Geomorphology — Quick Revision

A last-week revision sheet for geomorphology. Prelims tests matching (landform to agent, boundary to example, discontinuity to depth); Mains tests explanation of distributions.

Interior of the Earth

Layer	Depth (approx.)	Remember
Crust	Oceanic ~5 km; continental ~30 km (up to ~70 km under the Himalaya)	Continental: granitic (sial); oceanic: basaltic (sima)
Mantle	To ~2,900 km	Asthenosphere (upper mantle, to ~400 km) — source of magma
Outer core	~2,900–5,150 km	Liquid iron–nickel; generates the magnetic field
Inner core	~5,150–6,371 km	Solid iron–nickel (pressure)

Discontinuity	Between
Conrad	Upper and lower continental crust
Mohorovičić (Moho)	Crust and mantle
Repetti	Upper and lower mantle
Gutenberg	Mantle and outer core
Lehmann	Outer and inner core

Seismic Waves

Wave	Type	Medium	Speed
P	Longitudinal	Solid, liquid, gas	Fastest
S	Transverse	Solid only	Slower
Surface (L, Rayleigh)	Along the surface	—	Slowest, most destructive

High-yield facts

Shadow zone for both P and S waves: 105°–145° from the epicentre; S waves are absent everywhere beyond 105°.
Continental drift — Alfred Wegener, 1912: Pangaea, Panthalassa, Laurasia, Gondwanaland; forces proposed were pole-fleeing and tidal.
Convection current theory — Arthur Holmes, 1930s.
Sea-floor spreading — Harry Hess, 1961: magnetic striping; ocean-floor rocks no older than ~200 million years.
Plate tectonics — 1967–68: seven major plates — Pacific, North American, South American, Eurasian, African, Indo-Australian, Antarctic.
India collided with Eurasia ~40–50 million years ago; the Himalaya are still rising. Deccan Traps volcanism began ~60 million years ago.

Plate Boundaries

Boundary	Crust	Example
Divergent (constructive)	Created	Mid-Atlantic Ridge; East African Rift; Iceland
Convergent ocean–ocean	Destroyed	Mariana Trench; Japan island arc
Convergent ocean–continent	Destroyed	Andes; Peru–Chile Trench
Convergent continent–continent	Crumpled	Himalaya; Alps
Transform (conservative)	Neither	San Andreas Fault

Minor plates: Cocos, Nazca, Arabian, Philippine, Caroline, Fiji, Juan de Fuca, Scotia.

Rocks

Type	Formation	Examples
Igneous	Cooling of magma/lava	Granite (intrusive), basalt (extrusive)
Sedimentary	Lithification of sediments	Sandstone, shale, limestone, coal — fossils
Metamorphic	Heat and pressure	Marble (limestone), slate (shale), quartzite (sandstone), gneiss (granite)

Landforms by Agent

Agent	Erosional	Depositional
River	V-valley, gorge, canyon, waterfall, pothole, incised meander	Alluvial fan, floodplain, levee, ox-bow lake, delta
Groundwater	Sinkhole, doline, uvala, lapies, cave	Stalactite, stalagmite, pillar
Glacier	Cirque, arête, horn, U-valley, hanging valley, fjord	Moraine, esker, drumlin, outwash plain
Wind	Deflation hollow, mushroom rock, yardang, inselberg	Barchan, seif, parabolic dune, loess
Waves	Cliff, wave-cut platform, cave, arch, stack	Beach, bar, spit, lagoon, tombolo

Common Traps

Traps to avoid

The outer core is liquid; the inner core is solid.
P waves are faster than S waves.
Wegener’s mechanism was rejected; his evidence survived.
Continent–continent collision gives few volcanoes.
Barchan horns point downwind; parabolic horns point upwind.
Weathering is in place; erosion includes transport.