Plate Tectonics: Terminology for UPSC Geography Optional

Plate tectonics is the organising theory of modern geomorphology: it explains where oceans open, mountains rise, volcanoes erupt and earthquakes strike. This post gathers its vocabulary — plates and their driving forces, the three margins, subduction-zone anatomy, collision scars and plumes — with the landform, volcanic and seismic signature of each.

Every entry opens with an exam-ready definition, then mechanism, named examples and a sketch line. UPSC asked candidates to compare plate boundaries in 2019 and to trace plate tectonics back to isostasy and continental drift in 2021; the 2014 Himalaya–Appalachian question and the 1998 volcanicity question rest on the same terms.

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Plates and What Moves Them

Plate Tectonics

Plate tectonics is the unifying theory that the Earth’s rigid outer shell is broken into about seven major and many smaller lithospheric plates that move over the weak asthenosphere, creating crust where they part, consuming it where they converge and sliding it past where they shear, with most earthquakes, volcanoes and mountains concentrated along their edges.

• Formation of the theory: it was assembled in the 1960s from three older ideas. Alfred Wegener‘s continental drift (1912) proved continents had moved; Arthur Holmes (1928–31) supplied mantle convection as a motor; Harry Hammond Hess (1962) proposed sea-floor spreading, confirmed by the magnetic stripes of Frederick John Vine and Drummond Hoyle Matthews (1963). John Tuzo Wilson named plates and transform faults (1965); Dan McKenzie and Robert L. Parker (1967), William Jason Morgan (1968) and Xavier Le Pichon (1968) described plate motion on a sphere.
• Debt to isostasy and drift: isostasy showed that the crust floats in gravitational balance on a yielding layer, so large lateral and vertical movements are physically possible; drift supplied the evidence of movement. Plate tectonics kept both but replaced Wegener’s sial ploughing through sima with whole lithospheric slabs riding the asthenosphere, and replaced his tidal and pole-fleeing forces with mantle-driven ones.
• Key features: rigid plate interiors; deformation concentrated at margins; constant surface area, since crust made at ridges equals crust consumed at trenches.
• Examples: the Atlantic has widened for about 180 million years along the Mid-Atlantic Ridge while the Pacific shrinks by subduction; India drifted north after leaving Gondwana and has been colliding with Asia since about 50 million years ago to raise the Himalaya (collision orogeny); the Appalachians record the Palaeozoic closure of the older Iapetus Ocean.
• Significance / limits: it explains the global belts of earthquakes and volcanoes, but stable-interior earthquakes such as Latur (1993) and broad diffuse boundaries such as Tibet show that plates are not perfectly rigid.
• Sketch: a ridge-to-trench cross-section showing lithosphere, asthenosphere, ridge axis, transform offset, trench, subducting slab and volcanic arc, with arrows of plate motion.

Lithospheric Plate

A lithospheric plate is a rigid slab of the lithosphere — crust together with the uppermost mantle — roughly 70–100 km thick beneath old oceans and 150–250 km beneath ancient continental shields, which moves as one coherent unit over the asthenosphere and deforms mainly along its boundaries rather than in its interior.

• Types: oceanic (Nazca, Cocos), composite ocean-and-continent (Indian, African) and microplates (Burma microplate).
• Key features: seven major plates — Pacific, North American, South American, Eurasian, African, Antarctic and Indo-Australian (now often split into Indian and Australian) — plus secondary plates such as Arabian, Philippine Sea, Caribbean and Juan de Fuca. The word “plate” in this sense was introduced by John Tuzo Wilson in 1965.
• Examples: the Indian Plate is bounded by the Himalayan collision front to the north, the Andaman–Sunda subduction zone to the east, the Owen Fracture Zone and Chaman Fault to the west, and the Carlsberg and Central Indian ridges to the south-west.
• Don’t confuse with: a continent — continents ride inside plates, and a single plate may carry ocean floor and land.

Ridge Push, Slab Pull and Mantle Convection

Ridge push, slab pull and mantle convection are the forces proposed to drive plates: ridge push is lithosphere sliding under gravity off the elevated, hot mid-ocean ridge; slab pull is the downward tug of a cold, dense slab sinking at a subduction zone; mantle convection is the slow thermal circulation of the mantle beneath the plates.

