Earth’s Interior, Seismic Waves and Geomagnetism: Terminology for UPSC Geography Optional

No borehole has gone deeper than about 12 km, yet we know the Earth has a crust, a mantle and a two-part core. That knowledge comes from seismic waves, which change speed and path at every internal boundary, and from the magnetic field generated in the liquid outer core. This post gathers the vocabulary of the layered Earth and its field.

Each entry gives an exam-ready definition, then mechanism, depths and velocities, examples and a sketch line. UPSC has asked about the role of seismic waves in studying the interior (2005), the relevance of seismic study to interior structure (2010) and geomagnetism with palaeomagnetism (2011); all three are answered from the entries below.

Quick Revision Table

Term	Meaning in one line	Example
Body waves (P-waves & S-waves)	Seismic waves travelling through the interior; P compressional, S shear	S-waves stopped at the core–mantle boundary
Surface waves (Love & Rayleigh waves)	Slow, large waves guided along the surface; arrive last, damage most	2004 Sumatra–Andaman waves circling the globe
Shadow zone	Belt of the globe receiving no direct P- or S-waves from an earthquake	P-wave gap at about 104°–140° from the epicentre
Seismic tomography	Three-dimensional imaging of the interior from millions of travel times	Indian lithosphere imaged beneath Tibet
Seismic discontinuity	Internal surface where wave velocity changes abruptly	Moho; Gutenberg; Lehmann
Conrad discontinuity	Boundary between upper and lower continental crust, 15–20 km deep	Eastern Alps, Austria
Mohorovičić (Moho) discontinuity	Crust–mantle boundary; P-waves jump to about 8 km/s	About 70 km beneath Tibet
Repetti discontinuity	Indian-syllabus name for the upper–lower mantle boundary near 660–700 km	Beneath subduction zones of the western Pacific
Gutenberg (Wiechert–Gutenberg) discontinuity	Core–mantle boundary at about 2,900 km	S-wave disappearance at 2,891 km
Lehmann discontinuity	Boundary of the solid inner core at about 5,150 km	Inferred from the 1929 Buller earthquake, New Zealand
Crust (continental & oceanic)	Thin, light outer shell above the Moho	Dharwar craton; Bay of Bengal floor
SIAL, SIMA & NIFE	Eduard Suess’s compositional shorthand for crust layers and core	Granite continents; basalt ocean floor
Lithosphere & asthenosphere	Rigid outer shell of crust plus top mantle; weak, ductile layer beneath	Indian Plate’s thin lithosphere
Low-velocity zone	Upper-mantle layer where seismic speeds drop a few per cent	Beneath the Pacific Ocean floor
Mantle	Rocky shell from the Moho to 2,891 km, about 84 per cent of Earth’s volume	Peridotite xenoliths in kimberlites
Mesosphere	Strong, rigid mantle below the asthenosphere	Lower mantle beneath 660 km
D″ layer	Lowermost 200–300 km of the mantle, highly heterogeneous	African and Pacific low-velocity provinces
Outer & inner core	Liquid iron–nickel shell around a solid iron–nickel sphere	Innermost inner core, 2023
Geothermal gradient	Rate at which temperature rises with depth	Puga Valley, Ladakh
Geomagnetism	Study of Earth’s magnetic field, its elements and variations	Alibag observatory, Maharashtra
Geodynamo	Self-sustaining generation of the field by outer-core convection	Field sustained for over 3 billion years
Magnetic declination & inclination (dip)	Horizontal angle between magnetic and true north; vertical angle of the field	Agonic line crossing Greenwich, 2019
Magnetic equator & geomagnetic poles	Line of zero dip; ends of the best-fit dipole axis	Thumba rocket station near the dip equator

Seismic Waves: Probes of the Interior

Body Waves (P-waves and S-waves)

Body waves are seismic waves that travel through the Earth’s interior rather than along its surface. They are of two kinds — primary (P) waves, which compress and stretch the rock in the direction of travel, and secondary (S) waves, which shear it at right angles — and the changes in their speed and path with depth are the main evidence for the Earth’s layered structure.

