Theories of Mountain Building – UPSC

• A mountain is an area of land that rises abruptly and steeply above its surroundings, with greater height and local relief than a hill and a restricted summit area.
• Orogeny (Greek oros, mountain + genesis, origin) is the process of mountain building: the folding, thrusting, faulting, metamorphism and uplift of rock belts, usually along the margins of continents.
  • The word was in use earlier, but Grove Karl Gilbert (1890) fixed its modern meaning by separating orogeny (mountain-making deformation of narrow belts) from epeirogeny (broad, gentle vertical movement of continents).
• Theories of mountain building try to explain the force that squeezes and lifts rock belts, the place where mountains form, their orientation and their periodic recurrence through geological time.
• The classical theories (1920s) grew around the idea of geosynclines; since the 1960s plate tectonics has explained mountain building as the result of plate convergence.

Mountains, Their Forms and the Classification of Theories

Forms of Mountain Relief

• Mountains occur at several scales, from a single ridge to a continental belt; the terms below are used for this hierarchy of mountain relief.

Form	Meaning	Example
Ridge	Linear, steep-sided high crest or spur; slopes may be symmetrical or one steep and one gentle	Shimla Ridge
Range	Linear line of mountains and hills with several ridges, peaks and valleys	Pir Panjal, Dhauladhar
Chain	Several long, narrow, roughly parallel ranges of different ages	Parallel ranges of the Andes
System	Several ranges of the same period, separated by valleys	Himalayan system (Greater, Lesser, Shiwalik)
Group	Several mountain systems in an irregular (unsystematic) pattern	Mountain knots of the Pamir
Cordillera	Spanish term for a continental-scale orogenic belt of many ranges, chains and systems	Western Cordillera of North America (Pacific to the Great Plains)

Types of Mountains by Origin

• Mountains are grouped by origin (genesis) as well as by age.
  • By age: Precambrian (e.g. Aravallis), Caledonian (Scandinavian and Scottish Highlands), Variscan or Hercynian (Urals, Harz) and Alpine or Tertiary (Alps, Himalaya, Rockies, Andes).
  • The classical theories below were built chiefly to explain the young fold mountains of the Alpine period.

Type	How it forms	Examples
Fold mountains	Compression folds and thrusts thick sedimentary belts	Himalaya, Alps, Andes, Rockies
Block (fault) mountains	Uplift of a block between faults (horst) beside sunken blocks (graben)	Vosges and Black Forest; Satpura (horst)
Volcanic mountains	Accumulation of lava and ash around vents	Fuji, Kilimanjaro; Barren Island
Dome mountains	Upwarping by intrusions or broad uplift	Black Hills (USA)
Residual (relict) mountains	Remnants of older ranges worn down by erosion	Aravallis, Parasnath

Features a Theory Must Explain

• Young fold mountains share a set of features, and every theory is judged by how many of them it can account for.
  • Thick marine sediments: thousands of metres of strata with shallow-water fossils, now standing high above sea level, as in the Tethyan sediments of the Zanskar and Tibetan Himalaya.
  • Linear or arcuate belts, usually along the margins of continents or between two landmasses, not scattered at random.
  • Intense compression: folds, reverse faults, thrusts and nappes that record large crustal shortening.
  • Metamorphism, granite intrusions and volcanism in the core of the belt.
  • Deep roots of light crust beneath the ranges, which keep them in isostatic balance.
  • Episodic recurrence: orogenies are separated by long quiet intervals, and each is followed by uplift and prolonged erosion.
  • Trend: some belts run west–east (Alps, Himalaya), others north–south (Rockies, Andes), and island arcs fringe some ocean margins.

