Geomorphology
What it is
Geomorphology is the science of landforms: how internal (endogenetic) forces build relief and external (exogenetic) forces wear it down. Unit I runs from continental drift and plate tectonics through weathering, denudation and the cycles of Davis and Penck to slopes, earth movements and geomorphic hazards. UGC-NET tests it mainly through theorist–concept and process–landform matching.
Core concepts
Continental Drift. Alfred Wegener (1912; book 1915) held that one supercontinent, Pangaea, ringed by the ocean Panthalassa, split into Laurasia in the north and Gondwanaland in the south across the Tethys Sea. Evidence: the jigsaw fit of South America and Africa (best at the shelf edge), the Glossopteris flora and the reptile Mesosaurus on lands now oceans apart, Permo-Carboniferous tillites across the Gondwana continents, and rock belts matching across the Atlantic. His driving forces, pole-fleeing (rotational) and tidal, were far too weak, so the theory was shelved until Arthur Holmes's convection currents (1930s) and ocean-floor data revived it.
Plate Tectonics. Harry Hess's sea-floor spreading (1960–62) makes crust at ridges and destroys it at trenches; Vine and Matthews (1963) confirmed it with symmetric palaeomagnetic stripes. The rigid lithosphere moves over the weak asthenosphere as seven major plates: Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic. Divergent (constructive) boundaries form ridges and rifts. Convergent (destructive) ones form island arcs (ocean–ocean), Andean ranges (ocean–continent) or the Himalaya (continent–continent, India meeting Asia roughly 40–50 million years ago). Transform (conservative) ones, such as the San Andreas, neither make nor destroy crust. Subduction earthquakes trace the Wadati–Benioff zone, and hotspots such as Hawaii (J. Tuzo Wilson, 1963) are intraplate.
Endogenetic and Exogenetic Forces. Endogenetic forces, driven by internal heat, are either slow diastrophic movements or sudden ones (earthquakes, volcanism). Diastrophism is epeirogenic (broad vertical uplift or subsidence) or orogenic (horizontal compression and tension, giving folds and faults). They create relief. Exogenetic forces, driven by solar energy and gravity, act through weathering, mass wasting and erosion by water, ice, wind and waves, and they reduce relief. Their work is gradation: degradation plus aggradation.
Denudation and Weathering. Denudation is the total lowering of land by weathering, mass wasting, erosion and transport; weathering alone is in-situ breakdown without transport. Physical weathering: frost wedging (water expands about 9 per cent on freezing), unloading (sheeting, exfoliation domes), thermal block and granular disintegration, and salt crystallisation. Chemical weathering: carbonation (CaCO3 + H2CO3 → Ca(HCO3)2, the basis of karst), oxidation, hydration (anhydrite to gypsum) and hydrolysis (feldspar to kaolinite). Chemical weathering dominates hot, humid climates and frost action cold, moist ones.
Geomorphic Cycle (Davis). W.M. Davis (1899): landscape is a function of structure, process and stage, after rapid uplift and a long still-stand. Youth has V-shaped valleys, waterfalls and rising relative relief, which peaks in early maturity. Maturity has graded rivers, widening valleys and lowering divides. Old age leaves floodplains, meanders and a peneplain with residual monadnocks. Slopes decline (downwearing). Renewed uplift brings rejuvenation: knickpoints, incised meanders, paired terraces. Hack (1960) later replaced stages with dynamic equilibrium.
Geomorphic Cycle (Penck). Walther Penck (Die morphologische Analyse, posthumous, 1924) let uplift and erosion act together, so form reflects the ratio of uplift rate to degradation rate. Waxing development (aufsteigende Entwicklung, uplift faster) gives convex slopes; uniform development (gleichförmige) gives straight slopes; waning development (absteigende) gives concave slopes. The initial surface is the Primärrumpf and the end surface the Endrumpf; an expanding dome leaves stepped Piedmonttreppen. Slopes retreat by backwearing, with slope replacement from below.
Theories and Process of Slope Development. G.K. Gilbert (1909) tied convex hilltops to soil creep. Alan Wood (1942) named four elements: waxing slope (convex crest), free face, constant (debris) slope and waning slope (concave). L.C. King applied them to semi-arid lands with parallel scarp retreat, leaving pediments that coalesce into a pediplain with inselbergs. Dalrymple, Blong and Conacher (1968) built a nine-unit landsurface model.
