Nature of Magma

Magma is a molten or partially molten, naturally occurring silicate material beneath the Earth’s surface.
It consists of:

Liquid phase → molten silicate melt.
Solid phase → crystals of early-formed minerals.
Gaseous phase → dissolved volatiles (H₂O, CO₂, SO₂, Cl, F).
When magma cools and solidifies at depth → intrusive igneous rocks (e.g., granite).
When erupted on the surface → lava forming extrusive rocks (e.g., basalt).

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Types of Magma (Based on Silica Content & Viscosity)

1. Felsic (Acidic) Magma

SiO₂ content: \~65–75%.
Rich in: Quartz, K-feldspar, Na-plagioclase.
Color: Light (granite, rhyolite).
Viscosity: High (thick, slow flow).
Gas content: High → explosive eruptions.
Example: Rhyolitic magma.

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2. Intermediate Magma

SiO₂ content: \~55–65%.
Rich in: Amphibole, biotite, Na-Ca plagioclase.
Color: Medium (diorite, andesite).
Viscosity: Moderate.
Example: Andesitic magma.

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3. Mafic (Basic) Magma

SiO₂ content: \~45–55%.
Rich in: Pyroxene, Ca-plagioclase, olivine.
Color: Dark (gabbro, basalt).
Viscosity: Low (fluid, flows easily).
Gas content: Low → quiet eruptions.
Example: Basaltic magma.

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4. Ultramafic Magma

SiO₂ content: <45%.
Rich in: Olivine, pyroxene.
Color: Very dark/greenish.
Viscosity: Very low.
Rare at the surface (more mantle-derived).
Example: Komatiite (ancient).

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Composition of Magma

Major elements: O, Si, Al, Fe, Mg, Ca, Na, K.
Silica (SiO₂): Controls viscosity and type of rock formed.
Volatiles (gases): H₂O, CO₂, SO₂, H₂S, Cl, F → influence explosiveness.
Trace elements: Give magma distinct geochemical signatures (used in petrology).

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Generation of Magma (Magma Genesis)

Magma forms due to partial melting of mantle and crustal rocks under different geological conditions.

1. Decompression Melting

Occurs when pressure decreases but temperature remains high.
Typical at mid-ocean ridges (divergent boundaries).
Produces basaltic magma.

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2. Flux Melting (Hydration Melting)

Water and volatiles lower the melting point of rocks.
Occurs at subduction zones (convergent boundaries).
Produces andesitic to rhyolitic magma.

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3. Heat Transfer Melting

Rising hot magma intrudes into the crust and melts surrounding rocks.
Common in continental rifts and hotspots.
Produces varied magma compositions.

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4. Partial Melting

Only part of the source rock melts, producing magma richer in silica than the parent rock.
Example: Mantle peridotite → basaltic magma.

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5. Crystal Fractionation and Assimilation (Magma Evolution)

Fractional crystallization: Early minerals crystallize and separate, changing magma composition.
Assimilation: Magma melts and incorporates surrounding crustal material.
These processes generate diversity in igneous rocks.

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In summary:

Nature → Molten rock with liquid, solid, and gas phases.
Types → Felsic, Intermediate, Mafic, Ultramafic (based on SiO₂).
Composition → Mainly silicates + volatiles + trace elements.
Generation → Decompression, flux, heat transfer, partial melting, and magmatic differentiation.