• Mechanism: new lithosphere cools, thickens and subsides away from the ridge, so gravity pushes it downslope; at trenches, old oceanic lithosphere, denser than the asthenosphere, pulls the plate behind it.
• Relative importance: Donald W. Forsyth and Seiya Uyeda (1975) showed that plates with long subducting edges — Pacific, Nazca, Cocos — move fastest, so slab pull is the dominant force; plates with little subducting edge, such as the Eurasian and African, move slowly.
• Mantle convection: Arthur Holmes pictured currents dragging passive plates; today plates are seen as the cold upper layer of convection itself.
• Examples: the Pacific Plate moves at roughly 7–10 cm a year; India raced north at up to 15–18 cm a year in the Late Cretaceous, then slowed sharply after collision.

Euler Pole

An Euler pole is the point where the imaginary axis about which one plate rotates relative to another cuts the Earth’s surface; by Leonhard Euler‘s fixed-point theorem, any movement of a rigid plate over a sphere can be described as a rotation about such an axis through the Earth’s centre.

• Mechanism: every point on the moving plate travels along a small circle about the pole; relative velocity is zero at the pole and greatest 90° away, so spreading or convergence rate varies along a single boundary.
• Key features: transform faults lie along small circles about the pole, so great circles drawn at right angles to them meet at the pole — the test applied to the equatorial Atlantic by William Jason Morgan (1968), after Dan McKenzie and Robert L. Parker (1967) had used it for the North Pacific.
• Examples: the Red Sea widens southwards because Arabia rotates away from Africa about a pole lying to the north-west.
• Sketch: a globe with the pole, concentric small circles, and ridge segments offset by transforms lying along those circles.

The Three Plate Margins

Divergent (Constructive) Plate Margin

A divergent (constructive) plate margin is a boundary along which two plates move apart and hot asthenosphere rises into the gap, melting by decompression to form new basaltic crust; it appears as a mid-ocean ridge in ocean basins and as a rift valley where a continent is being split.

• Mechanism: as mantle rises beneath the axis, pressure falls faster than temperature, so peridotite partially melts to tholeiitic basalt; magma fills dykes and erupts as pillow lava; the new crust cools, contracts and subsides as it moves away, which is why ridges stand 2–3 km above the abyssal plains.
• Stages: continental rift (East African Rift) → narrow sea with an oceanic floor (Red Sea, Gulf of Aden) → broad ocean with a central ridge (Atlantic). In the Afar region of Ethiopia, the 2005 Dabbahu rifting episode opened a dyke about 60 km long by up to 8 m within weeks.
• Landform signature: axial rift valleys on slow ridges, ridge flanks cut by normal faults, volcanic islands where supply is high, and horsts and grabens on land. Iceland straddles the Mid-Atlantic Ridge, whose Þingvellir fissures mark the boundary on land.
• Volcanic signature: quiet, effusive fissure eruptions of low-viscosity basalt, such as the Reykjanes Peninsula eruptions in Iceland (Fagradalsfjall from 2021; the Sundhnúkur crater row near Grindavík from December 2023).
• Seismic signature: frequent, shallow, moderate earthquakes from normal faulting, generally shallower than about 20 km.
• Examples: Mid-Atlantic Ridge (slow, about 2–5 cm a year in total); East Pacific Rise (fast, up to about 15 cm a year); Carlsberg and Central Indian ridges in the Arabian Sea and Indian Ocean.
• Sketch: cross-section of a ridge with rising asthenosphere, magma chamber, axial rift, symmetrical magnetic stripes and arrows pointing apart.

Convergent (Destructive) Plate Margin

A convergent (destructive) plate margin is a boundary where two plates move towards each other and lithosphere is consumed — by subduction of the denser plate into the mantle where oceanic lithosphere is involved, or by crustal shortening and thickening where two buoyant continents meet.

