Geology, the atmosphere, oceans, and the climate system.
A study reference, not a substitute for primary sources.
Updated 2026-09-08.
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Structure of the Earth
Crust — outermost layer; continental crust is thicker (~30–50 km) and less dense (granite-rich, felsic); oceanic crust is thinner (~5–10 km) and denser (basalt-rich, mafic).
Moho (Mohorovičić discontinuity) — boundary between crust and mantle; discovered by seismic velocity changes (Andrija Mohorovičić, 1909).
Mantle — ~2,900 km thick; mostly solid silicate rock but flows plastically over geologic timescales. The asthenosphere (upper mantle, ~100–350 km) is partially molten and allows plate movement.
Outer core — liquid iron-nickel; its convection generates Earth’s magnetic field via the geodynamo.
Inner core — solid iron-nickel despite temperatures of ~5,000–6,000 °C; extreme pressure keeps it solid; radius ~1,220 km.
Seismic waves — P-waves (primary, compressional, travel through solids and liquids); S-waves (secondary, shear, travel only through solids). The liquid outer core blocks S-waves, revealing its liquid state.
Shadow zone — region ~103°–142° from an earthquake epicenter that receives no direct S- or P-waves, indicating the liquid outer core.
Gutenberg discontinuity — boundary between the mantle and the outer core at ~2,900 km depth; named for seismologist Beno Gutenberg, who identified it from P-wave travel-time data in 1914; marks the transition from solid silicate mantle to liquid iron-nickel outer core.
Love waves — surface seismic waves that move the ground horizontally perpendicular to the direction of wave propagation; they are faster than Rayleigh waves and cause significant lateral shaking damage.
Rayleigh waves — surface seismic waves that roll the ground in an elliptical retrograde motion (like ocean waves); slower than body waves; responsible for much of the shaking felt during earthquakes.
Inge Lehmann — Danish seismologist (1888–1993) who discovered Earth’s solid inner core in 1936 by analyzing anomalous P-wave arrivals in the shadow zone; her paper “P′” is among the most important in geophysics.
Plate Tectonics
Theory — Earth’s lithosphere (crust + rigid upper mantle) is divided into ~15 major tectonic plates that move over the asthenosphere; formalized in the 1960s, building on Alfred Wegener’s continental drift hypothesis (1912).
Driving mechanisms — ridge push (new oceanic crust pushes plates apart at mid-ocean ridges) and slab pull (subducting dense oceanic crust drags plates); mantle convection plays a role.
Divergent boundaries — plates move apart; creates mid-ocean ridges (e.g., Mid-Atlantic Ridge) or continental rift valleys (e.g., East African Rift). New oceanic crust forms by seafloor spreading.
Convergent boundaries — plates move together; produces subduction zones (oceanic sinking beneath continental or oceanic crust, generating ocean trenches, volcanoes, and earthquakes) or collision zones (two continents: mountain building, e.g., Himalayas from India–Eurasia collision).
Transform boundaries — plates slide horizontally past each other; produces strike-slip faults (e.g., San Andreas Fault, California).
Pangaea — supercontinent that began breaking up ~175–200 Ma. Earlier supercontinents include Rodinia (~1.1 Ga) and Columbia/Nuna.
Paleomagnetism — alternating bands of normal and reversed magnetic polarity symmetrically flanking mid-ocean ridges provided key evidence for seafloor spreading (Vine-Matthews-Morley hypothesis, 1963).
Hot spots — mantle plumes stationary relative to plates; the Hawaiian Island chain formed as the Pacific Plate moved over the Hawaiian hot spot.
Hawaiian-Emperor seamount chain — the Hawaiian Islands grade northwest into a chain of progressively older seamounts that bends sharply northward at the Emperor chain (~47 Ma); the bend records a change in Pacific Plate motion direction; Kauai is older than Hawaii (the Big Island), which sits over the active hot spot.
Wadati-Benioff zone — the inclined plane of seismicity that defines a subducting oceanic plate as it descends into the mantle; earthquakes occur down to ~670 km; named independently by Japanese seismologist Kiyoo Wadati (1920s) and American seismologist Hugo Benioff (1950s).
Wilson cycle — the full life cycle of an ocean basin: continental rifting → new ocean forms → ocean widens → subduction begins → ocean narrows → continents collide → suture zone; named for J. Tuzo Wilson; explains why ancient orogenic belts contain ophiolites and blueschist.
Vine-Matthews-Morley hypothesis — the 1963 proposal by Frederick Vine, Drummond Matthews, and independently Lawrence Morley that symmetric magnetic-reversal stripes on the seafloor flanking mid-ocean ridges record seafloor spreading and geomagnetic reversals; provided the decisive evidence for plate tectonics.
Geomagnetic reversal — periodic swap of Earth’s magnetic north and south poles; recorded in the palaeomagnetic signature of cooling oceanic crust; the current normal polarity epoch (Brunhes chron) began ~780,000 years ago; reversals are recorded in the geomagnetic polarity time scale (GPTS).
Gondwana — the southern supercontinent that included South America, Africa, Antarctica, Australia, the Indian subcontinent, and the Arabian Peninsula; began to break up after Pangaea split (~180 Ma); named by Eduard Suess.
Laurasia — the northern supercontinent (North America, Europe, most of Asia) that formed when Pangaea split; separated from Gondwana by the Tethys Sea.
Rodinia — Precambrian supercontinent assembled ~1.1 Ga and rifted apart ~750–700 Ma; preceded Pangaea; its reconstruction is debated but Australia and Antarctica are placed near the core.
