Unraveling the Mystery: The Generation of Felsic Magma in Island Arcs

Felsic magmas, also known as acid magmas, are characterized by high silica content and low iron and magnesium content. They are typically associated with explosive volcanic eruptions and are responsible for the formation of many of the world’s largest calderas and ignimbrites. Felsic magmas are commonly produced in subduction zones, where one tectonic plate is forced beneath another. Island arcs are one type of subduction zone known to be a major source of felsic magmas. In this article we will explore how felsic magmas are produced at island arcs.

What are island arcs?

Island arcs are long, curved chains of islands formed by the subduction of one oceanic plate beneath another. They are typically found in the Pacific Ocean, where the Pacific plate is subducting beneath the Eurasian, Philippine, and Australian plates. Island arcs are characterized by active volcanic activity and earthquakes, and are often associated with the formation of felsic magmas.

The process of subduction begins when one tectonic plate is forced beneath another. As the oceanic plate descends, it is subjected to increasing pressure and temperature. This causes the water and other volatiles trapped in the plate to be released, which in turn lowers the melting point of the overlying mantle wedge. As a result, partial melting occurs and magma is produced.

Felsic magma production at island arcs

Felsic magmas are produced at island arcs by a process known as fractional crystallization. As the magma rises to the surface, it begins to cool and solidify. The minerals with the highest melting points crystallize first, leaving a more felsic magma with a higher silica content. This process can be repeated several times, resulting in a magma that is even more felsic than the original.

Another process that can contribute to the production of felsic magmas in island arcs is assimilation. This occurs when magma comes into contact with surrounding rocks and incorporates them into the magma. If the assimilated rocks have a higher silica content than the magma, the resulting magma will be more felsic.

The role of water

Water plays a crucial role in the production of felsic magmas at island arcs. As the subducting oceanic plate releases water and other volatiles, these fluids are transported into the overlying mantle wedge. The presence of water lowers the melting point of the mantle, making it easier for partial melting to occur. In addition, water can help facilitate the rise of magma to the surface by reducing its viscosity.
The water content of the magma itself can also affect its composition. Felsic magmas produced at island arcs tend to have a high water content, which can contribute to their explosive nature. The water in the magma can evaporate and expand rapidly during an eruption, leading to explosive volcanic activity.

Conclusion

In summary, island arc felsic magmas are produced by a combination of partial melting, fractional crystallization, and assimilation. Water plays a critical role in the production of these magmas, both through its ability to lower the melting point of the mantle and through its effect on the viscosity and explosive nature of the magma. Understanding the processes that lead to the production of felsic magmas in island arcs is important for predicting volcanic activity and mitigating the risks associated with volcanic eruptions.

FAQs

1. What is felsic magma?

Felsic magma, also known as acidic magma, is a type of magma that is characterized by its high silica content and low iron and magnesium content. It is typically associated with explosive volcanic eruptions and is responsible for the formation of many of the world’s largest calderas and ignimbrites.

2. How are felsic magmas produced at island arcs?

Felsic magmas are produced at island arcs through a combination of partial melting, fractional crystallization, and assimilation. The subducting oceanic plate releases water and other volatile substances, which are transported into the mantle wedge above it. The presence of water lowers the melting point of the mantle, making it easier for partial melting to occur. As the magma rises to the surface, it begins to cool and solidify, and minerals with the highest melting points will crystallize first, leaving behind a more felsic magma with a higher silica content. This process can be repeated multiple times, resulting in a magma that is even more felsic than the original.

3. What is fractional crystallization?

Fractional crystallization is a process that occurs when magma cools and solidifies. The minerals with the highest melting points will crystallize first, leaving behind a more felsic magma with a higher silica content. This process can berepeated multiple times, resulting in a magma that is even more felsic than the original.

4. How does assimilation contribute to the production of felsic magmas?

Assimilation occurs when magma comes into contact with the surrounding rocks and incorporates them into the magma. If the assimilated rocks have a higher silica content than the magma, the resulting magma will be more felsic. This process can contribute to the production of felsic magmas at island arcs.

5. What role does water play in the production of felsic magmas at island arcs?

Water plays a crucial role in the production of felsic magmas at island arcs. As the subducting oceanic plate releases water and other volatile substances, these fluids are transported into the mantle wedge above it. The presence of water lowers the melting point of the mantle, making it easier for partial melting to occur. Additionally, water can help to facilitate the ascent of magma to the surface by reducing its viscosity. The water content of the magma itself can also have an impact on its composition, as felsic magmas that are produced at island arcs tend to have a high water content, which can contribute to their explosive nature.

6. What is an island arc?

An island arc is a long, curved chain of islands that is formed by the subduction of an oceanic plate beneath another plate. They are typically found in the Pacific Ocean, where the Pacific Plate is subducting beneath the Eurasian Plate, the Philippine Plate, and the Australian Plate. Island arcs are characterized by active volcanic activity and earthquakes, and are often associated with the formation of felsic magmas.

7. Why is understanding the production of felsic magmas important?

Understanding the production of felsic magmas is important for predicting volcanic activity and mitigating the risks associated with volcanic eruptions. Felsic magmas are associated with explosive volcanic eruptions, and can cause significant damage to surrounding areas. By understanding the processes that lead to the production of felsic magmas, scientists and geologists can better predict when and where volcanic activity is likely to occur, and take appropriate measures to protect people and property in the affected areas.