Recent Views on Mountain Building – UPSC

• Orogeny is the process of mountain building: the folding, faulting, metamorphism, magmatism and uplift that turn belts of the crust into mountain ranges.
  • The term was coined by the American geologist Grove Karl Gilbert (1890), who used it for the fold belts of the Alps and the Rockies.
  • Submarine ridges and seamounts form by different processes and are not treated as orogens.
• Two views have shaped modern thinking:
  • The geosynclinal view: mountains are squeezed out of thick sediment piles laid down in long, subsiding troughs.
  • The plate tectonic view (from the 1960s): mountains rise where lithospheric plates converge, by subduction, collision and accretion.
• Earlier theories of mountain building rested on contraction, drift and convection (Kober, Jeffreys, Joly, Daly, Holmes).

Geosynclinal View of Mountain Building

Meaning of Geosyncline

• The early globe is pictured as rigid masses (ancient continental nuclei) surrounded by mobile belts of water receiving heavy sediment; these belts are geosynclines.
• Definition: geosynclines are long, narrow and shallow water depressions marked by sedimentation and subsidence.
  • James Alfred Steers (1932): long, relatively narrow depressions that subsided while sediment accumulated in them.
• Characteristics:
  • Shallow but deeply filled: sediments of the young fold mountains (Rockies, Andes, Alps, Himalaya) are many kilometres thick yet carry shallow-sea fossils, so the floor must have sunk as it filled.
  • Mobile: their location, shape and extent changed through geological time.
  • Bordered by two rigid forelands, whose erosion supplies the fill.

Evolution of the Concept

James Hall (1859) and James Dwight Dana (1873)

• James Hall, working in the Appalachians, showed that the folded rocks are shallow-water marine sediments far thicker than strata of the same age inland.
  • The trough floor subsided under the load, so water depth stayed nearly constant.
• James Dwight Dana named such a sinking trough a geosyncline: a long, narrow, shallow and sinking sea bed, much longer than wide.
• The idea is due to Hall and Dana; the theory of how geosynclines develop is due to Haug.

Émile Haug (1900)

• Émile Haug treated geosynclines as relatively deep-water belts, much longer than wide, lying between rigid masses.
• On his Mesozoic palaeogeographic map he placed:
  • Five rigid masses: North Atlantic, Sino-Siberian, Africa–Brazil, Australia–India–Madagascar and Pacific.
  • Four geosynclines between them: Rockies, Ural, Tethys and Circum-Pacific.
• Sedimentation: coarse material at the shallow margins, fine material in the centre; the margins shift with marine transgression and regression.
• Not every geosyncline completes the cycle; some fill for ages without yielding mountains.
• Criticism:
  • The map shows land masses far larger than the geosynclines, with no account of where they went after the Mesozoic.
  • Deep-water geosynclines clash with the shallow-sea fossils of fold mountains.

Later Refinements

Stages of Geosynclinal Orogeny

• Leopold Kober (1921) set the geosynclinal cycle in three stages, driven by contraction of a cooling Earth that pushes the forelands together.

Lithogenesis

• Creation, sedimentation and subsidence: the forelands are eroded, their debris settles on the geosyncline floor, and the load deepens it to take more sediment.

Orogenesis

• Squeezing and folding: converging forelands compress the fill into ranges.
  • Moderate compression folds only the edges into two marginal ranges (Randketten), leaving an unfolded median mass (Zwischengebirge) between them.
  • The Tethys, squeezed between the European and African forelands, gave the Alpine system, with its nappes and overthrusts.

Gliptogenesis

• Gradual rise and denudation: the ranges continue to rise while weathering and erosion lower them; unloading triggers isostatic rebound.

Limitations of the Geosynclinal View

• No adequate force: cooling contraction cannot shorten the crust by hundreds of kilometres, and radioactive heat undercuts a cooling Earth.
• It explains where sediment gathered, not why forelands move, nor the one-sided thrusting of nappes.
• Deep-sea drilling showed that many “geosynclinal” rocks are ocean-floor and continental-margin deposits; the geosyncline survives only as a descriptive term.

Plate Tectonic View of Mountain Building

Basis of the View

• Plates are rigid slabs of lithosphere (crust plus uppermost mantle), about 100 km thick, moving over the weaker asthenosphere.
  • J. Tuzo Wilson (1965) described a network of plates bounded by ridges, trenches and transform faults; Dan McKenzie and Robert Parker (1967), W. Jason Morgan (1968) and Xavier Le Pichon (1968) worked out plate motion on a sphere.
  • Evidence came from palaeomagnetism and sea-floor spreading.
• Boundaries are divergent (constructive), convergent (destructive) and transform (conservative).
• Core idea: fold mountains form at convergent boundaries, where compression crumples margin sediments, thickens the crust and adds magma.
  • John F. Dewey and John M. Bird (1970) grouped mountain belts into cordilleran (subduction) type and collision type.
• Driving force: mantle convection was long held to move the plates; slab pull of sinking lithosphere is now seen as the main force, aided by ridge push and mantle flow.

Orogeny at Convergent Margins

Ocean–Ocean Convergence: Island Arcs

• The denser plate descends along a Benioff (Wadati–Benioff) zone, marked by a deep trench.
• At about 100 km depth the slab releases water, which lowers the melting point of the overlying mantle wedge; the magma rises to build a volcanic arc some distance behind the trench.
  • Arc lavas are mainly andesitic, as partial melts mix with crust and ocean-floor sediment.
• Japan (Honshu): the Pacific plate subducts at the Japan Trench; the Sea of Japan is a back-arc basin.
  • Its twin belts of rock, high-pressure–low-temperature on the ocean side and high-temperature–low-pressure on the arc side, are the paired metamorphic belts described by Akiho Miyashiro (1961).
  • Mountains here are mostly volcanic, built by repeated eruption rather than folding alone.
• Indian example: Barren Island, India’s only active volcano, sits on the Andaman arc where the Indian plate subducts beneath the Burma plate.
  • Silent for about 140 years after 1852, it erupted in 1991 and has been intermittently active since, with a fresh phase from 2022.
  • The Andaman–Nicobar ridge itself is an uplifted accretionary prism of scraped-off sediments.

