Three fundamentally different ways plates interact — each producing distinct geological features
Convergent: plates move together. Three subtypes: oceanic-oceanic (one subducts → island arc + trench), oceanic-continental (oceanic subducts → Andes, Cascades + trench), continental-continental (neither subducts → collision mountains: Himalayas, Alps). Divergent: plates pull apart. Oceanic: mid-ocean ridges (MAR (Mid-Atlantic Ridge), EPR (East Pacific Rise)). Continental: rift valleys (East African Rift, Basin and Range). Transform: plates slide past each other. No crust created or destroyed. San Andreas Fault, Alpine Fault (New Zealand), Dead Sea Transform. Only shallow earthquakes. No volcanism.
Convergent
Crash — mountains, trenches, volcanoes
Divergent
Drift — ridges, rift valleys
Transform
Twist — faults, earthquakes only
Subduction
Subduction: denser oceanic plate sinks under less dense plate → ocean trench + volcanic arc + earthquakes.
Subduction Zones
Where oceanic plates dive into the mantle — and why they generate the world's most powerful earthquakes
Subduction: oceanic crust (denser, basalt) descends beneath continental or oceanic crust. Trench: topographic low — Mariana Trench (11 km deep). Benioff zone: earthquakes along the descending slab, down to 700 km. Dehydration: water released from subducting slab → lowers melting point of mantle wedge → magma rises → volcanic arc. Volcanic arcs: continental (Andes, Cascades) or island (Japan, Philippines, Aleutians). Subduction drives plate tectonics ('slab pull' is strongest force). Examples: Pacific Ring of Fire — almost all active volcanism and large earthquakes.
Seafloor Spreading
Seafloor spreading: magma rises at mid-ocean ridges, cools, moves away — magnetic stripes prove it.
Seafloor Spreading
Harry Hess's 1962 insight — and the magnetic evidence that made it irrefutable
Harry Hess (1962): 'Essay in Geopoetry' — seafloor spreading at mid-ocean ridges. New ocean floor created at ridges, old floor consumed at subduction zones. Evidence: symmetric magnetic stripes (Vine and Matthews, 1963) — as basalt cools at ridge, records current magnetic field orientation → alternating normal/reversed stripes either side of ridge. Age: increases with distance from ridge. Ocean floor: never older than ~200 Ma (continuously recycled). Youngest, hottest ocean floor: at ridge crests (mid-ocean ridge basalt — MORB (Mid-Ocean Ridge Basalt)). Mid-Atlantic Ridge: Atlantic widening ~2.5 cm/yr.
Hot Spots
Hot spots: stationary mantle plumes create chains of volcanoes as plate moves over them. Hawaii-Emperor chain.
Deep mantle plumes: anomalously hot material rising from core-mantle boundary (D'' layer). Plate moves over stationary plume → chain of progressively older volcanoes. Hawaiian-Emperor chain: oldest (Emperor Seamounts, ~80 Ma, now subducted) → youngest (Big Island, currently over plume). Bend in chain (~47 Ma): plate motion direction changed. Other hot spots: Yellowstone (continental), Iceland (on MAR + hot spot → unusually thick crust), Galápagos, Tristan da Cunha, Réunion (triggered Deccan Traps?). Hot spots are exceptions — most volcanism at plate boundaries. Plume heads: can trigger flood basalts (Deccan Traps, Siberian Traps).
Wilson Cycle
Wilson Cycle: rift → young ocean → mature ocean → subduction → collision → suture. ~500 million years per cycle.
The Wilson Cycle
The complete cycle of ocean opening and closing — the engine of continental drift
J. Tuzo Wilson (1966): continents separate and rejoin repeatedly. Stages: (1) embryonic rift (East African Rift), (2) young ocean (Red Sea), (3) mature ocean (Atlantic — spreading, passive margins), (4) declining ocean (Pacific — subduction dominates spreading), (5) terminal ocean (Mediterranean — closing), (6) suture zone (Himalayas — ocean fully closed, continents collide). Timescale: ~300–500 million years per complete cycle. Next cycle already starting: East African Rift may open new ocean. Atlantic will eventually close → Pangaea Proxima (~250 Ma future).
