Topographically there is commonly a depression, a trench, marking the boundary on the sea floor. The subducted plate is heated by the hot asthenosphere and eventually melts. Some of the melt rises to form volcanoes. Because oceanic lithosphere is consumed at subduction zones, there is no very ancient seafloor.
Subduction zones are very geologically active.
Accretionary wedge A mass of sediments and oceanic lithosphere scraped from subducting plate and plastered onto the edge of the overriding plate. Subduction begins Continents grow as isolated continental fragments collide producing larger, more stable continental masses. Primitive island-arcs accreted to margins of continents. Anatomy of a Continent Craton Tectonically stable interior region of a continent composed of an ancient crystalline shield surrounded by platform of younger sedimentary rock Shield Stable continental region consisting of exposed Precambrian igneous and metamorphic rocks. Most continental lithospheric growth had occurred by about 3.5 billion years ago.