Answer: The rocks formed in rift valleys and at mid-ocean ridges are primarily mafic and not felsic due to the melting of the underlying mantle rocks. Mafic rocks are rich in magnesium (Mg) and iron (Fe) minerals, while felsic rocks are rich in feldspar and silica (SiO2).
Rift valleys and mid-ocean ridges are areas where tectonic plates are moving apart, creating a gap or a rift. As the plates separate, the underlying mantle rocks experience reduced pressure, leading to decompression melting. The mantle rocks have higher concentrations of mafic minerals, such as pyroxene and olivine, which melt at higher temperatures compared to felsic minerals.
Since the mantle is the primary source of molten material during rift valley formation and mid-ocean ridge spreading, the resulting magma is mafic in composition. This magma then rises to the surface, cools, and solidifies to form mafic igneous rocks like basalt. Felsic rocks, on the other hand, typically result from partial melting of continental crust, which is not the dominant process in these geologic settings.
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