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).

[Illustration:

Heezen _et al._, PL. 16

FIGURE 1. WESTERN BERMUDA RISE

FIGURE 2. CENTRAL BERMUDA RISE

FIGURE 3. EAST-CENTRAL BERMUDA RISE

FIGURE 4. EASTERN BERMUDA RISE

REPRESENTATIVE PDR RECORDS FROM BERMUDA RISE

Depth in fathoms.]

[Illustration:

Heezen _et al._, PL. 17

PDR RECORD, BERMUDA SCARP ZONE

Depth in fathoms.]

[Illustration:

Heezen _et al._, PL. 18

FIGURE 1. HIGH FRACTURED PLATEAU

FIGURE 2. UPPER STEP

PDR RECORDS MID-ATLANTIC RIDGE

Depth in fathoms.]

[Illustration:

Heezen _et al._, PL. 19

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.

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 felt in Bermuda before instrumental recording was available

to permit location of their epicenters. Two quakes occurred in the

abyssal plain northwest of the Cape Verde Islands, and two were located

near Theta Gap northwest of Cape Finistere. All other earthquakes

of the central part of the ocean basin are associated with the

Mid-Atlantic Ridge or its eastern extension.

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, overlying 3-4 km of oceanic crustal rocks (6.5 km/sec);

beneath this the sub-M mantle rocks show a velocity of about 8.1

km/sec. This pattern has been observed by most workers in the abyssal

floors of other oceans (Raitt, 1957; Hill, 1956).

Officer, Ewing, and Wuenschel (1952) and Katz and Ewing (1955) have

reported on the structure of the Bermuda Rise. The topographic

change from abyssal floor to the Bermuda Rise is accompanied by a

corresponding change in crustal structure (Fig. 35_b_ and _d_).

A typical column measured on the Bermuda Plateau is shown in Figure

35_d_. Here the sub-M velocity appears to be lower, or possibly a new

intermediate-velocity layer is inserted between the oceanic crust

and the true mantle. In the Bermuda Rise seismic velocities in the

oceanic crust differ somewhat from the typical abyssal-floor values.

In the Bermuda Apron and adjacent parts of the Bermuda Plateau (Fig.

35_c_) above the oceanic crust a thick (up to 4 km) section of 4.5

km/sec velocity is found which has been quite reasonably identified as

sedimentary and volcanic rocks.

Seismic-reflection measurements in the smoother parts of the

ocean-basin floor fall into two general groups. Reflection records from

the oceanic rises generally show a succession of reflections which

can be correlated for considerable distances. Reflection records from

the abyssal plains in general show many reflections which are usually

impossible to correlate even between adjacent shots. This difference

has been explained by Ericson, Ewing, and Heezen (1952) in terms of

the distribution of turbidity-current deposits. In the abyssal plains

relatively frequent turbidity flows have deposited an alternating

sequence of clays and silts which return a great number of reflections

to the reflection seismograph. In contrast the rises receive only

pelagic sedimentation, and thus the layering of their sedimentary cover

is simple and widespread since it relates to major changes in pelagic

sedimentation of past geologic ages.

[Illustration: FIGURE 35.--_Crustal sections in various physiographic

provinces, determined by seismic-refraction measurements_

(a). Eastern New York; Katz and Ewing (1955)

(b). Western Sohm Abyssal Plain; Station A 172-28, from Katz and Ewing

(1955)

(c). Bermuda Apron; Station A 172-20 from Katz and Ewing (1955) and

Officer _et al._ (1952)

(d). Bermuda Plateau; Station A 172-19, from Katz and Ewing (1955)

(e). Nares Abyssal Plain; personal communication from J. I. Ewing

(f). Mid-Atlantic Ridge; personal communication from J. I. Ewing]

MID-OCEANIC RIDGE

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.

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 that the ridge is the area enclosed by the 4000-meter

contour (2250 fathoms). Shepard (1948) states that its "average depth

is about 1500 fathoms, but it rises about 1000 fathoms above deeper

zones on either side."

Tolstoy and Ewing (1949) and Tolstoy (1951) in general limit the ridge

to depths of less than 2500 fathoms, although in one part of the text

Tolstoy and Ewing limit it to less than 2240 fathoms, and Tolstoy

(1951) implies that the ridge extends to 2900 fathoms. In the present

paper the Mid-Atlantic Ridge is considered as a morpho-tectonic unit

defined in terms of morphology, and therefore its definition is not

based on a closed isobath.

The Mid-Atlantic Ridge is that portion of the Mid-Oceanic Ridge system

which lies within the limits of the Atlantic Ocean. It consists of a

broad, fractured median arch or swell which occupies approximately

the center third of the ocean. Its crest lies near the median line of

the ocean, and its lateral boundaries are formed by scarps[2] which

lie near the axes of maximum depth of the eastern and western basins.

Adjacent to the Mid-Atlantic Ridge both to the east and to the west is

the abyssal floor (usually abyssal hills) of the ocean-basin floor.

[2] Scarps have been seen on every recorded crossing. If they should

be absent on a future crossing, it is expected that a major change in

gradient will be found which will serve as a consistent definition for

the ridge boundary.

