abyssal plain

Broad, relatively flat expanse of sea floor lying 3–6 km/2–4 mi below sea level. Abyssal plains are found in all the major oceans, and they extend from bordering continental rises to mid-oceanic ridges. Abyssal plains are covered in a thick layer of sediment, and their flatness is punctuated by rugged low abyssal hills and high sea mounts.

Underlain by outward-spreading, new oceanic crust extruded from ridges, abyssal plains are covered in deep-sea sediments derived from continental slopes and floating microscopic marine organisms. The plains are interrupted by chains of volcanic islands and seamounts, where plates have ridden over hot spots in the mantle, and by additional seamounts, which were originally formed in oceanic ridge areas and transferred to the deep as the oceanic crust subsided. Otherwise, the abyssal plains are very flat, with a slope of less than 1:1000.