Earth Movements: Folding and Faulting. An anticline (upfold) has the oldest beds at its core, a syncline (downfold) the youngest. Folds may be symmetrical, asymmetrical, overturned (both limbs dip one way), isoclinal or recumbent; a nappe is a recumbent fold thrust far forward (Alps, Himalaya). In a normal fault (tension) the hanging wall drops; in a reverse fault (compression) it rises; a thrust is a low-angle reverse fault; a strike-slip fault moves sideways (the right-lateral San Andreas). A graben (Rhine, Narmada) is a down-dropped block, a horst (Vosges, Black Forest) an upstanding one.
Earth Movements: Seismicity. Reid's elastic rebound theory (after the 1906 San Francisco earthquake) explains fault rupture. Energy leaves the focus (hypocentre); the epicentre lies above it. P waves (compressional, fastest) cross solids, liquids and gases. S waves (transverse) cross solids only. The liquid outer core stops S waves beyond about 105° from the epicentre and, by refracting P waves, leaves a P-wave shadow between roughly 105° and 145°. Love and Rayleigh surface waves arrive last and do most damage. Foci are shallow (under 70 km), intermediate (70–300 km) or deep (to about 700 km, only in subduction zones). Magnitude (Richter, 1935; moment magnitude for large events) measures source energy; intensity (Modified Mercalli, I–XII) measures local effects.
Earth Movements: Vulcanicity. Viscosity rises with silica. Low-silica basaltic magma erupts quietly as shield volcanoes (Mauna Loa) or fissure flood basalts (Deccan Traps); silica-rich magma is explosive and builds composite cones (Fuji, Vesuvius). Eruptions grade Hawaiian, Strombolian, Vulcanian, Pelean (nuée ardente, Mont Pelée 1902) and Plinian (Vesuvius, 79 CE); summit collapse makes a caldera. Intrusions are the batholith (deep, granitic), laccolith (domed, concordant), lopolith (saucer), phacolith (in fold crests or troughs), sill (horizontal, concordant) and dyke (steep, discordant). Barren Island is India's only active volcano.
Landform Occurrence and Causes of Geomorphic Hazards. Earthquakes bring shaking, liquefaction, landslides and tsunamis, and can be reservoir-induced (Koyna, 1967). Volcanoes add pyroclastic flows, ash and lahars (Nevado del Ruiz buried Armero in 1985). A landslide occurs when driving force exceeds shear strength, so the factor of safety (resisting ÷ driving) falls below 1; triggers are rain-raised pore pressure, undercutting, road cuts, deforestation and shaking. Movements range from creep and solifluction through slumps and mudflows to rockfalls. Snow avalanches, loose-snow or slab, release mostly on 30°–45° slopes after heavy snowfall, wind loading or warming. The Himalaya and Western Ghats are India's main landslide zones.
Worked example
Comparing earthquake magnitudes. Each Richter step multiplies wave amplitude by 10. The Gutenberg–Richter relation log E = 1.5M + 4.8 (E in joules) multiplies energy by 10^1.5, about 31.6, per step. Compare M 5.0 with M 7.0.
- Magnitude difference: 7.0 − 5.0 = 2.0.
- Amplitude ratio: 10^2 = 100.
- Energy ratio: 10^(1.5 × 2) = 10^3 = 1,000.
- Check: log E is 7.5 + 4.8 = 12.3 for M 5.0 and 10.5 + 4.8 = 15.3 for M 7.0; the logs differ by 3.0, so the ratio is again 1,000.
| Magnitude difference | Amplitude ratio | Energy ratio |
|---|---|---|
| 1 | 10 | about 31.6 |
| 2 | 100 | 1,000 |
| 3 | 1,000 | about 31,600 |
So an M 7 releases about a thousand times the energy of an M 5.
Common traps
- Swapping Davis and Penck: Davis has time-bound stages, slope decline and a peneplain; Penck has the uplift–erosion ratio, slope replacement and an Endrumpf.
- Equating magnitude with intensity: magnitude is one source value per earthquake; intensity varies from place to place.
