Magma – Nature, Types, Composition and Generation

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## **Nature of Magma**

* Magma is a **naturally occurring molten rock material**, generated within the Earth’s crust or upper mantle.
* It is a **complex, multicomponent system** consisting of:

  1. **Liquid phase** – silicate melt (major component).
  2. **Solid phase** – suspended crystals of early-formed minerals.
  3. **Gaseous phase** – dissolved volatiles (H₂O, CO₂, SO₂, Cl, F, etc.).
* Magma is the parent material for **all igneous rocks**.

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## **Types of Magma (Based on Silica Content & Composition)**

1. **Basaltic Magma**

   * SiO₂ \~ 45–52%
   * Rich in Fe, Mg, Ca; low in K, Na
   * Low viscosity, high temperature (1100–1200 °C)
   * Low volatile content → less explosive eruptions
   * Rocks: basalt, gabbro

2. **Andesitic Magma**

   * SiO₂ \~ 52–63%
   * Intermediate in composition
   * Moderate viscosity and temperature (\~900–1100 °C)
   * Moderate volatile content → moderately explosive
   * Rocks: andesite, diorite

3. **Rhyolitic (Granitic) Magma**

   * SiO₂ \~ 63–77%
   * Rich in K, Na; low in Fe, Mg
   * High viscosity, low temperature (\~700–900 °C)
   * High volatile content → very explosive eruptions
   * Rocks: rhyolite, granite

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## **Composition of Magma**

### **Major Elements**

* Oxygen (O), Silicon (Si), Aluminum (Al), Iron (Fe), Calcium (Ca), Magnesium (Mg), Sodium (Na), Potassium (K), Titanium (Ti).

### **Minor Elements**

* Phosphorus (P), Manganese (Mn).

### **Trace Elements**

* Nickel (Ni), Chromium (Cr), Copper (Cu), Zinc (Zn), Rare Earth Elements (REEs).

### **Volatiles (Gases)**

* Water vapor (H₂O), Carbon dioxide (CO₂), Sulfur dioxide (SO₂), Chlorine (Cl), Fluorine (F).
* Control eruption style and crystallization.

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## **Generation of Magma**

Magma is produced mainly by **partial melting** of mantle or crustal rocks.

### **Mechanisms of Magma Generation**

1. **Decompression Melting**

   * Occurs when hot mantle material rises and pressure decreases.
   * Example: Mid-ocean ridges, continental rifts.

2. **Flux Melting (Hydration Melting)**

   * Water and volatiles lower the melting point of rocks.
   * Example: Subduction zones → andesitic magmas.

3. **Heat-Induced Melting**

   * Addition of heat from mantle plumes or nearby magmatic intrusions causes crustal melting.
   * Example: Granitic magmas in continental crust.

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## **Geological Significance of Magma**

* Source of **igneous rocks** (extrusive & intrusive).
* Drives **plate tectonics** and volcanic activity.
* Responsible for formation of **ore deposits** (Fe, Ni, Cu, Au, REEs).
* Provides insights into **Earth’s interior composition** and **thermal evolution**.