Introduction
The rock cycle is a central conceptual framework for interpreting how the three principal rock types—igneous, sedimentary and metamorphic—transform into one another over geologic time. It frames these transformations as responses to disequilibrium: rocks are carried into new physical, chemical or biological environments by internal and external forces (for example, tectonic transport, burial, uplift, weathering or interaction with hydrospheric processes), and in doing so they undergo breakdown, recrystallization, melting or lithification that yields different rock types. This entry treats the rock cycle as a transitional concept of geologic time; however, the underlying source material has been noted to lack sufficient inline citations (flagged January 2014), and readers should consult primary literature for detailed evidentiary support.
Mechanistically, examples of disequilibrium-driven change are straightforward: an extrusive igneous rock such as basalt can be chemically and physically degraded at Earth’s surface by atmospheric and hydrologic agents, whereas the same material may be returned to a molten state if it is conveyed to depth by subduction and subjected to higher temperatures and pressures. Plate tectonics and the global water cycle repeatedly expose crustal material to varying pressure–temperature–composition regimes, providing the ongoing fluxes that underpin the cyclical conversion among rock types.
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The rock cycle has heuristic and biogeochemical importance because it clarifies the genetic relationships among rock classes and the processes that interconvert them; on planets with life, these geologic transformations also participate in long-term biogeochemical cycling by transferring elements among lithosphere, hydrosphere, atmosphere and biosphere. Individual minerals can record specific formation conditions within this cycle—for example, diamond forms under very high pressures and temperatures in mantle or deeply buried metamorphic environments, and its occurrence within igneous or metamorphic host rocks preserves evidence of those conditions.
A practical aspect of the igneous component of the cycle is the architecture of magmatic systems. Common intrusive and extrusive features include magma chambers or batholiths, dykes, laccoliths, pegmatites, sills and stratovolcanoes, which together constitute the plumbing and emplacement products of magmatism. Textural and structural relationships within and between intrusions reveal temporal and process information: younger intrusions commonly cut older ones; fragments of older country rock may be entrained as xenoliths or preserved as roof pendants; thermal alteration of adjacent rock produces contact metamorphic aureoles; and emplacement of laccoliths can mechanically dome and uplift overlying strata. These features and processes provide the observational basis for reconstructing the history of igneous activity within the broader rock‑cycle framework.
Transition to igneous rock
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When rocks are buried deeply enough to experience sufficient heating and pressure, their minerals may melt to produce magma; subsequent cooling and solidification of that molten material yields igneous rock. If magma crystallizes while still emplaced beneath the surface, the slow loss of heat favors the growth of large, visible mineral grains, producing intrusive (plutonic) rocks such as granite. By contrast, magma erupted as lava onto the surface cools rapidly in contact with the atmosphere or water, constraining crystal growth and forming fine‑grained extrusive (volcanic) rocks; basalt is a common product of such rapid cooling. Extremely rapid quenching can arrest crystallization entirely and produce natural volcanic glass (obsidian).
Thus, the cooling history and emplacement environment principally determine igneous texture: prolonged cooling at depth yields coarse crystals, rapid cooling at or near the surface yields fine crystals, and instantaneous quenching produces a glassy fabric. Importantly, any rock class—igneous, sedimentary, or metamorphic—can be remelted and recycled into new igneous material, reflecting the continuous recycling inherent in the rock cycle.
Secondary changes (epigenetic alteration)
Epigenetic alteration comprises low-temperature, low-pressure mineralogical modifications driven by fluid–rock interaction; these secondary processes commonly rework specific mineral groups across whole rock bodies and often operate concurrently within a single lithology. They alter primary assemblages and hence the physical and chemical behavior of rocks during subsequent stages of the rock cycle.