Seafloor spreading — the process by which new oceanic crust is created at mid-ocean ridges as magma wells up, cools, and spreads symmetrically outward; proposed by Harry Hess (~1960–1962); confirmed by palaeomagnetic stripes.
Major tectonic plates
Plate
Notes
Pacific
Largest; mostly oceanic; subducts under surrounding plates
North American
Includes most of North America and part of the North Atlantic
Eurasian
Colliding with Indian Plate (Himalayas)
Indian
Convergent with Eurasian; spreading at Carlsberg Ridge
Antarctic
Surrounds Antarctica; bounded mostly by divergent boundaries
African
Rifting along East African Rift
Nazca
Subducting under South American Plate (Andes)
Rocks and the Rock Cycle
Igneous rocks
Origin — solidification of magma (underground) or lava (surface).
Extrusive (volcanic) — cool quickly at the surface; fine-grained or glassy. Examples: basalt (most common volcanic rock; oceanic crust), obsidian (volcanic glass), pumice (vesicular, floats on water), rhyolite (felsic extrusive).
Felsic vs mafic — felsic rocks (granite, rhyolite) are silica-rich, lighter colored, lower density; mafic (basalt, gabbro) are magnesium/iron-rich, darker, denser.
Sedimentary rocks
Origin — compaction and cementation (lithification) of sediments, or precipitation from solution.
Clastic — fragments of other rocks: shale (clay-sized particles, most common sedimentary rock), sandstone (sand-sized), conglomerate (rounded gravel-sized clasts), breccia (angular clasts).
Chemical/biochemical — limestone (CaCO3, often from marine organisms; includes chalk and coquina); rock salt (halite, evaporite); chert (silica); coal (compressed organic matter).
Significance — fossils form almost exclusively in sedimentary rock; sedimentary layers (strata) record geologic history.
Metamorphic rocks
Origin — pre-existing rocks transformed by heat, pressure, or hydrothermal fluids without melting.
Contact metamorphism — heat from nearby magma intrusion. Regional metamorphism — large-scale heat and pressure, typically at convergent boundaries.
Metamorphic facies — sets of mineral assemblages that form under specific pressure-temperature conditions, regardless of rock composition; defined by indicator minerals: zeolite facies (low P-T), greenschist facies (chlorite, epidote), blueschist facies (high-P, low-T; subduction zones), amphibolite facies (staurolite, kyanite), granulite facies (high-T, lower-P), and eclogite facies (very high P; garnet + omphacite).
Index minerals — minerals used to track metamorphic grade in pelitic (shale-derived) rocks; in order of increasing temperature: chlorite → biotite → garnet → staurolite → kyanite → sillimanite (Barrow’s zones, Scottish Highlands).
Ophiolite — a sequence of ocean-floor rocks (deep-sea sediments, pillow basalts, sheeted dikes, gabbro, peridotite) thrust onto continental crust during collision; marks the suture of ancient oceans; classic examples in Oman (Semail) and Cyprus (Troodos).
The rock cycle
Cycle — igneous rock is weathered to sediment → lithification → sedimentary rock → metamorphism → metamorphic rock → melting → magma → igneous rock. Any rock type can transform to any other.
Weathering — mechanical (frost wedging, abrasion) breaks rock without changing composition; chemical (oxidation, hydrolysis, carbonation) alters mineral composition. Erosion transports weathered material.
Minerals and the Mohs Scale
Mineral definition — naturally occurring, inorganic, solid, with a definite chemical composition and crystalline structure.
Silicates — most abundant mineral group; built on SiO4 tetrahedra. Includes feldspar, quartz, mica, pyroxene, amphibole, olivine.
Feldspar — most abundant mineral in Earth’s crust; two main types: orthoclase (potassium feldspar) and plagioclase (sodium/calcium).
Quartz — SiO2; very common; hardness 7; varieties include amethyst, chert, flint.
Native elements — gold, silver, copper, sulfur, diamond (carbon), graphite (carbon, different polymorph).
Bowen’s reaction series — N.L. Bowen’s 1922 framework describing the sequence in which silicate minerals crystallize from a cooling basaltic magma; the discontinuous series (olivine → pyroxene → amphibole → biotite mica) and the continuous series (Ca-plagioclase → Na-plagioclase) both converge on orthoclase feldspar, muscovite mica, and quartz at the low-temperature end; explains the compositional diversity of igneous rocks.
Olivine — first mineral to crystallize in Bowen’s series; Mg2SiO4 to Fe2SiO4 solid solution; green, high density; dominant in the upper mantle; the gem variety is peridot.
Silicate structures — SiO4 tetrahedra linked in different arrangements: isolated (nesosilicates, e.g., olivine), single-chain (inosilicates, e.g., pyroxene), double-chain (amphibole), sheet (phyllosilicates, e.g., mica, clay), and framework (tectosilicates, e.g., quartz, feldspar); degree of polymerization increases viscosity in melts.
Mica group — sheet silicates with perfect basal cleavage; muscovite (white/colorless, K-Al mica) and biotite (black, K-Mg-Fe mica) are the main rock-forming varieties; forms under medium-grade metamorphism and in granitic rocks.
Pyroxene — single-chain inosilicate; important in mafic and ultramafic rocks; augite is the most common variety; orthopyroxene (enstatite-ferrosilite) and clinopyroxene families.
Amphibole — double-chain inosilicate; hornblende is the common dark rock-forming variety; distinguished from pyroxene by 60°/120° cleavage angles (vs. pyroxene’s ~90°) and hydroxyl content.
