Magma – Definition, Components, Properties and it’s Origin

Magma definition

Magma is naturally occurring molten or semi-molten rock material, generated within the Earth from which igneous rocks are derived through solidification and related processes. Magma is found beneath the surface of the Earth. Magma is typically composed of silicate minerals such as feldspar, mica, and quartz, as well as various dissolved gases, such as water vapor, carbon dioxide, and sulfur dioxide.

Magma is an important component of the Earth’s geologic system, and plays a key role in the formation of many types of rocks and mineral deposits. The study of magma and volcanic activity is an important area of research in the fields of geology, petrology, and volcanology.

Physical and chemical properties

Magma is a complex fluid with a range of physical properties that depend on its composition, temperature, and pressure. These physical properties of magma can have important effects on the behavior of volcanic eruptions, the formation of igneous rocks, and the generation of mineral deposits. Understanding these properties is therefore critical for the study of volcanology and economic geology. Here are some of the main physical properties of magma:

1. Viscosity: Viscosity is a measure of the resistance of a fluid to flow. Magma has a high viscosity due to its high content of silicate minerals and dissolved gases, which make it thick and sticky. High-viscosity magma is more likely to build up pressure and cause explosive volcanic eruptions.
2. Density: Magma is less dense than the surrounding rocks and fluids at depth, which allows it to rise towards the surface. However, the density of magma can vary depending on its composition and temperature.
3. Temperature: Magma is very hot, with temperatures that can range from 700 to 1300 degrees Celsius. The temperature of magma depends on its composition, with more silicate-rich magmas generally having lower temperatures.
4. Pressure: The pressure of magma is high due to the weight of the overlying rock and fluids. As magma rises towards the surface, the pressure decreases, which can cause it to expand and release dissolved gases.

The composition of magma refers to the type and amount of minerals and gases present in the fluid. Magma is typically composed of silicate minerals such as quartz, feldspar, and mica, as well as dissolved gases such as water vapor, carbon dioxide, and sulfur dioxide.

The chemical composition of magma can vary widely depending on the type of rocks that are being melted and the geological setting in which the magma is formed. However, the chemical composition of magma is primarily determined by the proportion of the major elements present, in. The most common elements found in magma include:

1. Silicon (Si)
2. Oxygen (O)
3. Aluminum (Al)
4. Iron (Fe)
5. Magnesium (Mg)
6. Calcium (Ca)
7. Sodium (Na)
8. Potassium (K)

The relative amounts of these major elements in the magma determine the type of magma and its physical and chemical properties. Magmas are classified based on their composition into four main categories: felsic, intermediate, mafic, and ultramafic.

Felsic magma

Felsic magmas are rich in silica (SiO₂) and have relatively high amounts of aluminum, potassium, and sodium. Felsic or silicic magmas have a silica content greater than 63%. They include rhyolite and dacite magmas. They are typically viscous and can produce explosive eruptions.

Intermediate magma

Intermediate magmas have lower silica content than felsic magmas but higher than mafic magmas. Specifically, intermediate or andesitic magmas contain 52% to 63% silica, and are lower in aluminium (Al) and usually somewhat richer in magnesium (Mg) and iron (Fe) than felsic magmas. Intermediate magmas are also commonly hotter, in the range of 850 to 1,100 °C (1,560 to 2,010 °F). Intermediate magmas show a greater tendency to form phenocrysts,^[2] Higher iron and magnesium tends to manifest as a darker groundmass, including amphibole or pyroxene phenocrysts.^[1]

Mafic magma

Mafic magmas are low in silica (52% to 45%) and have higher amounts of iron and magnesium than felsic and intermediate magmas. They are less viscous than felsic and intermediate magmas and are associated with effusive eruptions. Mafic magmas generally erupt at temperatures of 1,100 to 1,200 °C. Most basalt lavas are of ʻAʻā or pāhoehoe types, rather than block lavas. Underwater, they can form pillow lavas, which are rather similar to entrail-type pahoehoe lavas on land.^[3]

Ultramafic magma

Ultramafic magmas are even lower in silica and are rich in iron and magnesium. Ultramafic magmas are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). Komatiites contain over 18% MgO, and are thought to have erupted at temperatures of 1,600 °C. Ultramafic magmas are thought to form in the upper mantle or at the boundary between the mantle and the crust.

