1. Partial Melting:
* Source Rock: Felsic magmas originate from pre-existing igneous or metamorphic rocks, typically granitic or sedimentary rocks. These rocks are often rich in quartz, feldspar, and mica, which are the primary minerals found in felsic magmas.
* Heat Source: The heat needed to melt these rocks can come from various sources:
* Subduction Zones: The sinking of oceanic plates (subduction) generates heat, which melts the overlying crust.
* Mantle Plumes: Hot, buoyant plumes of magma rising from deep within the Earth can melt the overlying crust.
* Crustal Extension: Stretching and thinning of the crust can expose rocks to lower pressure, reducing their melting point.
* Melting Process: The process of partial melting involves the melting of only a portion of the source rock. This is because different minerals have different melting points. Quartz and feldspar melt at lower temperatures than other minerals like olivine or pyroxene. Therefore, the first minerals to melt are silica-rich, resulting in a felsic magma.
2. Fractional Crystallization:
* Magma Ascent: As the felsic magma rises towards the surface, it cools and begins to crystallize.
* Mineral Separation: Minerals crystallize from the magma in a specific order, based on their melting points. Early-formed minerals like olivine and pyroxene are denser than the surrounding magma and sink to the bottom. This leaves behind a magma that is even richer in silica and aluminum.
* Enrichment: This process of removing early-formed minerals enriches the remaining magma in felsic components, making it even more felsic.
3. Other Factors:
* Assimilation: As the magma rises, it may assimilate surrounding rocks, incorporating their chemical components into the magma.
* Mixing: Felsic magmas can also mix with other types of magmas, like mafic magmas, resulting in a hybrid magma composition.
Summary:
Felsic magmas are formed through a combination of partial melting, fractional crystallization, and other processes like assimilation and mixing. The initial partial melting of silica-rich source rocks produces a felsic magma, which is further enriched in felsic components through fractional crystallization as it ascends.
