ABYSSAL HILLS
Definition and distribution.—An abyssal hill is a small hill that rises from the ocean-basin floor and is from a few fathoms to a few hundred fathoms in height and from a few hundred feet to a few miles in width. The term abyssal hills province is applied to those areas of the ocean-basin floor in which nearly the entire area is occupied by hills—that is, the province lies at approximately the depth of the adjacent abyssal plain but lacks a smooth floor. Isolated abyssal hills and groups of abyssal hills also occur in the abyssal plains.
Figure 25.—Index chart, location of abyssal hills profiles illustrated in Figures 26 and 27
Limits of abyssal hills provinces shown by dotted lines.
Figure 26.—Eleven profiles, Western Atlantic Abyssal Hills Province
40:1 vertical exaggeration; positions shown on Figure 25.
Abyssal hills are found along the seaward margin of most abyssal plains and probably occur in profusion in basins isolated from adjacent land areas by ridges, rises, or trenches. In the North Atlantic the abyssal hills form two strips parallel to the Mid-Atlantic Ridge for virtually its entire length. The Bermuda Rise is bordered on the southeast by abyssal hills which join with the strips adjoining the Mid-Atlantic Ridge (Fig. 25). Southeast of the Bermuda Rise the abyssal plain is absent, and consequently the Western Atlantic Abyssal-Hills Province exceeds 500 miles in width.
Figure 27.—Eight profiles, Eastern Atlantic Abyssal Hills Province
Vertical exaggeration 40:1; positions shown on Figure 25.
In the North Atlantic the axes of maximum depth on the eastern and on the western sides of the Mid-Atlantic Ridge lie in the abyssal-hills province. This pattern probably continues through the South Atlantic, Indian Ocean, and South Pacific oceans. Individual abyssal hills are identical to the smaller hills which rise from the steps of the Mid-Atlantic Ridge and are probably of the same origin. The abyssal hills and the Lower Step of the Mid-Atlantic Ridge can be distinguished topographically only by the contrast in level.
Regional description.—The abyssal hills within the area of the physiographic diagram are illustrated by profiles WH-1-11 and EH-1-8 reproduced in Figures 26 and 27 respectively.
WESTERN ATLANTIC: The abyssal-hills province ranges from a few miles to more than 125 miles in width from the north edge of the map to the Southeast Newfoundland Ridge. Along profile WH-1 the province consists of about a dozen hills 3-4 miles wide and 50-100 fathoms high, while in WH-2 the hills are 4-10 miles in width, and some exceed 200 fathoms in height. The abyssal-hills province appears to pinch out at the Southeast Newfoundland Ridge. Southwest of the Southeast Newfoundland Ridge the abyssal hills form a belt 60-120 miles wide which skirts along the north edge of Corner Rise. In this area, as illustrated by WH-3, -4, and -5, the largest abyssal hills nowhere exceed 250 fathoms in height. The individual hills range from 3 to 12 miles wide; maximum depths between individual hills frequently exceed the depth of the adjacent abyssal plain by 50-100 fathoms.
Along the northwest corner of Corner Rise the abyssal-hills province pinches out. South of Corner Rise the province appears on both the east and the west sides of the Sohm Abyssal Plain, and isolated abyssal hills become more numerous within the abyssal plain. In profile WH-6 the abyssal hills within the province reach 300 fathoms in height and are 3-5 miles wide. A few higher peaks occur such as the two in profile WH-6, one 500 fathoms high and 20 miles wide and the other 700 fathoms high and 15 miles wide. In addition eight isolated hills 100 to 175 fathoms high rise from the plain in the same profile. To the south, the frequency of hills within the plain increases, and the hills province widens. In profile WH-7 the province increases to 35 miles in width on the east side of the Sohm Plain and to 100 miles in width on the west side. The smaller individual hills in the plain are 200 fathoms high, in contrast to the average of 100 fathoms in WH-6. Twenty-one individual hills rise from the plain in WH-7, in contrast to 15 in WH-6. In WH-8 the abyssal plain is represented by a flat-floored valley only 12 miles wide; the remaining 500-mile width of the abyssal floor is occupied by abyssal hills. Individual hills are 300 to 600 fathoms high and 5 to 10 miles wide (Fig. 28; Pl. 10). If we filled all the depressions on WH-8 with 100 fathoms of sediments the profile would closely resemble WH-7.