Common types of epigenetic alteration include silicification, kaolinization, serpentinization, uralitization, chloritization and epidotization. Silicification replaces original minerals with microcrystalline or crystalline silica and is most typical in felsic hosts (e.g., rhyolite) though it can affect some ultramafic, serpentine-bearing bodies. Kaolinization results from chemical breakdown of feldspars to kaolinite (with residual quartz and other clay phases) and is especially pronounced in coarse‑grained felsic plutons such as granites and syenites. Serpentinization entails hydration of olivine to serpentine minerals, commonly producing magnetite, and is diagnostic of peridotites and widespread in mafic lithologies. Uralitization produces hornblende at the expense of primary augite, converting originally pyroxene‑dominated mafic rocks into hornblende‑rich assemblages. Chloritization converts ferromagnesian phases (for example augite, biotite, hornblende) into chlorite and is frequent in mafic intrusives and extrusives (diabases, diorites, greenstones). Epidotization forms epidote from precursors such as biotite, hornblende, augite or plagioclase and commonly accompanies the chloritization–uralitization suite in mafic rocks. Together, these alterations reflect the pervasive role of low‑grade metasomatism in modifying mineralogy prior to metamorphic or sedimentary reworking.
Transition to metamorphic rock
Metamorphism comprises the physical and chemical reworking of pre‑existing rocks driven by elevated temperature and pressure, producing new mineral assemblages, textures and chemical distributions while remaining below the point of melting. These changes reflect recrystallization, pressure‑driven realignment of minerals and fluid‑assisted chemical exchange rather than igneous fusion.
Two principal settings dominate. Regional metamorphism affects extensive volumes of crust during orogenic processes: crustal thickening, continental collision and attendant deformation create conditions of high pressure and temperature across broad belts. Rocks so altered commonly develop foliation—planar fabrics or compositional banding—formed by directional stress, mineral segregation and recrystallization during ductile deformation. By contrast, contact metamorphism is highly localized around igneous intrusions; the thermal pulse from magma recrystallizes adjacent country rock to form a thermally zoned aureole, typically with effects that diminish away from the intrusion and with textures that are often non‑foliated.
Fluid interactions also play a significant role. Magmatic fluids can introduce or remove chemical components (metasomatism), producing mineral assemblages that cannot be explained by pressure and temperature alone. Any lithology—igneous, sedimentary or previously metamorphosed rock—can act as a protolith, so metamorphic processes have the potential to modify the mineralogy, texture and chemistry of virtually any rock given suitable temperature, pressure and fluid regimes.
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Rocks exposed at Earth’s surface are subject to weathering (in-place disintegration) and erosion (removal and transport), processes that break intact bedrock into solid fragments and mobilize dissolved constituents. Mechanical and chemical weathering yield a spectrum of particle sizes (from sand to clay) alongside ions carried in solution; the solid fragments are deposited when transport energy falls, while dissolved material can be transported long distances before precipitating.
These sediments and solutes preferentially accumulate in sedimentary basins, where repeated deposition buries earlier layers. Progressive burial subjects loose detritus to compaction and chemical alteration—diagenetic processes that expel pore fluid, promote mineral growth, and cement grains together—thereby converting unconsolidated sediment into coherent sedimentary rock. Although an individual sand grain preserves the mineral identity of its source, the compacted and cemented aggregate constitutes a distinct rock type.
Sedimentary rocks form by three principal pathways. Clastic (or detrital) sedimentary rocks arise from the lithification of physically transported fragments derived by the breakdown of preexisting rocks; such deposits may also incorporate organic debris (for example, plant fragments). Biogenic sedimentary rocks result from the accumulation and compaction of biologically produced material—frequently preserving fossils—such as shells, skeletal fragments, and organic matter. Chemical (precipitate) sedimentary rocks develop when dissolved ions, mobilized by weathering, concentrate and precipitate from solution—commonly by evaporation—and are subsequently lithified. Together, these pathways record the surface processes that recycle rock material into new sedimentary strata.
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Plate tectonics — the Wilson cycle
In a landmark 1967 paper, J. Tuzo Wilson proposed a cyclical model for the long‑term evolution of ocean basins and continental margins that has since been labeled the Wilson cycle. Wilson framed a repeatable sequence in which continental rifting generates new oceanic crust and seafloor spreading, allowing a basin to widen until tectonic convergence initiates subduction; continued consumption of oceanic lithosphere culminates in continental collision and suturing, after which the cycle may begin anew. He illustrated this sequence using the modern Atlantic as a working example of basin formation and eventual demise.
Developed amid the wider acceptance of plate tectonics, the Wilson cycle supplied a practical schema linking observable tectonic stages to recurrent changes in basin geometry and plate‑boundary configuration. Its integration into tectonic thinking reoriented how geoscientists understand the rock cycle by embedding rock formation, metamorphism, and material recycling within the dynamics of moving plates, thereby establishing plate tectonics as the primary mechanism driving global lithospheric evolution.
