Paleomagnetism Earth ’ s magnetic field has reversed many times in its history magnetic minerals in lava record direction of Earth ’ s magnetic field when the lava hardened records exist on every continent pattern of magnetism near ridges: bands alternate normal and reversed magnetism mirror image pattern is found around ridges If sea-floor spreading had never occurred, all magnetism would be all normal or all reversed, the same as Earth ’ s original magnetic field

Subduction occurs when a sea-floor plate collides with another plate and is forced down into the mantle granitic sea-floor sediments melt and form low-density granitic magma magma rises to form volcanoes the rest of the basaltic plate melts deeper into the mantle deep ocean trenches and volcanoes are always associated with subduction when 2 sea-floor plates collide, a volcanic island arc forms examples:Aleutian Islands, Alaska Japan The Antilles in the Caribbean Indonesia if there ’ s a continent next to a trench, volcanoes will be among the mountains at the continent ’ s edge examples: Cascades in the NW U.S. and SW Canada Andes in South America