Geology — Plate Tectonics, Geophysics, and Seismology

Plate Tectonics

The history of continental drift theory:

• Alfred Wegener (1912): proposed continental drift: the supercontinent Pangaea evidence: matching coastlines across the Atlantic, identical fossil distributions Glossopteris: a plant fossil common to southern-hemisphere continents rejected in his lifetime: he couldn’t explain the mechanism of movement
• Seafloor spreading (Harry Hess, 1962): mid-ocean ridges: create new oceanic crust trenches: destroy oceanic crust (subduction)
• Paleomagnetic stripes: symmetric magnetic reversal records on either side of a ridge the Vine-Matthews hypothesis (1963): decisive evidence for seafloor spreading
• Plate tectonics established (late 1960s): the unifying theory takes shape

Plate types and boundaries:

• Earth’s surface: about 15 major plates (Eurasian, Pacific, North American, South American, African, Indo-Australian, Antarctic, Cocos, Nazca, Caribbean, Philippine, Arabian, Juan de Fuca, Scotia)
• Continental plate: granitic, lower density, thick and old
• Oceanic plate: basaltic, higher density, thin and young

Plate boundary types:

• Divergent boundary: mid-ocean ridge: creates oceanic crust the Mid-Atlantic Ridge, the East Pacific Rise hydrothermal vents: distinctive ecosystems continental rift: the start of continental breakup the East African Rift (the Great Rift Valley) the Red Sea, the Gulf of Aden: young oceans forming
• Convergent boundary: ocean-ocean convergence: volcanic arcs and trenches (the Marianas, Tonga) subducting oceanic crust → an andesitic volcanic arc ocean-continent convergence: an Andean-type orogenic belt the Nazca and South American plates: the Andes Mountains, the Peru-Chile Trench active earthquakes and stratovolcanoes continent-continent collision: a Himalayan-type orogenic belt the Indian and Eurasian plates → the Himalayas (from ~50 Ma) crustal thickening, uplift, reverse faulting
• Transform boundary: horizontal motion, friction, active shallow earthquakes the San Andreas Fault: between the North American and Pacific plates Atlantic transform faults: segment the mid-ocean ridge

Mechanisms of plate motion:

• Mantle convection: driven by radioactive decay heat + primordial heat slow asthenospheric convection (a few cm/year)
• Slab pull: the cooled, heavy oceanic plate sinks under its own weight the most widely accepted primary driving force
• Ridge push: the ridge’s elevation causes gravitational sliding
• Hotspots: a fixed, deep mantle heat source the Hawaiian hotspot: records plate direction and speed Yellowstone, Iceland, the Galápagos

The Wilson cycle:

• Supercontinent breakup → ocean formation → plate convergence → supercontinent reassembly
• A cycle of roughly 400–500 million years
• Rodinia (1.3 Ga ago) → Gondwana → Pangaea (300 million years ago) → the present

Orogeny (mountain building):

• The Caledonian orogenic belt: Scotland, Scandinavia
• The Variscan orogenic belt: central Europe, the Carboniferous period
• The Alpine-Himalayan orogenic belt: from the Cenozoic to today
• The circum-Pacific orogenic belt: the Rockies, the Andes

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Geophysics

Earth’s internal structure:

• Crust: continental crust: 30–70 km thick · granitic (SIAL: Si, Al) oceanic crust: 5–10 km thick · basaltic (SIMA: Si, Mg)
• Mantle: 84% of Earth’s volume lithosphere: crust + rigid uppermost mantle (~100 km) asthenosphere: 100–200 km · partially molten, mobile transition zone: 410–660 km · mineral phase changes lower mantle: 660–2,900 km
• Outer core: 2,900–5,100 km · liquid iron-nickel generates Earth’s magnetic field (the dynamo theory)
• Inner core: 5,100–6,371 km · solid iron-nickel solidified by pressure, at a temperature of about 5,000–6,000°C

Seismic waves and probing Earth’s interior:

