Here's why:

* Peridotite: This is a dense, dark-colored igneous rock rich in olivine and pyroxene. It's found in the Earth's mantle.

* Serpentinization: When peridotite is exposed to water, a process called serpentinization occurs. This involves chemical reactions between the minerals in peridotite and water, resulting in the formation of serpentine minerals.

So, serpentinization is essentially the transformation of peridotite into serpentine.