1. Fractional Crystallization:
* As magma cools, different minerals crystallize at different temperatures.
* Minerals with higher melting points (e.g., olivine, pyroxene) crystallize first and settle to the bottom of the magma chamber.
* The remaining magma becomes enriched in elements that make up minerals with lower melting points (e.g., quartz, feldspar).
* This process, called fractional crystallization, leads to the formation of different rock types from the same magma.
2. Assimilation:
* As magma rises through the crust, it may incorporate surrounding rocks (country rocks).
* This process, called assimilation, changes the chemical composition of the magma and can lead to the formation of igneous rocks with different mineral compositions.
3. Mixing:
* Magma bodies can mix with other magmas of different compositions.
* This mixing creates a new magma with a composition that falls between the two original magmas, leading to the formation of igneous rocks with unique mineral assemblages.
Example:
A basaltic magma (mafic) with a high content of magnesium and iron can undergo fractional crystallization. Early crystallization of olivine and pyroxene will leave a magma richer in silica, sodium, and potassium. This residual magma can then solidify into a more felsic rock, such as andesite or rhyolite, depending on the extent of crystallization and the involvement of other factors.
Therefore, the composition of an igneous rock is not solely determined by the initial composition of the magma but is also influenced by the processes that occur during its journey from the mantle to the surface.
