5.3 Volcanic Arcs
Fire Above the Descending Slab
Volcanic arcs are chains of volcanoes that form parallel to subduction trenches, typically 100–300 km landward of the trench axis. They represent the surface expression of one of Earth's most important magmatic processes: flux melting of the mantle wedge, triggered by water released from the dehydrating slab. Arc volcanism produces the most explosive eruptions on Earth and is the primary mechanism by which continental crust is generated from mantle material.
There are approximately 40,000 km of volcanic arcs globally, hosting about 90% of the world's subaerial active volcanoes. The "Ring of Fire" around the Pacific Basin is the most prominent expression, accounting for ~75% of global volcanic arc length.
Slab Dehydration & Flux Melting
Arc magmatism is fundamentally a consequence of water transport into the mantle. Oceanic lithosphere entering subduction zones carries water in three reservoirs: (1) pore water in sediments and crust (~1–3 wt%), (2) structurally bound water in hydrous minerals formed during seafloor hydrothermal alteration (chlorite, amphibole, serpentine), and (3) water in the hydrated slab mantle (serpentinized peridotite). The total water flux into subduction zones is estimated at ~1012 kg/yr.
As the slab descends, progressive metamorphism drives dehydration reactions. The most volumetrically significant is the breakdown of antigorite (a high-temperature serpentine polymorph):
Antigorite dehydration reaction:
Antigorite → Olivine + Enstatite + H2O
This reaction occurs at approximately 600–700°C and 2–3 GPa (~100–150 km depth). The Clausius-Clapeyron equation governs the P-T slope of this reaction:
\[ \frac{dP}{dT} = \frac{\Delta S}{\Delta V} = \frac{\Delta H}{T \, \Delta V} \]
where $\Delta S$ is the entropy change, $\Delta V$ is the volume change, and $\Delta H$ is the enthalpy change of the dehydration reaction. The positive Clapeyron slope means the reaction temperature increases with pressure (depth).
The released water migrates upward into the overlying mantle wedge, where it dramatically lowers the solidus temperature of peridotite. This process — flux melting(or fluid-fluxed melting) — is the primary mechanism of arc magma generation:
Effect of water on peridotite solidus:
\[ T_{\text{solidus}}^{\text{wet}} \approx 1000°\text{C} \quad \text{vs.} \quad T_{\text{solidus}}^{\text{dry}} \approx 1300°\text{C} \]
Water reduces the solidus by ~300°C at upper-mantle pressures. Since the mantle wedge temperature (brought to ~1300–1400°C by corner flow) exceeds the wet solidus but is below the dry solidus, melting only occurs where slab-derived fluids are present.
Arc-Trench Gap & Arc Spacing
The arc-trench gap — the horizontal distance from the trench to the volcanic front — ranges from ~100 to ~300 km and is controlled by two factors: the slab dip angle and the depth at which major dehydration occurs (~100 km). For a slab dipping at angle$\delta$, the arc-trench gap $d$ is approximately:
Arc-trench gap geometry:
\[ d \approx \frac{z_{\text{dehydration}}}{\tan \delta} \]
where $z_{\text{dehydration}} \approx 100\text{--}150$ km and $\delta$ is the mean slab dip in the upper 150 km. Steep slabs (Mariana, $\delta \approx 60°$) produce narrow gaps (~60–80 km); shallow slabs (Cascadia, $\delta \approx 15°$) produce wide gaps (>200 km).
Along the arc, individual volcanic centers are spaced at remarkably regular intervals of approximately ~70 km (de Bremond d'Ars et al., 1995). This regularity has been attributed to a Rayleigh-Taylor instability in the partially molten zone at the base of the arc crust: a gravitationally unstable layer of buoyant partial melt develops periodic upwellings with a characteristic wavelength controlled by the layer thickness and viscosity contrast.
~70 km
Mean Volcano Spacing
100–300 km
Arc-Trench Gap Range
~100 km
Dehydration Depth
Calc-Alkaline Magma Series
Arc magmas are dominantly of the calc-alkaline series, which is the geochemical fingerprint of subduction-related magmatism. The series evolves through fractional crystallization and assimilation:
Basalt (~48–52 wt% SiO2)
Primary arc basalts are generated by partial melting of the hydrated mantle wedge. They are enriched in large-ion lithophile elements (LILE: K, Rb, Ba, Sr) and depleted in high-field-strength elements (HFSE: Nb, Ta, Ti) relative to MORB, reflecting the fluid-mobile vs. fluid-immobile character of these elements during slab dehydration.