• Types: ocean–ocean — the older, denser plate sinks, building a trench and island arc (Mariana, Izu–Bonin, Aleutians); ocean–continent — oceanic lithosphere sinks beneath a continent, building a trench and continental volcanic arc (Nazca Plate beneath South America, raising the Andes); continent–continent — continental crust is too light to sink far, so it crumples and stacks (India against Eurasia, raising the Himalaya and the Tibetan Plateau).
• Landform signature: trenches, arcs, fold-and-thrust belts and plateaus such as Tibet and the Altiplano.
• Volcanic signature: explosive andesitic to rhyolitic eruptions above subduction zones; the slab does not simply melt — water released from it at about 100 km lowers the melting point of the overlying mantle wedge (flux melting). Collision zones such as the Himalaya have almost no active volcanoes.
• Seismic signature: the widest range of focal depths, from shallow to about 700 km along the Wadati–Benioff zone, and the largest megathrust earthquakes.
• Don’t confuse with: the typology of the mountains that result — Andean-type versus Himalayan-type orogeny is covered under collision and subduction orogeny; this entry describes the margin itself.

Transform Fault (Conservative Margin)

A transform fault is a strike-slip plate boundary along which two plates slide horizontally past each other, so crust is neither created nor destroyed; it “transforms” motion from one ridge or trench segment to another and, in the oceans, offsets mid-ocean ridges into staggered segments.

• Coined by: John Tuzo Wilson (1965); Lynn R. Sykes (1967) confirmed from earthquake first motions that slip on ridge-offsetting transforms is opposite to the sense suggested by the offset.
• Key features: active only between offset ridge ends; aligned along small circles about the Euler pole; shallow earthquakes, no volcanism; on land, linear valleys, offset streams and sag ponds.
• Examples: San Andreas Fault, California — right-lateral boundary between the Pacific and North American plates, which slipped up to about 6 m in 1906; Alpine Fault, New Zealand; Dead Sea Transform. Around India: the Owen Fracture Zone (India–Arabia) and Chaman Fault (Pakistan–Afghanistan) on the west, and the right-lateral Sagaing Fault of Myanmar on the east (2025 Myanmar earthquake).
• Don’t confuse with: an ordinary strike-slip fault within a plate — a transform ends abruptly against other boundaries and its displacement is plate-scale.

The three margins compared

Fracture Zone

A fracture zone is the inactive, linear scar on the ocean floor that continues beyond the ends of an active transform fault; across it the two sides now move together as one plate but differ in age, depth and heat flow, so it preserves a record of former ridge offsets and plate-motion directions.

• Key features: escarpments and troughs thousands of kilometres long; a depth step because crust of different ages has subsided by different amounts; hardly any earthquakes.
• Examples: Romanche Fracture Zone in the equatorial Atlantic; Mendocino and Clarion fracture zones in the North Pacific.
• Significance: fracture zones trace the direction of past plate motion and are used to reconstruct Euler poles and continental break-up.
• Don’t confuse with: the transform fault itself — only the segment between ridge ends is active. The Owen “Fracture Zone” in the Arabian Sea is, despite its name, an active transform boundary.

Triple Junction

A triple junction is the point where three plate boundaries meet; it is classified by the boundary types involved — R for ridge, T for trench, F for transform — and it persists only when the relative velocities of the three plates allow the geometry to remain stable as they move.

• Coined by: analysed by Dan McKenzie and William Jason Morgan (1969), who showed that some junctions are stable and others migrate or evolve.
• Types: RRR — the Afar triple junction where the Nubian, Somali and Arabian plates separate along the Red Sea, Gulf of Aden and East African Rift, and the Rodrigues triple junction where the Central, Southwest and Southeast Indian ridges meet; TTT — the Boso triple junction off central Japan, the only one of its kind; FFT — the Mendocino triple junction off northern California.
• Significance: when one arm stops opening it becomes a failed rift (aulacogen), such as the Benue Trough of Nigeria, which can later host thick sediment and hydrocarbons.

Anatomy of a Subduction Zone

Subduction

Subduction is the process by which one lithospheric plate, usually older and denser oceanic lithosphere, sinks beneath another at a convergent margin and descends into the mantle under its own weight, recycling ocean floor and generating the arc volcanism and great earthquakes that ring the Pacific.

