Here's why:

* Mafic rocks are rich in magnesium (Mg) and iron (Fe), which are the key elements in dark silicate minerals like pyroxene and olivine.

* Plagioclase feldspar is a common mineral in both mafic and intermediate rocks.

Examples of mafic rocks include:

* Basalt: A common volcanic rock found on the ocean floor and in some land areas.

* Gabbro: A coarse-grained intrusive equivalent of basalt.

Let me know if you have any other questions!