Classification of Mountain-Building Theories

• The classical theories differ mainly in the direction of the primary crustal movement and in the force they invoke.
• Horizontal-movement theories hold that sideways compression buckles sediments into mountains.
  • Contraction theories: the earth shrinks as it cools, and the shrinkage supplies the compression (Leopold Kober, Harold Jeffreys).
  • Drift and mobile-substratum theories: moving continents or a moving substratum supply the compression (Frank Bursley Taylor, Alfred Wegener, John Joly, Reginald Daly, Arthur Holmes, and later plate tectonics).
• Vertical-movement theories hold that uplift comes first and folding follows as the raised mass slides under gravity.
  • Erich Haarmann (1930): oscillation theory, with bulges (“geotumours”) raised by deep forces and folds formed by gravity sliding off their flanks.
  • Reinout Willem van Bemmelen (1930s): undation theory, with wave-like vertical swells of the crust as the primary movement.

Group	Primary movement	Driving force	Theories
Contraction	Horizontal	Cooling and shrinking of the earth	Kober (orogen), Jeffreys (thermal contraction)
Drift / mobile substratum	Horizontal	Tidal force, gravity, radioactive heat, convection	Taylor, Wegener, Joly, Daly, Holmes
Vertical movement	Vertical, then gravity sliding	Deep-seated uplift	Haarmann (oscillation), van Bemmelen (undation)
Modern	Horizontal	Plate motion (slab pull, ridge push, mantle flow)	Plate tectonics

Contraction Theories

Geosynclinal Orogen Theory of Kober

• Leopold Kober (1883–1970), an Austrian geologist, set out the theory in Der Bau der Erde (1921), an account of the structure of the earth’s surface.
• His aim was to link the ancient rigid masses with the mobile zones of sedimentation between them, and to explain the formation, history and evolution of mountains from those mobile zones.
  • He defined orogenesis as the process that links rigid masses with geosynclines: mountains grow out of geosynclines under the pressure of rigid masses.
  • He regarded the rigid masses as the foundation stones of the continents, which have grown by adding mountain belts to their margins.

Orogenic Force

• Kober was a contractionist: the contraction of a cooling earth is the motive force.
  • Contraction sets the rigid masses (forelands) moving horizontally towards each other, and they squeeze, buckle and fold the sediments lying between them.
  • In his view, contraction has continued since the earth’s origin and has supplied the force for each period of mountain building, each passing through much the same sequence of events.

Basis: Kratogen, Orogen and Orogenic Periods

• Kratogen: the old rigid masses or forelands (Hans Stille later shortened the word to kraton).
  • Examples: Canadian Shield, Baltic Shield and Russian Platform, Siberian Shield, Chinese Massif, Peninsular India, African Shield, Brazilian mass, and the Australian and Antarctic masses.
• Orogen: the geosyncline, the mobile zone of water where mountains are built.
  • Kober’s geosynclines are long and wide seas marked by sedimentation and subsidence, whereas Émile Haug’s geosynclines were comparatively narrow depressions.
  • His scheme combined the geosynclinal ideas of James Hall and James Dwight Dana, as developed by Haug, with his own view of orogenesis.
  • He also placed a mid-Pacific geosyncline between north and south Pacific forelands, which later foundered to form the Pacific Ocean.
• Six periods of mountain building:
  • Three in the Precambrian, about which little is known.
  • Caledonian, completed by the end of the Silurian.
  • Variscan (Hercynian), culminating in the Permo-Carboniferous.
  • Alpine, completed in the Tertiary.
  • In India, the Aravallis are the worn-down relic of a Precambrian orogeny, while the Himalaya belong to the Alpine period.

Mechanism: Three Stages

1. Lithogenesis (preparatory stage)
   • Geosynclines form, bordered by the forelands.
   • Rivers erode the forelands and deposit the waste in the geosyncline (sedimentation); the growing weight depresses its floor (subsidence).
   • The twin processes allow enormous thicknesses of sediment to accumulate.
2. Orogenesis (mountain-building stage)
   • Contraction drives the two forelands towards each other; the compression squeezes and folds the sediments.
   • With moderate compression, only the marginal sediments are folded into two border ranges, the Randketten (marginal ranges), one on each side.
     • The central part stays unfolded as the Zwischengebirge (“betwixt mountains”) or median mass.
   • With intense compression, the whole geosyncline is folded, the forelands close in, and recumbent folds and nappes develop, making the structure complex.
   • The later part of this stage also brings volcanism and intense metamorphism.
3. Gliptogenesis (uplift and denudation stage)
   • The mountains rise gradually, and a long period of subaerial erosion wears them down to areas of subdued relief.