Embryonic
East African Rift — opening
Young ocean
Red Sea — early stage
Mature ocean
Atlantic — spreading
Declining
Pacific — subduction > spreading
Terminal
Mediterranean — closing
Suture
Himalayas — ocean gone
Alfred Wegener and Continental Drift
Wegener (1912): fit of continents + fossil correlations + ancient climates → continental drift. Rejected without mechanism.
Alfred Wegener and Continental Drift
The geologist who was right before anyone believed him — and what ultimately vindicated him
Alfred Wegener (1912): Die Entstehung der Kontinente und Ozeane. Evidence: jigsaw fit of continents (S. America + Africa), fossil correlations across oceans (Glossopteris plant, Mesosaurus reptile), matching rock sequences, ancient climate indicators (coal in Antarctica, glacial deposits in tropics). Rejected: no known mechanism to move continents through ocean floor. Died 1930 on Greenland expedition. Vindicated by: seafloor spreading (Hess, 1962) + paleomagnetism + deep-sea drilling. Modern consensus (plate tectonics, 1967): confirmed by multiple independent lines of evidence. First scientific revolution in geology.
Pangaea and Supercontinents
Pangaea (~300 Ma): last supercontinent. Gondwana (S) + Laurasia (N). Before: Rodinia (~1 Ga). Future: Pangaea Proxima.
Supercontinents
The repeated assembly and breakup of Earth's landmasses throughout geologic history
Pangaea ('all Earth'): assembled ~300 Ma, began breaking up ~180 Ma. Gondwana: S. America, Africa, Antarctica, Australia, India (broke from Pangaea). Laurasia: N. America, Europe, Asia. Tethys Sea: ancient ocean between. Before Pangaea: Rodinia (~1.1 Ga, assembled ~1.2 Ga, broke up ~750 Ma). Columbia/Nuna (~1.8 Ga). Pattern: supercontinents form every ~300–500 Ma (Wilson Cycle). Future: Pangaea Proxima (Atlantic closes, ~250 Ma). Evidence: matching geology + fossils on separated continents, paleomagnetic data, mountain belts as suture zones.
Earthquakes and Plate Tectonics
95% of earthquake energy released at plate boundaries. Subduction = largest quakes. Transform = frequent moderate quakes.
Earthquakes and Plate Boundaries
The connection between plate tectonics and where and why earthquakes happen
Convergent (subduction): megathrust faults — largest earthquakes ever recorded. Chile 1960 (Mw 9.5), Alaska 1964 (Mw 9.2), Tohoku 2011 (Mw 9.1 → tsunami). Deep Benioff zone earthquakes. Divergent: shallow, moderate earthquakes (rifting, normal faulting). Transform: shallow earthquakes along fault length. San Andreas: creep + locked segments. 1906 San Francisco (Mw 7.9). Dead Sea Transform: historically active (Jericho repeatedly destroyed). Intraplate earthquakes: rare but can be large (New Madrid Seismic Zone, USA — 1811–12 Mw ~7.5–8.0 sequence).
Volcanoes and Plate Tectonics
Volcanism at: divergent (basaltic, effusive), subduction (explosive andesite/rhyolite), hot spots (basaltic). No volcanism at transforms.
Volcanoes and Plate Tectonics
Why volcanoes occur where they do — and why the magma type differs by tectonic setting
Divergent boundaries: decompression melting (pressure drops as mantle rises → melt). Basaltic MORB magma — low silica, low viscosity, effusive eruptions. Iceland: on mid-ocean ridge + hot spot. Subduction zones: fluid-fluxed melting (water from subducting slab lowers mantle melting point). Andesitic/rhyolitic magma — high silica, high viscosity, explosive (Pinatubo 1991, St. Helens 1980). Ring of Fire: ~75% of Earth's volcanoes. Hot spots: decompression melting above plume. Basaltic but can be explosive (yellowstone — rhyolitic). No volcanism at transform boundaries: no plate creation or subduction, no melting.