• Coined by: borrowed from Alpine geology, where André Amstutz used it in 1951.
• Mechanism: oceanic lithosphere older than a few tens of millions of years is denser than the asthenosphere; basalt converts to denser eclogite at depth, increasing the pull; dehydration of the slab near 100 km depth triggers flux melting in the mantle wedge above.
• Types: Seiya Uyeda and Hiroo Kanamori (1979) contrasted Mariana-type subduction — old, steep slab, weak coupling, back-arc extension — with Chilean-type — young, gently dipping slab, strong coupling, compression and giant earthquakes.
• Examples: Pacific Plate beneath Japan; Nazca Plate beneath South America; Indian Plate beneath the Burma and Sunda plates along the Andaman–Sunda Trench, site of the 2004 earthquake (megathrust earthquake).
• Sketch: a slab dipping from a trench beneath an arc, showing the forearc, accretionary wedge, 100 km depth line, rising magma and volcano.

Wadati–Benioff Zone

A Wadati–Benioff zone is an inclined, planar band of earthquake foci that dips from an oceanic trench beneath the overriding plate to depths of as much as 700 km, outlining the subducting slab; it is named after Kiyoo Wadati, who mapped deep foci beneath Japan in the 1920s–30s, and Victor Hugo Benioff, who described such zones worldwide in 1949–55.

• Key features: shallow thrust events on the plate interface, intermediate and deep events within the slab; dip ranges from gentle (central Chile) to nearly vertical (Mariana); beneath north-east Japan the zone is doubled into two parallel planes.
• Significance: direct evidence that lithosphere descends deep into the mantle; the arc lies where the zone is about 100 km deep.
• Examples: beneath Japan and the Tonga–Kermadec arc; the Hindu Kush intermediate-depth zone whose tremors are felt across north India; the eastward-dipping zone beneath the Indo-Burman ranges of Manipur and Myanmar.
• Don’t confuse with: the entire convergent margin — it is only the seismically active slab. Depth classes of earthquakes are defined under focal depth.

Oceanic Trench

An oceanic trench is a long, narrow, steep-sided depression of the deep-ocean floor, usually 6–11 km deep, formed where oceanic lithosphere bends down into a subduction zone; trenches are the surface trace of convergent margins and the deepest places on Earth.

• Formation: the downgoing plate flexes into the subduction zone, raising a gentle outer rise seaward and a steep inner wall.
• Key features: asymmetrical V-shaped cross-section; arcuate plan convex towards the subducting ocean; floors may be bare or filled with turbidites where large rivers feed sediment.
• Examples: Mariana Trench, whose Challenger Deep reaches close to 11 km; Peru–Chile (Atacama) Trench; Japan and Kuril trenches; the Sunda (Java) Trench and its northern continuation, the Andaman Trench, west of the Andaman and Nicobar Islands.
• Don’t confuse with: the axial rift valley of a mid-ocean ridge — trenches mark destruction of crust, not creation.

Accretionary Wedge (Accretionary Prism)

An accretionary wedge is a wedge of deformed, thrust-stacked ocean-floor sediment and slivers of crust scraped off the downgoing plate and plastered on to the leading edge of the overriding plate at a subduction zone; it grows outward and upward with time and may rise above sea level as an outer-arc ridge.

• Formation: the overriding plate acts like a bulldozer blade; sediments are sliced off along stacked thrusts, while some are dragged down and mixed into a chaotic mélange.
• Key features: landward-dipping imbricate thrusts, folded mud and sandstone, ophiolite slices and fluid seeps.
• Examples: Makran coast of Pakistan and Iran, one of the thickest wedges; Barbados; the Andaman and Nicobar Islands, which are the emergent outer-arc wedge, while Barren Island and Narcondam form the volcanic inner arc; the Indo-Burman (Arakan) ranges.
• Significance: splay faults within wedges can raise tsunamis; overpressured mud escapes as mud volcanoes, as at Baratang in the Andamans.

Island Arc

An island arc is a curved chain of volcanic islands built on the overriding plate about 100–300 km behind an oceanic trench, where magma generated in the mantle wedge above a subducting slab rises to the surface; arcs are typically convex towards the ocean from which the subducting plate comes.

• Formation: ocean–ocean subduction; flux melting at about 100 km depth; andesitic stratovolcanoes grow and coalesce into islands.
• Key features: trench, forearc, volcanic front and back-arc in parallel belts; the island “festoons” of east Asia.
• Examples: Aleutian, Kuril, Mariana and Lesser Antilles arcs; the Sunda arc; India’s arc volcanoes, Barren Island (active) and Narcondam, above the Indian Plate subducting in the Andaman Sea.
• Don’t confuse with: a continental volcanic arc (Andes, Cascades) built on a continent. Japan began as an island arc but now rests on continental fragments rifted from Asia.
• Sketch: plan and section of trench, arc and back-arc basin with the dipping slab.