• P-waves (primary, longitudinal, compressional): particles vibrate back and forth along the ray, as in sound. They are the fastest waves — about 6 km/s in continental crust, about 8 km/s just below the Moho and about 13.7 km/s at the base of the mantle — and they pass through solids, liquids and gases. On entering the liquid outer core they slow abruptly to about 8 km/s.
• S-waves (secondary, transverse, shear): particles move perpendicular to the ray. They travel at roughly 0.55–0.6 of P-wave speed (about 3.5 km/s in the crust, 7.3 km/s at the base of the mantle) and cannot cross a liquid, which has no rigidity.
• Mechanism: wave speed depends on rigidity and incompressibility relative to density. With depth, pressure raises rigidity faster than density, so waves speed up and their paths bend into arcs concave towards the surface. At sharp boundaries they are reflected and refracted, locating each discontinuity.
• What they revealed: Richard Dixon Oldham separated P, S and surface phases (1900) and inferred a core (1906); Andrija Mohorovičić found the crust–mantle boundary (1909); Beno Gutenberg fixed the core at about 2,900 km (1913); Inge Lehmann found the solid inner core (1936). The S-wave shadow proves the outer core is liquid; P-waves reflected off the inner core confirm its sharp edge.
• Examples: the 1909 Kupa valley earthquake in Croatia; India’s national network of the National Center for Seismology, whose S–P arrival intervals locate each epicentre.
• Sketch: a circular Earth section with the epicentre at the top, curved P and S rays through the mantle, P rays bending at the core and S rays stopping at 2,900 km, both shadow zones labelled.

Surface Waves (Love and Rayleigh Waves)

Surface waves are seismic waves guided along the Earth’s surface and outer layers, generated when body waves reach the surface; they are slower than P- and S-waves and arrive last, but their large amplitude and long duration make them the most destructive part of the shaking. Older texts call them L (long-period) waves.

• Love waves: shear the ground horizontally from side to side at right angles to the direction of travel; predicted by Augustus Edward Hough Love (1911). They are slightly faster than Rayleigh waves and cannot travel through water.
• Rayleigh waves: roll the ground in a backward (retrograde) ellipse in the vertical plane, like an ocean swell; predicted by John William Strutt, Lord Rayleigh (1885).
• Key features: energy decreases with depth; long-period surface waves can circle the globe; their dispersion (longer periods sample deeper, faster rock) is used to estimate crustal thickness.
• Examples: surface waves from the December 2004 Sumatra–Andaman earthquake were recorded circling the Earth several times.
• Don’t confuse with: a tsunami, which is a water wave set off by sea-floor displacement (tsunami).

Shadow Zone

A shadow zone is the part of the Earth’s surface, measured as angular distance from an earthquake, that receives no direct P- or S-waves because the core bends or blocks them; the direct P-wave shadow lies at about 104°–140° from the epicentre, and direct S-waves are absent at all distances beyond about 104°.

• Mechanism (P-waves): at the core–mantle boundary P-wave speed falls sharply, so rays are refracted towards the centre and emerge beyond 140°, leaving a gap.
• Mechanism (S-waves): S-waves cannot pass through the liquid outer core, so everything beyond about 104° lies in their shadow.
• Significance: the size of the shadow gives the core’s radius (about 3,480 km) and the S-wave shadow proves a liquid outer core; faint P arrivals inside the P shadow led Inge Lehmann to the solid inner core.
• Examples: for an imaginary earthquake at the North Pole, the P-wave shadow would be a belt between about 14°S and 50°S latitude, and no direct S-waves would arrive anywhere south of about 14°S.
• Sketch: Earth section with epicentre, rays at 104° and 140°, shaded P shadow band and the wider S shadow.

Seismic Tomography

Seismic tomography is the technique of building three-dimensional images of the Earth’s interior from the travel times of millions of seismic waves crossing it along different paths, rather as a medical CT scan images the body; regions where waves travel faster are usually colder and more rigid, and slower regions hotter or partly molten.

