 · Inventory
  • Seven majors, several minors, all in motion
  • The lithosphere is broken into roughly fifteen plates. The seven major ones cover most of the surface; the minors sit at busy intersections.
  • Major: Pacific, North American, South American, African, Eurasian, Indo-Australian, Antarctic
  • Minor: Cocos, Nazca, Caribbean, Arabian, Philippine, Juan de Fuca, Scotia
  • Oceanic crust is thin (~7 km), dense, dark basalt — young (< 200 Ma).
  • Continental crust is thick (~35 km), light granite — ancient, with rocks up to 4 Ga old.
  • Fig. 3 — Schematic plate map; red lines = boundaries
Slide 06

Boundaries

  • 06 · Three Kinds of Edge
  • Where plates meet, things happen
  • Divergent — plates pull apart; magma rises, new crust forms (Mid-Atlantic Ridge, East African Rift)
  • Convergent — plates push together; one dives under (subduction) or both crumple (collision)
  • Transform — plates slide past horizontally, neither created nor destroyed (San Andreas)
  • All the violence Earth offers above the weather — earthquakes, volcanoes, mountain belts, tsunamis — concentrates along these thin lines.
  • Fig. 4 — Three boundary geometries
Slide 07

Subduction

  • 07 · Diving Plates
  • Where ocean meets continent and loses
  • Oceanic crust is dense. When it meets continental crust, it bends and dives — typically at angles of 30 to 60 degrees — descending hundreds of kilometers into the mantle.
  • Trench at the surface (deepest spots in the ocean)
  • Water released from sinking slab triggers melting above
  • Magma rises through the overlying plate → volcanic arc
  • Deep "Wadati-Benioff" earthquakes track the slab down
  • Examples: the Andes (Nazca ↓ South America), the Cascades (Juan de Fuca ↓ N. America), Japan (Pacific ↓ Eurasia).
  • Fig. 5 — Subduction-zone cross-section
Slide 08

Collision

  • 08 · Mountain Building
  • India hits Asia · the Himalayas
  • When two continental plates collide, neither will subduct — both are too buoyant. Instead the crust crumples and stacks, doubling its thickness and rising into mountains.
  • India broke from Gondwana ~140 Ma ago
  • Slammed into Asia ~50 Ma ago at unprecedented speed (~15 cm/yr)
  • Continues pushing north today at ~5 cm/yr
  • Himalayas still rising at ~1 cm per year
  • Mt Everest grows taller each year. Erosion shaves it back. The mountain is a balance between tectonic uplift and the patient work of water and ice.
  • Fig. 6 — Continental collision: India + Eurasia
Slide 09

Hotspots

  • 09 · Anomalies
  • Volcanoes that don't follow the rules
  • Most volcanism happens at plate boundaries. But Hawaii sits in the middle of the Pacific Plate. Yellowstone sits in the middle of North America. Why?
  • The leading explanation: mantle plumes — narrow columns of hot rock rising from deep in the mantle, perhaps from the core-mantle boundary 2,900 km down. The plate slides over the stationary plume, creating a chain of progressively older volcanoes.
  • Hawaii: Big Island (active) → Maui → Oahu → Kauai → Emperor Seamounts (80 Ma)
  • Yellowstone: hotspot track across Snake River Plain; supereruptions every ~600 ka
  • Iceland: hotspot superimposed on a mid-ocean ridge
  • Fig. 7 — Stationary plume, moving plate
Slide 10

Earthquakes

  • 10 · Stick and Slip
  • Stress accumulates · stress releases
  • Plates don't glide smoothly. They lock against each other, deform elastically, and then fail. The accumulated strain releases in seconds as a rupture propagates along the fault — that's an earthquake.
  • The San Andreas Fault is the textbook transform boundary, running ~1,200 km up California. The Pacific Plate slides northwest past the North American Plate at ~3-4 cm/yr. Locked sections store decades of strain, then release in M7+ events.
  • 1906 San Francisco — M7.9, fault offset up to 6 m
  • 1989 Loma Prieta — M6.9, collapsed Bay Bridge segment
  • The southern segment hasn't released since ~1857; it is overdue
  • "It is not if, but when." — USGS, on the next great San Andreas rupture.
  • Fig. 8 — California's transform boundary
Slide 11

Past Supercontinents

  • 11 · Deep Time
  • The pieces have assembled before
  • Plate motion runs in cycles. Roughly every 400-600 Myr the continents collect into a single supercontinent, then rift apart and disperse, only to reconvene on the far side of the globe.
  • Pangaea (~335-175 Ma) — the famous one; broke apart in the Jurassic, opening the Atlantic
  • Pannotia (~600 Ma) — short-lived, late Precambrian
  • Rodinia (~1.1-0.75 Ga) — assembled deep in the Proterozoic
  • Columbia / Nuna (~1.8-1.5 Ga) — even older
  • Kenorland (~2.7 Ga) — Archean assembly, edges of geologic memory
  • We can read Pangaea directly from matching coastlines and rocks. Rodinia is reconstructed from paleomagnetic data, mountain belts, and zircon ages — increasingly fuzzy back through time.
Slide 12

Pangaea Ultima

  • 12 · Looking Forward
  • ~250 million years from now
  • If current motions continue, the continents will reassemble into a new supercontinent in roughly a quarter-billion years. Geologists have proposed several possible configurations:
  • Pangaea Ultima (Scotese) — Atlantic closes, Americas slam back into Eurasia/Africa
  • Amasia — continents collect over the North Pole as the Pacific closes
  • Aurica — both Atlantic and Pacific close; new ocean opens elsewhere
  • A 2023 study suggests Pangaea Ultima would be hostile to mammals — a hot, dry interior with CO₂ levels driving surface temperatures past mammalian survival limits. Long after we're gone.
  • Fig. 9 — Possible Pangaea Ultima configuration
Slide 13

Closing

  • 13 · References & Further Reading
  • Continents adrift on stone — go deeper
  • References
  • Wegener, A. Die Entstehung der Kontinente und Ozeane (1912/1929)
  • Vine, F. & Matthews, D. "Magnetic Anomalies over Oceanic Ridges" — Nature (1963)
  • Wilson, J. T. "A new class of faults and their bearing on continental drift" — Nature (1965)
  • Morgan, W. J. "Rises, Trenches, Great Faults, and Crustal Blocks" — JGR (1968)
  • Le Pichon, X. "Sea-Floor Spreading and Continental Drift" — JGR (1968)
  • Farnsworth et al. "Climate extremes likely to drive land mammal extinction during next supercontinent assembly" — Nature Geoscience (2023)
  • Watch
  • YouTube: Plate Tectonics Explained
  • Overview lectures, animations, documentaries
  • YouTube: Wegener & Continental Drift
  • The history of the idea — from heresy to orthodoxy
  • "The Earth tells its own history in stone. We have only to learn the language."
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