Andesite Line

The Andesite Line is the petrologic boundary that rings the Pacific basin, separating the andesitic, silica-rich volcanic rocks of the surrounding subduction-zone arcs from the basaltic rocks of the true ocean interior. Inside the line the seafloor and its islands, such as Hawaii, are basaltic ocean crust and hot-spot output; outside it the island arcs and continental margins, the Aleutians, Japan, the Marianas, the Andes, erupt andesite and more evolved lavas. The line traces the geographic limit of subduction-related volcanism and marks the boundary of the genuine Pacific oceanic plate against the surrounding convergent margins.