Back-Arc Basin

A back-arc basin is a sea basin that opens behind an island arc, on the side away from the trench, by extension and small-scale sea-floor spreading within the overriding plate; it forms where the subducting slab sinks and “rolls back”, pulling the trench and arc oceanward and stretching the region behind them.

• Mechanism: slab rollback, typical of Mariana-type subduction of old, steep slabs; the arc splits and a spreading centre develops behind it.
• Key features: thin crust, high heat flow, hydrothermal vents and shallow earthquakes.
• Examples: Mariana Trough and Lau Basin (Pacific); Sea of Japan, which opened in the Miocene and detached Japan from mainland Asia; Tyrrhenian Sea (Mediterranean); the Andaman Sea, with an active back-arc spreading centre east of the Andaman–Nicobar arc.
• Don’t confuse with: a foreland basin, which forms in front of a mountain belt by crustal loading, not behind an arc by stretching.

Active and Passive Continental Margins

An active continental margin coincides with a plate boundary — usually a subduction zone or transform — and is marked by earthquakes, volcanoes, a narrow shelf and young mountains; a passive continental margin lies inside a plate, far from any boundary, formed by ancient rifting and marked by a broad shelf, thick sediment wedge and little seismicity.

• Active (Pacific-type) examples: western South America (Andes above the Peru–Chile Trench), Japan; transform-active: California.
• Passive (Atlantic-type) examples: eastern Americas and western Africa; both coasts of peninsular India — the east coast rifted from Antarctica–Australia in the Early Cretaceous, the west coast from Madagascar and later the Seychelles, leaving the Western Ghats escarpment.
• Key features: passive margins carry thick sediment prisms that host petroleum (Mumbai High, Krishna–Godavari basin); active margins carry trenches and arcs.
• Significance: the shallow-water shelf wedge of an old passive margin corresponds to the classical miogeosyncline (geosyncline).

Collision Scars and Continental Growth

Obduction and Ophiolite

Obduction is the thrusting of a slice of oceanic lithosphere up and over a continental margin, instead of its sinking by subduction; the emplaced slice is an ophiolite — a sequence of deep-sea sediment, pillow basalt, sheeted dykes, gabbro and mantle peridotite that preserves a complete section of former ocean floor on land.

• Coined by: “ophiolite” was introduced by Alexandre Brongniart (1813); Gustav Steinmann (1927) grouped serpentinite, pillow lava and radiolarian chert as the “Steinmann trinity”; the modern definition dates from the 1972 Penrose Conference; “obduction” was coined by Robert G. Coleman (1971).
• Mechanism: often at the onset of collision, when a buoyant continental margin dragged into a trench is overridden by ocean floor.
• Examples: Semail (Samail) ophiolite of Oman, the largest and best exposed, formed in the Late Cretaceous and thrust on to the Arabian margin; Troodos, Cyprus; in India, the Nidar and Spongtang ophiolites of Ladakh along the Indus–Tsangpo suture, the Naga–Manipur ophiolite belt and the Andaman ophiolite.
• Significance: ophiolites mark vanished oceans such as Neo-Tethys and carry chromite and copper ores.
• Sketch: a column of the ophiolite sequence, and a section of an ocean slab thrust over a continental margin.

Suture Zone

A suture zone is a narrow belt, often only a few kilometres wide, marking the join between two formerly separate continental blocks after the ocean between them has closed by subduction; it is recognised by ophiolites, mélange, high-pressure blueschist and arc rocks squeezed along a major fault.

• Formation: the last stage of ocean closure — subduction consumes the ocean, arcs and microcontinents collide, and the continental margins weld together along the former trench.
• Examples: Indus–Tsangpo (Yarlung Zangbo) Suture Zone from Ladakh across southern Tibet, recording the closure of Neo-Tethys and the joining of India to Asia; the Shyok Suture north of the Kohistan–Ladakh arc; the Iapetus Suture beneath the Solway Firth between Scotland and England.
• Landform link: its crushed rocks erode into longitudinal valleys, so the upper Indus and Yarlung Tsangpo follow the suture.
• Don’t confuse with: the Main Central and Main Boundary thrusts, which are younger faults inside the Indian Plate, not sutures (Himalayan thrust system).