Exaggerated profiles are useful in bringing out the major morphological

characteristics of deep-sea topography. For some purposes, however,

it is desirable to study the topography in profiles with no vertical

exaggeration. Two such profiles are shown in Figures 37-41. The

position of the two profiles is indicated in Figure 36. A very good

idea of the individual slopes can be gained from a study of these

natural-scale profiles, but the province boundaries are very difficult

to identify.

A typical cross profile at 40:1 exaggeration is shown in Figure 42, a

typical oceanic cross section, in which each physiographic province of

the Mid-Atlantic Ridge is labelled.

[Illustration: FIGURE 36.--_Index to natural-scale Mid-Atlantic Ridge

profiles reproduced in Figures 37-41_]

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 crest of the Mid-Atlantic

Ridge consist of (1) the Rift Valley (or Valleys); (2) Rift Mountains;

and (3) High Fractured Plateau (Fig. 43). The Azores Plateau, which

forms part of the crest, presents additional problems and is discussed

separately.

RIFT VALLEY: The most striking feature on an average profile across

the Mid-Atlantic Ridge is a deep notch or cleft in the crest of the

ridge. In a small percentage of the sounding profiles two or three such

valleys are present, and on a few profiles no notable depressions are

observed. On an average profile the floor of the valley lies at about

2000 fathoms, while the adjacent peaks average about 1000 fathoms below

the sea surface. The width of the valley between the crests of the

adjacent peaks ranges between 15 and 30 miles, and the depth of the

valley floor beneath the highest adjacent peak ranges from 700 to 2100

fathoms. The width of the valley 500 fathoms above its floor ranges

from 5 to 22 miles. The range in observed depths of the valley is 1150

to 2850 fathoms in the area of the physiographic diagram. The adjacent

peaks range from 500 to 1300 fathoms within the same area (excluding

the area near the Azores) (Fig. 47).

[Illustration: FIGURE 37.--_Natural-scale Mid-Atlantic Ridge profile

1A_

Slope corrections have been applied to profiles 1 and 2. The method is

described by Elmendorf and Heezen (1957).]

[Illustration: FIGURE 38.--_Natural-scale Mid-Atlantic Ridge profile

1B_]

[Illustration: FIGURE 39.--_Natural-scale Mid-Atlantic Ridge profile

1C_]

[Illustration: FIGURE 40.--_Natural-scale Mid-Atlantic Ridge profile

2A_]

[Illustration: FIGURE 41.--_Natural-scale Mid-Atlantic Ridge profile

2B_]

[Illustration: FIGURE 42.--_Type profile, provinces of the Mid-Atlantic

Ridge_]

[Illustration: FIGURE 43.--_Tracing of PDR record, Rift Valley, Rift

Mountains, High Fractured Plateau, and Upper Step_]

[Illustration: FIGURE 44.--_Tracing of PDR record, Western Rift

Mountains_]

Twenty-six crossings of the Rift Valley are shown in Figure 45. The

profiles can be divided into three groups: (1) single well-developed

rift valley; (2) several well-developed deep valleys; (3) no

particularly deep central valley. Most of the profiles (20) fall into

the first class; the second class is represented by 5, and only 1 falls

in the third class.

The topography of the floor of the rift is rough. In no instance has a

flat floor been observed. Where the valley is widest mountains a few

hundred fathoms high protrude from its floor.

RIFT MOUNTAINS: The steep walls flanking the rift each form one side of

a large rough-sided block. They might be considered as tilted blocks

whose facing slopes form the Rift Valley. The back or outer slope of

the Rift-Mountains Province is generally broken into mountains as much

as 500 fathoms high and 10 miles wide (Fig. 44). The lateral limit of

the Rift-Mountains Province is reached when the average slope of the

sea floor flattens markedly. Because of the high local relief it is

sometimes difficult to pick the boundary of the Rift Mountains, but in

almost all recorded profiles the approximate position of the boundary

is unmistakable.

[Illustration: FIGURE 45.--_Twenty-six rift valley profiles,

Mid-Atlantic Ridge_

Position of profiles shown on Plate 23]

[Illustration: FIGURE 46.--_Five representative profiles, crest and

western flank of Mid-Atlantic Ridge_]

HIGH FRACTURED PLATEAU: The High Fractured Plateau is adjacent to

the Rift Mountains on either side of the ridge (Fig. 42). The local

relief is about 400 fathoms from peak to adjacent valley, and the

distance from peak to peak ranges from 8 to 20 miles. In contrast to

the adjacent flank provinces there are no filled intermontane valleys,

and the valleys are deeper and narrower than in the adjacent Upper Step

Province. Within the limits of the physiographic diagram, the average

depth of the High Fractured Plateau ranges from 1500 to 1900 fathoms.

* * * * *

_Flank provinces._--Between the outer margin of the High Fractured

Plateau provinces and the level of the ocean-basin floor lies a

succession of parallel provinces, known as the Upper Step, the Middle

Step, and the Lower Step. The limits of these provinces are the least

well defined of all the provinces described in this paper. The flanks

of the ridge are characterized by rough topography (Pl. 18). Peaks

of more than 200 fathoms high occur at a frequency of about 15 per

100 miles. Some of the valleys between peaks are smooth, particularly

in the provinces south of the Azores Plateau. The flanks of the

Mid-Atlantic Ridge are broken by scarps which seem to persist for

relatively long distances parallel to the trend of the crest (Pl. 20).

