SEAMOUNTS OF THE OCEAN-BASIN FLOOR

A seamount is defined as any isolated elevation which rises more than 500 fathoms above the sea floor. Those seamounts which lie entirely on oceanic rises have been described as part of the rise topography. Now we will describe the seamounts of the abyssal floor.

KELVIN SEAMOUNT GROUP: An impressive row of large conical peaks runs from the vicinity of Georges Bank for 600 miles toward the northeast tip of the Bermuda Rise. A profile plotted at natural scale which crosses five of the largest seamounts is reproduced in Figure 34. The line of seamounts runs across the continental rise, abyssal plain, and the Bermuda Rise, seemingly little affected on crossing the province boundaries. The larger seamounts such as Kelvin Seamount are about 2000 fathoms higher than the adjacent plain and are as much as 35 miles in diameter at the base. Those which rise from the continental rise or the abyssal plain do so abruptly, suggesting that their bases are partially buried. The tops of most of these seamounts lie between 550 and 850 fathoms, and at least a few are flat-topped. They are thus very similar to the flat-topped guyots of the Pacific in size, shape, and the range of depths of their flat summits (Hamilton, 1956). Photographs taken on these seamounts showed rock, ripples and live solitary corals.

CARYN SEAMOUNT: A small conical peak, 1000 fathoms high, whose base is 8 miles in diameter, lies in the abyssal plain west of the Bermuda Rise (36° 45´N.). A magnetic survey of the peak has been published by Miller and Ewing (1956). They found an exceptionally large anomaly associated with the peak, which clearly showed its volcanic origin. Cores from the peak contained manganese nodules, altered volcanic rocks, and Upper Cretaceous to Recent fossils, which shows that it is at least as old as Upper Cretaceous. The base of the peak rises from the abyssal plain which extends out from the Hudson Submarine Delta.

HORSESHOE SEAMOUNT GROUP: About 300 miles due west from the Straits of Gibraltar an impressive group of seamounts lies in a horseshoe-shaped arc. Several of these, most notably Ampere and Josephine seamounts, rise to less than 100 fathoms. Josephine Seamount is the largest of the group and lies along an east-west topographic trend (the Azores-Gibraltar Ridge). In the southern half of the Horseshoe Group the individual seamounts appear to be coalescing cones similar to the seamounts of the Kelvin Group. Seamounts of the northern half, although imperfectly known, seem to be elongated east-west. The southern half of the group appears to resemble volcanic cones, while in the northern half tectonic deformation seems to have played a larger part. The seamounts have been cored, dredged, and photographed by a Lamont Observatory expedition. The sediments obtained from the seamounts are middle Tertiary to Recent (Sutton et al., 1957).

REPRESENTATIVE PDR RECORDS FROM BERMUDA RISE

Depth in fathoms.

PDR RECORD, BERMUDA SCARP ZONE

Depth in fathoms.

PDR RECORDS MID-ATLANTIC RIDGE

Depth in fathoms.

Area of each photograph is about 6 by 8 feet.

Plate 19.—OCEAN-BOTTOM PHOTOGRAPHS; MID-ATLANTIC RIDGE

Figure 1. (Station T1-9, photo 42) Depth 1410 fathoms, location 48° 38´N., 28° 48´W., on Western Rift Mountains, Mid-Atlantic Ridge. In this plate Figures 1, 2, and 3 are closely spaced photographs less than 100 feet apart.

Note the coarse-grained light and dark gravel in winnow row in the foreground and the light-colored clay or ooze bottom in background. Of the sixty photographs taken at this location three resembled Figures 2 and 3, several resembled Figure 1, and in the remainder the ocean floor was composed entirely of the light and dark gravel.

Figure 2. (Station T1-9, photo 43) Location about 50 feet from photo shown in Figure 1, same depth and position. The dark color of the rocks may be due to a coating of manganese dioxide. However, the rocks may actually be composed of dark material; no dredgings were taken here. All dredgings from Mid-Atlantic Ridge have brought up basalt and in some cases also gabbroic and serpentinized rock. It seems likely that the rock in Figure 2 and 3 of this plate is basalt. Note the abundance of sessile life on the rocky areas in contrast to its absence in the gravel-covered areas.

Figure 3. (Station T1-9, photo 45) Location about 100 feet from photo shown in Figure 2, same depth and position. Rock bottom on the Western Rift Mountains, Mid-Atlantic Ridge.

Figure 4. (Station V4-7, photo 27) Depth 500 fathoms, location 37° 25´N., 31° 10´W., Western Rift Mountains, Mid-Atlantic Ridge, south of the Azores Plateau.

Figure 5. (Station T1-54, photo 15) Depth 2100 fathoms, location 23° 07´N., 43° 45´W., Upper Step (eastern side), Mid-Atlantic Ridge. Apparatus in lower right attached to camera. Note tracks of crawling animal in left side of photo and small holes in bottom sediment. Photograph contrasts sharply with those from Rift Mountains in lack of current or oscillation ripples and rock outcrops.

Figure 6. (Station T1-14, photo 6) Depth 2130 fathoms, location 46° 04´N., 17° 43´W., Lower Step (eastern side), Mid-Atlantic Ridge. Note fecal pellets, mounds, and small holes. Again lack of evidence of strong currents or outcrops is notable.

Positions of stations shown on Plate 30

Photographs of the seamounts generally show winnowed and rippled sediment and rock (Pl. 15). Virtually all loose sediment is being removed from the seamounts. The horseshoe, open to the east, encloses an abyssal plain which is, judging from the sea-floor gradients, fed largely from the east by turbidity currents originating in the Straits of Gibraltar and the Gulf of Cadiz areas.

MILNE SEAMOUNT: Early charts of the western Atlantic showed an extensive bank of 20,000 square miles rising from the center of Newfoundland Abyssal Plain. At about the center of this area an exceptionally high peak has been discovered by cable ships, and the old name Milne Seamount has been assigned to this peak. It rises more than 2500 fathoms above the abyssal floor. Two seamounts of similar size have been found north and south of the Milne Seamount by workers on the R. V. Atlantis (Fig. 30).

As the abyssal floor is better surveyed many more isolated seamounts will undoubtedly be discovered, and they may reveal tectonically significant patterns.