Felsic magma, which forms the basis for common rocks like granite and rhyolite, represents a highly evolved form of molten rock found deep within the Earth’s crust. This magma is characterized by a high content of silica, typically exceeding 65% by weight, alongside elevated levels of lighter elements such as sodium and potassium. Conversely, it has a low concentration of iron and magnesium. The chemical makeup of felsic magma gives it a relatively low density and a high viscosity, meaning it is thick and sticky. This high viscosity traps volatile gases, which is why felsic magma is often associated with explosive volcanic eruptions when it reaches the surface.
Source Rocks and Necessary Conditions
The primary source material for felsic magma is the continental crust, which is naturally richer in silica than the Earth’s mantle. Crustal rocks, such as sedimentary and metamorphic types, contain the necessary proportions of quartz and feldspar. For these solid rocks to melt, a combination of high temperature and specific chemical conditions must be met.
The immense pressure deep within the crust raises the melting point of most dry rock. However, the presence of water and other volatile compounds, such as carbon dioxide, acts as a flux. These volatiles infiltrate the rock and significantly lower the temperature required for melting to begin, a process known as flux melting. This allows the continental crust to melt at achievable geological temperatures, typically ranging between 650°C and 950°C.
The Process of Partial Melting
The formation of felsic magma begins with partial melting, where only a fraction of the source rock is converted into liquid. Rocks are composed of various minerals, each having a unique melting temperature. The minerals with the lowest melting points melt first, and these are predominantly the silica-rich components like quartz and potassium feldspar.
As melting proceeds, the initial liquid produced is always chemically different from the original solid rock, being significantly enriched in silica. The early melt selectively extracts the lighter, lower-temperature components, leaving behind a solid residue composed of higher-temperature, iron and magnesium-rich minerals. This selectivity ensures that the resulting magma is inherently felsic, having a silica content often greater than 70%, even if the source rock was mafic.
Magma Evolution Through Differentiation
Felsic magma often evolves from a less-evolved parent magma through magmatic differentiation. One effective way this occurs is through fractional crystallization, which operates as the magma begins to cool within a chamber. As the temperature drops, the densest, most mafic minerals, such as olivine and pyroxene, crystallize first and sink to the bottom of the chamber.
The removal of these early-forming crystals strips the remaining liquid melt of iron, magnesium, and calcium, causing the magma’s composition to become progressively richer in silica. This process can transform an intermediate or mafic parental magma into a highly felsic one. Another process, known as assimilation, contributes to this evolution by incorporating and melting the surrounding silica-rich country rock into the rising magma body.
Geological Settings for Felsic Magmatism
Felsic magma formation is linked to specific tectonic environments where the necessary heat and volatile conditions are met. Subduction zones, where one tectonic plate slides beneath another, are major sites for felsic magmatism. Here, water released from the subducting oceanic slab rises into the overlying crust, causing flux melting that facilitates magma generation.
The resulting magma often ascends slowly through the thick continental crust, providing ample time for fractional crystallization and assimilation to refine its composition. Felsic magma also forms in areas of continental collision, where crustal thickening pushes rock deep enough to undergo partial melting due to increased temperatures. Additionally, in regions of continental rifting or hotspots, high heat flow from the mantle can melt lower crustal rocks, producing substantial volumes of felsic melt.