Abyssal Plains

October 6, 2007
By Damond Benningfield

A tongue-in-cheek study a few years ago found that Kansas is flatter than a pancake. For their next work, the geographers who compared the two might want to study the ocean floor. Great stretches of it, called abyssal plains, are the flattest places on Earth.

These plains may cover almost a third of Earth’s surface -- about as much as all the exposed land combined. They’re found between the edges of the continents and great underwater mountain ranges.

Abyssal plains consist of beds of volcanic rock topped with sediments that are up to thousands of feet thick. Most of the sediments wash off the continents, and are carried to the depths by dense currents. Over time, the sediments spread out to provide a smooth, level surface. Abyssal plains are most common in the Atlantic; in the Pacific, deep trenches around the continents trap most of the sediment before it reaches the open ocean.

At depths of thousands of feet, there’s absolutely no light. The water is near freezing, and the pressure is hundreds of times greater than at the surface. Even so, many species have adapted to the harsh conditions. They eat a “snow” of dead organic matter that falls from the upper layers of the ocean. Because of the cold, they have slow metabolisms, so they don’t need to eat very often -- generally, only once every few months.

Just how flat are these plains? In most cases, their slopes are less than one foot in a thousand. Or to put it another way -- flatter than a pancake.

Copyright 2007.  The University of Texas Marine Science Institute