Median Masses and the Alpine–Himalayan Belt

• Kober applied his model to the Tethys geosyncline, which lay between the European (Angara) foreland in the north and the African (Gondwana) foreland in the south.
  • In Europe, the northward push of Africa folded the Betic Cordillera, Pyrenees, Provence ranges, Alps proper, Carpathians, Balkans and Caucasus towards the north.
  • The southward push of Europe folded the Atlas, Apennines, Dinarides and Hellenides towards the south.
  • In Asia, Angaraland and Gondwanaland moved together to fold the Tethys sediments; the Himalaya, Zagros and Burmese ranges were folded southward, while the Kunlun and ranges of Yunnan–Annam were folded northward.
  • The west–east Alpine trend breaks in north-east India, where the ranges swing south as the Burmese (Purvanchal) arc; the Asiatic Alpine belt runs from Asia Minor to the Sunda Islands.
• Median masses between ranges of opposite fold direction take three forms:

Form	Median mass	Bordering ranges
Plateau	Tibetan Plateau	Kunlun (north), Himalaya (south)
Plateau	Iranian Plateau	Elburz, Zagros
Plateau	Anatolian Plateau	Pontic, Taurus
Plateau/basin	Great Basin (Basin and Range)	Wasatch, Sierra Nevada
Plain	Hungarian (Pannonian) Plain	Carpathians, Dinaric Alps
Sea	Western Mediterranean (Corsica and Sardinia as remnants)	Pyrenees–Provence ranges, Atlas
Sea	Caribbean Sea	Central American and West Indian ranges

Kratogenic Movements

• Besides orogenic movements in the geosynclines, Kober recognised kratogenic movements within the rigid masses themselves.
  • Orogenic movements fold and metamorphose geosynclinal sediments to great depths.
  • Kratogenic movements produce rifts, fractures and block mountains in the forelands, and only shallow folding of any cover sediments.
  • He attributed to them the superficial folds of the Jura, the Rhine and East African rift valleys and the horsts of Central Europe.
• The Narmada–Son and Damodar rift structures of Peninsular India are fractures of this kind within a stable block.

Evaluation

• Strengths
  • The model explains the two-sided, symmetrical structure of many belts, with opposite fold directions and a central mass.
  • The idea that mountains form from thick geosynclinal sediments passed into almost every later theory, including early plate-tectonic accounts.
  • Its basic geometry of two blocks converging and crushing the zone between them survives in the plate-tectonic view of continent–continent collision.
• Weaknesses
  • The force of contraction is far too small to fold ranges as large as the Alps, Himalaya, Rockies and Andes.
  • The assumption that both forelands move was long disputed.
    • Eduard Suess held that only one side moves: the moving backland is thrust over the stable foreland; for the Himalaya, Angaraland moved south against a stationary Gondwanaland.
    • Plate tectonics settles the question differently: what matters is relative convergence, and for the Himalaya it is mainly the Indian plate moving north.
  • It suits west–east ranges (Alps, Himalaya) but fails for north–south ranges along the Pacific (Rockies, Andes), which have no opposing foreland.
  • Current view: the Tibetan Plateau is not an unfolded median mass but thickened, deformed crust built of accreted blocks; “median masses” are now read as microcontinents, back-arc basins or extensional basins.

Thermal Contraction Theory of Jeffreys

• Harold Jeffreys (1891–1989), a British geophysicist and mathematician, set out the theory in The Earth: Its Origin, History and Physical Constitution (1924).
• He aimed to explain the major relief features (continents, ocean basins, mountains, island arcs and festoons), above all the origin and distribution of mountain systems.
  • His reasoning was mathematical; he rejected continental drift because he found no force (tidal or pole-fleeing) strong enough to move continents.

