Geomorphology is the science that studies the origin, evolution and form of landforms on the Earth's surface. The word comes from three Greek words – geo (Earth), morphe (form) and logos (study). Mountains, plateaus, valleys, plains, deltas and coastlines are all shaped by forces working from inside and outside the Earth. This first part of the series explains the basic concepts, the forces that make landforms, the origin of the Earth's crust, the interior of the Earth and geomagnetism – with short tricks to remember everything.
Geomorphology Series (5 parts):
Part 1 – Basics, Forces, Earth's Crust, Interior and Geomagnetism
Part 2 – Geosynclines, Continental Drift, Isostasy, Plate Tectonics and Mountain Building
Part 3 – Volcanicity, Earthquakes and Tsunamis
Part 4 – Geomorphic Cycles, Denudation Chronology, Channel Morphology, Erosion Surfaces and Slopes
Part 5 – Applied Geomorphology, Economic Geology and Environment
Fundamental Concepts of Geomorphology
- Uniformitarianism – "The present is the key to the past." The physical processes working today (rivers cutting valleys, waves eroding cliffs) also worked in the past. Proposed by James Hutton (1785), the father of modern geology, and popularised by Charles Lyell.
- Structure, process and stage (time) – According to W.M. Davis, every landform is a function of these three. Geological structure is a dominant control factor in the evolution of landforms.
- Most landscapes are polycyclic – they carry marks of more than one cycle of erosion (W.D. Thornbury, Principles of Geomorphology, 1954, which lists ten fundamental concepts).
- Climate matters – different climates produce different sets of landforms (climatic geomorphology).
Factors Controlling Landform Development
| Factor | How it shapes landforms | Example |
|---|---|---|
| Rock type (lithology) | Hard rocks resist erosion and stand high; soft rocks wear down into lowlands. Permeability and jointing also matter. | Granite forms tors and inselbergs; limestone forms karst (caves, sinkholes) |
| Geological structure | Folds, faults, dips and joints guide rivers and slopes. | Trellis drainage in folded areas; rift valleys along faults |
| Climate | Controls the type and speed of weathering and the main agent of erosion. | Deserts – wind landforms; humid tropics – deep chemical weathering; cold regions – glacial landforms |
| Tectonics | Uplift, subsidence, folding and faulting create relief and renew erosion. | Himalaya; Deccan plateau |
| Time (stage) | Landforms change from youth to maturity to old age. | V-shaped valley (young) to broad plain (old) |
| Vegetation | Protects the soil and slows erosion; its removal speeds up erosion. | Gullies and badlands after deforestation (Chambal ravines) |
| Human activity | Mining, dams, urbanisation and farming change landforms quickly. | Open-cast mines, reservoirs, reclaimed land |
| Base level | The lowest level to which a river can erode (sea level). A change in base level renews or slows erosion. | River terraces after uplift |
Short trick: "Raju Sends Clever Teachers To Village Hostels" → Rock, Structure, Climate, Tectonics, Time, Vegetation, Human activity.
Endogenetic and Exogenetic Forces
Landforms are the result of a constant contest between two sets of forces. Endogenetic (internal) forces come from inside the Earth and generally create relief – they raise mountains and plateaus. Exogenetic (external) forces work on the surface and generally wear down relief and fill up low areas.
Short trick: "Endo builds, Exo levels." Endo = Enter (inside); Exo = Exit (outside).
Endogenetic Forces
Source of energy: heat from radioactive decay, primordial heat left over from the Earth's formation, and friction from the Earth's rotation and tides.
| Type | Sub-type | Description |
|---|---|---|
| Slow (Diastrophic) forces | Epeirogenic ("continent-building") | Slow vertical uplift or subsidence of large areas without much folding. Example: raised beaches, submerged forests, uplift of plateaus. |
| Orogenic ("mountain-building") | Horizontal movements causing folding (compression) and faulting (compression or tension). They create fold mountains, block mountains and rift valleys. | |
| Sudden forces | Earthquakes and Volcanism | Act within seconds to days; cause destruction and create new landforms such as volcanic cones and fault scarps. |
Short trick: Epeiro = Elevate/Extensive (broad up and down); Oro = Oro = mountain (fold and fault).
Exogenetic Forces (Gradation)
Source of energy: the Sun (which drives climate, wind and the water cycle) and gravity (which pulls material downslope).
The overall work of exogenetic forces is called gradation, which has two parts – degradation (wearing down of high land) and aggradation (building up of low land by deposition).
- Weathering – breaking down of rocks in place, without movement.
