Abyssal plains and abyssal hills cover about 25 of the surface of Earth. What are the ...

Abyssal hills and plains cover about 25 of the surface of the Earth. Abyssal plains are extraordinarily flat expanses of the ocean floor consisting of layers of sediment. Most of the sediment layers originated from shallow water or continental margins, and in some areas can be over 1000 meters thick. Abyssal plains lie between 3,700 and 5,500 meters beneath the surface, and are located between the continental margins and oceanic ridges. In some areas, the sediment layer may not be thick enough to cover the basaltic oceanic floor, especially near the edges of the ridges. In these areas, small abyssal hills are formed as sediment covers extinct volcanoes that once spewed molten rock. Abyssal hills are usually only about 200 meters high and are most commonly associated with seafloor spreading areas. They form when newly formed crust moves away from the center of a ridge, stretches, and cracks. Some blocks of the crust drop to form valleys, and others remain higher as hills.

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