• P waves (compressional waves): pass through solids, liquids, and gases speed: 5–8 km/s in the crust · 8–14 km/s in the mantle
• S waves (shear waves): pass only through solids (not liquids) travel at 57–60% of the speed of P waves S waves cannot pass through the outer core → evidence the outer core is liquid
• The Moho discontinuity: the crust-mantle boundary a sharp jump in seismic-wave speed (from 6 to 8 km/s) discovered by Andrija Mohorovičić (1909)
• The Gutenberg discontinuity: the mantle-outer core boundary (2,900 km) P-wave reflection and refraction, S waves blocked
• The Lehmann discontinuity: the outer core-inner core boundary (5,100 km) P-wave speed increases again

Seismic tomography:

• Recording many earthquakes → produces a 3D image of mantle structure
• Cool, sinking slabs: appear as fast, blue-colored regions
• Hot mantle plumes: appear as slow, red-colored regions
• Confirms slab descent to depths of thousands of kilometers

The geomagnetic field:

• A dipole field: the geomagnetic north and south poles (about 11° offset from the geographic poles)
• Cause: electrical currents in the liquid iron-nickel outer core (the dynamo theory) mantle convection + Earth’s rotation + the Coriolis effect
• Magnetic reversals: have occurred hundreds of times in geologic history the most recent: the Matuyama-Brunhes reversal, 780,000 years ago the field’s current strength is declining (a possible sign of a coming reversal?)
• Paleomagnetism: iron minerals in rock record the magnetic-field direction as they cool used to measure seafloor spreading and plate-movement speed
• Magnetic anomalies: used in ore-deposit exploration and archaeological dating
• Geomagnetic storms: interaction with solar wind, causing power and communication disruptions

Gravity anomalies:

• Earth’s standard gravity: varies with latitude
• Free-air anomaly: corrected for elevation
• Bouguer anomaly: corrected for topographic mass negative anomaly: mountain ranges (light crust with a root) positive anomaly: trenches (dense mantle exposed)
• Isostasy: the principle of crustal equilibrium Airy model: equilibrium through crustal-thickness variation Pratt model: equilibrium through crustal-density variation
• Applications: studying crustal structure, resource exploration

Geothermal heat:

• The geothermal gradient: rises roughly 25–30°C per km
• Heat sources: radioactive isotope decay (U, Th, K) + primordial heat
• Uses of geothermal energy: Iceland: geothermal power and district heating (90% of its energy) New Zealand, the Philippines, Kenya EGS (enhanced geothermal systems): artificial geothermal reservoirs

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Seismology

Earthquake mechanism:

• Elastic rebound theory: Harry Reid (1906 San Francisco earthquake) strain energy accumulates in rock → reaches a limit → fault slip → elastic rebound
• Hypocenter (focus): the point where seismic energy is released
• Epicenter: the point on the surface directly above the hypocenter
• Focal depth: shallow earthquakes (0–70 km): 75% of total energy · greatest damage intermediate-depth earthquakes (70–300 km) deep-focus earthquakes (300–700 km): within subducting slabs

Seismic wave types:

• Body waves: propagate through Earth’s interior P waves, S waves
• Surface waves: propagate along the surface Love waves: horizontal shear, an S-wave type Rayleigh waves: elliptical particle motion surface waves: large amplitude, cause the most damage

Earthquake magnitude and intensity:

• Richter magnitude (ML): measures the maximum amplitude on a seismograph a logarithmic scale: each unit increase = 10x the amplitude, 32x the energy
• Moment magnitude (Mw): today’s standard Mw = (2/3) log M0 − 10.7 based on fault area, slip displacement, and rock rigidity
• Modified Mercalli intensity (MMI): a I–XII scale reflects local ground conditions and describes the degree of damage
• Notable historical earthquakes: 1906 San Francisco: M7.9 · fire damage 1960 Valdivia, Chile: M9.5, the largest ever recorded 1964 Alaska: M9.2 2004 Sumatra: M9.1 · the Indian Ocean tsunami 2011 Tohoku, Japan: M9.0 · Fukushima

Earthquake hazards:

• Direct damage: surface rupture, structural collapse
• Tsunami: triggered by submarine earthquakes, volcanic eruptions, or landslides in the open ocean: low wave height, high speed (800 km/h) near the coast: wave height surges (tens of meters) · energy concentrates the 2004 Indian Ocean tsunami: about 220,000 deaths early warning: the DART system, the Pacific Tsunami Warning Center
• Liquefaction: loose, saturated sandy soil: seismic shaking → instant loss of strength causes building subsidence and buried structures to float upward 1964 Niigata, 1995 Kobe, 2011 Christchurch
• Slope instability: earthquake-triggered landslides and rockfalls
• Fire: gas-line and electrical damage causing fire (Kobe, 1995)

