The floor of the Gulf of Alaska is a smooth plain sloping gently to the southwest. Thirty-five major submarine mountains, ranging in relief from 3500 to 12,400 feet, are known to rise above the plain. The seamounts appear to be of two types and have been separated into two topographic provinces.
In one province the symmetry, slope angles, and alinement of the seamounts indicate that they are volcanos. Most of the shoaler seamounts in this province have flat tops at a depth of 400–500 fathoms. The flat tops, as much as 8.5 miles wide, apparently were produced by wave truncation near sea level. One seamount situated on the axis of the Aleutian Trench has a flat top at a depth of 1380 fathoms; it appears to owe its unusually great depth to subsidence related to the formation of the trench.
The seamounts in the second province are elongate and are found on low ridges. Profiles are asymmetrical and irregular; none of the seamounts are flat on top, although four are much shoaler than the flat-topped seamounts in the first province. By reason of their topography and their relation to the trend of a Pliocene-Pleistocene orogenic belt, the seamounts are thought to be orogenic mountains.
The geological history of southern Alaska suggests that the eastern part of the Aleutian Trench originated in early Tertiary time. It also suggests that an earlier trench, now filled with sediment, existed in Mesozoic time at the present site of Kodiak Island and the Kenai Peninsula. The eastern part of the Aleutian Trench has an asymmetrical, V-shaped profile unlike the profiles of other major oceanic trenches. The unusual profile suggests that this part of the Trench may have a deep filling of sediment.