Mohs hardness scale
Hardness
Mineral
Common reference
1
Talc
Softest; feels soapy
2
Gypsum
Fingernail (~2.5)
3
Calcite
Copper coin (~3)
4
Fluorite
5
Apatite
Steel knife blade (~5.5)
6
Orthoclase feldspar
Steel file (~6.5)
7
Quartz
Scratches glass
8
Topaz
9
Corundum
Ruby and sapphire
10
Diamond
Hardest natural mineral
Other mineral properties — cleavage (flat breaking along planes), fracture (irregular breaking), luster (metallic, vitreous, resinous, pearly), streak (color of powdered mineral), specific gravity.
Ore minerals
Hematite — Fe2O3; primary ore of iron; red-brown streak; major constituent of banded iron formations.
Magnetite — Fe3O4; strongly magnetic iron ore; black, high density; also a minor constituent of many igneous rocks.
Bauxite — not a single mineral but a mixture of aluminum hydroxide minerals (gibbsite, boehmite, diaspore); the principal ore of aluminum; forms by intense tropical weathering.
Cassiterite — SnO2; principal ore of tin; very high specific gravity; historically mined in Cornwall and Southeast Asia.
Iron pyrite — FeS2; “fool’s gold”; brassy yellow, cubic crystals; not an iron ore of economic importance but widespread in sedimentary and hydrothermal settings.
Cinnabar — HgS; primary ore of mercury; bright red (“vermilion”); associated with volcanic and hydrothermal activity.
Galena — PbS; principal ore of lead (and a source of silver); very high specific gravity; perfect cubic cleavage; metallic luster.
Dolomite — CaMg(CO3)2; carbonate mineral (and rock name); not typically mined as a metal ore, but used as a refractory material, flux in steelmaking, and agricultural lime.
Geologic Time Scale
Eons — four: Hadean (formation of Earth ~4.6 Ga to 4.0 Ga), Archean (4.0–2.5 Ga; first life), Proterozoic (2.5 Ga–538 Ma; first eukaryotes, first multicellular life), Phanerozoic (538 Ma–present; complex animal life).
“Cambrian Explosion” — rapid diversification of animal body plans at the base of the Phanerozoic (~538 Ma).
Phanerozoic eras and periods (selected)
Era
Period
Ma (start)
Notes
Paleozoic
Cambrian
538
Explosion of animal phyla
Ordovician
485
Marine invertebrates; ends in mass extinction
Silurian
444
First vascular land plants
Devonian
419
“Age of Fishes”; first tetrapods
Carboniferous
359
Vast coal swamps; first reptiles
Permian
299
Ends in largest mass extinction (~96% species lost)
Mesozoic
Triassic
252
First dinosaurs and mammals
Jurassic
201
Dinosaurs dominate; first birds
Cretaceous
145
Flowering plants; ends with K-Pg extinction
Cenozoic
Paleogene
66
Mammals diversify
Neogene
23
Grasslands spread; hominids appear
Quaternary
2.6
Ice ages; Homo sapiens
Tertiary Period — informal/obsolete term for the interval now formally divided into the Paleogene (66–23 Ma) and Neogene (23–2.6 Ma); still encountered in older literature and some applied geology contexts.
Biostratigraphy — the use of fossil assemblages (index fossils) to correlate and date rock strata; developed systematically in the early 19th century by William Smith and Georges Cuvier; complements radiometric (absolute) dating.
Big Five mass extinctions — (1) Ordovician-Silurian (~444 Ma); (2) Late Devonian (~372 Ma); (3) Permian-Triassic (“The Great Dying,” ~252 Ma, ~90–96% species); (4) Triassic-Jurassic (~201 Ma); (5) Cretaceous-Paleogene (K-Pg, ~66 Ma, ~75% species, includes non-avian dinosaurs).
K-Pg extinction cause — Chicxulub impactor (Yucatán Peninsula, ~10 km asteroid) confirmed by iridium layer at the K-Pg boundary (Alvarez hypothesis); Deccan Traps flood volcanism contributed.
Burgess Shale — exceptionally preserved Cambrian Lagerstätte (~508 Ma) in British Columbia, Canada; discovered by Charles Walcott (1909); preserves soft-body anatomy of early animals including Anomalocaris, Opabinia, Hallucigenia, and Wiwaxia; reinterpreted by Harry Whittington, Simon Conway Morris, and Derek Briggs in the 1970s–1980s; central to debates over Cambrian diversity and Stephen Jay Gould’s Wonderful Life.
Cambrian explosion — rapid diversification of animal body plans beginning ~538 Ma, producing representatives of most modern animal phyla within a geologically brief span (~20 million years); causes debated (oxygen rise, ecological interactions, developmental toolkit evolution); also documented in Chengjiang biota (Yunnan, China, ~520 Ma).
Permian-Triassic extinction — the largest mass extinction (~252 Ma); killed ~90–96% of marine species and ~70% of terrestrial vertebrate species; nicknamed “The Great Dying”; likely caused by Siberian Traps volcanism (CO2, SO2 release), ocean anoxia, and global warming; recovery took ~5–10 million years.
Chicxulub crater — ~180 km diameter impact structure beneath the Yucatán Peninsula and Gulf of Mexico; formed ~66 Ma; confirmed as the K-Pg impactor by iridium anomaly, shocked quartz, and tektites; proposed by Luis and Walter Alvarez (1980); confirmed by drilling in the 1990s.
Radiometric dating — uses known decay rates of radioactive isotopes (e.g., U-238→Pb-206 for old rocks; K-40→Ar-40; C-14 for organic material up to ~50,000 years); provides absolute ages of rocks.