Spreading ridges
At mid-ocean divergent margins, upwelling mantle undergoes partial melting in a shallow zone beneath the ridge, producing newly generated basaltic magma that forms the primary igneous component of oceanic crust. As the adjacent plates separate, this freshly formed crust is transported laterally away from the ridge axis, effecting continuous accretion of oceanic lithosphere. The young, hot crust is permeated by fractures that permit seawater to circulate; heated by magmatic heat, this circulating fluid acts as a hydrothermal agent that modifies the rock’s chemistry and mineralogy. These hydration- and cooling-driven alterations constitute retrograde metamorphism of the nascent oceanic crust, representing an early secondary change in the lithospheric rock cycle as the material is carried away from the ridge.
Subduction zones
New oceanic lithosphere is produced at mid‑ocean ridges as basaltic magma cools and solidifies; this relatively dense plate then moves laterally by sea‑floor spreading until it converges with another plate and begins to descend at a subduction zone. Progressive burial of the downgoing basaltic slab subjects it to increasing pressure and temperature, driving mineralogical re‑equilibration to high‑pressure assemblages (notably eclogite) and thereby increasing slab density. Concurrent breakdown of hydrous minerals within the slab liberates water and other volatiles, which migrate upward into the overlying mantle wedge; the added volatiles lower the peridotite solidus and trigger flux partial melting.
Buoyant melts generated in the volatile‑enriched mantle rise through the overriding lithosphere and produce volcanism concentrated in island arcs or continental volcanic belts. Spatial patterns in lava chemistry commonly reflect source and magmatic evolution: melts erupted near the arc front tend to be less silica‑rich, whereas more silicic compositions occur farther from the trench, consistent with deeper melt generation and greater degrees of fractional crystallization and differentiation for evolved magmas. In some tectonic scenarios, fragments of the metamorphosed slab and upper mantle are not recycled into the deep mantle but are emplaced onto continental margins (obduction), producing ophiolite complexes that preserve slices of oceanic lithosphere and high‑pressure rocks such as eclogite at the surface.
Surface and near‑surface volcanic products of arcs are rapidly subjected to mechanical and chemical weathering; climate strongly modulates the rate and style of erosion. The resulting volcaniclastic and clastic detritus is transported into adjacent forearc and back‑arc basins, where continued burial, compaction, and cementation convert loose sediment into lithified sedimentary rocks. These stratified deposits thereby record the volcanic, erosional and tectonic history of the subduction system.
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Continental collision
In the terminal phase of the Wilson cycle, closure of an ocean culminates in convergence between continental masses or accreted terranes. Because continental crust is composed of relatively low-density, silica-rich rocks, colliding lithospheres resist subduction; buoyancy forces prevent descent into the mantle and concentrate strain at the plate boundary. The resulting compression produces intense crustal shortening, thickening and uplift, driving an orogenic episode that constructs a mountain belt across the collision zone.
Deformation during collision combines brittle and ductile processes: layers and crustal blocks are folded and transported as nappes and thrust sheets, while faults accommodate displacement at a range of levels within the crust. These structural mechanisms collectively stack, fragment and uplift rock packages, producing the large-scale architecture of the orogen.
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Crustal thickening and burial elevate pressures and temperatures over broad regions, producing regional (barrovian-style) metamorphism rather than localized contact aureoles. All pre-existing lithologies—igneous, volcanic, sedimentary and earlier metamorphic rocks—are variably recrystallized and reequilibrated, so the metamorphic fabric of a collision zone records both inherited histories and the latest collisional conditions.
The complex spatial and lithological tapestry of collisional orogens thus reflects the interplay of lithospheric buoyancy, sustained compressional tectonics, metamorphic re-equilibration and repeated structural reworking. The result is mountain belts composed of juxtaposed, deformed and recrystallized terranes that preserve a composite record of convergent tectonism.
Continental collision produces high, uplifted mountain belts that are immediately subjected to surface denudation; once elevated, these ranges enter an erosion-dominated regime that progressively reduces topographic relief and generates large volumes of clastic detritus. Mechanical and chemical breakdown of the mountain-source lithologies—metamorphic, igneous and relict sedimentary rocks—yields sediment that is transported downslope and dispersed into adjacent depositional systems.
