Geological research suggests that the South American Plate is moving westward away from the Mid-Atlantic Ridge: "Parts of the plate boundaries consisting of alternations of relatively short transform fault and spreading ridge segments are represented by a boundary following the general trend."
As a result, the eastward-moving and more dense Nazca Plate
is subducting under the western edge of the South American Plate, along the continent's Pacific coast, at a rate of 77 mm (3.0 in) per year.
The collision of these two plates is responsible for lifting
the massive Andes Mountains
and for creating the numerous volcanoes
(including both stratovolcano's and shield volcanoe's) which are strewn throughout them.