Law of superposition — in undisturbed strata, older layers lie below younger ones (Steno, 17th century).
Index fossils — fossils of organisms with wide geographic range and narrow time range, used to correlate and date rock layers.
Earthquakes and Volcanoes
Earthquakes
Cause — sudden release of elastic strain energy when rocks along a fault rupture and slip.
Focus (hypocenter) — the point underground where rupture begins. Epicenter — the point on the surface directly above the focus.
Richter scale — originally local magnitude (ML) based on maximum wave amplitude on a seismograph; logarithmic (each integer ~32x more energy). Saturates for great earthquakes (>~8).
Moment magnitude (Mw) — measures total energy released; does not saturate; the modern standard for large earthquakes. Numerically similar to Richter at moderate magnitudes.
Modified Mercalli scale — measures intensity (I–XII) based on felt effects and damage at a location, not energy released.
Ring of Fire — belt of frequent earthquakes and volcanoes around the Pacific basin, associated with subduction zones.
Seismograph / seismometer — instrument that records seismic waves; three stations needed to triangulate the epicenter.
Tsunami — ocean wave triggered by seafloor displacement (large undersea earthquake, submarine landslide, or volcanic eruption); wavelength hundreds of kilometers; devastating at shore.
Volcanoes
Shield volcano — broad, gently sloping; low-viscosity basaltic lava flows; e.g., Mauna Loa (Hawaii). Least explosive.
Composite volcano (stratovolcano) — steep, symmetrical; alternating lava and pyroclastic layers; high-viscosity silicic magma; e.g., Mount St. Helens, Fuji, Pinatubo. Most explosive.
Cinder cone — small, steep; built from pyroclastic fragments; e.g., Parícutin (Mexico, appeared 1943).
Caldera — large depression formed when a magma chamber empties and the overlying rock collapses; e.g., Yellowstone Caldera (supervolcano).
Pyroclastic flow — fast-moving current of hot gas and volcanic matter; the deadliest volcanic hazard; reached 700 °C+ (e.g., Vesuvius 79 CE, Mount Pelée 1902).
Volcanic explosivity index (VEI) — logarithmic scale 0–8 for eruption size; Pinatubo 1991 ≈ VEI 6, Toba ~74,000 BP ≈ VEI 8.
~75% of atmospheric mass; weather occurs here; temperature decreases with altitude
Stratosphere
12–50 km
Ozone layer (~15–35 km); temperature increases with altitude
Mesosphere
50–85 km
Coldest layer; meteors burn up here
Thermosphere
85–600 km
Very hot (absorbed UV/X-ray); aurora; ISS orbit
Exosphere
>600 km
Gradually merges with space
Composition — ~78% nitrogen (N2), ~21% oxygen (O2), ~0.93% argon (Ar), ~0.04% CO2, plus trace gases and variable water vapor.
Ozone layer — absorbs most UV-B and UV-C radiation; depletion caused by chlorofluorocarbons (CFCs); Montreal Protocol (1987) phased out CFCs; ozone hole over Antarctica is slowly recovering.
Tropopause / stratopause / mesopause — boundary layers between each atmospheric zone.
Ionosphere — the ionized upper portion of Earth’s atmosphere, roughly coinciding with the upper mesosphere and thermosphere (~60–1,000 km altitude); solar UV and X-ray radiation ionize gas molecules, creating free electrons and ions; divided into D layer (~60–90 km, daytime only), E layer (~90–150 km), and F layer (~150–600 km); critical for shortwave (HF) radio propagation — the ionosphere refracts radio waves back to Earth’s surface, enabling long-distance communication; disrupted by solar storms and geomagnetic disturbances; the ISS orbits within the ionosphere; causes GPS signal delays.
Lapse rate — rate of temperature decrease with altitude in the troposphere; dry adiabatic lapse rate ~10 °C/km; moist ~6 °C/km (latent heat released by condensation).
Weather and Climate
Atmospheric dynamics
Pressure systems — high pressure (anticyclone): air sinks, clear skies, winds rotate clockwise in the Northern Hemisphere. Low pressure (cyclone): air rises, clouds and precipitation, winds rotate counterclockwise in the Northern Hemisphere (reversed in Southern Hemisphere, Coriolis effect).
Coriolis effect — deflection of moving air (and water) due to Earth’s rotation; deflects right in the NH, left in the SH.
Trade winds — persistent winds blowing toward the equator from subtropical highs (~30°N/S), deflected westward by Coriolis; NE in NH, SE in SH.
Jet streams — fast, narrow air currents in the upper troposphere at ~30° and ~60° latitude; the polar jet stream steers mid-latitude weather systems; winds typically 120–250 km/h; exploited by transatlantic aviation (faster eastbound flights).
Föhn / Chinook winds — warm, dry downslope winds on the leeward side of mountains; air rises on the windward side losing moisture (and latent heat), then descends and compresses adiabatically, arriving warmer and drier than when it started; called Föhn in the Alps and Chinook in the Rocky Mountains; can rapidly melt snow (“snow eater”).
Orographic lift — forced ascent of air over a mountain range; causes adiabatic cooling, cloud formation, and precipitation on the windward side; rain shadow on the leeward side.
Cyclone vs. anticyclone — a cyclone is a low-pressure system with inward-spiraling winds (counterclockwise in NH); an anticyclone is a high-pressure system with outward-spiraling winds (clockwise in NH); the terms apply to mid-latitude systems as well as tropical cyclones (hurricanes/typhoons).
