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The Geological Society of America Special Paper 65 THE FLOORS OF THE OCEANS I. The North Atlantic Text to Accompany the Physiographic Diagram of the North Atlantic
The Geological Society of America Special Paper 65 THE FLOORS OF THE OCEANS I. The North Atlantic Text to Accompany the Physiographic Diagram of the North Atlantic
BY Bruce C. Heezen, Marie Tharp, and Maurice Ewing Lamont Geological Observatory (Columbia University) Palisades, New York April 11, 1959 Made in the United States of America TEXT PAGES COMPOSED AND PRINTED BY THE WILLIAM BYRD PRESS COLLOTYPE PLATES PRINTED BY THE MERIDEN GRAVURE COMPANY LINE PLATES PRINTED BY WILLIAMS AND HEINTZ BOUND BY RUSSELL-RUTTER COMPANY PUBLISHED BY THE GEOLOGICAL SOCIETY OF AMERICA ADDRESS ALL COMMUNICATIONS TO THE GEOLOGICAL SOCIETY OF AMERICA 419 WEST 117 STREET, NEW
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NOTE
NOTE
Physiographic Diagram: Atlantic Ocean, Sheet 1 (Plate 1) Copies of the Physiographic Diagram: Atlantic Ocean, Sheet 1 are available unfolded so that each will be suitable for wall mounting. The diagram is therefore not physically inserted or attached to this volume although it forms the basic part of the paper. [Pg vi] [Pg vii]...
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ACKNOWLEDGEMENTS
ACKNOWLEDGEMENTS
The studies of submarine topography at the Lamont Geological Observatory have been supported by the United States Navy Bureau of Ships under Contract NObsr 64547. The expeditions which obtained topographic data were supported by the Office of Naval Research under Contracts N6 onr 27124 and N6 onr 27113 and the Bureau of Ships under Contract NObsr 64547. Three cruises were supported by the National Geographic Society, the Woods Hole Oceanographic Institution, and Columbia University. Financial su
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ABSTRACT
ABSTRACT
The Physiographic Diagram: Atlantic Ocean, Sheet 1, which portrays the North Atlantic between 17° and 50° North Latitude, is the first of a projected series of diagrams. The diagram is based on continuous echo-sounding traverses made by research vessels. The relief shown on the profiles was sketched in perspective using the technique introduced by Lobeck. Between sounding profiles the relief is speculative, based on extrapolation of trends noted in the profiles. The area of the diagram is divide
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PART 1. PREPARATION OF THE PHYSIOGRAPHIC DIAGRAM
PART 1. PREPARATION OF THE PHYSIOGRAPHIC DIAGRAM
Several steps are involved in the preparation of a marine physiographic diagram. The raw data consist of continuously recorded echograms and lists of positions of the research ship. Echograms are profiles of ocean depth, automatically plotted against time (Luskin et al. , 1954). The first step is to read and tabulate the depth at each peak, trough, or change of slope. These readings are plotted on a chart (1:1,000,000) as a series of closely spaced soundings. Depth profiles are plotted against d
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INTRODUCTION
INTRODUCTION
Descriptions of physical features of the earth's surface are found in the earliest-known writings. However, the systematic classification of land forms is relatively recent and followed the development of the science of physical geology. The natural topographic divisions of the continents have been classified into physiographic provinces according to several similar systems (Lobeck, 1939; Fenneman, 1938; Atwood, 1940; and others). These systems take into account form and age of the relief, as we
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NOMENCLATURE AND CLASSIFICATION OF DEEP-SEA RELIEF
NOMENCLATURE AND CLASSIFICATION OF DEEP-SEA RELIEF
Before the advent of continuously recorded echo-sounding profiles, and their revelation of the texture of the sea-floor relief, classification and nomenclature of submarine topography were based on broad closed isobaths. We can characterize the older system as the bathymetric system of nomenclature in contrast to that employed in this paper, which we can call a geomorphic or textural system. The terms "basin" and "deep" used in the older literature are usually defined by closed 3000-, 4000-, or
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UNITS OF DEPTH AND SLOPE
UNITS OF DEPTH AND SLOPE
On the profiles and echograms the vertical scale is in units of echo-sounding time rather than in units of true depth. In other words, all depths are calculated under the assumption that the vertical sound velocity is 800 fathoms per second. Considering that the sound travels to the bottom and back, the calculation is based on 400 fathoms per second of lapsed time. Since the average vertical velocity is, within the area covered, always slightly less than 800 fathoms per second, the true depth is
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CONTINENT AND OCEAN
CONTINENT AND OCEAN
The two first-order morphologic divisions of the earth's crust are continent and ocean. The oceans can be divided into a few major divisions which are in turn subdivided into categories of physiographic provinces and then into individual provinces. The area of the present study is composed of the three major divisions shown in Figure 9: continental margin, ocean-basin floor, and mid-oceanic ridge. The discussion and description of the physiographic provinces of the North Atlantic will follow the
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DEFINITION AND GENERAL CATEGORIES
DEFINITION AND GENERAL CATEGORIES
The continental margin includes those provinces of the continents and of the oceans which are associated with the boundary between these two first-order features of the earth. General categories. —In most areas three parallel categories of provinces can be distinguished in the continental margin (Fig. 10). The relatively flat portions of the submerged continental platform constitute category I. These provinces are: continental shelf, epicontinental marginal seas ( e. g. , Gulf of Maine), and con
