Sediment on the ocean floor will often be scraped from the subducting plate. This causes the sediment to accumulate as a mass of material called an accretionary wedge (accretionary prism), which attaches itself to the upper plate. Volcanic island arcs or seamounts may collide with the continent, and as they are of relatively light material (i.e. low density) they will often not be subducted, but are added to the side of the continent.