The earth's most common topographical feature: abyssal hills
Typical depth profiles revealing the dimensions of the abyssal hills; the earth's most common but least understood terrain.
All of the continents put together do not cover even half the area occupied by the abyssal hills, which dominate 60-70% of our planet's surface. We hear a lot about the endless steppes of Asia and the immensity of the Sahara, but who writes about the earth's dominant geomorphological feature? The abyssal hills are left out of the textbooks because so little is known about them. They are hidden under 3 kilometers of water. Individual abyssal hills rise 100-2,000 meters from the ocean floor; they are about 7-15 kilometers across; their lengths are not well known. We know them mainly from fathometer readings. If we could see them visually, they would probably look like the Appalachians from an airplane; that is, wash-board-like. Although heavily draped with pelagic sediments, the abyssal hills are believed to have been shaped by faulting and volcanic activity. K. MacDonald and coworkers at the University of California, Santa Barbara, now believe that both processes are involved:
"On the basis of data obtained last January on a series of dives along the East Pacific Rise (EPR), a ridge that runs off the coast of Central and South America, researchers theorize that the linear ridges of abyssal hills, which decorate the ocean floor somewhat like ribs on a washboard, are formed by an interplay between both volcanic eruptions and faulting action."
(Anonymous; "Geologists Offer New Theory on Origin of Abyssal Hills," Eos, 75:234, 1994. Background source: Fairbridge, Rhodes W., ed.,; The Encyclopedia of Oceanography, New York, 1966.)
Comment. But why would 60-70% of the earth's surface be wrinkled like this? The theory proposed above is too general to be very helpful.