• Mechanism: recorded arrivals are compared with times predicted by a standard Earth model; delays and advances along intersecting rays map velocity anomalies of 1–3 per cent.
• Coined by: the global method was developed from the late 1970s, notably by Adam Marian Dziewoński, who with Don Lynn Anderson also published the Preliminary Reference Earth Model (1981).
• Examples: fast slabs of subducted lithosphere under the western Pacific and beneath the Tethyan collision belt; the Indian lithosphere underthrusting southern Tibet; two giant slow regions beneath Africa and the Pacific at the base of the mantle.

Discontinuities

Seismic Discontinuity

A seismic discontinuity is a surface or thin zone inside the Earth at which the velocity of seismic waves changes abruptly, reflecting and refracting them, because of a change in chemical composition, in mineral structure (phase change) or in physical state (solid to liquid).

• Types by cause: compositional (Moho — crustal rock to peridotite; Gutenberg — silicate to iron); phase-change (410 km and 660 km, where olivine collapses into denser structures); change of state (Lehmann — liquid to solid iron).
• Types by sharpness: first-order (sharp, a few kilometres thick, as at the core–mantle boundary) and gradational (spread over tens of kilometres).
• Examples: in order of depth — Conrad (15–20 km, continents only), Moho (5–70 km), 410 km, Repetti or 660 km, Gutenberg (2,891 km), Lehmann (5,150 km).
• Sketch: a depth–velocity graph with P and S curves and each discontinuity marked as a step.

Conrad Discontinuity

The Conrad discontinuity is the boundary within the continental crust, usually at 15–20 km depth, between an upper crust in which P-waves travel at about 6 km/s and a lower crust in which they travel at about 6.5–7 km/s; it was traditionally read as the change from granitic sial to more basic, denser rock.

• Coined by: Victor Conrad, the Austrian seismologist, from records of Austrian earthquakes of 1923 and 1927.
• Key features: absent beneath oceans and blurred in many continental regions, it is not global like the Moho.
• Modern view: deep seismic profiling suggests it often marks a change in metamorphic grade or in rock behaviour (brittle upper crust over ductile lower crust) rather than a simple sial–sima contact.
• Don’t confuse with: the Moho, which lies beneath the whole crust.

Mohorovičić (Moho) Discontinuity

The Mohorovičić discontinuity (Moho) is the boundary between the crust and the mantle, at which P-wave velocity rises sharply from about 6.5–7 km/s to about 8 km/s; it lies about 5–10 km below the ocean floor, about 35–40 km beneath the average continent and about 70 km beneath Tibet and the high Himalaya.

• Coined by: Andrija Mohorovičić, the Croatian seismologist, from the Kupa valley earthquake of 8 October 1909, whose distant records showed two sets of P- and S-waves — one direct through the crust, a faster one refracted along a denser layer below.
• Key features: it marks a change of rock from crustal basalt, gabbro and granite to mantle peridotite; its depth mirrors the relief above, deepest under mountains, as the Airy model of isostasy predicts.
• Examples: beneath the Indian shield the Moho lies at roughly 35–40 km; beneath the Ladakh–Tibet region it deepens to around 70 km; ophiolites expose an ancient oceanic Moho on land (ophiolite).
• Significance: no borehole has reached it; the Kola Superdeep Borehole in Russia stopped at 12,262 m, still in continental crust.
• Sketch: a crustal cross-section from ocean to mountain showing the Moho shallow under oceans and a deep root beneath mountains.

Repetti Discontinuity

The Repetti discontinuity is the name used in Indian syllabi for the boundary between the upper and lower mantle, placed at about 660–700 km depth; in modern global Earth models this corresponds to the 660-km discontinuity, where seismic velocities and density rise as mantle minerals change into denser, high-pressure forms.