These scarps or scarp zones break the ridge into a succession of units

here called steps (Fig. 46). An alternative solution also seriously

considered by the writers is that the steps might more correctly be

considered as a series of tilted blocks which could be referred to as

ramps. The difference between the two solutions can be appreciated by

inspecting Figures 42 and 46.

The writers must emphasize that the term "terraces" of Tolstoy is in no

sense the same as the term "step" used in this paper. Tolstoy defined

his terraces as "a succession of smooth shelves, each from 1 to 50

miles in width," which occupy a zone "200-300 miles" wide. Features

fitting this definition are called "intermontane basins" in this paper,

following a suggestion made by Heezen et al. (1951).

The location of smooth-floored intermontane basins is shown on Plate

20. They are found only in the area extending about 8° southwest of

the Azores. Small arrows indicate the slope of the smooth floors. In

general all the valleys slope away from the crest of the ridge. Steps,

on the contrary, are a succession of average levels separated by scarps

or scarp zones and in general are not smooth except that a few basins

may be filled. However, this filling is limited to a small area south

of the Azores.

The flanks are divided into three steps: upper, middle, and lower. The

division of anything into three parts is suspicious, whether it be a

geologic period or a physiographic region. Such divisions usually are

later replaced as more information is obtained. This is probably true

of the three steps. We are more confident of the significance of the

boundaries shown on the province chart than of the uniqueness of the

enclosed areas, because each boundary marks a major scarp or scarp

zone. The steps are defined in part in any limited area on the basis of

their mean depth. In general the Upper Step ranges from 1650 to 2300,

the Middle Step from 2250 to 2500, and the Lower Step from 2350 to

2800 fathoms. Just as the maximum depth of the ridge and the axis of

maximum depth of the basin vary with distance along the ridge (or with

latitude), so the steps vary in depth and width (Fig. 47).

[Illustration: FIGURE 47.--_Axial profile of the Mid-Atlantic Ridge_

(a). Distance between province boundaries of the eastern and western

flanks of the Mid-Atlantic Ridge measured along parallels of latitude.

(b). Width of the Rift Valley measured at 250 fathoms and at 500

fathoms above the valley bottom and between the highest peaks of the

Eastern and the Western Rift Mountains. Width in miles measured at

right angles to the trend of the ridge.

(c). The depth below sea level of the Rift Valley, the Rift Mountains

and the western and eastern axes of maximum depth. The depths shown on

this graph are in uncorrected echo-sounding fathoms.]

* * * * *

_Azores Plateau._--The Azores Plateau is an area of 52,000 square

miles of sea floor, surrounding the Azores Islands, where the

depth is less than 1000 fathoms. The Azores Islands are oriented

south-southeast-north-northwest along a topographic trend which strikes

off toward the Straits of Gibraltar. This topographic connection

between the Azores Plateau and the southern Iberian Peninsula has been

called the Azores-Gibraltar Ridge, which the present writers consider

as a poorly developed mid-oceanic ridge of the same general class as

the Mid-Atlantic Ridge. The Azores Plateau itself merges with the Rift

Mountains of the Mid-Atlantic Ridge. The sea-floor topographic trends

of the eastern part of the plateau are parallel to the known tectonic

and volcanic trends of the Azores Islands (Agostinho, 1937). In the

western part of the Azores Plateau trends are north-south parallel to

the main trends of the Mid-Atlantic Ridge. Although both Wüst (1940a)

and Tolstoy (1951) have published contour charts of the Azores Plateau,

it remains largely a mystery whether the trends of the eastern Azores

Plateau cross or join the axial trends of the Mid-Atlantic Ridge. The

Azores Plateau or bulge is generally considered as a highly fractured

tectonic uplift in which vulcanism has played a comparatively small

part (Cloos, 1939).

* * * * *

_Azores-Gibraltar Ridge._--An ill-defined irregular ridge runs from

the eastern end of the Azores Plateau to the Straits of Gibraltar.

Largely on the basis of its seismicity we infer that this ridge is

structurally and topographically similar to the Mid-Atlantic Ridge. The

few existing topographic profiles across this feature suggest that the

earthquake belt is associated with a rift valley of the same general

type as the central Rift Valley of the Mid-Atlantic Ridge. Depths in

this rift appear to reach 2300-2800 fathoms, and the depth of the tops

of the adjoining mountains range from 1600 to 2000 fathoms. The flank

provinces are even less well developed.

* * * * *

_Atlantis-Plato-Cruiser-Great Meteor Seamount Chain._--South of the

Azores a chain of great seamounts branches off from the High Fractured

Plateau and crosses the Upper and Middle steps in a nearly north-south

direction. These seamounts--Atlantis, Plato, Cruiser, and Great

Meteor--in general have broad, nearly flat summits at depths of 100-250

fathoms. The largest one, Great Meteor, was discovered by workers on

the METEOR in 1937. This seamount, 60 miles across at its base, rises

majestically more than 2600 fathoms above the floor of the ocean.

Sands and calcareous rocks have been dredged from the summits, and

Tertiary sediments have been obtained from the flanks of the seamounts.