How fast plates actually move — and how we measure it in real time
Average plate motion: ~2–10 cm/yr (fingernail growth rate). Fastest: Pacific plate ~10 cm/yr. Slowest: Antarctic, African plates ~1–2 cm/yr. Mid-Atlantic Ridge: ~2.5 cm/yr (spreading rate = total rate ÷ 2 per side). Atlantic has opened ~3,000 km since Pangaea breakup. GPS (GNOME, etc.): precisely measures plate motion in real time — confirms geological rates. VLBI (Very Long Baseline Interferometry) (very long baseline interferometry): radio astronomy technique also measures plate motion. SLR (satellite laser ranging). Hotspot tracks confirm past rates: Hawaiian chain bends show change ~47 Ma. GPS also detects strain buildup at fault zones → earthquake forecasting.
Mountain Building (Orogeny)
Orogeny: mountain building from plate collision, subduction, or accretion. Himalayas (India-Asia), Alps, Appalachians (ancient).
Mountain Building
How plate tectonics builds the world's mountains — and then destroys them
Collision orogeny: two continental plates collide — neither subducts (density too low). Himalayas: India-Asia collision (~50 Ma, still ongoing). Tibet Plateau: thickened crust, average 5 km elevation. Alps: Africa-Europe collision. Appalachians: ancient (Acadian, Alleghenian orogenies) — correlate with Caledonian Mountains of Scotland → same when Pangaea was assembled. Subduction orogeny: Andes (oceanic-continental) — compression, magmatic arc. Accretionary orogeny: terranes (exotic crustal fragments) accreted to continental margins — much of western North America built this way. Isostasy: erode mountains → root rises (mountain building and erosion = dynamic equilibrium).
Paleomagnetism
Paleomagnetism: rocks record Earth's magnetic field when they form. Magnetic stripes at ocean ridges proved seafloor spreading.
Paleomagnetism
How frozen magnetic signatures in rocks proved continental drift and seafloor spreading
When basalt cools below Curie temperature (~580°C for magnetite), magnetic minerals align with Earth's field and are 'locked in.' Apparent polar wander: continents moved — magnetic pole positions recorded in rocks trace the path. Magnetic reversals: field flips polarity irregularly (~every 200,000–300,000 yr average). Vine-Matthews-Morley (1963): symmetric magnetic stripes either side of mid-ocean ridges — normal and reversed polarity alternating. Proved seafloor spreading. GPTS (Geomagnetic Polarity Time Scale) (Geomagnetic Polarity Time Scale): sequence of reversals calibrated by radiometric dating — used to date ocean sediments and correlate globally. Last reversal: Brunhes-Matuyama ~780,000 ya.
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🎓 Common Exam Questions
Q: What are the three types of plate boundaries and what landforms and hazards does each create?
A: Convergent (Crash): Oceanic-continental: oceanic plate subducts. Creates: ocean trench (Mariana, Peru-Chile), volcanic arc on overriding plate (Andes, Cascades, Japan), megathrust earthquakes (2011 Tohoku M9.1, 1960 Chile M9.5). Oceanic-oceanic: one subducts. Creates: trench, island arc (Japan, Aleutians, Philippines). Continental-continental: neither subducts (both buoyant). Creates: fold mountains (Himalayas, Alps, Appalachians), no volcanoes, deep crustal earthquakes. Divergent (Drift): mid-ocean ridges (seafloor spreading, volcanic, shallow earthquakes), continental rift valleys (East African Rift — may become future ocean). Transform (Tear): lateral motion, no creation or destruction of crust. Creates: strike-slip faults (San Andreas = Pacific-North American boundary), shallow earthquakes, no volcanoes. 95% of all earthquake energy is released at plate boundaries.
Q: What evidence proved continental drift and who were the key scientists?