• Coined by: it is named after William C. Repetti, an American Jesuit seismologist of the Manila Observatory; the name is rare in international literature, so quote the depth and the 660-km label together.
• Mechanism: ringwoodite breaks down into bridgmanite and ferropericlase, a phase change rather than a change of bulk composition.
• Key features: it forms the base of the mantle transition zone (410–660 km); the deepest earthquakes, down to about 700 km, stop near it; some subducting slabs stall on it, while others sink through it.
• Examples: slabs of the western Pacific flatten at this level beneath Japan and eastern China in tomographic images.

Gutenberg (Wiechert–Gutenberg) Discontinuity

The Gutenberg discontinuity is the boundary between the rocky mantle and the metallic outer core, at about 2,900 km depth (2,891 km in modern models), at which S-waves disappear, P-wave velocity drops from about 13.7 to about 8 km/s and density jumps from about 5.5 to about 10 g/cm³ — the largest density contrast inside the Earth.

• Coined by: Richard Dixon Oldham first inferred a core from seismic records (1906); Beno Gutenberg, working at Göttingen, calculated its depth at about 2,900 km (1913, published 1914). His teacher Emil Wiechert had argued for a dense iron core in the 1890s, hence the name Wiechert–Gutenberg.
• Key features: also called the core–mantle boundary (CMB), it is a sharp compositional boundary overlain by the D″ layer.
• Significance: it explains the shadow zones; heat flowing across it drives mantle convection and possibly plumes (mantle plume).
• Don’t confuse with: a velocity increase — P-waves slow down here; the Lehmann discontinuity lies deeper, within the core.

Lehmann Discontinuity

The Lehmann discontinuity is the boundary between the liquid outer core and the solid inner core, at about 5,150 km depth, where P-wave velocity rises from about 10.3 to about 11 km/s and weak shear waves can again propagate; it is also called the inner-core boundary.

• Coined by: Inge Lehmann, the Danish seismologist, in 1936, from faint P arrivals recorded within the shadow zone after the 1929 Buller earthquake in New Zealand, which she explained by reflection from a solid inner core.
• Mechanism: iron freezes here because pressure raises its melting point above the local temperature; as the Earth cools, the inner core grows.
• Key features: latent heat and light elements released by freezing drive outer-core convection, powering the geodynamo.
• Don’t confuse with: a second, weaker “Lehmann discontinuity” at about 220 km in the upper mantle beneath some continents, which she also identified; in exam answers the name normally means the inner-core boundary.

Layers of the Earth

Crust (Continental and Oceanic)

The crust is the Earth’s outermost solid shell, lying above the Moho and forming less than 1 per cent of the planet’s volume. It is of two contrasting kinds: thick, light, ancient continental crust of broadly granitic composition, and thin, dense, young oceanic crust of basalt and gabbro.

• Continental crust: 25–70 km thick (average about 35–40 km); average density about 2.7 g/cm³; P-wave speed about 6–6.5 km/s; rocks up to about 4 billion years old (Acasta Gneiss, Canada).
• Oceanic crust: 5–10 km thick (about 7 km typical); density about 3.0 g/cm³; layered as sediments, pillow basalt, sheeted dykes and gabbro; nowhere older than about 200 million years in open oceans because it is recycled by subduction.
• Key features: continents stand high because thick, light crust floats higher on the mantle — the basis of isostasy and of the Earth’s two dominant levels, continental platforms and deep ocean floors.
• Examples: the Dharwar and Singhbhum cratons of peninsular India hold continental crust more than 3 billion years old; the floor of the Bay of Bengal is Cretaceous oceanic crust buried under the Bengal Fan.
• Sketch: a section from ocean floor to continent showing thin oceanic and thick continental crust above the Moho.

SIAL, SIMA and NIFE

SIAL, SIMA and NIFE are the names Eduard Suess gave to the three compositional shells of the Earth: SIAL (silica and aluminium), the light granitic rock of the continents; SIMA (silica and magnesium), the denser basaltic rock beneath the continents and forming the ocean floor; and NIFE (nickel and ferrum), the metallic core.