Photographs of the tops and of the flanks to a depth of 1600 fathoms

show ripple marks. This group is described in a paper by Heezen, Ewing,

Ericson, and Bentley (in press).

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 that virtually all that do not are within about 1° of

the Rift Valley. All seismic activity therefore is limited to the crest

provinces, and probably virtually all the activity is concentrated

within the Rift-Valley Province. A line of epicenters runs from the

Rift Valley near Flores Island of the Azores toward the Straits of

Gibraltar.

* * * * *

_Sediments and physiographic provinces of the Mid-Atlantic

Ridge._--The Mid-Atlantic Ridge is the major site of undisturbed

pelagic sedimentation of the Atlantic because of its isolation from

down-slope movements starting on the continental margin. However,

turbidity currents must form near the shores of oceanic islands and

the edges of shallow banks and probably contribute sediments to the

intermontane valleys. Photographs taken on the Rift Mountains and

on the sides of major seamounts show scour marks and ripple marks,

indicating considerable winnowing and scour by deep-ocean currents (Pl.

19). This sediment carried from the tops of peaks and deposited on the

steep mountain sides probably slumps occasionally and forms turbidity

currents which flow to the adjacent valley floors (Pl. 28). For this

reason cores taken in intermontane valleys and near the higher peaks

will have considerable interlayering of turbidity-current deposits, and

much of the sides and crests of individual high mountains is bare rock.

In the Rift Mountains true pelagic sediments are only occasionally

found. It is striking to note that in coring and bottom photography

bare-rock slopes are found most commonly in the Rift Mountains and High

Fractured Plateau, but flat-floored intermontane basins are absent in

these provinces. This must indicate either that the topography of the

Rift Mountains is very new or that the sediment eroded from the crest

provinces is carried all the way to the Upper Step Province where it is

deposited in the intermontane basins.

* * * * *

_Rocks of the Mid-Atlantic Ridge._--Our knowledge of the lithology of

the Mid-Atlantic Ridge comes from three sources: (1) rocks dredged from

the sea floor, (2) detrital rock fragments found in sediment cores, and

(3) rocks exposed on the islands of the Ridge.

Some of the earliest rock dredging on the Mid-Atlantic Ridge was done

in 1885 by the TALISMAN expedition. In 1949 Furon (1949) reported

the occurrence of fossil trilobites in dredge samples which had been

stored for more than half a century in a French Museum. One dredging

was made in the High Fractured Plateau of the eastern Atlantic at 42°

21´N., 17° 12´W., in 4255 meters depth (2330 fathoms). Furon believes

that the material was _in situ_ and therefore proof of early Paleozoic

outcrops on the Mid-Atlantic Ridge. The abundant evidence of glacially

rafted rocks even as far south as 30° N. casts serious doubt on this

conclusion, but nevertheless the possibility that the material might

have been _in situ_ must be considered.

The Mid-Atlantic Ridge Expedition of 1947 led by Ewing made a number

of successful rock-dredge hauls on the Mid-Atlantic Ridge. The most

successful hauls were made in the Rift Valley and on the adjacent Rift

Mountains at about 30° N. Lat. The specimens have been described by

Shand (1949), who reported olivine gabbro, serpentine, basalt, and

diabase. One limestone of probably Tertiary age was collected the

same year but has not been described. The suite of crystalline rocks

obtained is similar to those found on oceanic islands elsewhere on the

Mid-Oceanic Ridge.

* * * * *

_Crustal structure and the Mid-Atlantic Ridge provinces._--The crustal

structure of the Mid-Atlantic Ridge provinces has been determined

at about 20 places by the seismic-refraction technique (Fig. 35f).

These studies, conducted by John I. Ewing and W. M. Ewing (in press),

have shown that the average crustal structure of the crest provinces

and Upper Step consists of 0.4 km of low-velocity sediment and 2.8

km of rock with a velocity of 5.1 km/sec overlying a substratum in

which the velocity is 7.3 km/sec. The thickness of the layer of

low-velocity sediment varies considerably from place to place. In

the crest provinces the 5.1 km/sec layer is commonly exposed. In the

flank provinces appreciable thicknesses (to 1 km) of sediment have

been measured. The sediment seems to thicken between major scarp

zones and ridges as if the sediment were collecting in longitudinal

basins parallel to the axis of the ridge. An insufficient number of

measurements have been made to determine whether these accumulations

correlate with the boundaries of individual intermontane basins or with

the limits of individual step provinces.

The structure of the abyssal floor, crest provinces, and flank

provinces is compared in Figure 35. The two higher-velocity layers

shown in the abyssal-floor sections (Nares and Sohm Abyssal Plain, Fig.

35), have been observed in all measurements made in these provinces.

The 6.7 km/sec layer is generally considered to be gabbroic, and

the 8.1 km/sec layer is by definition the earth's mantle. In the

Mid-Atlantic Ridge section the upper high-velocity material has

an average velocity of 5.1 km/sec and is generally identified as

basaltic rock. The velocity of the underlying material (7.3 km/sec) is

intermediate between the velocity of oceanic crustal rocks (6.7 km/sec)

and that of mantle rocks (8.1 km/sec), as observed both beneath the

continents and beneath the abyssal floor.

