Physical & Human Geography Systems

Geography examines spatial patterns of physical and human phenomena — how processes shape landscapes, distribute populations, and create interconnected systems across scales. This pack covers geomorphology, climatology, population dynamics, urbanization, and spatial analysis.

## Plate Tectonics & Geomorphology

Earth's lithosphere is divided into ~15 major TPC (tectonic plates) riding on the partially molten AST (asthenosphere). Three boundary types drive most geologic activity:

DVG (divergent): plates separate, magma rises. Mid-ocean ridges (MOR — e.g., Mid-Atlantic Ridge) create new OCR (oceanic crust) via seafloor spreading at 2-15 cm/year. Continental rifting (e.g., East African Rift) eventually splits landmasses.

CNV (convergent): plates collide. Ocean-ocean → SUB (subduction) of denser plate → volcanic island arc (e.g., Japan). Ocean-continent → SUB → volcanic continental arc + trench (e.g., Andes). Continent-continent → ORG (orogenesis/mountain building) without SUB (e.g., Himalayas — Indo-Australian colliding with Eurasian plate, uplift ~5mm/year still ongoing).

TRN (transform): plates slide laterally. Strike-slip faults (e.g., San Andreas). Earthquakes without volcanism.

WTH (weathering) breaks down rock in place: mechanical (frost wedging, thermal expansion, root prying, salt crystal growth), chemical (hydrolysis of feldspars, dissolution of CaCO3 by carbonic acid, oxidation of iron minerals), and biological (lichen acids, root chemical exudation). WTH rates depend on climate — chemical WTH dominates in warm, wet environments; mechanical in cold, dry.

ERS (erosion) transports weathered material via water, wind, ice, and gravity. Fluvial systems are the dominant ERS agent globally. River channel morphology reflects discharge, sediment load, and gradient: braided channels (high sediment, steep gradient), meandering (lower gradient, cohesive banks, point bars and cut banks), and straight (rare, structurally controlled).

The DVC (Davis cycle of erosion) model: youth (steep V-valleys, waterfalls, rapids), maturity (wider floodplains, meanders), old age (peneplain — nearly flat erosion surface). While oversimplified, it captures the tendency toward graded profiles where ERS and DEP (deposition) equilibrate.

GLC (glacial) landscapes: continental ice sheets and alpine glaciers carved much of the Northern Hemisphere topography. GLC ERS produces cirques, arêtes, horns, U-shaped valleys, hanging valleys, fjords. GLC DEP creates moraines (lateral, medial, terminal, ground), drumlins, eskers, outwash plains, and kettle lakes. Till (unsorted) vs. stratified drift (sorted by meltwater).

## Climate Systems

Global atmospheric circulation is driven by differential solar heating. The tricellular model: HDL (Hadley cell — 0-30° latitude, thermally direct), FRL (Ferrel cell — 30-60°, thermally indirect), PLC (Polar cell — 60-90°, thermally direct). Rising air at the ITCZ (Intertropical Convergence Zone) and 60° produces low pressure and precipitation; sinking air at 30° (subtropical high — STH) and poles produces high pressure and aridity.

The CRS (Coriolis effect) deflects moving air right in NH and left in SH, producing the trade winds, westerlies, and polar easterlies. This combines with pressure gradients to create prevailing wind patterns.

KGC (Köppen-Geiger classification) categorizes climates by temperature and precipitation: A (tropical — no cold season), B (arid — evaporation > precipitation), C (temperate — mild winters), D (continental — severe winters), E (polar). Second letters specify precipitation patterns; third letters specify temperature details. Example: Cfb = temperate oceanic (London), Dfa = humid continental hot-summer (Chicago), BWh = hot desert (Sahara).

MSN (monsoon) systems: seasonal wind reversal driven by differential heating of land and ocean. Summer monsoon — low pressure over heated continent draws moist ocean air inland, producing heavy rainfall. Winter monsoon — high pressure over cooled continent pushes dry air seaward. South Asian monsoon is the most dramatic, delivering 70-80% of India's annual rainfall in June-September.

ORG (orographic) precipitation: air forced upward by topography cools adiabatically → condensation → precipitation on windward slope. Leeward side experiences RSH (rain shadow) — descending, warming air suppresses precipitation. Examples: Cascades (Seattle wet, eastern Washington dry), Himalayas (Cherrapunji monsoon vs. Tibetan plateau desert).