Hadley cell — equatorial air rises, moves poleward at altitude, sinks at ~30° latitude; drives tropics and subtropics. Ferrel cell (~30°–60°) and polar cell (>60°) complete the three-cell circulation model.
ITCZ (Intertropical Convergence Zone) — band of low pressure and heavy rainfall near the equator where NH and SH trade winds converge.
Fronts and precipitation
Cold front — cold air advancing under warm air; steep, produces cumulonimbus clouds and intense but brief precipitation.
Warm front — warm air advancing over cold air; gradual, produces stratus clouds and steady, prolonged precipitation.
Stationary / occluded fronts — stationary: front not moving; occluded: cold front catches a warm front, lifting the warm air.
Rain shadow — the dry leeward side of a mountain range; moist air rises and precipitates on the windward side, descends dry on the leeward side.
Types of precipitation — rain, snow, sleet (frozen rain), freezing rain (supercooled rain freezing on contact), hail (ice pellets grown in thunderstorm updrafts).
Climate classification
Köppen system — five main climate groups: A (tropical, hot and humid year-round), B (arid/semi-arid, deserts), C (temperate/humid mesothermal, e.g., Mediterranean, humid subtropical), D (continental/humid microthermal, cold winters), E (polar).
Mediterranean climate — dry summers, mild wet winters; “Csb/Csa” in Köppen; found on west coasts at ~30°–45° latitude.
Cloud types (Luke Howard classification) — cirrus (wispy ice-crystal clouds at high altitude, >6 km); cumulus (puffy, convective, fair-weather heaps); stratus (flat, layered, low-altitude); nimbus prefix or suffix (rain-bearing: nimbostratus, cumulonimbus); additional genera include altostratus, altocumulus, stratocumulus, cirrocumulus, cirrostratus; Luke Howard named them in 1803.
Cumulonimbus — the “king of clouds”; a tall, dense convective cloud extending from the lower troposphere to the tropopause (“anvil top”); produces thunderstorms, heavy rain, hail, lightning, and tornadoes.
Hurricane (tropical cyclone) — organized rotating tropical storm with sustained winds ≥119 km/h (74 mph); called hurricane in the Atlantic/E. Pacific, typhoon in the W. Pacific, cyclone in the Indian Ocean; powered by warm ocean water (≥26 °C) and release of latent heat; the eye is calm; the eyewall has the most intense winds; weakens rapidly over land or cool water.
Saffir-Simpson Hurricane Wind Scale — five-category scale for Atlantic/E. Pacific hurricanes based on maximum sustained wind speed: Cat 1 (119–153 km/h) through Cat 5 (≥252 km/h); does not rate storm surge or rainfall; developed by Herbert Saffir and Robert Simpson (~1971).
Enhanced Fujita (EF) scale — revised 2007 scale for tornado intensity based on damage indicators; EF0 (65–85 mph) through EF5 (>200 mph estimated); replaced the original Fujita scale (F-scale, devised by Tetsuya “Ted” Fujita); the strongest tornadoes (EF4–EF5) are responsible for ~70% of tornado fatalities.
Tornado formation — most tornadoes form from supercell thunderstorms with a rotating updraft (mesocyclone); wind shear changes wind speed and direction with height, creating horizontal vortices that are tilted vertical by the updraft; Tornado Alley (central US) is the most tornado-prone region globally.
Fog types — radiation fog (surface cools overnight by radiating heat, common in valleys); advection fog (warm moist air moves over a cold surface, e.g., San Francisco Bay); upslope fog (orographic); sea fog; evaporation/steam fog.
Monsoon — seasonal reversal of winds and associated heavy precipitation; most prominent in South Asia; driven by differential heating of land and ocean.
Albedo — fraction of solar radiation reflected by a surface; snow/ice ~0.8–0.9; forests ~0.1–0.2; oceans ~0.06; affects climate feedback.
Oceanography
Ocean composition — average salinity ~3.5% (35 ppt); dominated by sodium (Na+) and chloride (Cl−); salinity varies by evaporation, precipitation, and freshwater input.
Ocean layers — surface (mixed) zone (warm, well-lit, wind-mixed, to ~200 m); thermocline (sharp temperature decrease, ~200–1,000 m); deep zone (cold, ~2–4 °C, below ~1,000 m; vast majority of ocean volume).
Thermohaline circulation — ocean circulation associated with density differences caused by temperature and salinity. Cooling and changes in salinity help produce dense waters that sink and connect surface and deep circulation. The Atlantic Meridional Overturning Circulation (AMOC) is the Atlantic component of the global overturning system; it is not a synonym for all ocean circulation. Winds and mixing also contribute to the wider circulation.1
Surface currents — driven by wind; shaped by Coriolis effect and continental boundaries. Clockwise gyres in NH, counterclockwise in SH. Key currents: Gulf Stream (warm, NW Atlantic), California Current (cold, E Pacific), Kuroshio Current (warm, W Pacific).
Upwelling — cold, nutrient-rich deep water rises to the surface (e.g., along the California coast, Peru coast); supports rich fisheries.
Tides — caused by gravitational pull of the Moon (primary) and Sun (secondary). Spring tides (highest range): Moon, Sun, Earth aligned (new and full moon). Neap tides (smallest range): Moon and Sun at right angles (quarter moon).
Waves — wind-generated surface waves; energy moves forward but water particles orbit in circles; wavelength, period, and amplitude relate to wind speed and fetch.
Ocean gyres — large rotating systems of ocean currents driven by wind and Coriolis; five major subtropical gyres (North/South Atlantic, North/South Pacific, Indian Ocean) rotate clockwise in the NH and counterclockwise in the SH; the North Pacific Subtropical Gyre collects floating plastic in the Great Pacific Garbage Patch.
