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, EPR). 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-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): 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): sequence of reversals calibrated by radiometric dating — used to date ocean sediments and correlate globally. Last reversal: Brunhes-Matuyama ~780,000 ya.