Andesite (~57–63 wt% SiO2)
Andesite is the most characteristic rock of volcanic arcs, named after the Andes. It forms by fractional crystallization of basalt (removal of olivine, pyroxene, and plagioclase) and/or by mixing of basaltic and rhyolitic magmas. Andesitic composition approximates the bulk composition of continental crust, making arcs the primary factory for continent formation.
Dacite & Rhyolite (63–77 wt% SiO2)
Silicic magmas form by extensive fractional crystallization and/or partial melting of existing arc crust. They are viscous, volatile-rich, and produce the most explosive eruptions on Earth. Large rhyolitic eruptions (ignimbrite flareups) can eject hundreds to thousands of cubic kilometers of magma in a single event.
Continental Arcs vs. Island Arcs
Continental Arcs
Built on thick continental lithosphere (e.g., Andes, Cascades, Japan). Continental arcs produce more silicic magmas on average because ascending basaltic melts must traverse ~30–70 km of continental crust, where they undergo extensive differentiation and assimilation (AFC processes). The Andes, with peaks exceeding 6,000 m, represent the archetype: the Altiplano-Puna plateau records ~300 Myr of arc magmatism and crustal thickening.
Island Arcs
Built on oceanic lithosphere (e.g., Mariana, Tonga-Kermadec, Lesser Antilles, Izu-Bonin). Island arcs produce more basaltic to basaltic-andesitic magmas because the thin oceanic crust provides less opportunity for differentiation. They represent an early stage of continent building. Over geological time, island arcs can accrete to continental margins (arc accretion), adding juvenile crustal material to the continent.
Paired Metamorphic Belts
Akiho Miyashiro (1961) recognized that Japanese metamorphic rocks form two parallel, contemporaneous belts with contrasting P-T conditions — a pattern he termed paired metamorphic belts. This juxtaposition is a diagnostic signature of ancient subduction zones:
High-P / Low-T Belt (Trench Side)
Located on the trench side of the arc, this belt records the high-pressure, low-temperature conditions of the subduction zone. Characteristic assemblages include blueschist (glaucophane + lawsonite) and eclogite (garnet + omphacite). In Japan, the Sanbagawa Belt is the type example, with peak conditions of ~0.8–1.2 GPa and 400–550°C.
High-T / Low-P Belt (Arc Side)
Located on the arc (back-arc) side, this belt records the high-temperature, low-pressure conditions of the magmatic arc. Characteristic assemblages include andalusite, sillimanite, cordierite, and migmatites, reflecting high heat flow from arc magmatism. In Japan, the Ryoke Belt is the type example, with peak conditions of ~0.3–0.5 GPa and 600–800°C.
Metamorphic field gradient contrast:
\[ \left(\frac{dT}{dP}\right)_{\text{HP/LT}} \approx 5\text{--}10 \; °\text{C/km} \quad \text{vs.} \quad \left(\frac{dT}{dP}\right)_{\text{HT/LP}} \approx 30\text{--}60 \; °\text{C/km} \]
The factor of ~5 difference in apparent thermal gradient between the two belts directly reflects the thermal structure of the subduction zone: cold slab at the trench vs. hot wedge beneath the arc.
Ignimbrite Flareups
Ignimbrite flareups are episodes of catastrophically intense silicic volcanism that punctuate the history of continental arcs. During a flareup, eruption rates increase by 5–10 times above background levels, producing enormous volumes of welded ignimbrite sheets (pyroclastic density current deposits) across areas of 104–105 km².
Sierra Madre Occidental, Mexico (~38–20 Ma)
The largest ignimbrite province on Earth, covering ~390,000 km² with up to 1 km thickness of welded tuffs. Estimated total volume: >300,000 km³. Eruption was associated with the rollback of the Farallon slab and subsequent opening of the Gulf of California.
Altiplano-Puna Volcanic Complex, Andes (~10–1 Ma)
Multiple caldera-forming eruptions produced ignimbrites with individual volumes exceeding 1,000 km³ (e.g., the Cerro Galán caldera, ~2,500 km³). The flareup is attributed to crustal thickening and thermal maturation of the arc, with a large low-velocity zone (the Altiplano-Puna Magma Body) still detectable seismically beneath the plateau.
Ignimbrite flareups are thought to require the accumulation of a large, thermally mature crustal magma reservoir. The triggering mechanism may involve delamination of dense lower crust or mantle lithosphere, allowing hot asthenosphere to rise and provide a thermal pulse that rapidly drives partial melting of the overlying crust.
Key Numbers
~40,000 km
Global Arc Length
~300°C
Solidus Depression by H2O
~90%
Subaerial Volcanoes in Arcs
>300,000 km³
Sierra Madre Occidental Vol.