Sources of the Contractional Force

• Cooling of the earth: heat lost by radiation from the surface makes the earth shrink, and the shrinking crust must shorten.
• Slowing of the earth’s rotation: as tidal friction slows rotation, the equatorial bulge flattens, shortening the equatorial circumference.
  • Rotation has indeed slowed: a day lasted only about 18 hours some 1.4 billion years ago, against 24 hours now; but the stress this produces is small.

Mechanism: The Level of No Strain

• The earth is built of concentric shells; cooling is effective only in the outer ~700 km, below which temperature and volume have hardly changed.
• Within this zone each upper shell cools and contracts before and more than the shell beneath it; once a shell reaches its cooling limit, the shell below begins to cool and contract.
• This creates three zones:
  • Upper zone of compression: the cooled outer shell is too large for the still-contracting shell below; it must collapse onto it, and the resulting horizontal compression causes buckling, folding and thrusting, forming mountains.
  • Level of no strain: an intermediate surface where contraction exactly fits the layer below; it migrates downward as the earth cools.
  • Lower zone of tension: the contracting layer is too small to fit over the hot interior; it is stretched and thinned, and cracks and fissures open and fill with molten material from below.
• Continued stretching below allows the outer shell to collapse further, so mountains already formed rise higher.
• Jeffreys worked out the amount of shrinkage mathematically from assumed temperature falls in the outer shell, and treated the resulting reduction of surface area as the total shortening that all mountain belts together had to share.

Period, Zones and Direction of Mountain Building

• Period: compressive and tensile stresses accumulate slowly until they exceed rock strength; folding and faulting then set in, and stop when the stresses are spent.
  • Hence orogenic periods alternate with long periods of quiescence.
• Zones: soft, plastic rocks yield by folding, while hard, brittle rocks fail by faulting into blocks; so mountain building is localised.
• Direction: cooling and contraction are stronger beneath the oceans than beneath the continents.
  • The compression therefore acts from the oceans towards the continents, at right angles to the coast, and builds ranges parallel to the ocean margins.
  • This fits the Rockies and Andes along the Pacific, but not the west–east Alps and Himalaya.
  • The Himalaya face no ocean at all, and the north–south Arakan Yoma of the Indo-Burman ranges lies inland of the Bay of Bengal rather than parallel to a vast ocean floor.

Evaluation

• The shortening is far too small: Holmes pointed out that the calculated reduction of surface area falls well short of what mountain building requires; the Alpine and Himalayan belts record hundreds of kilometres of crustal shortening.
• The idea of the earth cooling shell by shell is not accepted, and the effect of slowing rotation on mountain building is negligible.
• It is hard to believe that contraction could be so great in the last ~200 million years as to raise all the Tertiary ranges (Alps, Himalaya, Rockies, Andes).
• Uniform contraction should give an even distribution of continents, oceans and mountains; instead they are unevenly distributed and mountains follow definite belts along continental margins.
• Contraction would produce many small puckers and minor folds, not a few long, massive ranges.
• It explains north–south Pacific ranges but not west–east ranges such as the Himalaya.
• Current view: the discovery of radioactive heating removed the premise of a steadily cooling, shrinking earth; the theory is now of historical interest only.

Drift and Radioactivity Theories

Radioactivity Theory of Joly

• John Joly (1857–1933), an Irish physicist and geologist at Trinity College Dublin, set out the theory in The Surface-History of the Earth (1925).
  • It is also called the thermal cycle theory; its main aim was a thermal history of the earth’s surface, but it also addressed mountain building and continental drift.
• Driving force: periodic expansion and contraction of the substratum as it melts and re-solidifies, driven by heat from radioactive elements; this causes transgression and regression of the seas.
  • Both Joly and Holmes used radioactive heat: Joly to melt and re-solidify the substratum, Holmes to drive convection in it.

