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

Understanding Magma: A Comprehensive Overview

• Definition of Magma: Magma, originating from the Greek word for 'thick ungent', represents a blend of molten or semi-molten rock, along with volatiles and solids. Typically, magma gathers in a magma chamber beneath the Earth's surface. It has the potential to solidify within the Earth or intrude neighboring rocks, possibly fueling volcanic activity.
• Magma Transformation: When magma emerges onto the Earth's surface as lava, it can lead to explosive reactions, giving rise to pyroclastic rocks.
• Types of Igneous Rocks: Around 700 variations of igneous rocks have been documented on Earth, hinting at a diverse array of magmas contributing to the formation of different rock types.
• Primary Magma: Initial magma composition, prior to any differentiation, is termed primary magma. This type typically contains over 10% MgO by weight and evolves into parental magma through differentiation processes. Primary magma is also known as primary melt.
• Primitive Magma: Derived from the mantle, primitive magma showcases minimal differentiation, offering insights into the mantle's composition and aiding in understanding magma evolution. It is also referred to as primitive melt.
• Parental Magma: This magma serves as the source from which various magma types originate through magmatic differentiation. Capable of generating all igneous rock series rocks, parental magma evolves from primary magma. It is also known as parental melt.

Unit 4: Magma Composition and Physical Properties

Introduction to Magma

• Magma Composition:

  Magma composition is directly derived through the melting of deep-seated primary magma.

Physical Properties of Magma

• Temperature

  Temperature plays a crucial role in the crystallization of magma. It is challenging to measure directly due to the dangers involved. However, remote control methods have allowed us to measure temperatures ranging from 1200°C to 650°C. Different types of magmas crystallize at varying temperatures:

  • Basaltic magma: 1000 to 1200°C
  • Andesitic magma: 800 to 1000°C
  • Rhyolitic magma: 800 to 650°C

• Viscosity

  Viscosity refers to the resistance of magma flow, which impacts its mobility. It is influenced by the composition and temperature of magma. Magmas with higher silica content, such as felsic magma, tend to have higher viscosity compared to mafic magma. Temperature also plays a role, as cooler magmas are more viscous. Viscosity is a critical factor in determining magma's eruptive behavior.

• Density

  The density of magma is around 2.65 gm/cm³, while the mantle's density is 3.3 gm/cm³. Magma rich in iron tends to be denser. Basalts are typically richer in Fe, Ca, and Ti, while rhyolites are richer in Na, Al, and Si. Basaltic magma has a density ranging from 2.65 to 2.80 gm/cm³, andesitic magma from 2.45 to 2.50 gm/cm³, and rhyolitic magma from 2.18 to 2.25 gm/cm³. Temperature and pressure also influence magma density.

• Volatiles/Gases

  Most magmas contain dissolved volatile gases, with water being a predominant constituent. As pressure decreases, magma rises towards the surface, leading to the formation of a separate gas phase. This gas behavior is akin to carbonated beverages—when pressure is released, gas escapes and forms bubbles. The presence of gas adds explosiveness to magma due to the expansion of gas volume.

Composition of Magma

• The mineralogical composition and texture of igneous rocks result from differences in magma composition and cooling conditions.
• Magma is primarily silicate and begins crystallization at high temperatures with a range of crystallization temperatures.
• Magma's fluidity varies due to volatile materials.

Components of Magma

• Magma is hot molten rock beneath the Earth's crust.
• The three states of magma include:
• 
  
  • The liquid portion, or melt, consists mainly of mobile ions of common elements like oxygen, silica, aluminum, iron, calcium, sodium, potassium, and magnesium.
  • The solid part includes silicate minerals that have crystallized from the melt.
  • Gaseous components are volatiles such as water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2), with minor constituents like HCl, H2S, HF, N2, Cl, F, and Br.

Types of Magma

• 'Dry' magmas contain no volatiles, while 'wet' magmas can have up to 15% dissolved volatiles which are released as gas when the magma moves towards the Earth.

Chemical Composition of Magma

• Magmas do not have a fixed composition and are described in terms of weight percentage of major oxides.
• The chemical composition includes elements like Silicon (Si), Titanium (Ti), Aluminum (Al), Iron (Fe), Magnesium (Mg), Calcium (Ca), Sodium (Na), Potassium (K), Hydrogen (H), and Oxygen (O), which together make up more than 99% of the constituents.
• In magma, these elements exist as electrically charged ions.

Chemistry and Mineralogy of Magma

Composition of Magma

• Positively charged cations (1 to 4 charge) and negatively charged oxygen anions (-2 charge) exist in magma.
• In magma, different charged ions tend to bond with each other to create electrically neutral molecules.
• SiO₂ and Al₂O₃ are the primary components of magma.
• Silicon-oxygen tetrahedron, consisting of a silicon atom surrounded by four oxygen atoms, is a fundamental structure in most silicates.
• Magma also contains elements like aluminum (Al), calcium (Ca), sodium (Na), potassium (K), iron (Fe), and magnesium (Mg) which bond with oxygen.
• Magma ranges from liquid to semi-liquid, forming clusters or short chains rather than a crystalline lattice.

Average Composition of Magma (Table 4.1)

• SiO₂ - 59%
• CaO - 5%
• Na₂O - 3.8%
• TiO₂ - 1%
• MnO - 0.1%
• Fe₂O₃ - 3%
• FeO - 3.5%
• K₂O - 3%
• P₂O₅ - 0.3%

Mineralogical Composition of Magma

• Al₂O₃ - 15%
• MgO - 3.5%
• H₂O - 1%
• CO₂ - 0.1%

Mineralogy of Magma

Formation of Silicate Minerals

• As magma cools and solidifies, silicate elements like SiO₂ and Al₂3 combine to form two main groups of silicate minerals.