Earthquake preparedness and prediction:

• Earthquake early-warning systems: P-wave detection → alert issued → seconds to tens of seconds before the S wave and surface waves arrive Korea: the Korea Meteorological Administration’s early-warning system Japan: among the world’s most advanced (J-Alert)
• Seismic design: base isolation: separating a building from the ground (lead-rubber bearings) damping: absorbing energy with dampers seismic reinforcement: strengthening rigidity Korea: seismic design has been mandatory for buildings six stories or taller since 2005
• Earthquake prediction: short-term prediction (days ahead): close to scientifically impossible medium-term prediction (decades ahead): seismic gaps, statistical recurrence intervals seismic-hazard maps: probabilistic seismic-hazard analysis
• Seismic-vulnerability assessment: site-response analysis: ground-amplification effects HAZUS: damage-estimation software

Volcanic activity and its relationship to earthquakes:

• Volcanic earthquakes: magma movement → high-frequency and long-period volcanic tremor
• Stratospheric volcanic ash: affects climate (Mount Pinatubo, 1991)
• Volcano types: shield volcanoes (Hawaii), stratovolcanoes (Mount Fuji)
• Korean peninsula seismic setting: intraplate earthquakes: relatively weak the Yangsan, Ulsan, and Gyeongju faults: their current activity is debated the 2016 Gyeongju M5.8 and 2017 Pohang M5.4 earthquakes: prompted a reassessment of seismic design the Pohang earthquake: possibly linked to water injection at a geothermal power plant (a potential induced earthquake)

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Frequently Asked Questions

Q. What was the decisive evidence for plate tectonics, and why wasn’t Wegener recognized in his lifetime? A. Wegener offered several lines of evidence for continental drift but could not explain “how” continents move, which led the geology establishment of his day to dismiss the idea. The physical laws known at the time made it seem impossible for continents to plow through the seafloor. Two discoveries proved decisive. First was seafloor spreading: oceanic exploration in the 1950s revealed the seafloor’s topography, leading Harry Hess to propose that ridges create new crust while trenches destroy it, a model confirmed by drilling results showing that seafloor crust grows older with distance from the ridge. Second was paleomagnetic striping: in 1963, Vine and Matthews identified symmetric magnetic-reversal stripes on either side of mid-ocean ridges, powerful evidence that the seafloor records the magnetic-field direction as it spreads. Combined with a mechanism for movement (mantle convection plus slab pull), these two discoveries established plate tectonics as geology’s unifying theory by the late 1960s — about 40 years after Wegener’s death (1930). The lesson from the history of science: even a correct conclusion is hard to accept without an explained mechanism, and once decisive evidence accumulates, a paradigm can shift quickly.

Q. Following the 2016 Gyeongju earthquake, could Korea experience an even larger earthquake? A. Korea sits in an “intraplate” region outside the main circum-Pacific seismic belt, making it relatively — but not absolutely — safe. Looking at the current seismic picture, the Korean peninsula has several active faults, including the Yangsan and Ulsan faults. The 2016 Gyeongju M5.8 was the largest since instrumental monitoring began, and the 2017 Pohang M5.4 is supported by scholarly opinion as a possible induced earthquake linked to water injection at a geothermal plant. Historical records note an estimated M6.5-plus earthquake at Gyeongju in 779 CE, and a substantial earthquake at Gangneung in 1681. Expert opinion is divided on the maximum expected earthquake, but an M7.0-plus event on the Korean peninsula cannot be ruled out. Japan’s stringent seismic-design standards reflect its position directly on a plate boundary, while Korea has historically applied comparatively lower standards. Since 2016, Korea has been strengthening its seismic-design codes and pursuing seismic-retrofit projects for older buildings. The southeastern region — Gyeongju, Pohang, and Busan — warrants particular caution given its proximity to the Yangsan fault zone. Practical precautions include enrolling in earthquake early-warning alerts, checking a building’s seismic-retrofit status, and knowing earthquake evacuation procedures.