Earth's internal structure β densities, compositions, and why we know what we can't see
Crust: oceanic (5β10 km, basalt, denser) and continental (25β70 km, granite, lighter). Moho: crust-mantle boundary (~7 km oceanic, ~35 km continental). Mantle: 2,900 km thick, silicate rock, solid but convects slowly. Outer core: liquid iron-nickel, ~2,250 km thick, generates magnetic field. Inner core: solid iron-nickel (~1,220 km radius), hot but solid (pressure). Total Earth radius: 6,371 km. We've only drilled ~12 km (Kola Superdeep Borehole). Everything below is inferred from seismic waves.
Crust
5β70 km β basalt (oceanic), granite (continental)
Moho
Crust-mantle boundary β seismic velocity jump
Mantle
2,900 km β silicate, convects slowly
Outer core
Liquid Fe-Ni β generates magnetic field
Inner core
Solid Fe-Ni β pressure keeps it solid
Seismic Waves
P-waves (compressional) travel through solids AND liquids. S-waves (shear) through solids ONLY. Shadow zones reveal liquid core.
Seismic Waves
How earthquakes reveal Earth's interior β the waves that mapped what we cannot see
P-waves (Primary): compressional β push-pull motion parallel to travel direction. Fastest, travel through everything (solid, liquid, gas). S-waves (Secondary): shear β motion perpendicular to travel. Slower, travel through solids only. S-wave shadow zone: 103Β°β142Β° from earthquake β liquid outer core blocks S-waves β proved outer core is liquid (Oldham, 1906; Lehmann inner core, 1936). Surface waves (Love and Rayleigh): along surface β most destructive. Richter vs moment magnitude: Richter outdated, Mw now standard.
The Moho
Moho (MohoroviΔiΔ discontinuity): crust-mantle boundary. Seismic waves speed up here. ~7 km oceanic, ~35 km continental.
The MohoroviΔiΔ Discontinuity
The sharp boundary between crust and mantle β discovered by seismic velocity jump
Andrija MohoroviΔiΔ (1909): noticed seismic waves from Croatian earthquake arrived faster than expected β a velocity discontinuity. At Moho: P-wave velocity jumps from ~6 km/s (crust) to ~8 km/s (mantle). Oceanic crust: ~5β10 km thick. Continental crust: ~25β70 km (thickest under mountain ranges β isostatic roots). Project Mohole (1960s): attempted to drill through oceanic crust to Moho β abandoned. IODP: modern drilling in ocean floor approaching Moho.
Isostasy
Isostasy: crust floats on denser mantle. Mountains have roots. Remove ice (glacial rebound) β crust rises.
Isostasy
The principle that crust floats in gravitational equilibrium on the mantle β like icebergs in water
Two models: Airy (mountains have deep roots β same density, varying thickness) and Pratt (varying density, flat base). Mountains: thick crustal root extends down into mantle (buoyancy). Erode mountains β 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 β measure departures from isostatic equilibrium. Ocean basins: thin dense oceanic crust sits low. Continental shelves: transition zones.
Earth's Magnetic Field
Geomagnetic field: liquid outer core convects + Earth's rotation β dynamo effect. Protects Earth from solar wind.
Earth's Magnetic Field
How the liquid outer core generates the magnetic shield that makes complex life possible
Dynamo theory: convection in liquid iron outer core + Coriolis effect from Earth's rotation β self-sustaining electromagnetic dynamo. Field strength: ~25β65 microtesla. Magnetic poles: not aligned with geographic poles (~11Β° offset currently). Magnetic declination: angle between magnetic and true north β varies by location. Magnetosphere: deflects solar wind. Without it: solar radiation strips atmosphere (like Mars). Magnetic reversals: occur every ~200,000β300,000 years on average (irregular). Last reversal: Brunhes-Matuyama ~780,000 ya. Recorded in ocean floor magnetic stripes.
Mantle Convection
Mantle convection: hot mantle rises, cools, sinks β drives plate tectonics. Whole mantle vs layered convection debated.
Mantle Convection
The slow churning of the mantle that drives all of plate tectonics
Mantle solid but flows on geological timescales (viscosity ~10Β²ΒΉ PaΒ·s β 10Β²Β² times more viscous than water). Heat sources: primordial heat (accretion) + radioactive decay (U, Th, K). Hot material: less dense β rises. Cool material: denser β 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) β hot spots (Hawaii, Iceland, Yellowstone). Slabs: cold subducted lithosphere sinking β pulls plates ('slab pull' > 'ridge push').
Earth's Heat Budget
Earth's heat: ~47 TW total. Half from radioactive decay (U, Th, K), half from primordial accretion heat.
Earth's Heat Budget
Where Earth's internal heat comes from β and why it's still hot 4.5 billion years later
Total heat flow: ~47 terawatts (TW). Sources: radioactive decay (~50%) β U-238, U-235, Th-232, K-40 in mantle and crust; primordial heat (~50%) β 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 β sustains dynamo. Geothermal gradient: ~25β30Β°C/km in continental crust. Geothermal energy: exploits this heat (Iceland, New Zealand, Kenya).
Two fundamentally different types of crust β different composition, age, and behavior at plate boundaries
Oceanic crust: basaltic, 5β10 km thick, density 3.0 g/cmΒ³, youngest ~180β200 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), 25β70 km, density 2.7 g/cmΒ³, 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 and Asthenosphere
Lithosphere: rigid outer layer (crust + upper mantle), ~100 km thick. Asthenosphere: partially molten, weak β plates slide on it.