Dark Silicate Minerals (Mafic)

• Rich in iron and/or magnesium, low in silica.
• Common minerals include olivine, pyroxene, amphibole, and biotite mica.

Light Silicate Minerals (Felsic)

• Contain nonferromagnesium minerals, rich in silica.
• Common minerals include quartz, muscovite mica, and feldspar group (orthoclase, microcline, plagioclase).
• Feldspars make up a significant portion of igneous rocks, with felsic minerals having higher silica content compared to mafic minerals.

Common Minerals Found in Igneous Rocks (Table 4.2)

• Felsic Minerals: Quartz, Orthoclase Feldspar, Plagioclase Feldspar, Muscovite (Mica Group)

Block 2: Igneous Petrology-II

• Gabbroic Rocks
• Mafic Minerals
• Olivine
• Pyroxene Group
• Amphibole Group
• Biotite (Mica)

Major Types of Magmas

Magmas can give rise to various rock types from a single magmatic province or eruptive center. Initially, scientists believed in distinct magma types corresponding to specific rock types like felsic, mafic, intermediate, and ultramafic. However, it is now widely accepted that there is a single homogeneous basaltic parental magma. N.L. Bowen in 1923 introduced the concept of primary magma, which can lead to different magma types. He proposed that all igneous rocks stem from a common parental magma, providing a logical explanation for rock diversification.

To comprehend magma, consider four pots of different molten chocolates on a stove: milk chocolate, bitter chocolate, dark chocolate, and resin chocolate. Just as these chocolates differ upon solidification, magmas vary in chemical composition. Geologists classify magmas into four major types based on silica content:

• Felsic magma: High in feldspar and silica content.
• Intermediate magma: Composition between felsic and mafic.
• Mafic magma: Rich in iron oxide and magnesium oxide.
• Ultramafic magma: Contains minimal silica compared to mafic magma.

While the magma is fundamentally basaltic, its mineralogical and chemical composition allows for categorization into different types. This differentiation in magma leads to the formation of various rock types through magmatic differentiation, a process diversifying magma to produce rocks with distinct compositions.

Unit 4 Summary: Chemical Composition and Physical Properties of Magma

Table 4.3: Summary of the chemical composition and physical properties of four magma types:

• Magma Types:
  • Felsic (Rhyolitic):
    • Chemical Composition: SiO2 65-75%, low in Fe, Mg, Ca and high in K, Na
    • Temperature: 650-800°C
    • Viscosity: High
  • Intermediate (Andesitic):
    • Chemical Composition: SiO2 55-65%, intermediate in Fe, Mg, Ca, Na, K
    • Temperature: 800-1000°C
    • Viscosity: Intermediate
  • Mafic (Basaltic):
    • Chemical Composition: SiO2 45-55%, high in Fe, Mg, Ca and low in K, Na
    • Temperature: 1000-1200°C
    • Viscosity: Low
  • Ultramafic (Picritic):
    • Chemical Composition: SiO2 38-45%, Fe-Mg > 8%, up to 32% MgO and lowest in K, Na
    • Temperature: Up to 1500°C
    • Viscosity: Very low

We have covered the physical properties, components, and composition of magma. Now, let's assess our understanding with some questions:

SAQ 1

• a) Define Primary magma.
• b) How does the composition of magma affect viscosity?
• c) Mention the three components of magma.
• d) What is a silicon-oxygen tetrahedron?

Volatile Content: High in Felsic, Low in Mafic

Concept of Magma: Take time to review your learning.

4.5 ORIGIN OF MAGMA

In Unit 3, we learned about the Structure and Composition of Earth and discovered that the Earth's crust varies in thickness, being about 35 km thick under continents and 7 km beneath oceans.

Natural magmas exhibit a wide range of chemical compositions, which follow a systematic pattern. Petrologists have dedicated significant efforts to understanding why there is a consistent range of magma compositions.

Geologists have studied this variation extensively, leading to the conclusion that the overall compositional range and systematic variation are intrinsic to the Earth's processes.

Igneous Petrology II

• Composition of Natural Magma

  The composition of natural magma primarily reflects two critical processes occurring within the Earth:

  • Partial melting: This process influences the composition of an initial magma formed deep within the Earth.
  • Fractional crystallization: This process alters the composition of magma as it moves towards the surface.

• Origin of Magmas

  Most magmas are believed to originate from:

  • Partial melting of the upper mantle under specific conditions.
  • Melting of rocks in the lower regions of the continental crust.

• Causes of Melting in Mantle and Lower Crustal Rocks

  Various processes can lead to the melting of rocks in the upper mantle and lower continental crust:

  • Increase in temperature.
  • Decrease in pressure.
  • Addition of fluids rich in H2O.

• Magma Formation and Crystallization

  Key points regarding magma formation and crystallization:

  • Magmas originate at different depths within the Earth's crust and upper mantle, possibly up to 250 km deep.
  • The Earth's crust consists of less dense materials, while the mantle is composed of denser rocks.
  • Most magmas forming igneous rocks are produced in the upper mantle, typically at temperatures ranging from 600°C to 1200°C.
  • As magma cools, minerals crystallize at different temperatures.