• Key features: SIAL has a density of about 2.7 g/cm³ and is rich in potassium, sodium and aluminium silicates; SIMA has a density of about 3.0 g/cm³ and is rich in magnesium, iron and calcium silicates; NIFE is iron with some nickel, density about 10–13 g/cm³.
• Significance: the scheme allowed continents to be pictured as light blocks floating in denser rock, the premise of continental drift and of isostasy.
• Modern view: SIAL corresponds broadly to the upper continental crust and SIMA to the oceanic crust; the mantle below them is peridotite, not basalt, and the core contains about 10 per cent lighter elements (oxygen, sulphur, silicon or hydrogen) besides iron and nickel.

Lithosphere and Asthenosphere

The lithosphere is the rigid outer shell of the Earth, made of the crust and the uppermost mantle, which behaves elastically and breaks into plates; the asthenosphere is the hot, weak, ductile layer of the upper mantle beneath it, which flows slowly over geological time and allows the plates to move and to rise or sink.

• Lithosphere thickness: from a few kilometres at mid-ocean ridges to about 100 km beneath old ocean floor and 150–250 km beneath ancient continental shields; its base roughly follows the 1,300 °C isotherm.
• Asthenosphere: extends from the base of the lithosphere to roughly 350 km; rock is close to its melting point and may hold a small fraction of melt; the term was introduced by Joseph Barrell (1914).
• Examples: a 2007 study led by Prakash Kumar of the National Geophysical Research Institute, Hyderabad, found the Indian lithosphere only about 100 km thick, about half that of other Gondwana fragments, and linked this to India’s rapid drift.
• Significance: plates are pieces of lithosphere (lithospheric plate); post-glacial rebound depends on asthenospheric flow (isostatic rebound).
• Sketch: a column showing crust and uppermost mantle as rigid lithosphere over a weak asthenosphere, with the Moho drawn inside the lithosphere.

Low-Velocity Zone

The low-velocity zone (LVZ) is a layer in the upper mantle, commonly between about 100 and 250 km depth, in which P- and especially S-wave velocities fall by a few per cent below those of the rock above, because the mantle there is close to its melting point and may contain a small fraction of melt.

• Coined by: Beno Gutenberg, who argued for it from the 1920s onward, hence “Gutenberg low-velocity zone”.
• Key features: it is best developed beneath oceans and tectonically active regions and weak or absent beneath old cratons; it largely coincides with the asthenosphere.
• Significance: its weakness decouples plates from the deeper mantle; it is the region where basaltic magma for ridges and hotspots begins to form.
• Examples: beneath the Pacific Ocean floor; beneath the Basin and Range Province of the western United States.

Mantle

The mantle is the thick shell of hot, dense silicate rock between the Moho and the core–mantle boundary at about 2,891 km, forming about 84 per cent of the Earth’s volume and about two-thirds of its mass; although solid, it creeps and convects at a few centimetres a year, driving plate motion.

• Composition: peridotite, dominated by olivine and pyroxene near the top; at depth, pressure converts these into denser minerals, with bridgmanite the most abundant mineral in the lower mantle.
• Divisions: upper mantle (Moho to 410 km); transition zone (410–660 km); lower mantle (660–2,891 km), ending in the D″ layer.
• Examples: mantle peridotite xenoliths in the kimberlite pipes of the Wajrakarur field, Andhra Pradesh; hydrous ringwoodite found in a Brazilian diamond (2014) suggests the transition zone stores water.
• Sketch: a quarter-section of the Earth with upper mantle, transition zone and lower mantle labelled by depth.

Mesosphere

The mesosphere, in the mechanical layering of the Earth, is the strong, rigid part of the mantle beneath the asthenosphere, extending to the core–mantle boundary at about 2,891 km; despite temperatures of thousands of degrees, the enormous pressure keeps its rock stiffer than the asthenosphere above.

