The Northern Volcanic Zone (NVZ) extends from Colombia to Ecuador and includes all volcanoes on the continental mainland of these countries. Of the volcanoes in this zone, 55 are located in Ecuador, while 19 are in Colombia. In Ecuador, the volcanoes are located in the Cordillera Occidental and the Cordillera Real while in Colombia they are located in the Western and Central Ranges. The Pliocene Iza-Paipa volcanic complex in Boyacá, in the Eastern Ranges is the northernmost manifestation of the Northern Andean Volcanic Belt. The volcanic arc has formed due to subduction of the Nazca Plate underneath western South America. Some volcanoes of the Northern Volcanic Zone, such as Galeras and Nevado del Ruiz that lie in densely populated highland areas, are significant sources of hazards. It has been estimated that crustal thickness beneath this region varies from around 40 to perhaps more than . Sangay is the southernmost volcano of the Northern Volcanic Zone.