Ewing and Ewing (in press) suggest that this intermediate velocity is

the result of a physical mixture of oceanic crustal rocks and mantle

rocks. To explain such large-scale mixing they propose that extensive

vulcanism and intrusion along the Mid-Atlantic Ridge have produced

an intermingling of the crustal and mantle rocks, and that this was

associated with convection cells in the deep mantle which supply large

quantities of basaltic magma and produce extensional forces on the

crust and upper mantle.

Nearly 20 crossings of the crest of the ridge have been made with the

total-intensity magnetometer towed behind research vessels employing

continuously recording echo sounders. A characteristic anomaly pattern

has been noted by Ewing, Heezen, and Hirshman (1957). The Rift Valley

is characterized by a large positive anomaly, while the adjoining Rift

Mountains show negative anomalies of 300 to 500 gammas (Fig. 48).

Free-air gravity anomalies over the crest provinces and Upper Step are

usually 30-50 mg positive, while the Rift Valley as measured in two

places gave free-air anomalies of -3 and -20 mg.

[Illustration: FIGURE 48.--_Profile of total magnetic intensity and

topography, Mid-Atlantic Ridge_

Soundings made with PDR. Magnetic measurements made with fluxgate

total-intensity magnetometer. Magnetic values in gammas relative to an

arbitrary zero.]

[Illustration: FIGURE 49.--_Physiographic provinces and trans-Atlantic

structure_

Based on scattered seismic-refraction measurements in the North

Atlantic which have been projected along province boundaries. The

topographic profile was pieced together from continuously recorded

echo-sounding profiles from New York to Spanish Sahara.

In the continental margin the upper layer represents the sedimentary

rock. The dashed symbol indicates the continental crustal rocks. The

lower layer represents oceanic crustal rocks. The mantle lies below the

lowest layer.]

A heat-flow measurement by E. C. Bullard in the Rift Valley province in

the North Atlantic indicated a value of about 7 × 10⁻⁶ cal./cm²/sec.

which is about 6 times the average value of 1.2 × 10⁻⁶ cal./cm²/sec

observed in the Lower Step and abyssal floor of the eastern Atlantic

(Bullard, 1954; Bullard _et al._, 1956).

High heat-flow values have also been observed on the Easter Island

Ridge of the Southeast Pacific, suggesting that the entire Mid-Oceanic

Ridge rift system may be so characterized.

An adequate synthesis and explanation of all these converging lines

of evidence has not yet been formulated. However, the correlation of

so many types of geophysical and geological data speaks favorably for

the validity and tectonic significance of the physiographic provinces

described here.

On the basis of the observed correspondence of crustal structure and

physiographic provinces a hypothetical trans-Atlantic structure section

was prepared (Fig. 49). Seismic-refraction measurements were projected

along province boundaries and plotted beneath an echo-sounding profile

from New York to Spanish Sahara. The black splotched areas represent

the 7.3 km/sec layer. This velocity intermediate between 8.1 km/sec

of normal mantle and 6.7 km/sec of normal oceanic crust is considered

(1) a mixture of the two normal layers; (2) a low-velocity part of the

mantle, or (3) a distinct crustal layer characteristic of mid-oceanic

ridges. The structure shown for the continental margin of Africa is

based on analogy with the structure of the continental margin of

northeastern United States. This procedure seems justified by the close

similarity of the continental-margin physiographic provinces of the two

areas.

* * * * *

_Origin of the Mid-Atlantic Ridge._--Of the many theories which have

been proposed for the origin of the Mid-Atlantic Ridge almost all

have been extremely speculative, and none has been based on any very

detailed knowledge of the feature. We are still a long way from having

a comprehensive knowledge of the Ridge. The various theories of origin

and their factual basis have been briefly reviewed by Tolstoy and

Ewing, who conclude that it is impossible to say if the feature is

primarily of folded or faulted origin. In a paper in press Heezen and

Ewing compare in detail the topography and seismicity of the African

rift valleys and the Rift Valley of the Mid-Atlantic Ridge. Their

conclusion is that the two areas are of basically the same structure,

and in fact both form parts of the same continuous structural feature.

Since the African rift valleys seem clearly to be the result of normal

faulting resulting from extension of the crust, Heezen and Ewing

conclude that the topography of the Mid-Atlantic Ridge is largely

the result of normal faulting. Whether the forces are the result of

horizontal extension or vertical uplift remains the most important

unsolved problem in connection with the origin of the continental as

well as the suboceanic rift-valley systems. Hess (1954) has proposed a

mechanism relating suboceanic uplift to expansion due to serpentization

of the upper mantle.

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 necessary to run the recorder without interruption, sending

pings once a second. Since a faulty pinging circuit or some accident

of geometry could conceivably send out two closely spaced pings, the

supposed sub-bottom echoes must be carefully checked to make sure that

they are not both bottom echoes from two closely spaced pings. If two

pings were being sent out the second echo would always symmetrically

underlie the bottom surface. If, however, the two surfaces show local

variations, it can be safely concluded that the deeper one is a true

sub-bottom echo. In order to observe sub-bottom echoes the sea floor

should be reasonably smooth since in rugged relief side echoes and

crossing "highlight" hyperbolas obscure any sub-bottom echoes which

might occur. Sub-bottom echoes in the Gulf of Maine have been well

described by Murray (1947). In local inshore areas prominent sub-bottom

echoes recorded by unmodified or slightly modified standard echo

sounders have been used to map basement rocks (Smith _et al._, 1952).

