Physical Geography II: Landforms, Tectonics, and Water Systems
The Andes, Himalayas, Ring of Fire, Great Rift Valley, Nile Delta — every one of these landmark features comes from plate tectonics or water. Learn the three plate boundary types and the major water systems and you have the vocabulary for most physical-geography questions.
Plate tectonics: three boundary types
Earth's outer shell (the lithosphere) is broken into tectonic plates that move over the ductile mantle below. Three boundary types produce three characteristic landform patterns:
- Divergent — plates move apart. Molten rock rises to fill the gap, creating new crust. Mid-ocean ridges (Mid-Atlantic Ridge running through Iceland) and continental rift valleys (East African Rift, Great Rift Valley).
- Convergent — plates collide. Three variations:
- Oceanic-continental → oceanic plate subducts under continental → volcanic mountain chain (Andes, Cascades)
- Oceanic-oceanic → subduction produces volcanic island arcs (Japan, Philippines, Aleutians)
- Continental-continental → both crust types crumple upward → tall non-volcanic mountains (Himalayas from India colliding with Eurasia)
- Transform — plates slide past each other. Faults, earthquakes, no volcanoes (San Andreas Fault, Anatolian Fault).
The Ring of Fire
Around the Pacific margin, a horseshoe-shaped zone of convergent boundaries and subduction zones produces the Ring of Fire — most of the world's active volcanoes and about three-quarters of its earthquakes. Chile, Cascades, Alaska, Kamchatka, Japan, Philippines, Indonesia, New Zealand all sit on it.
Major landform types
- Fold mountains — Himalayas, Andes, Rockies, Alps
- Plateaus — Tibetan Plateau, Colorado Plateau, Ethiopian Highlands
- Plains — US Great Plains, Russian Plain, Ganges Plain
- Basins — Amazon Basin, Congo Basin, Tarim Basin
- Rift valleys — East African Rift (still widening)
- Deltas — Nile, Ganges-Brahmaputra, Mekong, Mississippi, Rhine
- Fjords — Norway, southern Chile, New Zealand, coastal British Columbia (glacially-carved coastal inlets)
Water systems and the hydrologic cycle
Earth's water is unevenly distributed:
- Oceans — 97% of Earth's water (saline)
- Ice sheets / glaciers — most of the freshwater
- Groundwater — most of the liquid freshwater
- Surface freshwater — lakes and rivers, small share of total
The hydrologic cycle moves water among these reservoirs via evaporation, transpiration, condensation, precipitation, runoff, and infiltration.
Major world rivers you should know
- Nile (Africa) — historically the longest, defined Egyptian civilization
- Amazon (South America) — highest discharge, largest drainage basin
- Yangtze / Chang Jiang (China) — third longest, Three Gorges Dam
- Mississippi-Missouri (US) — drains most of continental US
- Yenisei / Ob / Lena (Siberia) — flow north into Arctic
- Ganges-Brahmaputra (South Asia) — supports dense populations, delta in Bangladesh
- Tigris-Euphrates (Middle East) — cradle of Mesopotamian civilization, transboundary dispute today
- Mekong (Southeast Asia) — six countries, transboundary dam disputes
Transboundary rivers = political disputes
When a river crosses international borders, upstream states can affect downstream states through dams, diversions, and pollution. The exam favors these cases:
- Nile — Ethiopia's Grand Ethiopian Renaissance Dam vs Egypt and Sudan
- Tigris-Euphrates — Turkey's GAP project vs Syria and Iraq
- Mekong — Chinese and Lao dams vs Vietnam and Cambodia
- Aral Sea — Soviet-era diversion of Amu Darya and Syr Darya destroyed the sea
Landforms are always changing
Weathering breaks down rock. Erosion transports the sediment. Mass wasting (landslides, mudflows) moves material downslope. Deposition rebuilds landforms elsewhere (deltas, alluvial fans, sand dunes). Every landscape is a temporary balance between construction and destruction.