Origin of Magma

Magma originates from the melting of rocks in the Earth’s mantle and crust. The process of melting occurs when rocks are subjected to high temperatures and/or a reduction in pressure, causing the minerals to melt and form a molten mixture of silicates, gases, and other components.

Decompression Melting

One common way that magma is formed is through the process of decompression melting. This occurs when hot mantle rock rises to shallower depths in the Earth’s crust, where the pressure is lower, causing the rock to melt. This type of melting can occur at divergent plate boundaries, where tectonic plates are moving away from each other and creating space for hot mantle material to rise.

Flux Melting

Another way that magma can form is through the process of flux melting. This occurs when fluids such as water, carbon dioxide, or other volatile compounds are added to rock, which can lower the melting point of the minerals and cause them to melt. This type of melting can occur at subduction zones, where an oceanic plate is being forced beneath a continental plate, causing water and other fluids to be released from the descending plate and adding to the overlying mantle wedge.

Magma can also form through the process of heat transfer. This occurs when magma intrudes into the crust and heats up the surrounding rock, causing it to melt. This type of melting can occur in areas where magma is able to rise from the mantle and penetrate the crust, such as at volcanic hotspots or in areas where there are deep-seated magma chambers.

The exact process by which magma forms can vary depending on a number of factors, including the type of rocks being melted, the temperature and pressure conditions, and the presence of fluids or other geological factors.

Crystallization of magma

Crystallization is the process by which magma cools and solidifies to form rocks. The rate at which magma cools and the chemical composition of the magma both play a role in determining the types of minerals that will crystallize from the magma.

The process of crystallization occurs in two main stages: primary crystallization and fractional crystallization.

During primary crystallization, the first minerals to crystallize from the magma are typically those that have the highest melting temperatures and the lowest solubility in the remaining liquid magma. These are often feldspars, pyroxenes, and olivine, and they tend to form small crystals that are suspended in the liquid magma.

As the cooling process continues, the remaining liquid magma becomes more concentrated with the elements and minerals that are still in solution, and the temperature decreases further. This leads to the second stage of crystallization, fractional crystallization. During fractional crystallization, the remaining minerals in the magma continue to crystallize in order of their solubility and melting temperature, forming larger and more well-defined crystals.

The specific order in which minerals crystallize from magma is determined by factors such as temperature, pressure, and chemical composition. By studying the minerals present in a rock, geologists can gain insight into the conditions under which the rock formed and the processes that shaped it.

In some cases, magma may cool and solidify quickly, forming volcanic glass or other non-crystalline structures. This can happen when it is rapidly ejected from a volcano and exposed to the cool atmosphere or water, causing it to rapidly cool and solidify before crystals have a chance to form.

Use of Magma in energy production

The Iceland Deep Drilling Project, while drilling several 5,000 m holes in an attempt to harness the heat in the volcanic bedrock below the surface of Iceland, struck a pocket of magma at 2,100 m in 2009. Because this was only the third time in recorded history that magma had been reached, IDDP decided to invest in the hole, naming it IDDP-1.^[4]

A cemented steel case was constructed in the hole with a perforation at the bottom close to the magma. The high temperatures and pressure of the magma steam were used to generate 36 MW of power, making IDDP-1 the world’s first magma-enhanced geothermal system.^[4]

References

[1] Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2^nd ed.). Cambridge, UK: Cambridge University Press. pp. 19–20.

[2] Takeuchi, Shingo (5 October 2011). “Preeruptive magma viscosity: An important measure of magma eruptibility”. Journal of Geophysical Research. 116 (B10): B10201.

[3] Schmincke, Hans-Ulrich (2003). Volcanism. Berlin: Springer. pp. 49–50.

[4] Wilfred Allan Elders, Guðmundur Ómar Friðleifsson and Bjarni Pálsson (2014). Geothermics Magazine, Vol. 49 (January 2014). Elsevier Ltd.