Basis

• Continents are of light sial (density about 2.67), resting on a substratum of basaltic sima (about 3.0); ocean floors largely lack sial.
• Radioactive elements (uranium, thorium and others) are richer in sialic continental rocks than in the sima.
  • Their heat output is tiny, but over long periods it accumulates.
• Heat produced within the sial is lost by radiation, so it does not accumulate in the continents.
• Under the continents, Joly put the base of a 30 km thick sial at about 1,050 °C; the melting point of basalt is about 1,150 °C.
  • No heat passes up from the sima, so the lower sima stores radioactive heat; under the oceans, heat from the upper sima escapes into the water, but the deeper sima still accumulates heat.
  • He calculated that 33 to 56 million years would supply the extra heat, including the latent heat of fusion, to melt the substratum.

Mechanism: Transgressional and Regressional Phases

• Transgressional phase (substratum molten)
  • The sima expands, the earth’s radius increases, and continents rise relative to the earth’s centre.
  • The molten sima is less dense, so the continents sink deeper into it; the sea spreads over the continental margins (transgression).
  • Sediments accumulate in these shallow marginal seas, which become geosynclines.
  • Expansion stretches the ocean floors; cracks open and basalt rises, forming oceanic islands.
  • Floating continents respond to the tidal force and drift westward; by occupying new positions they let the stored heat escape, ending the phase.
• Regressional phase (substratum re-solidifies)
  • The substratum cools from the top down, regains its density, and the earth’s radius shrinks back.
  • The oceans withdraw from the continental margins (regression), exposing the marine sediments.
  • The ocean floors, which had expanded most, now contract most and press on the continental margins.

Two Processes of Mountain Building

• Horizontal compression: the contracting floors of two neighbouring oceans squeeze the soft marginal sediments into folds and nappes.
  • The pressure begins even before the sima has fully solidified.
• Vertical uplift: when the whole substratum has solidified, the continents rise and lift the folded belt by isostatic recovery.
• Consequences:
  • Mountains form along continental margins facing oceans.
  • The greatest mountains should face the greatest ocean, as the Rockies and Andes face the Pacific.
  • Orogeny is cyclic: each cycle of melting and re-solidification is a “revolution”, and quiescent periods separate the orogenic phases.

Evaluation

• The theory is simple and appealing, and gives a connected story of many surface features; critics regarded it as a descriptive history of the earth rather than a tested theory.
• Harold Jeffreys was its strongest critic:
  • Seismic evidence of his time suggested a sial only about 16 km thick; at that depth the temperature would fall far short of the melting point, and the cycle could not start.
  • No known force is adequate to drive continents westward.
  • Once melted by radioactive heat, the substratum could not re-solidify, so there could be no repeated cycles.
• Regular, equally spaced orogenies are not borne out; orogenies recur, but irregularly.
• The rule “greatest mountains face greatest oceans” holds for the Pacific, but mountains do not line both margins of every continent, and none parallel the Atlantic coasts; the west–east Alpine–Himalayan belt is unexplained.
• Its geosynclines receive sediment but do not subside, which cannot account for the great sediment thickness of the Alpine ranges.
• Knowledge of the amount and distribution of radioactive elements at depth was very limited, so the whole thermal budget of the theory rested on assumptions.
• Current view: the mantle is solid (it transmits S-waves) and does not periodically melt; radioactive heat is real and important (geoneutrino measurements put it at roughly half of the earth’s heat flow), but it drives slow solid-state convection, not melting cycles.

Sliding Continent Theory of Daly

• Reginald Aldworth Daly (1871–1957), a Canadian-American geologist at Harvard, set out the theory in Our Mobile Earth (1926).
• His main aim was to explain the origin, successive upheavals, distribution, orientation and extent of fold mountains.
• Driving force: gravity alone; mountains form as continental masses slide downhill into geosynclines. He invoked no tidal or other force.
• Daly was an early supporter of continental drift; in Strength and Structure of the Earth (1940) he pictured a rigid outer shell resting on a weak, slippery layer, an idea close to the later lithosphere and asthenosphere.

