3.1 Magma composition and properties
2 min read•july 22, 2024
, the molten rock beneath Earth's surface, is a complex mixture of liquid rock, crystals, and gases. Its composition and properties, including and gas content, play a crucial role in determining volcanic activity and eruption styles.
From to , magma compositions vary widely, affecting their behavior during ascent and eruption. Understanding these differences helps geologists predict volcanic hazards and interpret the Earth's geological history.
Magma Composition and Properties
Components of magma
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- Molten rock beneath Earth's surface composed of liquid rock, crystals, and dissolved gases
- Silicate melt made up of silicon, oxygen, and various cations (Al, Fe, Mg, Ca, Na, K)
- Solid mineral grains suspended in the melt can include olivine, pyroxene, amphibole, feldspars, and quartz
- Dissolved gases () such as water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2) play a crucial role in magma behavior and
Factors in magma properties
- Magma viscosity measures its resistance to flow and is influenced by composition, , and dissolved gas content
- Higher (SiO2) content and alkali metal content (Na and K) lead to higher viscosity
- Higher temperatures result in lower viscosity while cooler magmas are more viscous and less fluid
- Higher dissolved gas content can lead to lower viscosity but as magma rises and pressure decreases, gases exsolve, increasing viscosity
Volatiles in magma behavior
- Dissolved gases in magma, primarily H2O and CO2, influence viscosity and density
- Exsolution of volatiles during magma ascent can drive explosive eruptions
- Eruption style depends on volatile content and magma viscosity:
- High volatile content and low viscosity lead to effusive eruptions with gentle lava flows and fountains
- High volatile content and high viscosity cause explosive eruptions with violent ejection of ash, pumice, and gas
- Low volatile content and high viscosity result in thick, slow-moving lava flows
Mafic vs felsic magma
- Magma compositions exist on a spectrum from mafic to felsic
- Mafic magmas have high magnesium (Mg) and iron (Fe) content, low silica (SiO2) content (45-52%), lower viscosity and temperature (900-1200℃), and typically form from partial melting of mantle peridotite (basalt and gabbro)
- Felsic magmas have high silica (SiO2) content (>63%), are enriched in lighter elements (K, Na, Al), have higher viscosity and lower temperature (700-850℃), and form from partial melting of crustal rocks or of mafic magmas (rhyolite and granite)
Key Terms to Review (17)
Basaltic magma: Basaltic magma is a type of low-viscosity molten rock that is rich in iron and magnesium, typically resulting from partial melting of the mantle. It forms primarily at divergent boundaries and hotspots, leading to the creation of basalt rock when it cools. Due to its lower silica content compared to other magmas, basaltic magma can flow more easily, influencing volcanic activity and landform development.
Eruption style: Eruption style refers to the manner in which volcanic eruptions occur, encompassing the physical characteristics and behavior of the eruptive events. This concept is closely linked to the composition and properties of magma, as variations in magma viscosity, gas content, and temperature can lead to different eruption styles, such as explosive or effusive eruptions. Understanding eruption styles helps predict volcanic behavior and potential hazards associated with different types of eruptions.
Felsic: Felsic refers to a category of igneous rocks and magmas that are rich in silica and light-colored minerals such as quartz and feldspar. This composition significantly influences the characteristics of Earth's crust, the formation of magma, and the classification of igneous rocks based on their mineral content and texture. Felsic materials are typically associated with continental crust and can lead to explosive volcanic eruptions due to their high viscosity.
Fractional crystallization: Fractional crystallization is a process where different minerals crystallize from magma at different temperatures, leading to the separation of solid minerals from the liquid phase. This method is crucial in understanding how the composition of magma changes over time as it cools and differentiates, ultimately influencing the types of rocks that form. By analyzing this process, one can gain insights into magma properties and the evolutionary paths of magmas as they undergo differentiation.
Intermediate: In geology, the term 'intermediate' refers to a specific category of magma composition that lies between mafic and felsic types. This classification indicates a balanced presence of silica and other minerals, resulting in unique characteristics that affect the behavior of the magma as well as the types of volcanic rocks formed from it. Intermediate magmas typically produce andesitic rocks, which have implications for volcanic activity and the nature of eruptions.
