how are island arc and continental arc magmas similar?

Project Fab

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19-03-2025

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The Shared Ancestry: Exploring the Similarities Between Island Arc and Continental Arc Magmas

Island arcs and continental arcs, while geographically distinct, share a fundamental geological process: subduction. This process, where one tectonic plate slides beneath another, drives the generation of magmas that ultimately build these volcanic mountain ranges. While the specifics differ due to the contrasting nature of the subducting and overriding plates, the resulting magmas display remarkable similarities in their composition, petrogenesis, and tectonic implications. Understanding these similarities is crucial for comprehending the complexities of plate tectonics and the evolution of Earth's crust.

Both island arc and continental arc magmas are predominantly andesite in composition, although a range of other magma types can be found associated with them. Andesite is an intermediate volcanic rock, characterized by its intermediate silica content (typically between 57% and 63% SiO2). This contrasts with the more silica-rich felsic rocks (like rhyolite) and the more mafic rocks (like basalt) found in other tectonic settings. This intermediate composition reflects the complex interplay of processes involved in magma generation at subduction zones.

The primary source of magma in both settings is the partial melting of the mantle wedge. As the subducting oceanic plate descends, it releases volatiles (primarily water) into the overlying mantle. This addition of water lowers the melting point of the mantle peridotite, causing partial melting to occur. The resulting magma is initially basaltic in composition, but as it ascends through the crust, it undergoes various processes that modify its composition.

One key similarity lies in the process of fractional crystallization. As the basaltic magma rises, it cools and minerals begin to crystallize. These minerals, particularly olivine and pyroxene, are denser than the remaining melt and sink to the bottom of the magma chamber. This process removes mafic components from the melt, progressively enriching it in silica and other incompatible elements. This enrichment leads to the formation of andesitic and even more felsic magmas in both island and continental arcs.

Another important process affecting both magma types is assimilation. As the magma ascends, it may interact with and melt surrounding crustal rocks. This process, known as crustal contamination or assimilation, incorporates the chemical constituents of the crustal rocks into the magma. In island arcs, this assimilation primarily involves the oceanic crust, while in continental arcs, it involves the much thicker and more diverse continental crust. This leads to variations in the chemical composition of the magmas, but the overall trend towards andesitic compositions remains.

The role of magma mixing also plays a significant role in both island and continental arcs. Magmas of different compositions, generated at different depths or through different processes, may mix together. This mixing can create a wider range of magma compositions, obscuring the primary magmatic sources but contributing to the overall diversity observed in these settings. The proportion of mixing and the types of magmas involved can vary, leading to some differences between island and continental arcs, yet the underlying principle remains the same.

Despite these similarities in the fundamental processes, some differences exist, primarily due to the distinct nature of the overriding plates. In island arcs, the overriding plate is oceanic crust, which is relatively thin and mafic. This leads to a higher proportion of basaltic magmas and a relatively less pronounced effect of crustal assimilation compared to continental arcs. The resulting volcanic rocks tend to be more homogeneous in composition, although variations still occur due to fractional crystallization and magma mixing.

In continental arcs, the overriding plate is continental crust, which is significantly thicker and more felsic. This thick continental crust allows for more extensive interaction between the ascending magma and the surrounding rocks. Crustal assimilation is far more pronounced, leading to a wider range of magma compositions, with a higher proportion of andesitic and even more felsic magmas (dacite and rhyolite). The resulting volcanic rocks are often more chemically heterogeneous, reflecting the complex interaction of different magmatic components and the assimilation of diverse crustal materials.

Furthermore, the tectonic setting itself influences magma generation and evolution. The angle of subduction, the rate of plate convergence, and the presence of pre-existing structures within the overriding plate all play a role in determining the characteristics of the magmas. These factors can lead to variations in magma composition, even within the same arc system, highlighting the dynamic and complex nature of subduction zones.

In conclusion, despite the geographical and geological differences between island and continental arcs, their magmas share remarkable similarities. Both are primarily generated by the partial melting of the mantle wedge due to the release of volatiles from the subducting slab. Subsequent processes like fractional crystallization, assimilation, and magma mixing further shape the composition of the magmas, leading to the prevalence of andesitic compositions. While the extent and nature of these processes vary depending on the characteristics of the overriding plate, the fundamental similarities in magma petrogenesis underscore the unifying role of subduction in shaping Earth's volcanic landscapes. Further research focusing on isotopic tracing, detailed geochemistry, and experimental petrology can continue to unravel the intricate details of these processes and refine our understanding of the shared ancestry between island arc and continental arc magmas.