A: Alfred Wegener (1912) proposed continental drift but was initially rejected because he had no mechanism: Evidence for drift: geometric fit of continents, fossil correlations (Mesosaurus, Glossopteris), ancient climate evidence (coal in Antarctica, glaciations in tropics), mountain belt correlations. Harry Hess (1962) provided the mechanism: seafloor spreading at mid-ocean ridges. Paleomagnetism (Vine, Matthews, Morley, 1963): magnetic stripes on seafloor confirmed spreading and provided a rate clock. Plate tectonics synthesized these into a unified theory by the late 1960s. VLBI and GPS directly measure plate motion at centimeters per year. The Wilson Cycle (J. Tuzo Wilson) added the concept of the complete ocean lifecycle. The revolution in Earth science occurred in just one decade (1960s) — arguably the greatest paradigm shift in geology since Darwin in biology.
Q: Explain seafloor spreading and paleomagnetism — how do they work together?
A: Seafloor spreading (Hess, 1962): magma rises at mid-ocean ridges through decompression melting (mantle upwells, pressure drops, melting occurs). Basaltic lava flows, cools, and is added to the plates on either side, moving away from the ridge. Oldest crust is farthest from ridge (maximum ~200 Ma — all older has been subducted). Paleomagnetism: as basalt cools below the Curie temperature (~580 degrees C for magnetite), magnetic minerals lock in the direction of Earth's magnetic field. Since Earth's field periodically reverses (every 200,000-500,000 years on average), successive eruptions at the ridge record alternating normal and reversed polarity. The result: symmetric stripes of alternating polarity parallel to the ridge axis on both sides. By correlating stripes with the GPTS (Geomagnetic Polarity Time Scale), we can date ocean floor precisely and calculate spreading rates: MAR ~2.5 cm/yr, EPR ~15 cm/yr — without drilling a single hole.
Q: What is the Wilson Cycle and what is the current status of each ocean basin?
A: Wilson Cycle (J. Tuzo Wilson, 1966): the complete lifecycle of an ocean basin over ~300-500 million years. Stages: (1) Embryonic rift: continental lithosphere stretches and thins — East African Rift today. (2) Young ocean: seafloor spreading begins, narrow ocean forms — Red Sea today. (3) Mature ocean: wide ocean with mid-ocean ridge — Atlantic Ocean today. (4) Declining ocean: subduction begins along margins — Pacific Ocean (Pacific is closing as plates subduct). (5) Terminal ocean: small remnant ocean — Mediterranean today (closing as Africa-Europe converge). (6) Suture: continents collide, ocean disappears — Appalachians formed when Iapetus Ocean closed. Current status: Atlantic = mature and opening (MAR spreading). Pacific = declining and closing (ring of subduction zones). Arctic/Indian Oceans = complex, intermediate. Mediterranean = closing (Alps, caused by Africa-Europe collision). Red Sea = youngest ocean, just opened. East African Rift = proto-ocean, just starting.
Q: What causes volcanic eruptions at subduction zones vs divergent boundaries?
A: Divergent boundaries: decompression melting. As mantle rises at the ridge, pressure decreases but temperature stays high — rock that was solid under high pressure partially melts (~1-5% melt fraction). Produces basaltic MORB (Mid-Ocean Ridge Basalt) — low silica (~50%), low viscosity, effusive eruptions, VEI 0-1. Subduction zones: flux melting. Water (and other volatiles) released from the subducting slab as it heats and dehydrates lowers the melting temperature of the overlying mantle wedge — causing it to melt even though temperature alone would not suffice. Melt rises through the overlying plate. As it ascends, it assimilates crustal material and evolves toward more silicic, water-rich composition (andesite, dacite, rhyolite). High water content creates: high viscosity magma that traps gas, leading to explosive eruptions (VEI 3-8), pyroclastic flows, lahars. Examples: Pinatubo 1991 (VEI 6), Krakatoa 1883 (VEI 6), Tambora 1815 (VEI 7 — deadliest volcanic eruption in recorded history).