Accreted Terrane

An accreted terrane is a fault-bounded crustal fragment — an island arc, oceanic plateau, seamount chain or microcontinent — that was carried on a plate and welded to a continent at a subduction zone, and whose rocks, fossils and palaeomagnetic latitude differ sharply from those of its neighbours.

• Coined by: the concept of “suspect terranes” was set out by Peter J. Coney, David L. Jones and James W. H. Monger (1980) for the North American Cordillera.
• Mechanism: buoyant fragments too thick to subduct jam the trench and are sliced on to the continent; subduction then steps outboard behind them.
• Examples: Wrangellia, a Triassic oceanic plateau now in Alaska and British Columbia; the Kohistan–Ladakh arc, an island arc caught between the Indian and Asian plates.
• Significance: continents grow laterally by accretion.

Wilson Cycle

The Wilson cycle is the repeated opening and closing of ocean basins — continental rifting, sea-floor spreading, subduction, ocean closure and continental collision — proposed by John Tuzo Wilson in 1966 from evidence that a proto-Atlantic ocean closed to form the Appalachian–Caledonian belt before the present Atlantic reopened along nearly the same line.

• Named by: Kevin C. A. Burke (1975), in Wilson’s honour.
• Stages: embryonic (East African Rift); young (Red Sea); mature (Atlantic); declining, with subduction on its margins (Pacific); terminal (Mediterranean); relic scar or geosuture (Indus–Tsangpo suture and the Himalaya).
• Examples: closure of the Iapetus Ocean built the Appalachians and the Caledonides of Scotland and Norway; closure of Neo-Tethys built the Alpine–Himalayan belt.
• Significance: it reinterprets the classical geosyncline as the sediment wedge of an ocean margin in one phase of the cycle, and answers the 2014 question on the Himalaya and Appalachians together.
• Sketch: six small sections from rift to collision in sequence.

Supercontinent Cycle

The supercontinent cycle is the episodic assembly of nearly all continental crust into a single landmass and its later break-up, recurring roughly every 400–600 million years as the Wilson cycles of many oceans act together; Pangaea is the most recent supercontinent, preceded by Rodinia and Columbia (Nuna).

• Coined by: developed in the 1980s by Thomas R. Worsley, R. Damian Nance and Judith B. Moody.
• Sequence: Columbia (about 1.8–1.5 billion years ago) → Rodinia (about 1.1 billion–750 million years ago) → Pangaea (assembled by about 300 million years ago, breaking up from about 200 million years ago) → a possible future supercontinent (proposals include Amasia and Pangaea Proxima).
• Mechanism: a supercontinent insulates the mantle beneath it until plumes rise and rifting begins; peripheral subduction later reassembles the continents.
• Significance: it drives long-term sea-level, climate and evolutionary change; India’s cratons sat in Rodinia and later Gondwana.

Plumes and Intraplate Activity

Hotspot and Mantle Plume

A hotspot is a long-lived centre of volcanism fed by a mantle plume — a narrow column of abnormally hot mantle rising, possibly from the core–mantle boundary — that stays nearly fixed while a plate moves over it, so the plate records a line of volcanoes that grow progressively older away from the active centre.

• Coined by: John Tuzo Wilson (1963) proposed hotspots to explain the Hawaiian chain; William Jason Morgan (1971) proposed deep mantle plumes.
• Mechanism: a plume head spreading beneath the lithosphere produces a flood basalt province; the narrower plume tail then builds a hotspot track.
• Examples: Hawaiian–Emperor chain, about 6,000 km long, whose bend dated to about 47 million years records a change in Pacific Plate motion, with Kīlauea active at the young end; Iceland, where a plume coincides with a ridge; Yellowstone. The Réunion plume erupted the Deccan Traps about 66 million years ago, and its track runs through the Lakshadweep–Maldives–Chagos ridge and Mauritius to the active Piton de la Fournaise on Réunion; the Kerguelen plume produced the Rajmahal Traps (about 117 million years) and the Ninetyeast Ridge.
• Debate: palaeomagnetic work since 2003 shows the Hawaiian hotspot itself drifted south during the Emperor phase, so hotspots are not perfectly fixed, and the depth of plume origin remains disputed.
• Sketch: a plate moving over a plume with volcanoes ageing away from it.