Axioms

• Much of the theory rests on axioms (assumed truths) that Daly did not elaborate.
• A primitive crust formed soon after the earth’s origin.
• It rose as three belts of rigid masses: north polar, equatorial and south polar domes.
• These were separated by depressions: the mid-latitude furrows (the northern one being the Tethys) and the primeval Pacific Ocean.
  • Their floors of primitive crust made them the first geosynclines.
• The crust, cool and crystalline, was denser than the substratum, which he pictured as hot, glassy basalt.
• Water covered about half the globe, and the domes sloped towards the furrows and the Pacific.

Mechanism

• Heat loss made the interior contract away from the crust, and the crust collapsed onto it under:
  • the weight of ocean water,
  • the weight of geosynclinal sediments, and
  • gravity, strongest under the oceans, which lie nearer the earth’s centre.
• Rivers carried sediment from the domes into the furrows and the Pacific; loading made the sea floors subside.
• The sinking floors exerted lateral pressure on the continents, which swelled into larger, higher domes.
  • As the domes grew, dense material flowed beneath them from under the oceans.
• When the pressure on the ocean floor exceeded its strength, the floor ruptured; the domes lost their support, and large continental blocks slid towards the geosynclines.
• The sliding blocks squeezed and folded the geosynclinal sediments into fold mountains.
  • The dense continental and oceanic blocks foundered in the lighter substratum and melted, and the added volume lifted the ranges further.
  • The light sediments floated and were folded more intensely; the greater the slide, the greater the mountains.
• Distribution:
  • West–east ranges (Alps, Himalaya) formed as the polar and equatorial domes slid into the Tethys furrow.
  • North–south ranges (Rockies, Andes) formed as the Americas slid towards the Pacific.
  • The island arcs and festoons of east Asia formed as Asia slid towards the Pacific, and their load created the deep trenches in front of them.
  • On this view the Himalaya record the slide of the equatorial dome, which included Peninsular India, northward into the Tethys furrow, and the Andaman–Nicobar arc would be a festoon of the same kind on the Bay of Bengal side.

Evaluation

• Wrong earth structure: a crust denser than the substratum contradicts seismology, which shows density increasing with depth; such a crust would sink, not float.
• Guesswork: why was the crust uneven from the start, why did the domes slope towards the furrows, how did the Pacific form? The theory gives no convincing answers.
• Distorted geosynclines: geosynclines are long, narrow and relatively shallow; Daly’s were whole oceans, which could never be filled with sediment.
• It expects mountains from every ocean, whatever its size, depth or sediment load.
• It does not explain when gravity stops pulling and the floors begin to rupture, so its events are piecemeal, not a coherent sequence.
• It assumes a distribution of land and sea that suits its own purpose; above all, the cause of the primary bulges that start the sliding is never shown.
• Current view: gravity is now seen as a real agent in orogens.
  • Gravity gliding and spreading of nappes and the collapse of over-thickened orogens are accepted processes.
  • Ridge push and slab pull, the main plate-driving forces, are themselves gravitational; Daly was right about gravity, wrong about the mechanism.

Thermal Convection Current Theory of Holmes

• Arthur Holmes (1890–1965), a British geologist, presented the theory to the Geological Society of Glasgow in January 1928 as “Radioactivity and Earth Movements” (published 1929).
• He sought a physical mechanism for continental drift as well as for mountain building, opposing the fixed continents and ocean basins of the contractionists.
  • It was the only classical theory to reconcile the rival ideas of mountain building and continental drift.
• Driving force: convection currents in the substratum, driven by heat from radioactive decay.

