The Nazca plate is a large tectonic plate that underlies the Pacific Ocean near the western coast of South America. It is subducting under (that is, being forced under) the South American plate. A sudden breaking in the rocks along a roughly 560–620-mile (900–1,000-km) stretch of the Nazca Plate caused the earthquake, which has been generally agreed to have had a magnitude of 9.5—the largest earthquake recorded in the 20th century.