Geology tricks that make Earth's structure click

CMOI (C=Crust, M=Mantle, O=Outer core, I=Inner core): Crust, Mantle, Outer core, Inner core. 'Crispy M&Ms Often Impress.'

Crust5–70 km — basalt (oceanic), granite (continental)

MohoCrust-mantle boundary — seismic velocity jump

Mantle2,900 km — silicate, convects slowly

Outer coreLiquid Fe-Ni — generates magnetic field

Inner coreSolid Fe-Ni — pressure keeps it solid

P-waves (compressional) travel through solids AND liquids. S-waves (shear) through solids ONLY. Shadow zones reveal liquid core.

Seismic Waves — P-waves (Primary): compressional — push-pull motion parallel to travel direction. Fastest, travel through everything (solid, liquid, gas). S-waves (Secondary): shear — motion perpendicular to travel. Slower, travel through solids only. S-wave shadow zone: 103°–142° from earthquake → liquid outer core blocks S-waves — proved outer core is liquid (Oldham, 1906; Lehmann inner core, 1936). Surface waves (Love and Rayleigh): along surface — most destructive. Richter vs moment magnitude: Richter outdated, Mw now standard.

Moho (Mohorovičić discontinuity): crust-mantle boundary. Seismic waves speed up here. ~7 km oceanic, ~35 km continental.

The Mohorovičić Discontinuity — Andrija Mohorovičić (1909): noticed seismic waves from Croatian earthquake arrived faster than expected → a velocity discontinuity. At Moho: P-wave velocity jumps from ~6 km/s (crust) to ~8 km/s (mantle). Oceanic crust: ~5–10 km thick. Continental crust: ~25–70 km (thickest under mountain ranges — isostatic roots). Project Mohole (1960s): attempted to drill through oceanic crust to Moho — abandoned. IODP: modern drilling in ocean floor approaching Moho.

Isostasy: crust floats on denser mantle. Mountains have roots. Remove ice (glacial rebound) → crust rises.

Isostasy — Two models: Airy (mountains have deep roots — same density, varying thickness) and Pratt (varying density, flat base). Mountains: thick crustal root extends down into mantle (buoyancy). Erode mountains → root rises (isostatic rebound). Glacial isostatic adjustment: Fennoscandia still rising ~1 cm/yr since Pleistocene ice melted (~10,000 ya). Gravity anomalies: free-air, Bouguer — measure departures from isostatic equilibrium. Ocean basins: thin dense oceanic crust sits low. Continental shelves: transition zones.

Geomagnetic field: liquid outer core convects + Earth's rotation → dynamo effect. Protects Earth from solar wind.

Earth's Magnetic Field — Dynamo theory: convection in liquid iron outer core + Coriolis effect from Earth's rotation → self-sustaining electromagnetic dynamo. Field strength: ~25–65 microtesla. Magnetic poles: not aligned with geographic poles (~11° offset currently). Magnetic declination: angle between magnetic and true north — varies by location. Magnetosphere: deflects solar wind. Without it: solar radiation strips atmosphere (like Mars). Magnetic reversals: occur every ~200,000–300,000 years on average (irregular). Last reversal: Brunhes-Matuyama ~780,000 ya. Recorded in ocean floor magnetic stripes.

Mantle convection: hot mantle rises, cools, sinks — drives plate tectonics. Whole mantle vs layered convection debated.

Mantle Convection — Mantle solid but flows on geological timescales (viscosity ~10²¹ Pa·s — 10²² times more viscous than water). Heat sources: primordial heat (accretion) + radioactive decay (U, Th, K). Hot material: less dense → rises. Cool material: denser → sinks. Two models debated: whole-mantle convection (660 km boundary permeable) vs layered convection (separate upper and lower mantle cells). Plumes: narrow columns of anomalously hot mantle rising from core-mantle boundary (D'' layer) → hot spots (Hawaii, Iceland, Yellowstone). Slabs: cold subducted lithosphere sinking — pulls plates ('slab pull' > 'ridge push').

Earth's heat: ~47 TW total. Half from radioactive decay (U, Th, K), half from primordial accretion heat.