Lava flow: A lava flow is a stream of molten rock that erupts from a volcano and moves across the Earth's surface. The characteristics of lava flows, including their viscosity and temperature, are largely determined by the composition of the magma from which they originate. Understanding these flows is essential for grasping how different types of volcanoes behave during eruptions and the resulting landforms they create.
Mafic: Mafic refers to igneous rocks and magmas that are rich in magnesium and iron, typically resulting in darker colors and denser materials. This term is significant because it helps classify rocks based on their mineral composition, which directly relates to their formation processes in Earth's internal structure, the characteristics of magma, and the textures found in various igneous rocks.
Magma: Magma is a molten rock material located beneath the Earth's surface that forms when rocks partially melt due to high temperatures and pressure. It plays a crucial role in the rock cycle as it can cool and solidify to become igneous rock, or it can erupt as lava, leading to the formation of new landforms and influencing Earth's dynamic systems.
Magma chamber: A magma chamber is a large underground reservoir of molten rock, or magma, located beneath the Earth's surface. This chamber plays a crucial role in the formation of volcanic eruptions, as the pressure builds within it, leading to potential eruptions at the surface. The characteristics of the magma within the chamber, including its composition and viscosity, significantly influence the type of volcanic activity that can occur.
Magma differentiation: Magma differentiation is the process by which magma evolves chemically and physically, leading to the formation of different types of igneous rocks from a single magma source. This process involves the separation of minerals from the melt, resulting in changes in composition, temperature, and viscosity of the remaining magma. Understanding magma differentiation helps explain the diversity of volcanic and intrusive rocks found in various geological settings.
Rhyolitic magma: Rhyolitic magma is a type of magma characterized by its high silica content, typically exceeding 70%, which makes it the most viscous type of magma. This high viscosity leads to a slow movement and thick lava flows, often resulting in explosive volcanic eruptions. Rhyolitic magma is commonly associated with continental crust and can form various volcanic features, including domes and calderas.
Silica: Silica, or silicon dioxide (SiO₂), is a mineral composed of silicon and oxygen that is one of the most abundant components in the Earth's crust. It plays a crucial role in determining the composition and properties of magma, as well as influencing weathering processes in rocks. Silica can exist in various forms, such as quartz, and affects the viscosity of magma and the rate of weathering in different environments.
Temperature: Temperature is a measure of the average kinetic energy of particles in a substance, indicating how hot or cold that substance is. In geology, temperature plays a crucial role in determining the behavior and properties of magma, as well as influencing metamorphic processes that affect the formation of rocks. Higher temperatures can lead to changes in mineral stability and the melting of rock, which are essential for understanding various geological phenomena.
Ultramafic: Ultramafic refers to igneous rocks that contain a very high percentage of magnesium and iron, typically with low silica content. These rocks are primarily composed of minerals like olivine and pyroxene, and their composition significantly influences the characteristics of magma and the types of igneous rocks that form during volcanic and tectonic activities.
Viscosity: Viscosity is a measure of a fluid's resistance to flow, which is influenced by the fluid's composition and temperature. In the context of magma, viscosity affects how easily it can move, its ability to erupt, and the style of volcanic activity that may occur. Higher viscosity means thicker magma that tends to trap gases, leading to more explosive eruptions, while lower viscosity allows for more fluid movement and less violent eruptions.
Volatiles: Volatiles are substances that can easily vaporize and exist as gases at surface temperatures and pressures. In the context of magma, volatiles like water vapor, carbon dioxide, and sulfur dioxide play crucial roles in determining the behavior of magma during its ascent to the Earth's surface, affecting everything from eruption style to the formation of volcanic rocks.
Volcanic rock: Volcanic rock is a type of igneous rock that forms from the rapid cooling and solidification of lava at or near the Earth's surface. These rocks can provide valuable information about the volcanic activity and magma composition from which they originated, as their texture and mineral content are influenced by the conditions of eruption and cooling.