Peridotite, the rock believed to make up much of Earth's upper mantle, has a composition that is ultramafic.

Here's why:

* Mafic: This term refers to rocks rich in magnesium (Mg) and iron (Fe). Mafic rocks are typically dark-colored.

* Ultramafic: This term denotes rocks that are even *more* enriched in magnesium and iron than mafic rocks.

Peridotite is classified as ultramafic because it is composed primarily of:

* Olivine: A silicate mineral containing Mg and Fe.

* Pyroxene: Another silicate mineral with Mg, Fe, and sometimes Ca.

These minerals give peridotite its dark, dense, and ultramafic character.