Magma production at continental rifts is a complex process that involves several geological phenomena related to the extension and thinning of the continental crust. Here's an overview of how magmas are produced in these settings and the reasons for the presence of both basalt and rhyolite:
Processes of Magma Production
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Tectonic Extension: Continental rifts occur where tectonic plates are pulling apart, leading to crustal stretching. As the crust extends, it becomes thinner, which can lead to decompression melting of the underlying mantle.
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Decompression Melting: As the continental crust thins, the pressure on the mantle beneath it also decreases. The mantle is mostly composed of ultramafic rock (rich in magnesium and iron), and as the pressure decreases, it can melt, leading to the formation of basaltic magma. This process is highly influenced by the temperature and composition of the mantle rocks.
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Heat Transfer: Hot mantle material can rise toward the surface, further melting rocks in the lower crust and upper mantle as it ascends. This heat helps to partially melt the continental crust itself, which can produce more silica-rich melts (such as rhyolite) as these crustal materials have a higher concentration of silica compared to the mantle.
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Formation of Magma Chambers: As basaltic magma ascends, it can accumulate in magma chambers within the crust. This storage allows for processes such as differentiation, where the composition of the magma can evolve due to fractional crystallization and assimilation of surrounding crustal material.
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Variable Sources: The different sources of magma contribute to the diversity of compositions. Basalt, which is low in silica, is commonly produced directly from the melting of peridotite in the mantle. In contrast, rhyolite, which is high in silica, can form from the melting of the continental crust and, in some cases, the differentiation of more basaltic magma that has undergone extensive crystallization and/or contamination with crustal materials.
Presence of Basalt and Rhyolite
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Basalt: The production of basalt in continental rift settings is mainly due to the melting of the upper mantle. This rock is formed primarily through partial melting of mantle peridotite and is characterized by its low silica content, which makes it more fluid and typically results in relatively low-viscosity lava flows.
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Rhyolite: Rhyolite forms when there is significant melting of the continental crust or when basaltic magma evolves within a magma chamber. As this basaltic magma interacts with the surrounding crust, it can assimilate silica-rich rocks, increasing its silica content to produce rhyolitic magma. Additionally, processes like fractional crystallization (where certain minerals crystallize out of the melt, leaving behind a more silica-rich residual liquid) can also lead to the formation of rhyolite.
Summary
In summary, both basalt and rhyolite can be found in continental rift settings due to the combination of mantle-derived basaltic melts from decompression melting and crustal-derived rhyolitic melts via differentiation and assimilation processes. The interplay between these different melting processes and the varying sources of magma results in the wide range of igneous rock types observed in continental rift environments.