Intraplate Volcanism

Intraplate volcanism is volcanic activity located far from any plate boundary, in the interior of a plate, fed by mantle plumes, local mantle upwelling or stretching of the lithosphere rather than by sea-floor spreading or subduction; it accounts for ocean-island chains, continental flood basalts and isolated volcanic fields.

• Key features: mostly alkali-rich basalts; no trench or Wadati–Benioff zone; age progressions where a plume is involved.
• Examples: Hawaii in the middle of the Pacific Plate; Canary Islands; the Cameroon Line of West Africa; the Tibesti massif of Chad; the Eifel volcanic field of Germany; the Deccan Traps, erupted in the interior of the Indian Plate.
• Significance: it was once an awkward exception to plate tectonics; the plume hypothesis turned it into supporting evidence, a point expected in the 1998 volcanicity question.
• Don’t confuse with: East African Rift volcanoes such as Kilimanjaro, which sit on an incipient divergent boundary.

Aseismic Ridge (Hotspot Track)

An aseismic ridge is a long, linear submarine ridge that, unlike a mid-ocean ridge, is nearly free of earthquakes because it is not a plate boundary; most are volcanic tracks of thickened oceanic crust built as a plate moved over a hotspot, so their age increases steadily along their length.

• Examples: Ninetyeast Ridge in the Bay of Bengal and eastern Indian Ocean, about 5,000 km long, left by the Kerguelen hotspot as India moved north; Chagos–Laccadive (Lakshadweep–Maldives) Ridge, the Réunion track; Walvis Ridge off Namibia, the Tristan hotspot track.
• Significance: they record the direction and speed of past plate motion.
• Don’t confuse with: a mid-ocean ridge, which is an active, seismic spreading boundary with a central rift and symmetrical magnetic stripes.

PYQs Built on These Terms

• The concept of Plate Tectonics has been derived from the Isostasy and Continental Drift Theory. Elaborate citing suitable examples. (2021)
• Compare and contrast different types of plate boundaries. (2019)
• State the concept of plate tectonics. How does it help in explaining the formation of the Himalayas and Appalachian Mountains? (2014)
• Discuss the concept of volcanicity and show how the theory of Plate Tectonics explains the mechanism of volcanism and volcanic eruptions. (1998)

Frequently Asked Questions

What are the three types of plate boundaries?

The three types are divergent, convergent and transform boundaries. At divergent boundaries plates separate and new crust forms, as along the Mid-Atlantic Ridge; at convergent boundaries plates collide and crust is subducted or thickened, as in the Andes and Himalaya; at transform boundaries plates slide past each other, as along the San Andreas Fault.

What is the difference between subduction and obduction?

Subduction sends oceanic lithosphere down into the mantle beneath another plate, while obduction pushes a slice of oceanic lithosphere up on to a continent. Subduction is the normal fate of old ocean floor and builds trenches and arcs; obduction is rarer, usually happens as collision begins, and leaves ophiolites such as Oman’s Semail and Ladakh’s Nidar complexes exposed on land.

Why are there no volcanoes along transform faults?

Transform faults have no volcanoes because plates there move sideways without pulling apart or sinking. Magma needs either decompression, as at ridges, or water-induced melting above a sinking slab, as at subduction zones. Along a transform neither process operates, so the San Andreas and Owen transforms produce earthquakes but no eruptions, except where short “leaky” segments open slightly.

How is a hotspot different from a plate-boundary volcano?

A hotspot volcano is fed by a mantle plume beneath the middle of a plate, whereas a plate-boundary volcano is fed by spreading or subduction. Hotspots leave age-progressive chains such as Hawaii–Emperor or the Réunion track from the Deccan Traps; boundary volcanoes line ridges and arcs, such as Iceland’s fissures or Japan’s stratovolcanoes.

How did plate tectonics form the Himalaya?

The Himalaya formed when the Indian Plate, after the oceanic floor of Neo-Tethys ahead of it had been subducted beneath Asia, collided with the Eurasian Plate about 50 million years ago. Indian continental crust was too buoyant to sink, so it was sliced into stacked thrust sheets, while ophiolites along the Indus–Tsangpo suture mark the vanished ocean. Convergence continues today.