* Magma is derived from existing rock: Magma is molten rock, and it forms when existing rocks melt. This means the chemical composition of the magma is initially similar to the parent rock.
* Silica is a major component of rocks: Silica (SiO2) is a fundamental building block of most rocks. It's present in varying amounts, but it's always part of the rock's overall composition.
* Melting and crystallization processes: When rocks melt, not all minerals melt at the same temperature. Some minerals, like those rich in silica, melt at higher temperatures than others.
* During the initial stages of melting, the resulting magma will have a lower silica content than the original rock because the silica-rich minerals haven't yet melted.
* As temperatures rise, more silica-rich minerals melt, increasing the silica content of the magma. However, this process cannot lead to a magma with higher silica content than the parent rock.
What can happen to silica content during magma evolution:
* Fractional crystallization: As magma cools, certain minerals crystallize out of the melt, leaving behind a magma with a different composition. In some cases, this process can lead to a magma that is *slightly* higher in silica than the original magma.
* Assimilation: Magma can sometimes interact with surrounding rocks, incorporating some of their minerals into its composition. If the assimilated rocks are rich in silica, this could increase the silica content of the magma.
* Mixing: Two magmas with different compositions can mix, resulting in a magma with a different silica content than either parent magma.
Key Point: While magma composition can change during its evolution, it's not possible to have a magma with a higher silica content than the original rock from which it formed *solely through melting*. The silica content of the magma is ultimately limited by the silica content of the parent rock.
