Continental rifting is a tectonic process where a continent is stretched and thinned due to extensional forces. This geological activity creates conditions conducive to magma production, leading to the formation of various igneous rocks, including basalt and rhyolite.
Processes of Magma Production at Continental Rifts
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Gravitational Pull and Lithospheric Stretching: As the continental crust is pulled apart, it experiences rifting, leading to a decrease in pressure on the underlying mantle. This reduction in pressure can cause the mantle material to melt, forming basaltic magma, which is typically low in silica.
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Decompression Melting: As the lithosphere thins and the asthenosphere is upwelled due to the extensional forces, the pressure on the mantle rocks decreases. This allows them to melt through a process known as decompression melting, primarily producing basaltic magma.
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Heat Transfer and Magma Differentiation: In addition to decompression, heat from the mantle can also contribute to the melting of the crust. If there are hotspots or areas of volcanic activity in the rift, the added heat can cause significant melting in the lower continental crust, which is richer in silica and can produce more evolved magmas.
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Assimilation and Fractional Crystallization: Basaltic magma generated from the mantle can ascend through the continental crust, where it may interact with and partially melt the surrounding rocks, particularly if they are more silica-rich. This process is known as assimilation. As basaltic magma rises and cools, it may crystallize certain minerals, leading to a more evolved composition, forming intermediate or even rhyolitic magma if sufficient silica is introduced.
Presence of Basalt and Rhyolite
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Basalt: This rock type is typically produced directly from the melting of the upper mantle through decompression melting, resulting in its relatively low viscosity and low silica content. Basaltic eruptions are common in rift settings because of the high-temperature magma flowing easily to the surface.
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Rhyolite: Rhyolite, which is high in silica, can form through the processes mentioned above: assimilation of crustal rocks by basaltic magma, fractional crystallization, and partial melting of the lower crust. As basaltic magma ascends, it can carry along pieces of continental crust, melt them, and become more silicic, resulting in the production of rhyolite.
Conclusion
Therefore, the combination of mafic (basalt) and felsic (rhyolite) magma in continental rift settings is a product of both the initial mantle melting processes and the subsequent interactions with the continental crust. This diversity in magma types reflects the complex geological interactions occurring during the rifting process, leading to distinct volcanic features and rock formations.