Lithosphere and Asthenosphere
The mechanical layers that make plate tectonics possible β distinct from the compositional layers
Lithosphere: brittle, rigid β includes crust AND uppermost solid mantle. Oceanic lithosphere: ~100 km thick. Continental: ~150β200 km. Plates ARE lithosphere. Asthenosphere: upper mantle, ~100β700 km, partially molten (~1% melt), weak and ductile β allows plates to move. Low-velocity zone (LVZ (Low Velocity Zone)): seismic waves slow here β 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).
Three major volcano types β each reflecting different magma composition and tectonic setting
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 0β8.
Earthquakes
Earthquake focus = hypocenter (underground). Epicenter = surface point above. Shallow (<70 km) most destructive.
Earthquakes
The seismic events that reveal both Earth's structure and hazards at plate boundaries
Focus (hypocenter): actual underground point of rupture. Epicenter: point on surface directly above. Shallow focus (<70 km): most destructive β energy close to surface. Intermediate (70β300 km), deep (>300 km): only at subduction zones. Seismic moment magnitude (Mw): replaces Richter. Each unit = ~32Γ more energy. Mw 9.0 = 1,000Γ 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 β ocean displacement.
Earth's Interior β Evidence
We've never drilled past 12 km. Everything deeper = inferred from seismology, meteorites, and high-pressure lab experiments.
Evidence for Earth's Interior
How geologists know what's inside Earth β without being able to go there
Methods: Seismology (shadow zones, travel times, waveforms β most powerful). Moment of inertia: Earth's rotation reveals mass distribution β 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 β 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 β measure heat production directly.
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🎓 Common Exam Questions
Q: What does CMOI stand for and describe each of Earth's four main layers?
A: CMOI = Crust, Mantle, Outer core, Inner core. Crust: thin outermost layer. Oceanic crust is 5-10 km thick, basaltic (density 3.0 g/cm3), young (maximum ~200 Ma), created at mid-ocean ridges. Continental crust is 25-70 km thick, granitic (density 2.7 g/cm3), ancient (up to 4 Ga). Mantle: 2900 km thick, silicate rock, solid but ductile over geologic time β convects very slowly. Upper mantle contains the asthenosphere (partially molten, where plates slide). Lower mantle is denser. Outer Core: 2200 km thick, liquid iron-nickel β generates Earth's magnetic field through the geodynamo. Inner Core: solid iron-nickel sphere about 1220 km radius β hotter than the outer core but pressure is so great it remains solid.
Q: How do P-waves and S-waves reveal Earth's interior structure?
A: P-waves (Primary/compressional): push-pull motion parallel to travel direction. Travel through solids AND liquids. Fastest seismic waves (~6-8 km/s in crust). S-waves (Secondary/shear): side-to-side motion perpendicular to direction. Travel through solids ONLY β cannot pass through liquids. The S-wave shadow zone (S-waves absent on far side of Earth from an earthquake) proved the outer core is liquid. P-waves are refracted (bent) at layer boundaries, revealing changes in density. Travel time anomalies reveal the core, mantle, and crust boundaries. Seismic tomography uses thousands of recorded waves to image 3D structures inside the Earth β like a CT scan of the planet.
Q: What is the difference between the lithosphere and asthenosphere and why does it matter for plate tectonics?
A: Lithosphere: rigid outer layer β includes the crust AND uppermost solid mantle. About 100 km thick under oceans (thickens with age, up to 200 km), 100-250 km under continents. This is what breaks into tectonic plates. Asthenosphere: the zone of the upper mantle just below the lithosphere. Partially molten (1-2% melt), ductile and weak over long time scales β plates slide over it. The LVZ (Low Velocity Zone) within the asthenosphere is where seismic waves slow because of partial melting. Key distinction from compositional layers: lithosphere includes upper mantle that has the same composition as the asthenosphere but behaves rigidly. The mechanical boundary (litho vs astheno) is more relevant for plate tectonics than the compositional boundary (crust vs mantle = the Moho).
Q: What is isostasy and what is isostatic rebound?
A: Isostasy: the crust is in gravitational equilibrium, floating on the denser mantle. Like icebergs floating in water β the portion above water is proportional to the root below. Mountains have deep crustal roots to compensate for their topographic height (Airy isostasy). Alternatively, variations in crustal density can compensate (Pratt isostasy β explains why ocean trenches are deep). Isostatic rebound: when a weight is removed (glacier melts, mountains erode), the crust slowly rises back toward equilibrium. Scandinavia is still rising at about 1 cm/year after the Pleistocene ice sheet melted ~10,000 years ago. Hudson Bay is also rebounding. Ancient shorelines found far above current sea level are evidence of isostatic rebound.
Q: What is the VEI (Volcanic Explosivity Index) and how do shield volcanoes differ from stratovolcanoes?
A: VEI (Volcanic Explosivity Index): logarithmic scale 0-8 based on volume of ejecta and eruption column height. VEI 0-2: gentle, effusive (Hawaiian volcanoes). VEI 4-5: major eruptions (Mount St. Helens 1980 = VEI 5). VEI 6: Pinatubo 1991 β caused global temperature drop of 0.5 degrees C. VEI 7-8: super-eruptions (Toba 74,000 years ago = VEI 8, nearly caused human extinction). Shield volcanoes: low-viscosity basaltic lava, broad gentle slopes, non-explosive, VEI 0-1. Examples: Mauna Loa, Kilauea (Hawaii). Form at hot spots and divergent boundaries. Stratovolcanoes (composite): high-viscosity silicic lava traps gas, builds pressure, explosive eruptions with ash clouds and pyroclastic flows. VEI 3-7. Examples: Mt. Fuji, Mt. Rainier, Pinatubo. Form at subduction zones.