Basis

• The outer earth has three layers: an upper granodiorite layer (about 10–12 km), an intermediate amphibolite layer (about 20–25 km) and a lower eclogite layer.
  • The crust comprises the upper and middle layers with the crystalline top of the lower layer (sial above, sima below); the substratum is the hot, mobile part of the lower layer. Sial is generally absent under the oceans.
• Radioactive elements are richest in the crust, but its heat escapes by conduction and radiation, so the crust does not heat up.
  • The substratum has fewer radioactive elements, but it cannot lose its heat; radioactive heat plus original heat accumulate and set up convection.
• The pattern of currents depends on:
  • Crustal thickness: the crust is thicker at the equator than at the poles, so currents rise under the equator and sink under the poles.
  • Uneven radioactivity: currents rising under continents are stronger than those under oceans, as continental crust is richer in radioactive elements.

Mechanism: Opening and Closing of Oceans

• Diverging currents (under rising columns) stretch, thin and rupture the crust; the pieces are carried apart and the gap becomes a sea or ocean.
  • Holmes attributed the opening of the Tethys to currents diverging under the equatorial crust.
• Converging currents (over falling columns), one from under a continent and one from under an ocean, meet near continental shelves; the crust is compressed and dragged down, forming a geosyncline, and the sea begins to close.
  • Currents later brought Laurasia and Gondwanaland together, closing the Tethys and folding its sediments into the Alpine–Himalayan ranges.
  • In Indian terms, the same pair of processes opens the Indian Ocean behind the northward-moving Indian block and closes the Tethys in front of it, where the Himalaya rise.
• Folding proceeds together with deposition and subsidence, as lateral pressure acts from both sides of the geosyncline.
• Convection is periodic, not steady: it waxes, wanes and starts again from new centres (Holmes, 1952).

Three Stages of the Convection Cycle

Stage	Duration	Convection	Result
First (lithogenesis)	Longest	Currents converge under the continental shelves	Geosyncline forms; sedimentation and subsidence; buried sediments metamorphosed (amphibolite to denser eclogite)
Second (orogenesis)	Short	Sharp acceleration as cold, dense material sinks and hot material rises	Maximum compression folds the geosynclinal sediments; mountain building begins
Third (gliptogenesis)	Waning	Currents weaken and stop as the rising column cools	Pressure is released; eclogite melts, expands and rises; mountains are uplifted until isostatic balance is reached

• In the first stage, the downgoing column becomes a column of increasing density, as metamorphism turns amphibolite into eclogite; part of the heat is used in metamorphism.
• In the third stage, the sunken, heavier material rises as the load on the falling column is removed, and uplift continues until isostatic equilibrium is restored.
• Holmes also used convection to explain median masses, rift valleys and volcanism, while admitting that some of his suggestions were speculative.

Support and Evaluation

• Contemporary support:
  • Felix Andries Vening Meinesz found belts of strong negative gravity anomalies over the trenches of the East Indies in the 1920s–30s, suggesting crust held down by descending currents.
  • David Tressel Griggs (1939) reproduced down-buckling of a crust over converging currents in a mechanical model.
• Criticism at the time:
  • Little was known of the substratum; rising and falling columns were unproven.
  • The radioactive heat in the substratum might be too little to drive currents, and rising currents lose heat to the crust by conduction.
  • Without enough heat there could be no horizontal flow, no falling column and hence no mountains.
  • The change of amphibolite (about 3.0) to eclogite (about 3.4) may not add enough density to sink the column; if eclogite does not sink, the falling column fills up and cannot accept more material.
  • Why should currents rise only at a few centres, not everywhere?
• Later vindication:
  • Seafloor spreading (1960s) showed currents rising and diverging under mid-ocean ridges, splitting plates and feeding fissure eruptions, as Holmes had imagined.
  • Seismic tomography now images cold slabs sinking deep into the mantle; the mantle is solid but convects by slow creep.
  • Holmes kept the idea alive in his Principles of Physical Geology (1944), and it became the first mechanism offered for plate motion when Harry Hess proposed seafloor spreading.
  • Current view: the mantle convects, but the plates are the active upper part of the system; they are driven mainly by slab pull, with ridge push and mantle drag secondary, rather than carried passively on currents as Holmes pictured.

