Final answer:
The line of volcanoes along the western coast of South America is formed due to subduction, where the Nazca Plate subducts beneath the South American Plate, leading to volcanism that creates volcanoes closer to the coastline.
Step-by-step explanation:
Along the western coast of South America, near location 4, the formation of the line of volcanoes is best explained by the geologic process known as subduction. Subduction occurs where the Nazca Plate is moving under the South American Plate. As the oceanic crust of the Nazca Plate subducts, it encounters increasing pressure and temperature, which causes minerals within the plate that contain water to release that water. This addition of water lowers the melting temperature of the mantle above the subducting slab, facilitating the melting of the mantle material and leading to volcanism. The volcanoes along this subduction zone are therefore closer to the coastline, as the melt is generated near where the oceanic plate begins its descent under the continental plate.