The primary sinks for this mountain-derived sediment are nearby ocean margins, shallow epicontinental seas, and continental depocenters such as foreland basins, coastal plains and inland basins, where thick accumulations of detritus accumulate through time. Progressive burial of these deposits increases overburden pressure and drives diagenetic processes; compaction and cementation convert unconsolidated sediments into lithified sedimentary rocks. Thus the original mountain lithologies are physically and chemically recycled: uplift and erosion produce sediment, transport delivers it to basins, and burial and diagenesis transform it into new sedimentary strata. The coupled processes of orogenic uplift, erosion, sediment transfer, basin deposition and lithification constitute a cyclical linkage between convergent plate tectonics and the stratigraphic architecture of adjacent marine and continental sedimentary provinces.
An evolving process
The plate-tectonic rock cycle operates over geologic time as a sequence of melting, magmatic differentiation and tectonic transport that progressively alters the composition and architecture of the lithosphere, producing a clear chemical separation between mantle-derived rocks and the silica-rich continental crust. Primary melt generation occurs mainly in two tectonic environments: decompression melting of upwelling mantle at mid‑ocean ridges and flux‑induced melting in the mantle wedge above subduction zones where slab-derived volatiles lower the solidus. In both settings the earliest partial melts are dominated by low‑melting‑point, silica‑ and volatile‑rich phases; because these components enter the melt first the resulting magmas are compositionally evolved relative to their source, driving chemical segregation between melt and residual solid.
Silicic, volatile‑rich magmas are relatively low in density compared with surrounding mantle and many mafic lithologies, which favours their ascent into and retention within the crust as intrusive bodies and volcanic deposits rather than wholesale recycling back into the mantle. Repeated cycles of partial melting, fractional crystallization and melt extraction therefore progressively segregate material within the mantle and between mantle and crust, producing a stratified lithosphere in which refractory residues remain at depth while buoyant silicic material accumulates at the surface. Because continental crust is more silicic and buoyant than mantle peridotite, it resists subduction and preferentially preserves the silicic fractions generated by magmatism; the net result over time is growth and increasing chemical differentiation of continental crust at the expense of a more mafic, mantle‑dominated reservoir.
The role of water
Water is the principal agent coupling weathering, transport, alteration, and rock formation within the rock cycle. At Earth’s surface, precipitation, soil moisture and groundwater drive both chemical decomposition (dissolution, hydrolysis and other aqueous reactions) and mechanical disintegration of bedrock. Many igneous, metamorphic and marine sedimentary minerals are thermodynamically unstable in near‑surface conditions, so they are preferentially broken down and converted to ions and clastic fragments by aqueous processes.
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Once produced, dissolved ions and particulate sediment are mobilized by running water. Rivers and surface runoff move enormous masses of detritus and solute from continental interiors toward oceanic and intracontinental depositional basins. In those sinks, continued burial, compaction, cementation and diagenetic alteration convert accumulated sediment back into sedimentary rock, thereby closing a major erosional–lithification pathway of the rock cycle.
Circulating water also mediates subsurface and seafloor alteration. Cold to hydrothermally heated seawater percolating through fractured basalt reacts with primary volcanic minerals, producing alteration assemblages (e.g., serpentinization and other hydrothermal metasomatic reactions) that fundamentally change the mineralogy and physical properties of the oceanic crust. In subduction zones, dehydration of the downgoing slab releases volatiles—principally H2O together with CO2 and other carbon‑bearing species derived from subducted marine carbonates and sediments—into the overlying mantle wedge and lower crust. These fluids lower solidus temperatures, promote partial melting of mantle and crustal rocks, and thereby facilitate arc magmatism.
Because carbonates and organic carbon subducted with oceanic sediments are liberated as volatiles during metamorphism and slab devolatilization, the carbon cycle is tightly coupled to rock‑cycle processes at convergent margins; carbon released from the slab contributes to magmatic and hydrothermal fluxes that return carbon to the surface and atmosphere. In sum, water and other volatiles exert both chemical (dissolution, hydrolysis, fluid‑mediated metamorphism) and physical (sediment and solute transport) controls that link weathering, transport, burial, metamorphism and melt generation across the global rock cycle.