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REGIONAL DESCRIPTION OF CONTINENTAL MARGIN
REGIONAL DESCRIPTION OF CONTINENTAL MARGIN
This discussion is based on continuously recorded echo-sounding traverses made by Lamont Geological Observatory expeditions. Profiles approximately perpendicular to the continental margin are reproduced in Plates 24 and 25. None is precisely perpendicular, and thus slight distortions of slopes and widths of the features are unavoidable. EASTERN NORTH AMERICA : Thirty-four profiles of the continental margin of eastern North America are presented in Plate 24. The positions of the profiles are indi
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BENCHES AND TERRACES OF THE CONTINENTAL MARGIN
BENCHES AND TERRACES OF THE CONTINENTAL MARGIN
The topography of the continental margin provinces is divided into a series of benches or terraces. The largest are the continental shelf and slope (continental terrace) and the upper and lower continental rise. Superimposed on each of these major features is a series of smaller benches and terraces which range from features a few miles wide to simple breaks in the gradient of the continental slope. Many of these features can be traced for hundreds of miles (Heezen et al. , in press); some are i
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GEOLOGY AND GEOPHYSICS OF CONTINENTAL MARGIN PHYSIOGRAPHIC PROVINCES
GEOLOGY AND GEOPHYSICS OF CONTINENTAL MARGIN PHYSIOGRAPHIC PROVINCES
Seismicity of the continental-margin provinces. —Plate 29 shows the distribution of epicenters in the North Atlantic. Except in the Puerto Rico Trench and the adjacent Antilles Arc, earthquakes are exceedingly rare in the continental-margin provinces of the North Atlantic. From the Bahamas through the Grand Banks the only earthquakes reported since 1910 are two near the Laurentian Channel. In the eastern Atlantic an earthquake belt crosses the continental margin near Gibraltar but does not seem
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GENERAL DESCRIPTION
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 portio
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ABYSSAL PLAINS
ABYSSAL PLAINS
General discussion. —An abyssal plain has been defined (Heezen, Ewing, and Ericson, 1954) as "an area of the deep-ocean floor in which the ocean bottom is flat and the slope of the bottom is less than 1:1000". Abyssal plains have been found in all oceans, characteristically at the foot of the continental rise. Koczy (1954; 1956) and Gaskell and Ashton (1954) have described the abyssal plain south of the Bay of Bengal. Koczy (1956) has described plains in the equatorial Atlantic on either side of
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ABYSSAL HILLS
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 aby
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OTHER MAJOR FEATURES OF THE ABYSSAL FLOOR
OTHER MAJOR FEATURES OF THE ABYSSAL FLOOR
Abyssal gaps. —If two adjacent but distinct abyssal plains have no through passage at or below the level of the higher plain, they are said to be separated by a sill, a ridge, or a rise, depending on the dimensions of the feature involved. However, several plains are connected by constricted passages. An abyssal gap is a constricted passage connecting two abyssal plains which, in the vicinity of the gap, lie at different levels. The sea floor slopes down continuously through the gap from the hig
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OCEANIC RISES
OCEANIC RISES
Definition and distribution. —An oceanic rise is a large area (measured in hundreds of miles), not connected to or included in a mid-oceanic ridge or connected to a continental rise, which rises a few hundred fathoms above the surrounding abyssal floor. The topography of an oceanic rise ranges from gentle to extremely rugged. Included in this classification is the Bermuda Rise and Corner Rise of the North Atlantic, the Rio Grande Rise of the South Atlantic, and a number of similar but unnamed fe
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SEAMOUNTS OF THE OCEAN-BASIN FLOOR
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 sea
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SEISMICITY OF THE OCEAN-BASIN FLOOR
SEISMICITY OF THE OCEAN-BASIN FLOOR
The ocean-basin floor provinces are virtually devoid of earthquakes of a size detectable at distant seismic observatories. Of course small earthquakes (< 5, Richter scale) would probably not be locatable in such remote regions. The virtual absence of larger shocks makes it improbable that many small ones occur there either. Two earthquakes have occurred in the Bermuda Rise, one near the west boundary and one in the scarp zone of the southeast Bermuda Rise (Pl. 29). Several quakes were fel
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OCEAN-BASIN FLOOR PROVINCES AND CRUSTAL STRUCTURE
OCEAN-BASIN FLOOR PROVINCES AND CRUSTAL STRUCTURE
The results of seismic-refraction measurements in the ocean-basin floor can be divided into two categories depending on whether the measurements were made (1) in the abyssal floor, or (2) on an oceanic rise. Measurements in the abyssal floor of the western Atlantic (Ewing, Sutton, and Officer, 1954) revealed the simple pattern shown in Figure 35 b and e —namely, that beneath 4-5 km of water lies .5-1 km of sediments and sedimentary rock with a compressional-wave velocity of about 2 km/sec, overl
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DEFINITION
DEFINITION
The third basic subdivision of the oceanic depression is the Mid-Oceanic Ridge, a continuous median ridge which runs the length of the North Atlantic, South Atlantic, Indian and South Pacific oceans, for more than 40,000 miles (Heezen and Ewing, in press). In the center third of the physiographic diagram a short segment of this world-encircling ridge is represented....
