级联(太圣海伦火山的火山范围和太Ranier的一部分)是“火山弧”。“火山弧”等等)。火山弧形式在一个常规的距离(和相当规律的间隔)俯冲带的后面。俯冲带地区密集的海洋地壳遁入更活跃的地壳(年轻的海洋地壳或大陆地壳)。在大洋板块开始生成液体由于变质反应发生在特定的压力和温度。这些液体导致地幔的部分熔融在板上面。(相反的经常教导,海洋地壳本身不会融化在这一点上——这是地幔上面。)这导致弧火山形成的表面在大约在海洋板达到~ 100公里深度(我可能mis-remembering确切数字)。不过,有一个有趣的故事瀑布,圣安德烈亚斯断层,内华达山脉。基本上,内华达山脉是古老的火山弧前海洋扩张脊试图撤去加州海岸。 (Search for the Farallon plate.) The ridge was too buoyant to subduct, so subduction stopped, shutting off the supply of magma to the volcanic arc. Because the motion of the Pacific Plate (on the other side of the spreading ridge that tried to subduct) was roughly parallel to the margin of North America, a strike slip boundary formed instead: The San Andreas Fault. Northward, there's still a remnant of the Farallon plate that's subducting beneath Northern CA, OR, WA, and BC. Therefore, the Cascades are still active arc volcanoes, while the Sierras are just the "roots" of the old arc.