Ocean Basin

Abyssal plains are relatively flat areas of the ocean basin with slopes of less than one part in a thousand. They tend to be found at depths of 13,000-16,000 ft (4,000-5,000 m). Oceanographers believe that abyssal plains are so flat because they are covered with sediments that have been washed off the surface of the continents for thousands of years. On the abyssal plains, these layers of sediment have now covered up any irregularities that may exist in rock of the ocean floor beneath them.

Abyssal plains found in the Atlantic and Indian Ocean tend to be more extensive than those in the Pacific Ocean. One reason for this phenomenon is that the majority of the world's largest rivers empty into either the Atlantic or the Indian Oceans, providing both ocean basins with an endless supply of the sediments from which abyssal plains are made.

Abyssal hills are irregular structures on the ocean floor that average about 825 ft (250 m) in height. They often occur over very wide stretches of the ocean floor and are especially common in the Pacific Ocean. Abyssal hills are probably just smaller versions of the volcanic features known as seamounts.