In the deep sea, sub-bottom echoes or "penetration" are observed most

frequently in the continental rise, oceanic rises, and the far edges of

the abyssal plains. Penetration is rare on the open continental shelf

and on the continental slope. As the depth increases, echoes are more

difficult to obtain, so that records from different depths cannot be

directly compared in reference to ease of penetration. It nevertheless

seems to be true that sub-bottom echoes are rare or absent on PDR

records from the parts of the abyssal plains closest to the continental

margin. Penetration in the continental rise is common but frequently

irregular and intermittent. One of the most persistent and uniform

sub-bottom reflecting horizons observed occurs on the outer ridge east

of the Bahamas (south of 30° N.) (Pl. 6).

Records from the abyssal plain immediately adjacent to the abyssal

hills (Pl. 13 Fig. 4) and from the flat-floored tongues in the abyssal

hills (Pl. 10) reveal some of the deepest and strongest sub-bottom

echoes. Good sub-bottom echoes are common in the Bermuda Plateau.

The sub-bottom reflecting layers frequently crop out, and the overlying

sediments thicken and thin, revealing apparently noticeable variations

in the rate of accumulation of sediments. Outcropping of sub-bottom

layers on the steeper slopes indicate slumping, while the deepening of

the sub-bottom reflecting horizon in valleys indicates a greater rate

of deposition. High-frequency sound is normally strongly attenuated

by transmission through sediments. The observation of sub-bottom

reflections with high-frequency sound pulses (12 kc) indicates (1) that

the surface sediment is uniform and is of low density, and (2) that

a fairly sharp density change occurs beneath this surface layer of

low-density material. In areas such as the outer ridge from 22° to 29°

N. Lat. and the southern Bermuda Rise, it can be safely assumed that

the upper layer consists of deep-sea red clay. Density measurements

on red clay have indicated values of 1.25 to 1.45. The lack of

sub-bottom reflections over the parts of the abyssal plains close to

the continents is attributed to the numerous sand and silt layers found

in the cores which reflect most of the sound. The occurrence of good

reflections beneath the outer edges of the abyssal plains could be

explained by either assuming that for a long geologic time no sand-or

silt-carrying turbidity current has reached this area, or that red clay

is deposited here much faster than elsewhere.

An extremely prominent sub-bottom reflector observed over a vast area

of the east tropical Pacific has been identified by coring with a

10-cm thick bed of white, vitreous ash. This suggests that sub-bottom

reflections found elsewhere may, in general, represent ash horizons.

This, of course, would presuppose ash falls so vast that some record

should have been preserved on land. There is no reason to assume that

there is but a single cause of deep-sea sub-bottom echoes.

The widespread occurrence of the sub-bottom interface on the deeper

isolated rises may be of great importance if it be interpreted as

evidence of a sudden change in sedimentation resulting in a change from

higher- to lower-density sediment. It is just conceivable, however,

that some unstable diagenetic process may cause a sudden increase in

compaction at a depth corresponding to the sub-bottom reflection.

The sub-bottom reflections in depths of 2600 fathoms on the southern

Bermuda Rise and the outer ridge is about .02 second after the bottom

echo, and this indicates a layer about 10 fathoms thick. At a rate of

deposition of 1 cm/1000 years this change in sediment type would have

occurred 20 million years ago.

In a remotely situated oceanic area the factors controlling whether red

clay or _Globigerina_ ooze is laid down are largely related to depth

and temperature of the bottom water. These two factors are related to

those which control the solubility of the carbonate and thus the type

of bottom deposit. Emiliani and Edwards (1953), from a study of oxygen

isotopes in benthic Foraminifera in Tertiary deep-sea sediments from

the eastern Pacific, concluded that the temperature of Pacific bottom

water decreased 8° C. from the Eocene to the present. This should have

caused a great increase in the solution of carbonate assuming other

factors unchanged. Sub-bottom reflections then may also be interpreted

as the result of a change in the temperature or the circulation of

bottom water in the deep basin. Extensive, basin-wide sub-bottom

reflectors, whether the result of vast beds of ash or widespread

changes in pelagic sedimentation, imply events of global importance.

The further investigation and identification of these reflectors should

produce data of far-reaching application in geology, climatology, and

paleo-oceanography.

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 irregularities of the upper continental rise. A close

correspondence of topography and distribution of recent sediments

is apparent. For example, deep-sea sands are found in the submarine

canyons and on the canyon deltas of the lower continental rise. The

continental slope appears to be a thinly veneered or bare outcrop of

Tertiary and Mesozoic sediments. Individual topographic benches can

be traced for many miles along the strike. On the basis of published

descriptions and dating of dredged rock, certain prominent benches are

identified as the outcrop pattern of various Cretaceous and Tertiary

formations. The lower continental rise can be directly traced into

the outer ridge at Cape Hatteras. The upper continental-rise and the

marginal-trench provinces lie between the abrupt continental slope and

the outer ridge. Seismic-refraction measurements in the continental

margin indicate the greatest thickness of sedimentary rocks under the

upper continental rise. Thus if we consider the initial form as an

unfilled depression it would have been remarkably similar to the form

of the present marginal trenches.