Earth's Heat Budget — Total heat flow: ~47 terawatts (TW). Sources: radioactive decay (~50%) — U-238, U-235, Th-232, K-40 in mantle and crust; primordial heat (~50%) — residual from accretion and differentiation 4.5 bya. Oceanic heat flow: higher (thin crust, mid-ocean ridges). Continental heat flow: lower (thick crust insulates). Hot spots: local high heat flow. Core cooling: inner core solidification releases latent heat → sustains dynamo. Geothermal gradient: ~25–30°C/km in continental crust. Geothermal energy: exploits this heat (Iceland, New Zealand, Kenya).

Oceanic crust: thin (5–10 km), dense (basalt, 3.0 g/cm³), young (< 200 Ma). Continental: thick, less dense (granite, 2.7 g/cm³), ancient.

Oceanic vs Continental Crust — Oceanic crust: basaltic, 5–10 km thick, density 3.0 g/cm³, youngest ~180–200 Ma (constantly recycled by subduction). MORB (Mid-Ocean Ridge Basalt) (mid-ocean ridge basalt): most common rock type on Earth's surface. Ophiolites: ancient oceanic crust now on land. Continental crust: granitic (felsic), 25–70 km, density 2.7 g/cm³, some cratons >3.8 Ga old (Jack Hills zircons ~4.4 Ga). Why ocean basins are low: denser oceanic crust sits deeper in mantle (isostasy). Why continents persist: lower density = can't subduct easily.

Lithosphere: rigid outer layer (crust + upper mantle), ~100 km thick. Asthenosphere: partially molten, weak — plates slide on it.

Lithosphere and Asthenosphere — Lithosphere: brittle, rigid — includes crust AND uppermost solid mantle. Oceanic lithosphere: ~100 km thick. Continental: ~150–200 km. Plates ARE lithosphere. Asthenosphere: upper mantle, ~100–700 km, partially molten (~1% melt), weak and ductile — allows plates to move. Low-velocity zone (LVZ (Low Velocity Zone)): seismic waves slow here → detected the asthenosphere. Mesosphere: below asthenosphere, stronger again (higher pressure). Distinction: lithosphere/asthenosphere is MECHANICAL (rigid vs weak). Crust/mantle is COMPOSITIONAL (chemical difference at Moho).

Shield volcanoes (basalt, gentle): Hawaii. Stratovolcanoes (andesite, explosive): Cascades. Calderas: collapsed magma chambers.

Volcano Types and Structure — Shield volcanoes: low viscosity basaltic lava, gentle slopes, non-explosive. Hawaiian type (hot spot). Mauna Loa: largest volcano on Earth by volume. Stratovolcanoes (composite): alternating lava and pyroclastic layers, steep slopes, highly explosive. Andesitic/rhyolitic magma (subduction zones): Mt. St. Helens, Pinatubo, Krakatoa. Cinder cones: small, steep, single eruption. Calderas: magma chamber collapse after eruption (Yellowstone, Crater Lake). Supervolcanoes: VEI (Volcanic Explosivity Index) 8, global climate impact (Toba ~74,000 ya). Volcanic explosivity index (VEI): logarithmic scale 0–8.

Earthquake focus = hypocenter (underground). Epicenter = surface point above. Shallow (<70 km) most destructive.

Earthquakes — Focus (hypocenter): actual underground point of rupture. Epicenter: point on surface directly above. Shallow focus (<70 km): most destructive — energy close to surface. Intermediate (70–300 km), deep (>300 km): only at subduction zones. Seismic moment magnitude (Mw): replaces Richter. Each unit = ~32× more energy. Mw 9.0 = 1,000× energy of Mw 7.0. Faults: normal (extension), reverse/thrust (compression), strike-slip (transform). San Andreas: right-lateral strike-slip. Elastic rebound theory (Reid, 1906): strain builds until fault slips. Tsunamis: vertical seafloor displacement → ocean displacement.

We've never drilled past 12 km. Everything deeper = inferred from seismology, meteorites, and high-pressure lab experiments.

Evidence for Earth's Interior — Methods: Seismology (shadow zones, travel times, waveforms — most powerful). Moment of inertia: Earth's rotation reveals mass distribution — denser interior than surface average (Cavendish experiment). Meteorites: iron meteorites = analog for Earth's core (same age, composition). High-pressure experiments: diamond anvil cells recreate core conditions. Geodesy: Earth's shape (oblate spheroid — equatorial bulge 21 km). Gravity measurements: GRACE satellite mapped density variations. Magnetic field: reveals liquid iron outer core. Neutrino geoneutrinos: detect radioactive decay deep in Earth — measure heat production directly.