Some researchers have hypothesized that the subducted portion of the Tehuantepec Ridge under Mexico is responsible for the existence of El Chichón volcano as well as the other volcanoes of what is called the Chiapanecan Volcanic Arc. Studies of the 1982 eruption of El Chichón indicated its erupted output was unusual compared to other volcanoes in the Trans-Mexican Volcanic Belt which lies well to the northwest. The anomaly is thought to result from the distinctive composition of the rocks which comprise the Tehuantepec Ridge.