The correct answer is 'subduction'. This is a geological process taking place at convergent boundaries of tectonic plates, known as subduction zones. Here, one plate will move under the another. The lower plate sinks into the mantle due to gravity. Subduction rates are typically very slow: typically centimeters per year. Subduction zones can be very unstable, and are the sites of earthquakes and volcanoes. Mountain ranges also often form as a result of subduction.