Which type of volcanism formed the andes in south america?

The correct answer is - composite volcanism.

The Andes Mountain Range in South America, is by far the longest mountain range in the region, as well as being the highest, and most dominant geographical feature in the western part of the South American continent.

This mountain range has formed somewhere between 6 and 10 million years ago, and it has formed as a result of the subduction of the Nazca tectonic plate under the South American tectonic plate. On the place of the subduction, a composite volcanic activity has emerged, which is typical for this type of plate boundaries, and helped into lifting upwards the surrounding area, thus the creation of the Andes.