- Mass wasting – downslope movement of rock and soil under gravity (creep, landslides, mudflows, rockfalls).
- Erosion – removal and transport of material by moving agents.
- Deposition – laying down of material when the agent loses energy.
Denudation = weathering + mass wasting + erosion + transportation. The agents are running water, groundwater, glaciers, wind and sea waves. Running water is the most important agent on Earth.
Types of Weathering
| Type | Process | Typical climate / example |
|---|---|---|
| Physical (mechanical) | Exfoliation – outer layers peel off like an onion due to daily heating and cooling | Hot deserts; exfoliation domes |
| Frost wedging – water freezes in cracks and expands about 9% | Cold and high mountain regions | |
| Salt weathering – growth of salt crystals in pores | Deserts and coasts | |
| Block / granular disintegration | Rocks with joints or coarse grains | |
| Unloading (pressure release) – removal of overlying rock lets the rock expand and crack | Granite domes | |
| Chemical | Solution – minerals dissolve in water | Rock salt, gypsum |
| Carbonation – CO2 in rainwater (carbonic acid) dissolves limestone | Karst landforms – caves, stalactites, stalagmites | |
| Hydration – minerals absorb water and swell | Anhydrite to gypsum | |
| Hydrolysis – chemical reaction of minerals with water | Feldspar to clay (kaolin) | |
| Oxidation – combination with oxygen | Rusting of iron minerals; red colour of laterite | |
| Reduction – removal of oxygen in waterlogged areas | Grey-blue soils in swamps | |
| Biological | Roots widening cracks, burrowing animals, acids from lichens and bacteria | All climates |
Short trick for chemical weathering: "She Can Hardly Hold Our Rings" → Solution, Carbonation, Hydration, Hydrolysis, Oxidation, Reduction.
Physical weathering dominates in dry and cold climates; chemical weathering dominates in hot and humid climates.
Origin and Evolution of the Earth and its Crust
Theories of the Origin of the Earth
| Theory | Proposed by | Main idea |
|---|---|---|
| Nebular Hypothesis | Immanuel Kant (1755); Pierre-Simon Laplace (1796) | The solar system formed from a slowly rotating, cooling cloud of gas (nebula) |
| Planetesimal Hypothesis | T.C. Chamberlin and F.R. Moulton (1904) | A passing star pulled matter from the Sun, which cooled into small bodies (planetesimals) that joined to form planets |
| Tidal Hypothesis | James Jeans and Harold Jeffreys (1919–1925) | A passing star raised a cigar-shaped tidal filament from the Sun, which broke into planets |
| Binary Star Hypothesis | H.N. Russell and others | The Sun had a companion star whose break-up formed the planets |
| Big Bang Theory (origin of the universe) | Georges Lemaître (1927); expansion shown by Edwin Hubble (1929) | The universe began about 13.8 billion years ago from an extremely hot, dense point and has been expanding since |
| Modern view (Solar Nebula / accretion) | Revised nebular models | Dust and gas around the young Sun collided and stuck together (accretion) to form planetesimals and then planets |
Short trick: "Kant Likes Nebula" (Kant–Laplace = Nebular); "Cool Men Plan" (Chamberlin–Moulton = Planetesimal); "Jeans Jumped on Tides" (Jeans–Jeffreys = Tidal).
How the Crust Evolved
- The Earth formed about 4.54 billion years ago by accretion and was largely molten due to impacts and radioactive heat.
- Differentiation: heavy iron and nickel sank to form the core; lighter silicates rose to form the mantle and crust.
- Degassing: gases escaped through volcanoes and formed the early atmosphere; water vapour condensed into the oceans.
- A giant impact with a Mars-sized body (often called Theia) is believed to have formed the Moon.
- The first stable crust formed and was recycled many times. The oldest known mineral grains are zircons from Jack Hills, Western Australia (about 4.4 billion years), and among the oldest known rocks is the Acasta Gneiss of Canada (about 4 billion years).
- Over time, continental crust grew through volcanic activity, plate movements and the welding of smaller pieces (cratons). India's ancient cratons include the Dharwar, Singhbhum, Bastar and Aravalli–Bundelkhand blocks.
Continental Crust vs Oceanic Crust
| Feature | Continental crust | Oceanic crust |
|---|---|---|
| Composition | Granitic (rich in silica and aluminium) – SIAL | Basaltic (rich in silica and magnesium) – SIMA |
| Thickness | 30–70 km (thickest under mountains) | 5–10 km |
| Density | About 2.7 g/cm3 | About 3.0 g/cm3 |
| Age | Up to 4 billion years | Generally less than 200 million years |
Short trick (Eduard Suess's layers): SIAL = Si + Al (crust/continents); SIMA = Si + Magnesium (ocean floor and lower crust); NIFE = Nickel + Fe (core).