Ocean–Continent Convergence: Cordilleran Mountains

• The heavier oceanic plate subducts; continental-margin sediments are squeezed, folded and thrust, while arc magma intrudes as granite batholiths.
• Andes: the Nazca plate subducts beneath South America; the range combines fold-thrust belts, a volcanic arc and the Altiplano plateau.
• Rockies: the Farallon plate subducted beneath North America; during the Laramide orogeny (about 80–55 million years ago) the slab flattened and deformation reached far inland.
• The rising batholiths are light and buoyant, adding further uplift to the range.
  • Indian example: the granite batholiths around Ranchi on the Chotanagpur plateau are read as roots of Precambrian mountain belts.

Continent–Continent Collision

• A convergent margin that consumes an entire ocean brings two continents together.
• Continental crust is too buoyant to sink far; the leading edge underthrusts the other plate, the crust stacks and thickens, and high ranges and plateaus rise.
• The line of collision is a suture, often marked by ophiolites (slices of old ocean floor).
• Alps: Africa (with the Adriatic microplate) closed the western Tethys against Europe; the complex collision explains the Alps’ complex nappe structure.
  • Africa still moves north, subducting beneath the Aegean (Hellenic) arc.

Accretion of Terranes

• Terranes are fault-bounded blocks with a geological history different from their neighbours: island arcs, seamounts, ocean-floor slices or microcontinents.
  • Carried on a subducting plate, they are too buoyant to sink and are docked (accreted) to the continent, making it grow outwards.
• Peter Coney, David Jones and James Monger (1980) showed that over 70% of the North American Cordillera is built of such “suspect terranes”, added mostly in Mesozoic to early Cenozoic time.
  • Wrangellia is the best-known example.
• Indian example: the Kohistan–Ladakh arc, an island arc of the Tethys, was accreted to Asia before and during the India collision; the Ladakh batholith is the root of that subduction arc.

Orogenic Cycles and the Wilson Cycle

• Plate tectonics explains old fold belts as well as young ones, which the classical theories could not.
• J. Tuzo Wilson (1966) proposed that oceans open and close repeatedly: the Wilson cycle of rifting, spreading, subduction and collision.

Mountains Classified by Genesis

Origin of the Himalaya

Stages of Himalayan Orogeny

• The Himalaya is the type example of collision orogeny, about 2,400 km long.

• Scale of shortening: India and Asia have converged by about 3,600 km since collision, yet only 500–900 km of shortening is found in the Himalaya itself.
  • The rest is taken up in Tibet and Central Asia, by subducted Indian lithosphere, and, on one view, by a two-stage collision in which a Tethyan microcontinent docked first and India proper arrived about 25–20 million years ago.
• Classical Tethyan sediments survive high up: the summit rocks of Everest are Ordovician marine limestone.

Structure of the Himalaya

• The range is a stack of crustal slices driven south over India along thrusts that root in one Main Himalayan Thrust:

• Molasse: coarse debris shed from the rising range into the foreland basin; the Siwaliks are older molasse, and the Indo-Gangetic plains are the foreland basin still filling.

The Himalaya Is Still Rising

• Continued convergence: GPS shows India moving north-north-east at about 4–5 cm/yr; roughly 2 cm/yr is absorbed across the Himalaya.
• Crustal thickening and isostasy: the thickened crust (up to about 70 km beneath Tibet) keeps buoying the range; erosion of deep valleys triggers isostatic rebound of the peaks.
  • A 2024 study linked 15–50 m of Everest’s height to rebound after the Arun river was captured by the Kosi system about 89,000 years ago; the massif rises by about 2 mm/yr.
  • Everest’s height was re-measured by Nepal and China in 2020 at 8,848.86 m.
• Earthquake cycle: strain stored between earthquakes is released on the thrusts.
  • The Gorkha (Nepal) earthquake of 2015 (Mw 7.8) lifted the foothills near Kathmandu but slightly lowered the High Himalaya, so height is built over many cycles and on several thrusts.
  • The Dingri (Tibet) earthquake of January 2025 (M7.1) on a north–south rift shows Tibet spreading east–west as it is squeezed.
  • The whole arc lies in India’s seismic Zones IV–V (2016 map, still in force).
• Hazards follow uplift: steep, rising slopes and young sediments feed landslides and floods, as at Joshimath, where land subsidence was reported in 2023.

Evaluation of the Plate Tectonic View

Merits

• Supplies a real driving mechanism (plate convergence) where the geosynclinal view had none.
• Explains the location of fold belts at plate margins, their arcuate trends, paired trenches and volcanic arcs, and their seismicity.
• Accounts for granite batholiths, ophiolites, paired metamorphic belts and accreted terranes within one framework.
• Explains cyclic orogeny through the Wilson cycle, including Precambrian and Palaeozoic belts.

Limitations and Current View

• Intraplate mountains (Tien Shan, Laramide Rockies) and wide deforming zones such as Tibet do not fit rigid-plate behaviour; continents deform as broad zones, not thin boundaries.
• The shortening deficit of the India–Asia collision is still debated.
• Climate and erosion are now seen as active partners in orogeny: focused monsoon erosion and crustal channel flow help exhume the Greater Himalaya, while plate convergence alone sets the frame.
• The geosyncline is reinterpreted as the passive-margin wedge, trench and foreland basin that plate convergence later deforms.