• Key features: some authors start it at the base of the asthenosphere (about 350 km), others at 660 km, equating it with the lower mantle.
• Significance: it is not static: tomography shows slabs sinking into it and broad upwellings rising through it, so convection involves the whole mantle.
• Don’t confuse with: the atmospheric mesosphere, the layer about 50–85 km above the ground. State “mantle mesosphere” in answers to avoid ambiguity.

D″ Layer

The D″ (D-double-prime) layer is the lowermost 200–300 km of the mantle, directly above the core–mantle boundary, where seismic velocities vary strongly from place to place; it is a thermal and chemical boundary layer in which heat from the core enters the mantle.

• Coined by: Keith Edward Bullen divided the Earth into layers lettered A to G in the 1940s and later split layer D (lower mantle) into D′ and D″.
• Key features: two large low-shear-velocity provinces (LLSVPs), each thousands of kilometres across, sit beneath Africa and the central Pacific; patchy ultra-low-velocity zones (ULVZs), only about 5–40 km thick, reduce S-wave speed by tens of per cent and may be partly molten or iron-enriched.
• Recent work: LLSVP margins are widely treated as plume roots (hotspot); a 2023 modelling study proposed that the provinces are remnants of Theia, the Moon-forming impactor — a hypothesis only.
• Sketch: the base of the mantle with two broad dome-shaped low-velocity piles and thin ULVZ patches on the core–mantle boundary.

Outer and Inner Core

The core is the Earth’s central metallic sphere, about 3,480 km in radius, divided into a liquid outer core (2,891–5,150 km), whose convecting iron–nickel alloy generates the magnetic field, and a solid inner core (5,150–6,371 km, radius about 1,220 km), kept solid by pressure of about 330–360 gigapascals despite temperatures of about 5,000–6,000 °C.

• Evidence: no S-waves cross the outer core; P-waves slow on entering it; reflections confirm a solid inner core; the Earth’s mean density (about 5.5 g/cm³), twice that of surface rocks, requires a dense metallic centre.
• Key features: the core holds about 16 per cent of the Earth’s volume and about 32 per cent of its mass; outer-core density rises from about 9.9 to 12.2 g/cm³.
• Innermost inner core: in 2023 Thanh-Son Phạm and Hrvoje Tkalčić of the Australian National University used waves that bounced through the Earth’s centre up to five times to argue for an innermost inner core about 650 km in radius, with a distinct crystal alignment, perhaps recording an early growth stage.
• Rotation debate: in 2023 Yi Yang and Xiaodong Song reported that the inner core’s rotation relative to the mantle paused around 2009, perhaps as part of a multi-decade oscillation; in 2024 Wei Wang and John Vidale reported it has turned slightly more slowly than the mantle (“backtracking”) since about 2010. Both findings remain debated.
• Sketch: concentric circles for mantle, outer core and inner core with depths 2,891 and 5,150 km and the state of each.

Geothermal Gradient

The geothermal gradient is the rate at which temperature increases with depth inside the Earth, averaging about 25–30 °C per kilometre in the upper continental crust; it arises from heat escaping from the interior, fed by radioactive decay of uranium, thorium and potassium and by heat left over from the planet’s formation.

• Variation: about 10–20 °C per km in old shields; 50 °C per km or more in rifts and volcanic belts.
• Key features: the gradient falls with depth — if 25–30 °C per km continued, the mantle would be molten. Estimated temperatures: about 1,300 °C at the base of the lithosphere, about 4,000 °C at the core–mantle boundary and 5,000–6,000 °C at the centre.
• Examples: the Kola Superdeep Borehole met about 180 °C at 12.2 km, hotter than predicted; in India, the Puga Valley of Ladakh, with hot springs and borax deposits, is the country’s most promising geothermal field; hot springs at Tattapani (Chhattisgarh) and Manikaran (Himachal Pradesh) mark local high gradients.
• Significance: it controls where rocks melt (magma) and geothermal energy potential.

