Re: Why is the abyssal plain so flat?

The plain is not perfectly flat, but it is much flatter than the rest of 
the ocean floor.  This is not because of currents, which are generally too 
slow in the deep ocean to do much reshaping of the bottom.  Instead, the 
plains form because of the tectonics of the ocean floor and because of the 
sedimentation that you mentioned.

The ocean floor is part of a sort of conveyer system where new oceanic 
lithosphere (hard outer shell of the earth) is created at mid ocean ridges 
and moves away from those ridges over time.  As it moves away, it cools and 
shrinks.  Since the same amount of stuff is now in a smaller space, the 
density increases and the lithosphere sinks into the gooey asthenospere.  
At first, the sinking is rapid, giving rise to the part of the ocean floor 
that slopes away from the mid-ocean ridges.  With time, the cooling slows 
down and the sinking slows down and the slope of the ocean floor decreases. 
 
At the same time, as soon as the sea floor is formed, sediments begin to 
very slowly accumulate on it.  In time, these sediments even out the 
irregular surface of the sea floor and leave a very even surface.  If you 
could step on it, you would find that the material at the surface is like 
soft mud, in fact its technical name is "ooze."

As you get close to a continent, much more sediment is available to be 
washed into the ocean and so a big pile of sediment develops and rises up 
from the abyssal plain.  This forms the continental rise, slope, and shelf. 

Almost all pictures of these features that you will find in books 
exaggerate the features to make a point.  The plains are shown as perfectly 
flat, but they are not, they are gently sloped.  The continental slope and 
rise are shown with fairly steep slopes (maybe 20-30 degrees), however, 
they are actually only a few degrees.  If the pictures were drawn to scale, 
it would be very hard to see any difference between rise and slope, for 
example.

David Smith
Department of Geology, Environmental Science and Physics
La Salle University, Philadelphia, PA