## Population Geography

Global population distribution is strikingly uneven. Four major clusters: East Asia, South Asia, Europe, eastern North America — together holding >70% of humanity. Distribution correlates with climate, water availability, soil fertility, and historical development trajectories.

DTM (Demographic Transition Model) describes population change through development stages: Stage 1 (high BR — birth rate, high DR — death rate, low growth), Stage 2 (declining DR, high BR, rapid growth — most of sub-Saharan Africa), Stage 3 (declining BR, moderate growth — India, Brazil), Stage 4 (low BR, low DR, stable — US, France), Stage 5 (BR below replacement, aging population, potential decline — Japan, Italy, South Korea).

TFR (total fertility rate): the average number of children per woman. Replacement level is ~2.1 in developed countries. Global TFR has halved from ~5.0 in 1950 to ~2.3 today. Key drivers of fertility decline: female education (strongest predictor), access to contraception, urbanization (children become economic cost rather than agricultural asset), child survival improvements (fewer births needed to achieve desired family size).

MGR (migration) reshapes population geography. Push factors: conflict, poverty, environmental degradation, persecution. Pull factors: economic opportunity, safety, family reunification, quality of life. Ravenstein's laws: most MGR is short-distance, step-wise (rural → town → city), and economically motivated. Counter-streams develop. International MGR corridors: Mexico→US, South Asia→Gulf States, Eastern→Western Europe.

## Urban Geography

URB (urbanization) — the proportion of population in urban areas — has crossed 56% globally and is projected to reach 68% by 2050. Fastest URB is in sub-Saharan Africa and South Asia. The urban transition is a one-way shift driven by rural-urban MGR and reclassification of settlements.

Urban models: CBD (central business district) theory — Burgess concentric zone (1925), Hoyt sector (1939), Harris-Ullman multiple nuclei (1945). Developing world cities often show inverse patterns: affluent core, squatter periphery.

PRI (primate city) pattern: one dominant city disproportionately larger than the second (e.g., Bangkok is 40x larger than Thailand's second city). Rank-size rule: the nth largest city has 1/n the population of the largest — more common in large, complex countries (US, China). PRI cities concentrate economic and political power, often at the expense of peripheral regions.

PRU (periurbanization): expansion of urban functions into surrounding rural areas — commuter belts, edge cities, exurban sprawl. Driven by automobile dependence, land cost differentials, and telecommunications. Environmental consequences: HAB fragmentation, increased VMT (vehicle miles traveled), impervious surface expansion, agricultural land loss.

SMC (smart city) concept: integrating IoT, data analytics, and digital infrastructure to optimize urban services — transport, energy, waste, governance. Critiques: surveillance concerns, digital divide, technosolutionism ignoring structural inequality.

## Spatial Analysis & GIS

GIS (Geographic Information Systems) integrates spatial data capture, storage, analysis, and visualization. Two data models: RST (raster — grid of cells, best for continuous phenomena like elevation, temperature) and VCT (vector — points, lines, polygons, best for discrete features like roads, boundaries, buildings).

Key analytical operations: overlay (combining layers — Boolean, weighted), buffer (proximity zones), network analysis (shortest path, service areas), interpolation (estimating values between known points — IDW, kriging), and hot spot analysis (spatial clustering — Moran's I for spatial autocorrelation, Getis-Ord Gi* for hot/cold spots).

Tobler's First Law of Geography: "Everything is related to everything else, but near things are more related than distant things." This SAC (spatial autocorrelation) principle underlies all geographic analysis. Positive SAC: similar values cluster (income, disease incidence). Negative SAC: dissimilar values adjacent (rare). Understanding SAC patterns reveals underlying processes.

RS (remote sensing) provides data at scales from local to global. Passive RS detects reflected/emitted EMR (electromagnetic radiation) — multispectral (Landsat, Sentinel-2), thermal, hyperspectral. Active RS transmits and receives signals — SAR (synthetic aperture radar, penetrates clouds), LiDAR (3D surface mapping). NDVI (normalized difference vegetation index) from near-infrared and red bands is the workhorse metric for vegetation health monitoring, agricultural assessment, and deforestation detection.