GENERAL DESCRIPTION
The second of the three basic topographic divisions of the oceanic depression is the ocean-basin floor. Included in this division are those provinces of the oceanic depression that are not included in the continental margin or the mid-oceanic ridge.
The ocean-basin floor is divided into three categories: (1) abyssal floor, (2) oceanic rises, and (3) seamounts and seamount groups. The first category includes two types of provinces, abyssal plains and abyssal hills, which occupy the deepest portion of the ocean-basin floor. Included in these provinces are such features as abyssal gaps and mid-ocean canyons. The second category includes the larger positive features of the ocean-basin floor, and the large seamounts and seamount groups fall in the third category. The landward limit of the ocean-basin floor is the 1:1000 gradient isopleth along the continental margin. Along the mid-oceanic ridge the boundary is taken as that scarp or scarp zone where the average level rises appreciably above the axis of maximum depth of the basin floor. Broad elevations which rise above the basin floor as isolated rises are termed oceanic rises and are included in this discussion even though they may be structurally more closely related to the mid-oceanic ridge or the continental margin.