Figure 28.—Tracing of PDR record, abyssal hills, southeast of Bermuda Rise
WH-9 and WH-10 are very similar to WH-8 except that individual hills seem to become broader and lower toward the south. The height of individual hills in WH-10 ranges from 100 to 250 fathoms. South of WH-8 the abyssal-hill province splits into two strips which border the Nares Abyssal Plain. The western strip lies below the eastern scarp of the Bermuda Rise Scarp Zone, and the eastern strip follows the Mid-Atlantic Ridge, sending a narrow strip westward along the northern edge of the outer ridge north of Puerto Rico. A number of hills 50 fathoms high and 2-5 miles wide are scattered over the eastern part of the Nares Abyssal Plain. Abyssal hills are rare in the abyssal plain west of 65° W. The abyssal-hill province along the southwestern Bermuda Rise is extremely narrow and intermittent. No abyssal hills have been recorded for the Hatteras Abyssal Plain, and only locally is the province developed along the western side of the Bermuda Rise. Locally abyssal hills are moderately well developed along the northern margin of the Bermuda Rise.
EASTERN ATLANTIC: The abyssal floor west of the Anglo-French shelf is extremely narrow; the abyssal plain and abyssal hills combined are only 80 miles wide. The abyssal-hills province is absent, but, as shown on EH-1 and EH-2 (Fig. 27), large hills rise from the seaward edge of the Biscay Abyssal Plain. These hills are 5-10 miles wide and 100-400 fathoms high. West of the Iberia Abyssal Plain (EH-3) the abyssal-hills province is wider and better developed than near the Biscay Abyssal Plain.
South of Gibraltar the abyssal floor widens southward; northwest of Cape Verde it is more than 200 miles wide. Profile EH-6 runs obliquely through the province and indicates an exaggerated width for the province. Individual hills are 5-10 miles wide and 100-600 fathoms high (Pl. 9, fig. 4). The abyssal-hills province is widest, and the hills are the highest where the depth is greatest, a relation similar to that observed on the opposite side of the Mid-Atlantic Ridge in the sector southeast of the Bermuda Rise.
Origin of abyssal-hills topography.—The topography of the abyssal hills is considered to represent the original surface buried beneath the abyssal plains and perhaps beneath the continental rise. This explanation agrees well with the smooth topography and the distribution of deep-sea sands in the abyssal plain. The origin of the hills topography is a more difficult problem. We have no rock samples from these provinces. Individual hills cannot be studied with present seismic-refraction techniques; they do not seem to have a characteristic magnetic anomaly; and Worzel has only recently developed a method of gravity measurement which can be applied to a feature as small as an abyssal hill. At present we have only topography as a basis of speculation of their origin. As far as we know, individual abyssal hills are not discernibly different from the smaller hills of the steps of the Mid-Atlantic Ridge or much of the topography of the oceanic rises, and thus there is no reason to assume that the abyssal hills have a different origin.
We have noted that abyssal hills range from 50 to 600 fathoms in height and with some exceptions from 4 to 10 miles in width. However, many of the hills are about 200 fathoms high and 5 miles wide. The hills are very steep-sided, and frequently the sounder simply records a series of "highlights" from a succession of peaks (Pl. 10; Pl. 9, fig. 4). Soundings are difficult to obtain in the abyssal-hills provinces because of the great depth and primarily the preponderance of steep slopes (Pl. 10). We do not know whether individual hills are conical or elongate or, in fact, if they run in narrow, sinuous ridges as suggested by Tolstoy (1951). Accurate, detailed mapping of portions of the abyssal-hills provinces on scales of approximately 1:20,000 or larger should provide additional limits on theories of origin. At present we cannot decide among vulcanism, faulting, or folding, although folding seems the least probable.