- Reversing fault motion: normal means tension and hanging wall down; reverse means compression and hanging wall up.
- Mixing up fold cores: anticlines expose the oldest rocks at the centre, synclines the youngest.
- Mixing up intrusions: sills and laccoliths are concordant; dykes cut across the bedding.
Speed technique
- Pin each cycle to its end surface: Davis a peneplain with monadnocks, Penck an Endrumpf, King a pediplain with inselbergs.
- Penck's slope rule: waxing convex, uniform straight, waning concave.
- Boundary to landform: divergent gives ridges and rifts, ocean–ocean island arcs, continent–continent fold ranges, transform strike-slip faults.
Check yourself
- Which fossil plant, found across the Gondwana continents, did Wegener cite for drift?
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Glossopteris — a land flora that could not have crossed the oceans now separating them. - At which stage of Davis's cycle is relative relief greatest?
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Early maturity — valleys are cut down but divides are not yet lowered. - In Penck's model, what slope form results when uplift outpaces degradation?
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Convex — waxing development, aufsteigende Entwicklung. - What is a block let down between two parallel normal faults, with an Indian example?
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A graben, or rift valley — the Narmada flows through one. - Which body waves cannot cross the liquid outer core?
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S waves — transverse waves cannot travel through liquids.
Try it: Geomorphology questions
Real questions from the NET Geography bank on exactly this skill. Pick an answer to see the full solution — the intuition, the worked steps, the faster methods and the traps.
In Walther Penck's model of slope development, when the rate of uplift exceeds the rate of degradation, the slopes that develop are:
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Answer: option C
Penck held that uplift and erosion act together, so slope form depends on the ratio of the rate of uplift to the rate of degradation.
When uplift is faster (waxing development, aufsteigende Entwicklung) the slopes become convex; a balance gives straight slopes, and slower uplift (waning development) gives concave ones.
So uplift outpacing degradation gives convex slopes, option C.
W.M. Davis described every landscape as a function of three variables. They are:
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Answer: option D
Davis (1899) proposed his geographical cycle of erosion, in which a landscape develops after rapid uplift followed by a long still-stand.
He summed up the controls on landform as structure, process and stage, or time, which lets a landscape be described as youthful, mature or old.
So Davis's three variables are structure, process and stage, option D.
Consider the following two statements: Statement I: In a syncline, the youngest rock beds lie at the core of the fold. Statement II: The Narmada flows through a graben, a block let down between parallel normal faults. In the light of the above statements, choose the correct answer from the options.
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Answer: option A
Statement I: a syncline is a downfold, so the beds laid down last sit at its centre; Statement I is true.
Statement II: the Narmada valley between the Vindhya and Satpura ranges is a rift valley, or graben, bounded by faults; Statement II is true.
So Statement I, "In a syncline, the youngest rock beds…", and Statement II are both true, option A.
Using the Gutenberg–Richter relation log E = 1.5M + 4.8 (E in joules), the energy released by a magnitude 8.0 earthquake is how many times that released by a magnitude 6.0 earthquake?
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Answer: option C
For M 8.0, log E = 1.5 × 8.0 + 4.8 = 16.8; for M 6.0, log E = 1.5 × 6.0 + 4.8 = 13.8.
The logs differ by 16.8 − 13.8 = 3.0, so the energy ratio is = 1,000; as a check, each step multiplies energy by ≈ 31.6, and 31.6 × 31.6 ≈ 1,000.
So the M 8.0 earthquake releases 1,000 times the energy of the M 6.0 earthquake, option C.
Consider the following two statements: Statement I: The magnitude of an earthquake varies from place to place, being greatest near the epicentre. Statement II: The Modified Mercalli scale, graded from I to XII, rates the observed effects of shaking at a particular place. In the light of the above statements, choose the correct answer from the options.
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Answer: option D
Statement I: magnitude is a single value for an earthquake, set by the energy released at the source, and it is intensity that varies from place to place; Statement I is false.
Statement II: the Modified Mercalli scale is an intensity scale of twelve grades, I to XII, based on observed effects on people, buildings and the ground; Statement II is true.
So Statement I, "The magnitude of an earthquake varies…", is false and Statement II is true, option D.
Answer above — every one shows its working.