There are three kinds of sea floor sediment: terrigenous, pelagic, and hydrogenous. Terrigenous sediment is derived from land and usually deposited on the continental shelf, continental rise, and abyssal plain. It is further contoured by strong currents along the continental rise. Pelagic sediment is composed of clay particles and microskeletons of marine organisms that settle slowly to the ocean floor. Some of these organic sediments are called calcareous or siliceous “oozes” because they are so thick and gooey. The clay component (or sometimes volcanic ash) is generally carried from land by wind and falls on the surface of the ocean. Pelagic sediment is least abundant on the crest of midoceanic ridges because of the active volcanism. Hydrogenous sediments are rich with minerals, such as manganese nodules, that precipitate from seawater on the ocean floor.