1. Source Rock Composition:
* Initial Chemistry: The starting material for magma is rock from the Earth's crust or mantle. The chemical composition of this source rock is the foundation for the magma's composition.
* Mafic Rocks (Basalt): Rich in magnesium and iron, these rocks form magma with a lower silica content (45-55%), making it more fluid and often associated with volcanic eruptions.
* Felsic Rocks (Granite): High in silica (60-75%), these rocks produce viscous magma, leading to explosive eruptions.
* Intermediate Rocks (Andesite): These rocks contain intermediate silica levels (55-65%), creating magma that falls between the two extremes.
2. Partial Melting:
* Melting Point: Different minerals melt at different temperatures. As rocks heat up, minerals with lower melting points will melt first, creating a magma with a composition different from the original rock.
* Degree of Melting: The amount of melting affects the composition. Partial melting produces a magma enriched in the elements that melted first, while extensive melting can lead to a magma closer to the original rock composition.
3. Crystallization and Fractional Crystallization:
* Crystallization: As magma cools, minerals start to solidify (crystallize) out of the melt, altering the magma's composition.
* Fractional Crystallization: If the early formed crystals are removed from the magma (e.g., by settling), the remaining magma will become enriched in the elements that remain in the liquid phase. This process can significantly change the magma's composition over time.
4. Assimilation:
* Contamination: As magma rises through the crust, it may interact with surrounding rocks, dissolving them into the melt and changing its composition. This process is called assimilation.
5. Mixing:
* Magma Mixing: Two or more magmas with different compositions can mix, producing a new magma with a blended composition.
6. Degassing:
* Volatile Loss: Magma contains dissolved gases (volatiles). As magma rises to the surface, pressure decreases, leading to the release of these gases. This process can change the magma's composition, making it more explosive.
7. Tectonic Setting:
* Subduction Zones: Magmas generated in these zones are often more complex, reflecting the mixing of different source rocks and assimilation processes.
* Mid-Ocean Ridges: Magmas formed here are typically basaltic, reflecting the melting of mafic rocks in the mantle.
* Hotspots: Magmas from hotspots can be highly variable, depending on the depth and composition of the mantle source.
In summary, the composition of magma is a dynamic process, influenced by the source rock, melting conditions, crystallization, assimilation, mixing, and the geological setting.
