Question

describes magma or igneous rock that is rich in magnesium and iron and that is generally dark in color

Explanation:

The description is about a type of magma or igneous rock rich in magnesium and iron, dark in color. This relates to geology (a subfield of Natural Science, specifically under Geography or Earth Sciences which can be part of the 'Others' or related subfields in Natural Science, but more precisely, Geology which is often associated with Geography in such categorizations or as a subfield of Natural Science). The term for this is "mafic" (rocks or magma rich in Mg and Fe, dark - colored).

Answer:

mafic