Magma Composition and Igneous Rocks: Exam Notes

Magma compositions can be altered through several processes, leading to the formation of different rock types:

Magmatic Assimilation: This occurs when a magma body incorporates and melts surrounding country rock. For example, if a mafic magma melts felsic country rock, the resulting mixture will be intermediate in composition.
Crystal Settling (Fractional Crystallization): As magma cools, certain minerals crystallize and settle out due of their density. For instance, iron and magnesium-rich crystals (which are heavy and dense) will settle, changing the composition of the remaining magma. This is analogous to potatoes falling out of soup, altering the soup's composition.
Magma Mixing: Magmas of different compositions (e.g., felsic and mafic magmas) can mix to produce a new melt with an intermediate composition.

Origin: Both diapirs and hotspots originate from the D'' (D double prime) layer within the Earth's interior.
Diapirs: These are described as