The seafloor is the floor of the oceans. The major features of the seafloor are the continental shelf, the continental rise, the abyssal floor, seamounts, oceanic trenches, and mid-ocean ridges. The abyssal or deep ocean floor is about 3 kilometers (1.8 miles) deep and is mostly made of basaltic rock covered with fine-grained (pelagic) sediment consisting of dust and the shells of marine organisms. Oceanic trenches are up to 11 kilometers (7 miles) deep, typically 50–100 kilometers (30–60 miles) wide and may be thousands of kilometers long. The slopes are usually asymmetrical with the steeper slope on the landward side and a more gentle slope on the side of the ocean basin. They are regarded as the site of plate subduction. Oceanic ridges are long, linear volcanic structures which tend to occupy the middle of seafloors; they are the sites of crustal spreading. See also seafloor spreading.