Here's why:

* Peridotite is a dense, dark-colored igneous rock, rich in magnesium and iron.

* The upper mantle lies beneath the Earth's crust and extends to a depth of about 410 kilometers. It's primarily made up of peridotite.

While peridotite is the dominant rock type in the upper mantle, there are also other rock types present in smaller amounts, such as pyroxenite and dunite.