Gulf Stream — warm, swift western boundary current in the North Atlantic; flows from the Gulf of Mexico northeastward to Europe; part of the wind-driven gyre system and feeds into the thermohaline circulation; moderates climate of Western Europe.
Mid-ocean ridges — continuous underwater mountain chain ~65,000 km long; form at divergent plate boundaries; the Mid-Atlantic Ridge runs nearly the full length of the Atlantic; the East Pacific Rise is faster-spreading and less rugged; the global mid-ocean ridge system is Earth’s longest mountain range.
Ocean trenches — deepest parts of the ocean; form where oceanic plates subduct; Mariana Trench (W. Pacific) contains Challenger Deep (~10,935 m), Earth’s deepest point; other examples: Tonga Trench, Japan Trench, Puerto Rico Trench.
Thermohaline circulation / AMOC — the “global conveyor belt”; cold, dense water sinks in the Labrador Sea and Nordic Seas, flows south as North Atlantic Deep Water (NADW), upwells in the Southern Ocean and Indian/Pacific Oceans; complete circuit takes ~1,000 years; freshwater input from melting ice sheets could weaken AMOC.
Spring and neap tides — spring tides occur at new and full moon (Sun, Moon, Earth aligned = syzygy); tidal range is maximum. Neap tides occur at first and third quarter moon (Sun and Moon at ~90°); tidal range is minimum; the Bay of Fundy (Nova Scotia) has the world’s largest tidal range (~16 m) due to resonance.
Turbidity — the cloudiness or haziness of a fluid caused by suspended particles (sediment, organic matter, microorganisms) that scatter light; measured in Nephelometric Turbidity Units (NTU) or Formazin Turbidity Units (FTU) using a nephelometer; high turbidity reduces light penetration (limiting photosynthesis), indicates elevated sediment or organic load, and can carry adsorbed contaminants; turbidity currents are underwater avalanches of dense, sediment-laden water that flow down continental slopes at high speed, depositing characteristic graded beds (turbidites) on the ocean floor; turbidites record episodic mass transport events and preserve a stratigraphic record of past earthquakes and slope failures.
Ocean acidification — absorption of atmospheric CO2 forms carbonic acid (H2CO3), lowering ocean pH; ocean surface pH has decreased from ~8.2 to ~8.1 since the industrial era (~0.1 units = ~26% increase in H+ concentration); threatens shell-forming organisms.
El Niño–Southern Oscillation (ENSO)
Normal conditions — trade winds push warm water westward; cold upwelling off Peru coast; thermocline tilts (deep in west, shallow in east).
El Niño — weakening or reversal of trade winds; warm water spreads east across the Pacific; SST anomaly ≥0.5 °C in Niño 3.4 region for ≥5 consecutive overlapping 3-month periods; causes droughts in Australia/SE Asia, flooding on Americas’ west coasts.
La Niña — enhanced trade winds; anomalously cool eastern Pacific; opposite precipitation effects; often follows El Niño.
Southern Oscillation — atmospheric pressure seesaw between the central-eastern Pacific (Tahiti) and Indian Ocean–western Pacific (Darwin); measured by the Southern Oscillation Index (SOI).
ENSO cycle — irregular period of ~2–7 years; significant global climate teleconnections.
The Climate System and Global Warming
Greenhouse effect and carbon cycle
Natural greenhouse effect — solar radiation passes through the atmosphere; Earth’s surface absorbs and re-emits infrared radiation; greenhouse gases (H2O, CO2, CH4, N2O, O3) absorb and re-emit IR, warming the surface. Without it, Earth’s mean surface temperature would be ~−18 °C instead of ~+15 °C.
Greenhouse gases (GHGs) — water vapor (largest natural contribution); CO2 (most important anthropogenic driver, long atmospheric lifetime ~centuries); CH4 (methane, ~28–36x CO2 global warming potential over 100 years, shorter lifetime); N2O (~273x CO2 GWP 100-yr); fluorinated gases (synthetic, very high GWP).
Carbon cycle — carbon moves among atmosphere, biosphere, soils, oceans, and lithosphere. Fast cycle: photosynthesis/respiration. Slow cycle: volcanic outgassing, carbonate-silicate weathering (Urey reaction), fossil fuel formation.
Milankovitch cycles — three periodic variations in Earth’s orbital parameters that affect solar insolation and drive ice ages: (1) eccentricity (ellipticity of orbit, ~100,000-year cycle); (2) obliquity (axial tilt, 22.1°–24.5°, ~41,000-year cycle); (3) precession (wobble of rotation axis, ~23,000-year cycle); proposed by Serbian mathematician Milutin Milanković in the 1920s; confirmed by deep-sea sediment core records in the 1970s (Hays, Imbrie, Shackleton 1976).
Pleistocene — the first epoch of the Quaternary period, spanning ~2.58 Ma to ~11,700 years ago; characterized by recurring glacial-interglacial cycles (ice ages); at the Last Glacial Maximum (LGM, ~20,000 years ago) ice sheets covered much of North America (Laurentide ice sheet) and northern Europe; sea levels were ~120 m lower, exposing land bridges (e.g., Beringia between Asia and North America); megafaunal extinctions (mammoths, mastodons, giant ground sloths) occurred at the end of the Pleistocene, likely from a combination of climate change and human hunting; succeeded by the Holocene interglacial (~11,700 years ago to present).
