new seamount in the north pacific ocean

American Geographical Society

New Seamount in the North Pacific OceanAuthor(s): Haven NicholsSource: Geographical Review, Vol. 40, No. 3 (Jul., 1950), pp. 457-461Published by: American Geographical SocietyStable URL: http://www.jstor.org/stable/211220 .

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NEW SEAMOUNT IN THE NORTH PACIFIC OCEAN HAVEN NICHOLS

A NEW seamount, at a depth of 665 fathoms (Figs. i and 2), was recorded at 5o0o0'N., I4I59 'W., some 400 nautical miles south- west of the Queen Charlotte Islands, on September 26, I949, by the

United States Coast and Geodetic Survey ship Explorer, Commander H. Arnold Karo. A "seamount" has been defined as a submarine mountain rising more than 500 fathoms (3000 feet) above the deep-sea floor.' A

guyot" is a seamount with a flattened top that is generally at depths of 5oo to 900 fathoms.2 The crest of the newly discovered feature is within the limiting depths of a guyot, but its profile as based on a single line of soundings does not indicate such a form of submarine relief (Fig. 3).

The track line of the Explorer is an interesting study in itself; for it pre- sents a profile across the Gulf of Alaska and the North Pacific Ocean. A brief narrative description of this track line follows.

The track line began on September 2I, I949, in depths of 9I fathoms at latitude 5IO 49' N., longitude I750 45' W., near Adak Island, Alaska, and followed an approximate great-circle course to the vicinity of Yaquina Head, Oreg. A relatively sharp escarpment appeared at 52? 05' N., 1710 20'

W., where the bottom sloped from 570 fathoms to I040 fathoms in less than five miles. The depth then increased gradually to about 2000 fathoms at 520 i5' N., I68? oo' W., then rapidly to 3940 fathoms at 520 23' N., i65? 53 ' W., in the Aleutian Trench, then decreased more slowly to 2500 fathoms at 520 2I' N., i6i? 02' W. Thereafter, with only a few exceptions, depths were within 5o fathoms of 2500 to 5I? i5' N., I480 20' W. Except on the seamounts to be discussed later, the depth then decreased gradually and uniformly to I400 fathoms at 450 I2' N., I250 25' W., then sharply to only 45 fathoms at 440 40' N., I240 23' W., about 20 miles west of Yaquina Head.

' K. T. Adams: Hydrographic Manual, U. S. Coast and Geodetic Suirvey Special Publ. No. 143, re- vised edit., 1942, p. 54.

2 H. H. Hess: Drowned Ancient Islands of the Pacific Basin, Amer. Journ. of Sci., Vol. 244, I946, PP. 772-791; reference on p. 772.

The following general references may be cited: Frederick Betz, Jr., and H. H. Hess: The Floor of the North Pacific Ocean, Geogr. Rev., Vol. 32, I942, pp. 99-II6; H. W. Murray: Submarine Mountains in the Gulf of Alaska, Bull. Geol. Soc. of America, Vol. 52, 194I, pp. 333-362; idem: Profiles of the Aleutian Trench, ibid., Vol. 56, 1945, pp. 757-78I; F. P. Shepard: Submarine Geology, New York, 1948; F. P. Shepard and K. 0. Emery: Submarine Topography off the California Coast: Canyons and Tectonic Interpretation, Geol. Soc. of America Special Paper No. 31, 194I.

> MR. NICHOLS is a cartographic engineer with the United States Coast and Geodetic Survey, Washington, D. C.



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SEAMOUNT IN THE NORTH PACIFIC 459

SOUNDING METHODS AND CONTROL

Celestial sights supplemented by loran fixes were used for horizontal control for the greater part of the voyage and were sufficient to establish location accurately within three or four miles.

Depths were obtained with an NMC supersonic recorder, a navigational instrument that employs the magnetostrictive properties of nickel in its projectors to transform electrical energy into acoustical energy and vice versa. The recorder is calibrated for a sound speed of 8oo fathoms a second in sea water. It has two projectors, one for shallow soundings (0-200 fathoms) and one for deep soundings (O-4000 fathoms). Each projector has a resonant frequency of i8 kilocycles and serves for both transmitting and receiving, either projector being placed in the circuit by a "range" switch. The mechanism speed is maintained close to its calibrated value by use of a special 60-cycle frequency in the circuit, which limits the error to i in 8ooo or gooo cycles.

The operation of the recorder is briefly explained here to account for the apparent loss of profile at the bottom edges of the fathograms (graphic records of sounded depths) in Figures 3, 4, and S. The recording paper moves from top to bottom, with the zero on the left. When the range switch is set to read 0-2000 fathoms, the stylus, moving from left to right across the paper, records the depth by two marks: an initial mark or contact at the extreme left, which is the impulse sent to the bottom of the ocean, and another mark when the impulse is returned. When the stylus reaches the extreme right, another impulse is sent but is not recorded, since the stylus is electrically disconnected on all return strokes from right to left. When the range switch is set to read 2000-4000 fathoms, the initial mark or impulse is recorded on the extreme right; the return of the impulse is marked on the next stroke from left to right. The last recorded impulse, added to 2000 fathoms, is the desired sounding.

DESCRIPTION

The fathogram and a profile of the part of the Explorer's track line with which this paper is concerned are shown in Figure 3. Hourly fixes begin with position I27E, depth 2205 fathoms, at 3 :oo a.m., September 26, and end with position I3 iE. The line continues 40 minutes beyond I3 iE, where the depth is also 2205 fathoms. Corrections to the nearest five fathoms have been applied for instrumental errors and the velocity of sound in sea water. Recorded depths on the seamount are believed to be accurate within Io



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SEAMOUNT IN THE NORTH PACIFIC 46I

fathoms. Two less pronounced seamounts can be noted on the fathogram between positions I27E and I29E.

On October 2, I947, a seamount was discovered by the United States Coast and Geodetic Survey ship Pathfinder. The fathogram of this is shown in Figure 4; it lies about 6o miles northwest of the Explorer's seamount and is charted at a depth of 462 fathoms; its profile seems to be that of a guyot.

The fathogram of a seamount recorded on September 2I, I949, also by the Pathfinder, is shown in F gure 5. Its position, controlled by celestial sights and hourly loran line-of-position fixes, is at latitude 480 3I' N., longitude I330 io' W. Its corrected least depth of 502 fathoms was recorded by an NMB-i recorder, which operates and records similarly to the NMC. It plots on the plan map (Fig. 2) very near to the 432-fathom sounding at 480 32' N., I330 I2' W. This latter sounding was recorded October 2, I939, by the United States Coast and Geodetic Survey ship Guide. Its position was controlled by dead reckoning; the depth was recorded by a 3 I2-J fathom- eter, a sonic, visual type that has no mechanism for recording a profile.

It should be noted that the character of submarine features in any given area usually has been determined only by a single track line; accurate corre- lation should not be attempted without further adequate data.



new seamount in the north pacific ocean

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