Geomagnetism

Geomagnetism

Geomagnetism is the study of the Earth’s magnetic field — its origin in the liquid outer core, its roughly dipolar shape, its measurable elements (declination, inclination and intensity) and its changes over hours, centuries and millions of years. At the surface the field resembles that of a bar magnet inclined at about 10° to the rotation axis.

Structure and elements

• Shape: about 80–90 per cent of the surface field is a dipole; the rest is irregular non-dipole field. Field lines leave the southern hemisphere, loop through space and re-enter in the northern hemisphere, so the pole in the Arctic is, physically, the south pole of the “Earth magnet” — which is why it attracts the north-seeking end of a compass.
• Elements: declination, inclination (dip) and total intensity, which ranges from about 25 microtesla near the equator (lower still over the South Atlantic Anomaly) to about 65 microtesla near the magnetic poles.
• History of ideas: William Gilbert (1600) showed the Earth behaves as a giant magnet; Carl Friedrich Gauss (1830s) proved mathematically that the field originates inside the Earth. A permanent magnet is impossible because the core is far above the Curie point, so the field must be continuously regenerated by the geodynamo.

Variations

• Internal: secular variation — slow drift of field features, broadly westward at about 0.2° of longitude a year; wandering of the magnetic poles; and, over longer spans, polarity reversals recorded in rocks (palaeomagnetism).
• External: daily variations and magnetic storms produced by electric currents in the ionosphere and magnetosphere under the solar wind. The G5 storm of May 2024, the strongest in about two decades, produced auroras photographed from Hanle in Ladakh.

Significance

• Uses: it shields the surface from the solar wind, underpins navigation (the World Magnetic Model), aids mineral exploration and makes palaeomagnetism possible.
• Examples: India’s Alibag observatory near Mumbai, successor to the Colaba observatory begun in 1841, is operated by the Indian Institute of Geomagnetism.
• Sketch: a globe with a tilted dipole axis, field lines, geographic and geomagnetic poles, and dip needles horizontal at the magnetic equator and vertical at the dip poles.

Geodynamo

The geodynamo is the process by which the Earth’s magnetic field is generated and maintained: convection of electrically conducting liquid iron in the outer core, twisted into columns by the Earth’s rotation (Coriolis force), moves through an existing magnetic field, induces electric currents, and these currents regenerate and amplify the field — a self-sustaining dynamo.

• Mechanism: convection is driven by core cooling (thermal) and by light elements released as the inner core freezes (compositional); without continuous motion the field would decay within about 20,000 years.
• Coined by: Joseph Larmor proposed a self-exciting dynamo for sunspots in 1919; Walter Maurice Elsasser and Edward Crisp Bullard developed the theory for the Earth in the 1940s–1950s. In 1995 a computer simulation of the core produced a spontaneous polarity reversal for the first time.
• Key features: the field has existed for well over 3 billion years; it fluctuates in strength and reverses polarity at irregular intervals.
• Examples: Mars lost its dynamo early and keeps only crustal magnetism; the weakening dipole and growing South Atlantic Anomaly are monitored by the European Space Agency’s Swarm satellites.
• Don’t confuse with: remanent magnetism, which is frozen into rocks and belongs to palaeomagnetism.

Magnetic Declination and Inclination (Dip)

Magnetic declination is the horizontal angle between true (geographic) north and the direction in which a freely suspended compass needle points, recorded as east or west; magnetic inclination (dip) is the vertical angle that a freely pivoted needle makes with the horizontal, zero at the magnetic equator and 90° at the magnetic dip poles.

• Mapping lines: isogonic lines join places of equal declination and the agonic line joins places of zero declination; isoclinic lines join places of equal dip.
• Key features: both change with place and time (secular variation). Declination at London swung from about 11° east in 1580 to about 24° west around 1820 and back; in 2019 the agonic line passed through Greenwich, so compasses there pointed to true north for the first time in about 360 years.
• Relation to latitude: for a dipole field, tan (inclination) = 2 × tan (magnetic latitude); this rule lets palaeomagnetists find the latitude at which an old rock formed (palaeolatitude).
• Examples: Survey of India topographical sheets print a declination diagram so users can convert compass bearings to true bearings.