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MID-ATLANTIC RIDGE
MID-ATLANTIC RIDGE
One can find references to the Mid-Atlantic Ridge in the scientific literature dating back more than 80 years. Before the advent of the echo sounder the lateral limits of the Mid-Atlantic Ridge were very difficult to define, and even now widely different definitions are used. Murray (1912) mentioned that the ridge lay in depths less than 2000 fathoms but pointed out that locally on the ridge depths exceeded 2000 fathoms. The Meteor expedition charts and profiles generally imply by their labeling
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PROVINCES OF THE MID-ATLANTIC RIDGE
PROVINCES OF THE MID-ATLANTIC RIDGE
The Mid-Atlantic Ridge was subdivided by Tolstoy and Ewing (1949) and Tolstoy (1951) into (a) "the central backbone or main range which is shallower than 1600 fathoms," and (b) "the flanks" or "the terraced zone" "between the 1600- and 2500-fathom isobaths." In this paper we use a similar but somewhat differently defined system by dividing the provinces of the Mid-Atlantic Ridge into two categories: (1) the crest provinces, and (2) the flank provinces. Crest provinces. —The provinces of the cres
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GEOLOGY AND GEOPHYSICS OF MID-ATLANTIC RIDGE PHYSIOGRAPHIC PROVINCES
GEOLOGY AND GEOPHYSICS OF MID-ATLANTIC RIDGE PHYSIOGRAPHIC PROVINCES
Seismicity of the Mid-Atlantic Ridge. —The earthquake epicenters instrumentally determined for the North Atlantic up to 1956 are shown in Plate 29. Nearly all earthquakes fall in the crest zone. Considering that determination of shocks is accurate only to within -½° to 1° +, it is quite surprising that the plotted epicenters form such a narrow belt. Investigation of the problem of the physiographic province most seismically active reveals that many epicenters actually plot in the Rift Valley and
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SUB-BOTTOM REFLECTIONS RECORDED ON PRECISION DEPTH RECORDER RECORDS AND PHYSIOGRAPHIC PROVINCES
SUB-BOTTOM REFLECTIONS RECORDED ON PRECISION DEPTH RECORDER RECORDS AND PHYSIOGRAPHIC PROVINCES
In some areas of the ocean PDR records show a reflecting surface a few fathoms below the bottom. Such horizons are observed only when the sounder is operated with a short (5-millisecond) ping length (in echo sounding the transmitted sound is called the ping, and its duration is called the ping length). When a long ping is used the first returning echo masks any subsequent echoes occurring less than about 10 fathoms after the first echo. To establish continuity of the lower horizon it is necessar
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SUMMARY OF PROVINCE CHARACTERISTICS
SUMMARY OF PROVINCE CHARACTERISTICS
The Atlantic Ocean floor consists of three major morphological divisions: (1) continental margin, (2) ocean-basin floor, and (3) Mid-Oceanic Ridge. The continental margin is formed by three categories of provinces which represent (1) the submerged continental platform, (2) the steep edge of the continental block, and (3) the raised or depressed edge of the ocean floor. The topographic detail of the continental margin is predominantly smooth except for the submarine canyons and minor irregulariti
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BIBLIOGRAPHY
BIBLIOGRAPHY
Agostinho, J. , 1937, Volcanic activity in the Azores: Bulletin Volcanologique, v. II (Series II), p. 380-400 Andrade, C. F. de , 1937, Os vales Submarinos Portugueses: Servicos Geologicos de Portugal, Lisboa, 235 p. Anonymous , 1935, General bathymetric chart of the oceans, Sheet A1: Information concerning the preparation of the third edition of Sheet A1; North Atlantic Ocean: Int. Hydro. Bur., Spec. Pub. 30, 30 p. ---- 1939, A summary of echo sounding apparatus: Int. Hydro. Bur., Monaco, Spec.
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