because melting of the subducting slab there produced felsic igneous rocks instead of volcanoes. Offshore volcanism also occurs in Chile. Intraplate volcanism generated from the Easter and Juan Fernández hotspots has formed many Chilean islands, including Isla Salas y Gómez, Easter Island, and the Juan Fernández Islands. Underwater volcanism occurs due to seafloor spreading along the Chile Ridge. Nearly 100 Quaternary (Pleistocene- or Holocene-age) independent volcanoes exist in the country, in addition to 60 volcanic complexes and caldera systems. Of the 200 historically active volcanoes in the Andean Range, 36 are found in Chile. Local setting Cerro Azul is part of the South Volcanic Zone, which runs through central and western Chile and extends south to Argentina. This range includes at least nine caldera complexes, more than 70 of Chile's stratovolcanoes and volcanic fields that have been active in the Quaternary, and hundreds of minor eruptive centres. The South Volcanic Zone is the most volcanically active region in Chile, and produces around one eruption per year. Its largest historical eruption was at Quizapu Crater, located on the north side of Cerro Azul's summit (see below), and its most active volcanoes are Llaima and Villarrica. Cerro Azul, just south of Descabezado Grande volcano, is part of the Descabezado Grande–Cerro Azul eruptive system, a volcanic field that comprises its two large namesake volcanic edifices and several smaller vents, including 12 Holocene calderas. Both volcanoes lie on top of the Casitas Shield, a plateau built of over 100 lava flows that erupted in at least 12 volcanic episodes during the Quaternary period. The upper lava layers are dated at 340,000 years. As with the majority of the Andean volcanoes, Cerro Azul is a stratovolcano, meaning that it consists of layers, or strata, of volcanic ash and lava flows. The cone of Cerro Azul has a total volume of about , and is a young feature, formed in the Holocene. It is made of agglutinated pyroclasts and some dacite–andesine lavas. The cone has a few volcanic craters; the majority of its eruptions in recorded history have originated from Quizapu Crater on the northern flank of Cerro Azul's cone. Other craters lying on the flanks of the main cone are Caracol ("Snail"), Crater los Quillayes, Crater la Resolana, and Crater sin Nombre ("Nameless Crater"). All of the craters lie between in elevation, except Quizapu, which is up the volcano. The summit of Cerro Azul is