Elements of the Earth's Crust
| Element | Share by weight (approx.) |
|---|---|
| Oxygen | 46.6% |
| Silicon | 27.7% |
| Aluminium | 8.1% |
| Iron | 5.0% |
| Calcium | 3.6% |
| Sodium | 2.8% |
| Potassium | 2.6% |
| Magnesium | 2.1% |
Short trick: "Only Silly Ants Find Candy, Never Kick Monkeys" → O, Si, Al, Fe, Ca, Na, K, Mg (in decreasing order).
For the whole Earth, the order changes: Iron (about 32%) > Oxygen (30%) > Silicon (15%) > Magnesium (14%).
Physical Conditions of the Earth's Interior
Sources of Information
- Direct sources: mines, deep drilling and volcanic material. The deepest hole ever drilled is the Kola Superdeep Borehole in Russia – about 12.26 km, a tiny fraction of the Earth's radius of 6,371 km.
- Indirect sources: seismic waves (the most important), gravity, magnetic field, meteorites (which have similar materials), and changes in temperature, pressure and density with depth.
Seismic Waves
| Wave | Nature | Travels through | Key point |
|---|---|---|---|
| P-waves (Primary) | Longitudinal (compressional) – particles vibrate in the direction of the wave | Solids, liquids and gases | Fastest; arrive first |
| S-waves (Secondary) | Transverse – particles vibrate at right angles to the wave | Only solids | Cannot pass through the liquid outer core |
| Surface waves (L-waves) – Love and Rayleigh waves | Move along the surface | Upper crust | Slowest but most destructive |
- S-wave shadow zone: S-waves are not recorded beyond about 105° from the epicentre – proof that the outer core is liquid.
- P-wave shadow zone: between about 105° and 145° from the epicentre, because P-waves bend (refract) at the core.
Short trick: P = Push and Pass through everything; S = Shake and only Solid. Surface = Slow but Smashing.
Layers of the Earth
| Layer | Depth | Nature | Density (g/cm3) |
|---|---|---|---|
| Crust | 0 – about 35 km under continents (5–10 km under oceans) | Solid; thinnest layer, under 1% of Earth's volume | 2.7 – 3.0 |
| Upper mantle (including the asthenosphere) | Up to about 660–700 km | Asthenosphere (about 100–400 km) is weak, partly molten and plastic – the main source of magma; plates move over it | 3.3 – 4.4 |
| Lower mantle | About 700 – 2,900 km | Solid, very hot rock; the mantle makes up about 84% of the Earth's volume | 4.4 – 5.6 |
| Outer core | 2,900 – 5,150 km | Liquid iron and nickel; source of Earth's magnetism | About 10 – 12 |
| Inner core | 5,150 – 6,371 km | Solid iron and nickel because of enormous pressure | About 13 |
The lithosphere (crust plus the rigid upper part of the mantle, about 100 km thick) is broken into plates. The average density of the whole Earth is about 5.5 g/cm3.
Discontinuities
| Discontinuity | Separates | Depth |
|---|---|---|
| Conrad | Upper crust (SIAL) and lower crust (SIMA) | Within the continental crust |
| Mohorovičić (Moho) – discovered by Andrija Mohorovičić, 1909 | Crust and mantle | About 35 km (continents), 5–10 km (oceans) |
| Repetti | Upper mantle and lower mantle | About 700 km |
| Gutenberg – Beno Gutenberg, 1913 | Mantle and outer core | About 2,900 km |
| Lehmann – Inge Lehmann, 1936 | Outer core and inner core | About 5,150 km |
Short trick (top to bottom): "Can Moho Repeat Gutenberg's Lesson?" → Conrad, Moho, Repetti, Gutenberg, Lehmann.
Temperature, Pressure and Density
- Temperature rises with depth – about 1°C for every 32 m in the upper crust (a geothermal gradient of roughly 25–30°C per km). The rate slows down deeper. The temperature of the core is about 5,000–6,000°C, close to that of the Sun's surface.
- Pressure increases with depth because of the weight of overlying rocks. At the centre it is about 3.6 million times the atmospheric pressure. This is why the inner core is solid despite its high temperature.
- Density increases from about 2.7 in the crust to about 13 in the inner core.
Fundamentals of Geomagnetism
The Earth behaves like a giant bar magnet. William Gilbert first stated this in his book De Magnete (1600). The magnetic field protects life by shielding the Earth from harmful solar and cosmic radiation.