Ice ages / glacial-interglacial cycles — Earth has experienced multiple glacial periods during the Quaternary (~2.6 Ma–present); the last glacial maximum (LGM) peaked ~20,000 years ago; ice sheets covered much of North America and Northern Europe; current interglacial (Holocene) began ~11,700 years ago; earlier major ice ages include the Marinoan “Snowball Earth” (~635 Ma).
Hydrologic cycle — the continuous movement of water: evaporation (ocean is dominant source) → atmospheric transport → precipitation → surface runoff/infiltration → groundwater → return to ocean; driven by solar energy; key reservoirs: oceans (97%), glaciers (~2%), groundwater, lakes/rivers, atmosphere.
Aquifer — an underground layer of permeable rock or sediment that stores and transmits groundwater; confined aquifers are under pressure (artesian wells); the Ogallala (High Plains) aquifer underlies eight US states and is being depleted faster than recharge.
Karst topography — landscape formed by dissolution of soluble rocks (limestone, dolomite, gypsum) by slightly acidic groundwater; features include sinkholes, caves, disappearing streams, springs, and tower karst; examples: Mammoth Cave (Kentucky), Carlsbad Caverns (New Mexico), Guilin (China).
Water table — the upper surface of the saturated zone in an unconfined aquifer; fluctuates seasonally; the capillary fringe lies just above it.
Pre-industrial CO2 — ~280 ppm for the preceding ~10,000 years (Holocene); ice cores confirm this.
Keeling Curve — continuous CO2 measurement at Mauna Loa Observatory, Hawaii, since 1958 (Charles David Keeling); passed 420 ppm in 2023; shows annual oscillation (NH growing season) superimposed on rising trend.
Observed climate change
Global mean temperature — approximately 1.1–1.2 °C above pre-industrial baseline (~1850–1900) as of the early 2020s; 2023 and 2024 are the warmest years on instrumental record.
Sea level rise — ~3.7 mm/year current rate (accelerating); driven by thermal expansion of seawater and melting of ice sheets/glaciers.
Arctic amplification — Arctic warming ~2–4x the global mean rate; sea ice extent declining; positive feedbacks include ice-albedo feedback.
Extreme events — increased frequency and intensity of heat waves, heavy precipitation events; intensification of tropical cyclones; lengthening fire weather seasons.
IPCC and policy context
IPCC — Intergovernmental Panel on Climate Change; established 1988 by UNEP and WMO; synthesizes published science in assessment reports (AR); does not conduct original research.
AR6 (2021–2022) — Sixth Assessment Report; concluded human influence is “unequivocal”; 1.5 °C above pre-industrial levels likely reached in the early 2030s without deep emissions cuts.
Paris Agreement (2015) — international treaty to hold warming to well below 2 °C and pursue 1.5 °C; nationally determined contributions (NDCs); not legally binding on emissions levels.
Feedback loops — positive: ice-albedo, water vapor, permafrost methane release; negative: Planck response (increased outgoing IR as temperature rises), lapse rate feedback (tropics). Net feedback is positive, amplifying warming.
Climate sensitivity — equilibrium climate sensitivity (ECS): expected global warming from a doubling of CO2 once equilibrium is reached; AR6 assessed likely range 2.5–4 °C, best estimate 3 °C.
Key Figures
Georgius Agricola — German scholar (1494–1555); De Re Metallica (1556) systematized mining, mineralogy, and metallurgy; often called the father of mineralogy.
James Ussher — Archbishop of Armagh; calculated from Biblical genealogies that Earth was created in 4004 BC (published 1650); exemplifies pre-scientific chronology that radiometric dating superseded.
Comte de Buffon (Georges-Louis Leclerc) — French naturalist; conducted cooling-sphere experiments (~1778) and estimated Earth’s age at ~75,000 years, far older than the Ussher date; early empirical challenge to scriptural chronology.
Georges Cuvier — French zoologist (1769–1832); founded vertebrate paleontology; championed catastrophism (Earth shaped by sudden violent events, each followed by new creation); recognized extinction as a fact from fossil evidence.
Eduard Suess — Austrian geologist (1831–1914); named Gondwanaland for the southern supercontinent (from rock and fossil correlations); also named the Tethys Sea and coined “biosphere.”
J. Tuzo Wilson — Canadian geophysicist (1908–1993); proposed transform faults and hot-spot theory; the Wilson cycle describes the opening and closing of ocean basins (rift → ocean → subduction → collision → suture).
Charles Lyell — established uniformitarianism in geology: “the present is the key to the past” (Principles of Geology, 1830–1833).
James Hutton — father of modern geology; recognized the deep age of Earth from rock cycle observations (18th century).
Andrija Mohorovičić — discovered the crust-mantle boundary from seismic data (1909).
Charles David Keeling — began continuous atmospheric CO2 measurements at Mauna Loa (1958); the record bears his name.
Svante Arrhenius — first to calculate quantitatively that doubling CO2 would warm Earth’s surface (~1896); underestimated feedbacks but got the sign right.
Luis and Walter Alvarez — proposed the asteroid impact hypothesis for the K-Pg extinction based on iridium layer evidence (1980).
Willi Köppen — developed the Köppen climate classification system (early 20th century).
William Smith — English geologist (1769–1839); produced the first nationwide geological map (of England, Wales, and part of Scotland, 1815); established the principle of faunal succession (strata can be identified by the fossils they contain); called the “Father of English Geology.”
Charles Richter — American seismologist (1900–1985); developed the Richter magnitude scale (ML) in 1935 with Beno Gutenberg; the scale was designed for Southern California earthquakes recorded on Wood-Anderson seismographs; largely replaced by the moment magnitude scale for large events.
