Magma composition plays a crucial role in shaping volcanic activity. From silica-rich felsic magmas to iron-rich mafic ones, the chemical makeup determines how a volcano behaves. Understanding these differences helps predict eruption styles and potential hazards.

Magma types vary in viscosity, temperature, and gas content. Felsic magmas are sticky and explosive, while mafic magmas flow easily. This diversity in composition leads to a wide range of volcanic landforms and eruption patterns worldwide.

Magma Composition

Primary Chemical Components and Silica Content

• Magma is composed primarily of silica (SiO2), along with varying amounts of other major oxides such as Al2O3, FeO, MgO, CaO, Na2O, and K2O
• Silica content in magma typically ranges from 45% to 75% by weight
  • Higher silica content corresponds to more felsic magmas (rhyolite)
  • Lower silica content corresponds to more mafic magmas (basalt)

Variability and Trace Elements

• The relative proportions of the major oxides in magma can vary significantly depending on the source material and the processes that have affected the magma during its formation and evolution
• Trace elements and volatiles are also present in magma in smaller quantities but can have significant effects on its behavior and properties
  • Volatiles include water (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)
  • Trace elements are chemical elements present in very small amounts (typically less than 0.1% by weight)

Magma Types by Composition

Silica Content and Classification

• Magmas are classified into four main types based on their silica content:
  • Felsic (>63% SiO2)
  • Intermediate (52-63% SiO2)
  • Mafic (45-52% SiO2)
  • Ultramafic (<45% SiO2)

Characteristics of Magma Types

• Felsic magmas, such as rhyolite, are rich in silica and alkali elements (Na and K) and have a light color due to the presence of quartz and feldspar minerals
• Intermediate magmas, such as andesite, have moderate silica content and are characterized by the presence of plagioclase feldspar and mafic minerals like hornblende and biotite
• Mafic magmas, such as basalt, have lower silica content and are enriched in magnesium, iron, and calcium, resulting in a darker color due to the abundance of mafic minerals like olivine and pyroxene
• Ultramafic magmas, such as komatiite, are extremely low in silica and rich in magnesium and iron, and are rarely found in modern volcanic settings

Magma Composition and Properties

Influence on Physical Properties

• Magma composition strongly influences its physical properties, such as viscosity, density, and temperature
• Felsic magmas tend to have higher viscosity, lower density, and lower temperature compared to mafic magmas due to their higher silica content and the presence of polymerized silica chains
• Mafic magmas have lower viscosity, higher density, and higher temperature due to their lower silica content and the presence of less polymerized silica chains

Role of Volatiles and Crystals

• The presence of volatiles in magma can significantly affect its viscosity and behavior
  • Higher volatile content generally leads to lower viscosity and more explosive volcanic eruptions
  • Volatiles can exsolve from the magma as it rises, forming bubbles and increasing the potential for explosive activity
• The crystal content of magma also influences its physical properties
  • Higher crystal content leads to increased viscosity and potentially slower magma ascent and eruption rates
  • Crystals can form within the magma as it cools and decompresses, altering its rheology and behavior

Mafic vs Intermediate vs Felsic Magmas

Mafic Magmas

• Mafic magmas are characterized by their low silica content (45-52% SiO2), high magnesium and iron content, and relatively high temperature (1000-1200°C)
  • They are typically less viscous and more fluid than felsic magmas
  • Mafic magmas are often associated with effusive eruptions and shield volcanoes (Hawaiian volcanoes)

Intermediate Magmas

• Intermediate magmas have moderate silica content (52-63% SiO2) and are transitional between mafic and felsic magmas in terms of their composition and properties
  • They have intermediate temperatures (800-1000°C) and viscosities
  • Intermediate magmas can produce a range of volcanic landforms and eruption styles (stratovolcanoes, lava domes)

Felsic Magmas

• Felsic magmas have high silica content (>63% SiO2), relatively low magnesium and iron content, and lower temperatures (700-800°C) compared to mafic and intermediate magmas
  • They are typically more viscous and less fluid
  • Felsic magmas are more commonly associated with explosive eruptions and stratovolcanoes (Mount St. Helens)