Magnetic Equator and Geomagnetic Poles

The magnetic (dip) equator is the irregular line on the Earth’s surface along which the field is horizontal (inclination zero); the geomagnetic poles are the two points where the axis of the best-fitting dipole meets the surface, while the magnetic (dip) poles are the places where the field is actually vertical.

• Three kinds of pole: geographic (ends of the rotation axis); geomagnetic (ends of the dipole axis, the northern one near north-west Greenland); dip poles (inclination 90°), which wander fastest because the non-dipole field shifts them.
• North dip pole drift: James Clark Ross located it on the Boothia Peninsula, Canada, in 1831. It has since crossed the Arctic Ocean towards Siberia, crossing the 180° meridian around 2017. Its speed rose from about 15 km a year in the early 1900s to about 50–60 km a year around 2000 and has since slowed to about 35 km a year.
• World Magnetic Model 2025: released in December 2024 by the United States National Centers for Environmental Information and the British Geological Survey and valid for 2025–2029, it tracks this drift for navigation; the south dip pole, off Antarctica, moves only about 9 km a year.
• Examples: the Thumba Equatorial Rocket Launching Station near Thiruvananthapuram was set up in 1963 because it lay close to the magnetic equator, ideal for studying the equatorial electrojet.
• Sketch: a globe showing the tilted dipole axis, geographic and geomagnetic poles, the dip pole and a wavy magnetic equator crossing the geographic equator.

PYQs Built on These Terms

• Write short note on Geomagnetism and Paleomagnetism. (2011)
• Bring out the relevance of seismic study in determining the structure of the interior of the earth. (2010)
• Write short note: Role of seismic waves in the study of earth’s interior. (2005)

Frequently Asked Questions

What is the difference between P-waves and S-waves?

P-waves are compressional waves that travel through solids, liquids and gases, while S-waves are shear waves that travel only through solids. P-waves are faster and arrive first; S-waves travel at roughly 0.6 of their speed. Because S-waves cannot cross the liquid outer core, their disappearance beyond about 104° from an epicentre revealed that the outer core is liquid.

Why is there a seismic shadow zone?

A shadow zone exists because the core blocks or bends seismic waves. P-waves slow and refract sharply on entering the liquid outer core, leaving a belt about 104°–140° from the epicentre with no direct P arrivals. S-waves are stopped entirely by the liquid, so no direct S-waves reach any point beyond about 104°.

Is the lithosphere the same as the crust?

No. The crust is a compositional layer above the Moho, while the lithosphere is a mechanical layer that includes the crust and the rigid uppermost mantle. The lithosphere is about 100 km thick on average — far thicker than the 7 km oceanic or 35–40 km continental crust — and it is the lithosphere, not the crust alone, that forms tectonic plates.

Is the Earth’s inner core solid or liquid?

The inner core is solid, although it is hotter than 5,000 °C, because immense pressure raises the melting point of iron above the local temperature. Inge Lehmann inferred it in 1936. Research since 2023 suggests an innermost inner core about 650 km in radius and small changes in the inner core’s rotation relative to the mantle, both still debated.

Why is the magnetic north pole moving towards Siberia?

The north magnetic pole moves because the flow of liquid iron in the outer core keeps changing the field it generates. Shifting patches of magnetic flux beneath Canada and Siberia have pulled it across the Arctic Ocean. Its speed peaked at about 50–60 km a year and has slowed to about 35 km a year, as tracked by the World Magnetic Model 2025.

What is the difference between the geographic and the magnetic poles?

Geographic poles are the fixed ends of the Earth’s rotation axis, while magnetic poles are where the magnetic field is vertical, and they wander. The geomagnetic poles, ends of the best-fitting dipole axis, lie about 10° from the geographic poles. This gap produces magnetic declination, which is why compass bearings must be corrected on maps.