Origin – the Geodynamo
The magnetic field is produced by the movement of liquid iron in the outer core. Convection currents in this electrically conducting fluid, twisted by the Earth's rotation (Coriolis effect), generate electric currents, which in turn produce the magnetic field. This self-sustaining process is called the geodynamo.
Key Terms
| Term | Meaning |
|---|---|
| Magnetic poles | Points where the field lines are vertical. They do not match the geographic poles and keep shifting; the north magnetic pole has been moving from northern Canada towards Siberia. |
| Magnetic declination | The horizontal angle between true (geographic) north and magnetic north. Important for navigation with a compass. |
| Magnetic inclination (dip) | The vertical angle that the field makes with the horizontal. It is 0° at the magnetic equator and 90° at the magnetic poles. |
| Field strength | About 25 microtesla near the equator to about 65 microtesla near the poles. |
| Magnetosphere | The region around the Earth controlled by its magnetic field; it deflects the solar wind. |
| Van Allen belts | Two belts of charged particles trapped by the field, discovered in 1958 by James Van Allen. |
| Aurora | Light display near the poles when charged particles hit the upper atmosphere – Aurora Borealis (north) and Aurora Australis (south). |
Short trick: Declination = Direction on the map (horizontal); Inclination = Incline / tilt (vertical).
Palaeomagnetism and Magnetic Reversals
- When lava cools, iron minerals such as magnetite line up with the Earth's magnetic field and get "frozen" (below the Curie point, about 580°C for magnetite). This preserved record is called palaeomagnetism.
- The Earth's field has reversed its polarity many times – the north and south magnetic poles swap places. The last full reversal, the Brunhes–Matuyama reversal, took place about 780,000 years ago.
- Symmetrical bands of normal and reversed magnetism on both sides of mid-ocean ridges (shown by Vine and Matthews, 1963) became key evidence for sea-floor spreading and plate tectonics (see Part 2).
- Palaeomagnetic data also show that the continents have moved over time (apparent polar wandering).
Quick Revision Points
- Uniformitarianism: James Hutton; "present is the key to the past".
- Davis: landform = structure + process + stage.
- Epeirogenic = continent-building (vertical); orogenic = mountain-building (horizontal).
- Carbonation forms karst landforms in limestone.
- Crust order: O > Si > Al > Fe; whole Earth: Fe > O > Si > Mg.
- Moho: crust–mantle; Gutenberg: mantle–core (2,900 km); Lehmann: outer–inner core (5,150 km).
- S-waves do not pass through the liquid outer core; S-wave shadow beyond 105°.
- Earth's magnetism: geodynamo in the liquid outer core.
- Dip is 90° at magnetic poles and 0° at the magnetic equator.
Practice MCQs
1. "The present is the key to the past" is the principle of:
(a) Catastrophism
(b) Uniformitarianism
(c) Isostasy
(d) Continental drift
2. Mountain-building movements are called:
(a) Epeirogenic
(b) Orogenic
(c) Isostatic
(d) Gradational
3. Which process forms caves and stalactites in limestone areas?
(a) Hydration
(b) Oxidation
(c) Carbonation
(d) Exfoliation
4. The most abundant element in the Earth's crust is:
(a) Silicon
(b) Iron
(c) Aluminium
(d) Oxygen
5. The discontinuity between the mantle and the core is:
(a) Moho
(b) Conrad
(c) Gutenberg
(d) Lehmann
6. Which seismic waves cannot pass through liquids?
(a) P-waves
(b) S-waves
(c) Love waves
(d) All of these
7. The Earth's magnetic field is mainly generated in the:
(a) Crust
(b) Asthenosphere
(c) Outer core
(d) Inner core
8. Magnetic inclination (dip) at the magnetic poles is:
(a) 0°
(b) 45°
(c) 90°
(d) 180°
9. Who gave the Planetesimal Hypothesis?
(a) Kant and Laplace
(b) Chamberlin and Moulton
(c) Jeans and Jeffreys
(d) Hubble
10. The SIMA layer is mainly composed of:
(a) Silica and aluminium
(b) Silica and magnesium
(c) Nickel and iron
(d) Iron and oxygen
Answers: 1 – (b) | 2 – (b) | 3 – (c) | 4 – (d) | 5 – (c) | 6 – (b) | 7 – (c) | 8 – (c) | 9 – (b) | 10 – (b)
Reference: NCERT Fundamentals of Physical Geography (Class XI); W.D. Thornbury, Principles of Geomorphology; Savindra Singh, Geomorphology.