The ocean-basin floor lies between the continental margin and the

Mid-Oceanic Ridge and consists of the deeper abyssal floor and

the elevated oceanic rises. On the abyssal floor adjacent to the

continental margins are found the flattest surfaces of the earth. These

abyssal plains apparently were built by turbidity-current deposits. The

unburied abyssal floor is represented by the abyssal hills. The oceanic

rises are broad uplifts which rise from the abyssal floor through a

series of scarps. Oceanic rises are covered with pelagic sediments

except locally near islands and seamounts. The crustal structure of

oceanic rises differs significantly from the typical abyssal floor in

having lower velocities and generally thicker crustal layers.

The Mid-Oceanic Ridge is a broad fractured arch whose axis follows the

median line of the ocean. It generally covers the center third of the

ocean. The ridge provinces are divided into crest provinces and flank

provinces. The crest provinces include (1) the Rift Valley, a long

axial cleft 15-30 miles wide and 500-1500 fathoms deep; (2) the Rift

Mountains which form the sides of the Rift Valley; and (3) the High

Fractured Plateau, a rugged plateau which borders the Rift Mountains.

The flank provinces consist, on each side, of roughly three steps

separated by large scarps. A seismic belt accurately follows the Rift

Valley. The topography of the Mid-Oceanic Ridge seems best explained by

extensive normal faulting. The mid-oceanic Rift Valley connects with

and is probably of the same origin as the African rift valleys.

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H. R., 1957, Geophysical investigations in the eastern

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RAITT, R. W., 1956, Seismic-refraction studies of the Pacific

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RICHARDS, H. J., AND RUHLE, J. L., 1955, Mollusks from a

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SCHOTT, G., 1942, Geographie des Atlantischen Ozeans: C.

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across the Peru-Chile trench: Deep Sea Research, v. 4, p.

