2.1 Magma Composition and Classification
3 min read•july 30, 2024
Magma composition plays a crucial role in shaping volcanic activity. From -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
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- 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 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 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)
Key Terms to Review (18)
Basaltic magma: Basaltic magma is a type of low-viscosity, high-temperature magma that primarily consists of basalt, a dark-colored volcanic rock rich in iron and magnesium. It is the most common type of magma produced by mantle melting and is associated with effusive eruptions, forming features like shield volcanoes and lava flows. Understanding basaltic magma is crucial for grasping how different types of magma behave and evolve in the Earth's crust.
Bowen's Reaction Series: Bowen's Reaction Series is a concept in geology that outlines the sequence of mineral crystallization from cooling magma. It describes how different minerals form at specific temperatures and how this affects the composition of igneous rocks. This series helps to understand the relationship between magma composition, mineral formation, and the resulting rock types, highlighting the dynamics of magma evolution and the diversity of igneous materials found in the Earth's crust.
Dissolved Gases: Dissolved gases refer to gases such as water vapor, carbon dioxide, sulfur dioxide, and others that are held within magma in a dissolved state under high pressure. These gases play a crucial role in magma's behavior and composition, influencing how it erupts and the type of volcanic activity that occurs. The concentration of these gases can vary significantly based on the magma's source and conditions during its ascent to the surface.
Effusive eruption: An effusive eruption is a volcanic event characterized by the gentle flow of low-viscosity lava, which results in the formation of broad, shield-shaped volcanoes. These eruptions are generally less explosive than other types, allowing lava to spread out over large areas, creating distinct landforms and contributing to the landscape's evolution.
Explosive eruption: An explosive eruption is a volcanic eruption characterized by the violent expulsion of magma, gas, and volcanic ash into the atmosphere. This type of eruption is typically associated with high-viscosity magma that traps gas, leading to intense pressure buildup and a sudden release, resulting in an explosive release of materials.
Fractional Crystallization: Fractional crystallization is the process by which different minerals crystallize from a cooling magma at different temperatures, leading to the separation of various components based on their chemical composition and melting points. This process significantly influences the composition of magma as it evolves, affecting everything from its physical properties to the types of volcanic products that eventually form.
High viscosity: High viscosity refers to a fluid's resistance to flow, indicating that it is thick and sticky. In volcanology, this term is crucial when discussing magma, as high-viscosity magma does not flow easily, leading to different eruption styles and formation of geological features compared to low-viscosity magma.
IUGS Classification: The IUGS Classification is a systematic approach developed by the International Union of Geological Sciences to categorize igneous rocks based on their mineral composition and texture. This classification helps geologists communicate effectively about volcanic materials and enhances the understanding of magma processes and eruption dynamics. It establishes a framework for analyzing various igneous rocks, allowing for clearer interpretation of geological history and volcanic activity.
Low Viscosity: Low viscosity refers to a fluid's ability to flow easily and resist deformation. In the context of magma, low viscosity is crucial because it allows the magma to move more freely within the Earth's crust and erupt more fluidly, often resulting in less explosive volcanic activity. This characteristic is primarily influenced by the composition of the magma, particularly its silica content and temperature, which dictate how easily it can flow and escape during eruptions.
Magma chamber: A magma chamber is a large underground pool of molten rock located beneath the Earth's surface, where magma accumulates and resides before it can erupt as lava. This chamber plays a crucial role in volcanic activity and is instrumental in determining the composition, behavior, and style of eruptions.
Magma differentiation: Magma differentiation is the process by which a single magma source evolves into different types of magma, resulting in variations in composition and physical properties. This process is influenced by factors such as temperature, pressure, and the presence of crystals that may settle out or react with the liquid magma, leading to diverse volcanic rock types.
Magma temperature: Magma temperature refers to the thermal state of molten rock beneath the Earth's surface, which plays a crucial role in determining its behavior and composition. The temperature of magma influences its viscosity, crystallization processes, and the types of minerals formed as it cools. Variations in magma temperature can also impact volcanic activity and the style of eruptions, making it a vital factor in understanding volcanic systems.
Olivine: Olivine is a common mineral found in the Earth's mantle and is primarily composed of magnesium and iron silicate. It plays a crucial role in understanding magma composition, generation processes, and the relationship between tectonics and magma. As a major component of ultramafic rocks, olivine can influence the properties of magma and provide insights into the conditions under which it forms.
Partial Melting: Partial melting refers to the process by which only a portion of a solid material, such as rock, melts while the rest remains solid. This process is crucial in the formation of magma and influences its composition, which directly affects volcanic activity and the types of rocks formed from cooled magma.
Rhyolitic magma: Rhyolitic magma is a type of high-silica, low-density magma that is typically characterized by a high viscosity and a tendency to produce explosive volcanic eruptions. Its composition often includes significant amounts of quartz and feldspar, making it less fluid compared to other types of magma. This unique composition affects its physical properties, behavior in magma chambers, and the nature of volcanic activity, especially at convergent plate boundaries.
Silica: Silica, or silicon dioxide (SiO₂), is a fundamental chemical compound found in many types of rocks and minerals, particularly those associated with volcanic activity. In the context of magma composition and classification, silica content plays a crucial role in determining the physical properties of magma, such as viscosity, melting temperature, and eruption style. The variation in silica levels within magma can influence whether an eruption is explosive or effusive, ultimately shaping the type of volcanic landforms that develop.
Solidus Temperature: The solidus temperature is the specific temperature at which a material begins to melt, transitioning from a solid state to a partially molten state. This term is crucial in understanding the behavior of magma, as it defines the point at which minerals start to melt and contributes to the classification of different types of magma based on their composition and thermal properties.
Volatiles: Volatiles are substances in magma that can easily evaporate or vaporize at surface conditions, primarily consisting of water vapor, carbon dioxide, sulfur dioxide, and other gases. These components play a crucial role in influencing magma behavior, eruption dynamics, and the characteristics of volcanic eruptions. The presence and concentration of volatiles significantly affect magma viscosity, the potential for explosive eruptions, and the overall chemistry of volcanic products.