Beno Gutenberg — German-American seismologist (1889–1960); identified the core-mantle boundary (Gutenberg discontinuity); co-developed the Richter scale; established the Gutenberg-Richter law relating earthquake frequency to magnitude.
Uniformitarianism — the principle that the same geological processes operating today operated in the past at the same general rates; championed by James Hutton and systematized by Charles Lyell; contrasts with catastrophism; the famous summary is Hutton’s “no vestige of a beginning, no prospect of an end” and Lyell’s “the present is the key to the past.”
Hutton’s unconformity — James Hutton’s observation at Siccar Point, Scotland (1788) of tilted Silurian greywacke overlain by horizontal Devonian sandstone; demonstrated an enormous time gap and the cyclical nature of geological processes; key evidence for deep time.
Harry Hess — American geologist (1906–1969); proposed seafloor spreading in his 1962 paper “History of Ocean Basins”; argued that new crust forms at mid-ocean ridges and old crust is destroyed at trenches; sometimes called the “Father of Plate Tectonics” alongside Wegener.
Alfred Wegener — German meteorologist and geophysicist (1880–1930); proposed continental drift in Die Entstehung der Kontinente und Ozeane (1915); cited matching coastlines, identical rock formations, and fossil correlations (e.g., Glossopteris flora, Mesosaurus); died on the Greenland ice sheet; his mechanism (an unknown force moving continents) was wrong, but his core hypothesis was vindicated by plate tectonics.
J. Harlen Bretz — American geologist (1882–1981) who proposed (1923) that the Channeled Scablands of eastern Washington were carved by catastrophic glacial outburst floods (Missoula Floods) from Glacial Lake Missoula; the idea was ridiculed for decades but ultimately confirmed; awarded the Penrose Medal in 1979 at age 96.
Marie Tharp — American geologist and cartographer (1920–2006) who mapped the ocean floor with Bruce Heezen; her 1952 discovery of the rift valley at the center of the Mid-Atlantic Ridge provided crucial evidence for seafloor spreading; her work was initially dismissed by Heezen, who called it “girl talk.”
Davis and Penck: landscape development — William Morris Davis’s cycle of erosion described progressive landscape change toward a peneplain. Walther Penck’s Die morphologische Analyse (1924) related landform development to interacting uplift and denudation. The familiar claim that Penck simply proposed parallel slope retreat is a disputed interpretation; slope replacement is a more careful description of his model.2
Mohorovičić discontinuity (Moho) — seismic boundary between crust and mantle. Its depth varies: roughly 6–7 km beneath the ocean floor and about 35 km beneath continents on average, with substantially thicker crust under some mountain belts. These are typical crustal thicknesses, not one fixed depth below sea level.3
Laurentia vs. Laurasia — Laurentia is the ancient continental nucleus associated with North America; Laurasia was the much larger northern assemblage of Pangaea, incorporating Laurentia and Eurasian blocks. The names refer to different geographic and tectonic scales.4
Great Oxygenation Event (GOE) — a major Paleoproterozoic rise in atmospheric oxygen, conventionally placed around 2.4 billion years ago. It was a prolonged Earth-system transition, not the first appearance of oxygenic photosynthesis or an instantaneous switch to modern oxygen levels; geological records preserve earlier oxygen signals as well as later fluctuations.5
Peridotite — the dominant rock of Earth’s upper mantle; composed mainly of olivine and pyroxene; intrinsically ultramafic (very low SiO2, <45%); occasionally brought to the surface as xenoliths in basaltic or kimberlite eruptions; kimberlite pipes (source of diamonds) are peridotite-bearing diatremes.
Granite — coarse-grained felsic intrusive rock; composed of quartz, orthoclase feldspar, plagioclase, and mica; forms the cores of mountain ranges (batholiths); characteristic rock of continental crust; specific gravity ~2.7.
Uniformly distributed mass extinctions — the Big Five are supplemented in quizbowl by noting each boundary: the Ordovician-Silurian extinction (~444 Ma) may have been triggered by a gamma-ray burst and glaciation; the Late Devonian (Kellwasser event, ~372 Ma) affected reef ecosystems; the Triassic-Jurassic (~201 Ma) was likely caused by the CAMP (Central Atlantic Magmatic Province) flood basalts.
Phreatic vs. magmatic eruption — phreatic (steam-blast) eruptions are caused by water flashing to steam when heated by magma; no new magma is expelled; can occur without warning (e.g., Ontake 2014); magmatic eruptions involve fresh magma reaching the surface.
Subduction — process by which denser oceanic lithosphere sinks into the mantle beneath lighter lithosphere; generates deep-focus earthquakes (down to ~670 km); produces island arcs or Andean-type volcanic arcs; slab dehydration releases fluids that lower the mantle wedge’s melting point, producing water-fluxed melting.
P-wave shadow zone detail — P-waves are refracted (bent) as they pass through the liquid outer core, creating a zone between ~103° and ~142° from the epicenter where direct P-waves do not arrive; S-waves are completely blocked from ~103° onward; first described by Richard Oldham (1906).
Glacial features — U-shaped valleys (glacial erosion vs. V-shaped river valleys); cirques (bowl-shaped bedrock basins where glaciers form); arêtes (sharp ridges between cirques); moraines (ridges of glacially deposited till): terminal/end, lateral, medial, ground; drumlins (streamlined hills formed under moving glaciers); kettles (depressions from melted ice blocks); eskers (sinuous ridges of glaciofluvial sediment).