Igneous Petrology – Magma Genesis
Igneous Petrology: Magma Genesis
How magmas are born, what makes them behave the way they do, and why the mantle melts where it does — the complete UPSC-ready treatment of magma genesis.
Physical-Chemical Properties of Magmas
Magma is not a simple liquid — it is a multiphase system whose behaviour (eruption style, crystallisation path, ascent rate) is dictated entirely by these properties.
What is magma?
Magma is naturally occurring, high-temperature molten or partly molten rock material generated within the Earth's crust or mantle, mobile enough to intrude or extrude, and from which igneous rocks crystallise on cooling. It is a three-phase system: a silicate melt (liquid) + suspended crystals (solid) + dissolved and/or exsolved volatiles (gas).
Chemical composition
Magmas are dominantly silicate liquids, classified by their SiO₂ content, which in turn governs almost every other physical property:
| Magma type | SiO₂ content | Example rock |
|---|---|---|
| Ultramafic | < 45% | Komatiite |
| Mafic (Basaltic) | 45–52% | Basalt |
| Intermediate | 52–63% | Andesite |
| Felsic (Silicic) | > 63% | Rhyolite / Granite |
Volatile content
Magmas carry dissolved volatiles — chiefly H₂O, along with CO₂, SO₂, Cl and F — held in solution under the confining pressure at depth. As magma ascends and pressure drops, volatiles exsolve as gas bubbles (vesiculation); the rate and violence of this exsolution, controlled largely by viscosity, is what determines whether an eruption is effusive or explosive.
Density
Mafic (basaltic) magmas are denser (≈ 2.6–2.8 g/cm³) than felsic magmas (≈ 2.2–2.4 g/cm³). The density contrast between magma and surrounding country rock controls buoyant ascent; magma may pond and fractionate at a level of neutral buoyancy — often near the crust–mantle (Moho) boundary — before resuming ascent.
Viscosity of Magmas
Viscosity — the internal resistance to flow — is the single most important control on how a magma erupts and what landform or rock texture it produces.
Controlling factors
- Silica (SiO₂) content: SiO₄ tetrahedra polymerise into long chains and networks in silica-rich melts, raising viscosity sharply. This is the single strongest control.
- Temperature: viscosity falls as temperature rises — a hotter melt flows more easily (inverse relationship).
- Dissolved water content: H₂O breaks Si–O–Si polymer bonds (depolymerisation), dramatically lowering viscosity — even a small increase in dissolved water can lower viscosity by orders of magnitude.
- Crystal content: suspended crystals mechanically impede flow, raising the effective (bulk) viscosity — described approximately by the Einstein–Roscoe relation.
- Pressure: a minor effect; higher pressure keeps volatiles dissolved, indirectly keeping viscosity lower until the magma decompresses.
Consequence for eruptive style
| Magma type | Viscosity | Typical behaviour |
|---|---|---|
| Basaltic | Low | Effusive, fast-moving lava flows (pāhoehoe, ʻaʻā); gas escapes easily |
| Andesitic | Intermediate | Moderately explosive; stratovolcano cone-building |
| Rhyolitic | High | Highly explosive (gas trapped) or forms thick, slow-moving domes |
Temperature of Magmas
Magma temperature is measured through mineral-pair geothermometry, melt-inclusion analysis, and direct lava measurement — and it falls steadily as a magma differentiates.
Typical eruption temperatures
| Magma type | Temperature range |
|---|---|
| Basaltic | 1100–1250°C (hottest — direct mantle-derived melt) |
| Andesitic | 950–1100°C |
| Rhyolitic / Granitic | 650–850°C (coolest — most evolved / differentiated) |
Why temperature falls with differentiation
As a basaltic parent magma undergoes fractional crystallisation (per Bowen's Reaction Series), early high-temperature mafic minerals (olivine, Ca-rich plagioclase, pyroxene) crystallise out first and are removed from the liquid. The residual melt progressively enriches in SiO₂, Na₂O and K₂O and correspondingly cools — meaning composition and temperature evolve together along a single differentiation trend, from hot mafic to cool felsic.
Liquidus & solidus
The liquidus is the temperature above which a magma is entirely liquid; the solidus is the temperature below which it is entirely solid. The interval between them is the crystallisation interval, during which melt and crystals coexist — this interval is generally wider for more silicic, differentiated magmas.
Origin of Primary Magmas
Not every magma erupted at the surface is "primary" — most have been chemically modified on the way up. Knowing the distinction is a frequently tested UPSC point.
Definition
A primary magma is one formed directly by partial melting of a source rock, and which has undergone no subsequent compositional modification — no fractional crystallisation, crustal contamination, or magma mixing — before being sampled or studied. It represents the unmodified composition of a partial melt straight from its source.
Conditions required for melting
A source rock begins to melt only when the ambient pressure–temperature conditions cross its solidus. This can be achieved by three broad routes — raising temperature, lowering pressure, or lowering the solidus itself by adding volatiles (detailed mechanism-by-mechanism in Section V).
Primary basaltic vs. primary granitic magma
Primary basaltic magma is common — generated directly by partial melting of mantle peridotite; basalts erupted at mid-ocean ridges are close to primary compositions. Primary granitic magma, by contrast, is rare in nature: most granite is the product of extensive fractional crystallisation of basaltic parent magma, or of partial melting (anatexis) of pre-existing crustal rocks rather than a simple, unmodified mantle melt.
Degree of partial melting
The percentage of source rock melted strongly controls magma composition. Low degrees of partial melting (a few %) yield melts strongly enriched in incompatible elements and alkalis (e.g. alkali basalt); higher degrees of partial melting (10–20%+) progressively dilute these enrichments, yielding more silica-poor, tholeiitic basaltic melts.
Low % melting → alkaline, enrichedHigh % melting → tholeiitic, depletedMantle Melting Mechanisms
The mantle is overwhelmingly solid. Three distinct mechanisms explain the exceptions — where and why it partially melts to generate primary magma.
1. Decompression melting (adiabatic upwelling)
Solid mantle rises — by convection at mid-ocean ridges, or within a buoyant mantle plume — fast enough that it loses very little heat (rises "adiabatically"). Because the peridotite solidus temperature drops faster with falling pressure than the rising mantle's own temperature drops, the upwelling material eventually crosses the solidus and begins to partially melt, even without any external heat input.
Mid-ocean ridgesMantle plumes / hotspots (e.g. Hawaii)2. Flux melting (volatile-induced melting)
At subduction zones, the descending oceanic slab releases water (and other volatiles) from hydrous minerals as it heats up. This water rises into the overlying mantle wedge and lowers the peridotite solidus temperature — melting is triggered without needing higher temperature or decompression. This is the dominant mechanism generating magma beneath volcanic arcs.
Subduction-zone volcanic arcs (e.g. Andes, Japan)3. Heat-induced melting
Direct addition of heat — for instance, hot basaltic magma "underplating" and ponding at the base of the crust, or heat conducted from an adjacent hot body — can raise crustal or mantle rock above its solidus. This mechanism is a relatively minor generator of primary mantle melt but is the principal driver of crustal anatexis (partial melting of crustal rocks to generate granitic magma).
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