Geology tricks that make plate tectonics click

Convergent = Crash (mountains/trench). Divergent = Drift (rift/mid-ocean ridge). Transform = Tear (strike-slip faults, earthquakes only).

ConvergentCrash — mountains, trenches, volcanoes

DivergentDrift — ridges, rift valleys

TransformTwist — faults, earthquakes only

Subduction: denser oceanic plate sinks under less dense plate → ocean trench + volcanic arc + earthquakes.

Subduction Zones — 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: magma rises at mid-ocean ridges, cools, moves away — magnetic stripes prove it.

Seafloor Spreading — 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: stationary mantle plumes create chains of volcanoes as plate moves over them. Hawaii-Emperor chain.

Hot Spots — 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: rift → young ocean → mature ocean → subduction → collision → suture. ~500 million years per cycle.

EmbryonicEast African Rift — opening

Young oceanRed Sea — early stage

Mature oceanAtlantic — spreading

DecliningPacific — subduction > spreading

TerminalMediterranean — closing

SutureHimalayas — ocean gone

Wegener (1912): fit of continents + fossil correlations + ancient climates → continental drift. Rejected without mechanism.

Alfred Wegener and Continental Drift — 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 (~300 Ma): last supercontinent. Gondwana (S) + Laurasia (N). Before: Rodinia (~1 Ga). Future: Pangaea Proxima.

Supercontinents — 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.

95% of earthquake energy released at plate boundaries. Subduction = largest quakes. Transform = frequent moderate quakes.

Earthquakes and Plate Boundaries — 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).

Volcanism at: divergent (basaltic, effusive), subduction (explosive andesite/rhyolite), hot spots (basaltic). No volcanism at transforms.

Volcanoes and Plate Tectonics — 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.

Fastest plate: Pacific ~10 cm/yr. Slowest: Antarctic ~2 cm/yr. Mid-Atlantic Ridge: ~2.5 cm/yr. GPS (Global Positioning System) confirms plate motion.

Plate Motion Rates and GPS — 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.

Orogeny: mountain building from plate collision, subduction, or accretion. Himalayas (India-Asia), Alps, Appalachians (ancient).

Mountain Building — 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: rocks record Earth's magnetic field when they form. Magnetic stripes at ocean ridges proved seafloor spreading.

Paleomagnetism — 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.