The South American Plate is in motion. "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." Moving westward away from the Mid-Atlantic Ridge. The eastward-moving and more dense Nazca Plate is subducting under the western edge of the South American Plate along the Pacific coast of the continent at a rate of 77 mm per year. This collision of plates is responsible for lifting the massive Andes Mountains and causing the volcanoes which are strewn throughout them.