Subduction is a geological process where one tectonic plate moves under another and sinks into the Earth's mantle. This typically occurs at convergent boundaries, where an oceanic plate meets a continental plate or another oceanic plate. The denser oceanic plate is forced downward, leading to the formation of deep ocean trenches and volcanic arcs. As the subducting plate descends, it melts and can cause magma to rise, resulting in volcanic activity. This process is responsible for many earthquakes and the creation of mountain ranges, such as the Andes in South America, which are formed by the collision of the Nazca Plate and the South American Plate.