Here's how it works:

* Subduction: The denser Nazca plate is being forced under the South American plate.

* Melting: The Nazca plate melts as it descends, creating magma.

* Volcanism: This magma rises and erupts through the surface, forming volcanoes like Nevado del Ruiz.

This process is called subduction, and it's responsible for the formation of many volcanic mountain ranges along the west coast of South America, including the Andes where Nevado del Ruiz is located.