Generation and Crystallization of Magmas | Geology Optional Notes for UPSC PDF Download

Magma: A Comprehensive Overview

Definition of Magma

• Magma is a mixture of molten or semi-molten rock, volatiles, and solids, derived from the Greek word for "thick ointment."
• It accumulates in a magma chamber beneath the Earth's surface.
• Magma can solidify below the surface or intrude into adjacent rocks, often driving volcanic activity.
• When magma reaches the surface as lava, it may lead to explosive eruptions, forming pyroclastic rocks.

Types of Igneous Rocks

• Approximately 700 varieties of igneous rocks have been identified.
• These variations arise from diverse magma compositions and conditions.

Primary Magma

• Definition: The original composition of magma before differentiation.
• Typically contains >10% MgO by weight.
• Evolves into parental magma through differentiation.

Primitive Magma

• Source: Derived directly from the Earth's mantle with minimal differentiation.
• Provides insights into mantle composition and magma evolution.

Parental Magma

• Acts as the source for various magma types through magmatic differentiation.
• Capable of forming all igneous rock series.
• Originates from primary magma.

Magma Composition and Physical Properties

Magma Composition

• Directly linked to the melting of deep-seated primary magma.
• Influenced by mineralogical and chemical variations.

Physical Properties of Magma

• Varies with magma type and is challenging to measure directly.
• Ranges from 650°C to 1200°C:
• Basaltic magma: 1000–1200°C.
• Andesitic magma: 800–1000°C.
• Rhyolitic magma: 650–800°C.

• Resistance to flow; impacts magma's mobility and eruption style.
• Felsic magma: High viscosity due to high silica content.
• Mafic magma: Low viscosity due to lower silica content.

• Typically ranges between 2.18 and 2.80 g/cm³, depending on magma composition.
• Basaltic magma: Denser, rich in Fe and Ca.
• Rhyolitic magma: Less dense, rich in Na and Si.

• Dissolved gases like H₂O, CO₂, and SO₂ significantly impact explosiveness.
• Gases expand as pressure decreases, causing eruptions.

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Components of Magma

• Composed of ions like oxygen, silica, aluminum, and others.

• Crystallized silicate minerals.

• Include water vapor, carbon dioxide, sulfur dioxide, and trace gases (e.g., HCl, HF).

Chemical Composition

• Magma composition varies, typically described by the weight percentage of major oxides.
• Primary oxides
  • SiO₂, TiO₂, Al₂O₃, Fe₂O₃, FeO, MgO, CaO, Na₂O, and K₂O.

• Felsic magma: High silica, low in Fe and Mg.
• Mafic magma: High in Fe, Mg, and Ca.

Mineralogical Composition

• Dark Silicate Minerals (Mafic)
  • Rich in Fe and Mg, low in silica.
  • Examples: Olivine, pyroxene, amphibole, biotite mica.
• Light Silicate Minerals (Felsic)
  • Rich in silica and Al.
  • Examples: Quartz, feldspar, muscovite mica.

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Types of Magma

• High in silica and feldspar.
• Viscous and erupts explosively.

• Moderate silica content.
• Intermediate viscosity and temperature.

• Low silica content, high Fe and Mg.
• Low viscosity, often leads to fluid lava flows.

• Extremely low silica content, very rich in Mg and Fe.
• Least viscous among magma types.

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Processes Influencing Magma

• Occurs in the upper mantle or lower crust.
• Influences the composition of the initial magma.

• Alters magma composition as it cools and crystallizes.

Causes of Melting

• Increase in temperature.
• Decrease in pressure.
• Addition of volatiles (e.g., water, CO₂).

Summary of Magma Properties

Felsic Magma

• SiO₂ Content: 65–75%
• Temperature: 650–800°C
• Viscosity: High
• Volatiles: High

Intermediate Magma

• SiO₂ Content: 55–65%
• Temperature: 800–1000°C
• Viscosity: Intermediate
• Volatiles: Moderate

Mafic Magma

• SiO₂ Content: 45–55%
• Temperature: 1000–1200°C
• Viscosity: Low
• Volatiles: Low

Ultramafic Magma

• SiO₂ Content: 38–45%
• Temperature: Up to 1500°C
• Viscosity: Very Low
• Volatiles: Very Low