From Classical Theories to Plate Tectonics

Plate Tectonics and Mountain Building

• Plate tectonics (1960s) explains mountain building by the convergence of lithospheric plates.
  • Mountains rise where plates converge, whether by subduction of oceanic lithosphere or collision of continents.
  • Divergent boundaries build mid-ocean ridges and rift valleys; transform boundaries mostly offset plates without building major ranges.

Convergence	Process	Mountain type	Example
Ocean–continent	Oceanic plate subducts beneath continent	Cordilleran (volcanic-arc) ranges	Andes, Cascades
Continent–continent	Buoyant continents collide after the ocean between them closes	Collision fold-thrust belts, high plateaus	Himalaya–Tibet, Alps
Ocean–ocean	One oceanic plate subducts beneath another	Island arcs with trenches	Japan, Mariana, Aleutians
Arc–continent	Island arc collides with a continent	Accreted mountains	New Guinea

• Ocean–continent margins: along western South America, subduction of the Pacific floor since the early Mesozoic has crumpled the marginal sediments, added volcanic arcs and uplifted the Andes; deformation grew as South America moved west.
• Continent–continent collision: the convergence of Africa and Europe closed the western Tethys and raised the Alps and the Atlas, much as India and Asia raised the Himalaya.
  • Erosion of the rising belt feeds flysch (deep-water sediments shed during folding) and later molasse (coarse continental debris in the foreland); the Shiwalik deposits are the Himalayan molasse.
• Ocean–ocean margins: arcs such as Japan are separated from the mainland by back-arc basins (the Sea of Japan), with a deep trench on the ocean side.
  • Eduard Suess had already recognised such arcs as continuations of the young fold ranges of the continents; plate tectonics explains them by subduction.
• Indian example: the Himalaya rose after India, drifting north across the Tethys at about 15–18 cm/yr, met Asia about 50 million years ago.
  • India still moves north-north-east at about 4–5 cm/yr, so the range remains tectonically active, as the Gorkha (Nepal) earthquake of 2015 and the Dingri (Tibet) earthquake of January 2025 show.

Comparative Evaluation of the Theories

Theory	Year	Force	Explains	Fails to explain
Kober (geosynclinal orogen)	1921	Contraction moves two forelands together	Two-sided ranges, median masses, west–east Alps–Himalaya	Adequate force; north–south Pacific ranges
Jeffreys (thermal contraction)	1924	Cooling contraction; slowing rotation	Ranges parallel to the Pacific; orogenic periods	Adequate shortening; west–east ranges; mountain belts
Joly (radioactivity)	1925	Radioactive melting and re-solidification of substratum	Marginal ranges; cycles; Pacific rim	Re-solidification; Atlantic coasts; Alpine–Himalayan belt
Daly (sliding continents)	1926	Gravity sliding of continental domes	Both west–east and north–south ranges; island arcs	Denser crust over lighter substratum; initial bulges
Holmes (convection)	1928–29	Radioactively driven convection	Drift, ocean opening and closing, geosynclines, uplift	Sinking of eclogite; source of heat (at the time)
Plate tectonics	1960s	Plate convergence (slab pull, ridge push, mantle flow)	Location, trend, age and structure of mountain belts	Details of forces and continental deformation still debated

What the Classical Theories Contributed

• Kober: the link between thick sedimentary belts and mountains, and the geometry of two converging blocks crushing the zone between them.
• Jeffreys: a quantitative approach, and the recognition that stress must accumulate before orogeny, giving orogenic periods.
• Joly: the importance of radioactive heat in the earth’s thermal history, and the link between sea-level change and sedimentation.
• Daly: the role of gravity in moving and sliding crustal masses.
• Holmes: mantle convection, which became part of the plate-tectonic explanation.
• The geosyncline concept itself is now obsolete; its thick sedimentary wedges are read as passive continental margins and accretionary wedges that are deformed when plates converge.