238-249

AUTHOR INDEX

Agostinho, J., 98, 109

Alldredge, L. R., 51, 111

Andrade, C. F. de, 5, 21, 37, 109

Armstrong, J. C., 36, 109

Ashton, W., 55, 110

Athearn, W. D., 60, 113

Atwood, W. W., 11, 109

Bartholomew, J. C., 5, 109

Beckmann, W. C., viii, 23, 66, 109, 112

Behm, A., 8

Bentley, C., 98, 109, 111

Berckhemer, H., 79, 112

Bourcart, J., 37, 49, 109

Bucher, W. H., viii

Bullard, E. C., 49, 52, 103, 109

Cloos, H., 98, 109

Cohee, G. V., 53, 112

Crary, A. P., 43, 110

Dahl, W. H., 43, 109

Davidson, M. J., viii, 52, 109

Day, A. A., 49, 109

Deacon, G. E. R., viii

de Smitt, V. P., 67, 109

Dietrich, G., 5, 109

Dietz, R. S., 8, 66, 109

Dorman, J., 66, 70, 74, 110

Drake, C. L., vii, 50, 52, 53, 109, 112

Edwards, G., 106, 110

Edwards, R. S., viii, 110, 112

Elmendorf, C. H., viii, 50, 85, 109

Emery, K. O., 5, 109

Emiliani, C., 106, 110

Ericson, D. B., vii, 21, 27, 33, 35, 47, 53, 55, 56, 57, 59, 60, 66,

70, 74, 75, 80, 95, 98, 110, 111

Ewing, J. I., 36, 81, 100, 110, 112

Ewing, W. M., vii, 21, 23, 25, 27, 33, 35, 36, 43, 47, 49, 51, 52, 55,

56, 57, 59, 60, 66, 70, 74, 75, 76, 78, 80, 83, 84, 95, 98, 100,

103, 109, 110, 111, 113

Fenneman, N. M., 11, 110

Fessenden, R., 8

Fisher, R. L., 60, 110

Florisson, 8

Frassetto, R., 36, 110

Frosch, J., 76, 112

Fuglister, F. C., vii

Furon, R., 99, 110

Gaskell, T. F., 49, 52, 55, 109, 110

Gibson, W. M., 66, 110

Gould, Capt. D., vii

Grousson, R., 39, 110

Gutenberg, B., 80, 110

Hamilton, E. L., 78, 110

Hamilton, G. R., vii

Heezen, B. C., viii, 5, 21, 23, 27, 33, 35, 36, 41, 46, 47, 50, 51,

53, 55, 56, 57, 59, 60, 66, 70, 74, 75, 80, 83, 85, 98, 100, 103,

109, 110, 111, 112

Hersey, J. B., vii, 111

Hess, H. H., 35, 103, 111

Hill, M. N., viii, 37, 49, 52, 80, 109, 111

Hirshman, J., viii, 100, 110

Hjort, J., 55, 56, 57, 83, 112

Hubbard, C., vii

Iselin, C. O'D., vii

Israel, H. G., 8, 111

Johnson, D., 21, 111

Johnson, G. L., III, vii

Johnson, H. R., vii, 110, 112

Jordan, G. F., viii

Karlson, Capt. A., vii

Karo, Admiral A., viii

Katz, S., 47, 80, 81, 111

Kay, M., 54, 111

Keller, F. J., 51, 111

Knott, S. T., 111

Koczy, F. F., 55, 111

Kohler, Capt. H., vii

Kuenen, P. H., 21, 111

Landisman, M., vii, 3, 8, 49, 56, 111

Lane, Capt. A. K., vii

Langevin, 8

Langseth, M., vii

Laughton, A. S., 49, 109

Lee, C. S., 36, 111

Lobeck, A. K., v, 1, 11, 111

Luskin, B., vii, 3, 8, 56, 111

Marie, P., 49, 109

Matthews, D. J., 13, 111

Maury, M. F., iv, 5, 111

Maxwell, A. E., 103, 109

Menard, H. W., 66, 74, 109, 111

Menzies, R. J., vii, 55, 56, 111

Meuschke, J. L., 51, 111

Miller, E. T., vii, 51, 52, 78, 109, 111

Mosby, O., 67, 112

Murray, H. W., 25, 105, 111

Murray, J., 55, 56, 57, 83, 112

Nafe, J. E., viii, 49, 50, 79, 112

Nelson, A. L., Cmdr. R.N. (R.), vii

Newell, N. D., 53, 112

Northrop, J., vii, 36, 46, 66, 70, 74, 76, 110, 112

Officer, C. B., 80, 81, 110, 112

Ovey, C. D., 12, 38, 113

Peterson, J. J., 46, 112

Pettijohn, F. J., 53, 112

Pike, Capt. J., vii

Press, F., viii, 23, 112

Pryor, J. S. N., Cmdr. R.N., viii

Raisz, E., v, 112

Raitt, R. W., 80, 112

Revelle, R. R., 103, 109

Richards, H. J., 112

Richter, C. F., 80, 110

Roberts, A., vii

Ruhle, J. L., 112

Rutherford, H. M., 43, 110

Schott, G., 112

Schüler, F., 112

Shand, S. J., 99, 112

Shepard, F. P., 19, 20, 23, 25, 53, 83, 112

Shurbet, G. L., 36, 52, 76, 112, 113

Simonson, K., vii

Sinclair, V., Captain U.S.N. (Ret.), vii

Small, H., 60, 113

Smith, D., vii

Smith, E. H., 67, 112

Smith, P. A., 5, 21, 25, 113

Smith, W. O., 105, 112

Soule, F. M., 67, 112

Spangler, W. B., 46, 112

Stetson, H. C., 25, 43-46, 50, 53, 112

Stocks, T., 112

Stommel, H., 50, 112

Sutton, G. H., vii, 37, 52, 53, 79, 80, 109,

110, 112

Swain, F. M., 46, 112

Swallow, J. C., 49, 50, 109, 113

Talwani, M., following page 122

Tolstoy, I., vii, 5, 22, 27, 57, 66, 76, 83, 84, 95, 98, 103, 113

Torphy, S. R., 25, 113

Treffenden, J. M., 112

Udintsev, G. B., 60, 113

Upham, W., 43, 113

Upson, J. E., 105, 112

Usher, Capt. F. S., vii

Van Santford, H., vii

Veatch, A. C., 5, 21, 25, 113

Vine, A. C., 43, 110

Washington, H. S., 113

Wirshup, M., vii

Wiseman, J. H. D., 12, 38, 113

Woollard, G. P., 43, 110

Worthington, L. V., 50, 113

Worzel, J. L., vii, 36, 43, 52, 65, 76, 109, 110, 112, 113

Wuenschel, P. C., 80, 81, 112

Wüst, G., 5, 12, 22, 50, 57, 98, 112, 113

Zeigler, J. M., 25, 60, 113

SUBJECT INDEX

Abyssal floor, 1, 15, 55

abyssal gaps, 1, 15, 66

abyssal hills, 1, 15, 55, 61

abyssal plains, 1, 15, 55

mid-ocean canyons, 1, 15, 66

Abyssal-floor topography, origin of, 74

Abyssal gaps

definition, 66

general, 1, 66

Theta Gap, 66, 72, 73, 74

Vema Gap 58, 66, 72, 74

Abyssal hills, 1, 61

definition, 61

distribution of, 61, 63

origin of, 65

PDR records of, 38

sub-bottom echos, 105

Abyssal plains

Balearic Abyssal Plain, 57

Bay of Bengal, abyssal plain in, 55

Biscay Abyssal Plain, 56, 57, 60

Blake-Bahama Abyssal Plain, 56, 57, 58, 59

Canary Abyssal Plain, 56, 61

Cape Verde Abyssal Plain, 57, 61

Cayman (Trench) Abyssal Plain, 56, 60

definition, 61

discovery of, 55

distribution of, 61, 63

general, 1, 15, 53

Hatteras Abyssal Plain, 56, 57, 58

Hispaniola-Caicos Abyssal Plain, 60, 65

Horseshoe Abyssal Plain, 57, 61

Iberia Abyssal Plain, 56, 57, 61

Madeira Abyssal Plain, 56, 61

Nares Abyssal Plain, 56, 57, 59

Newfoundland Abyssal Plain, 56

nomenclature, 55

Old Bahama Abyssal Plain, 60

Puerto Rico (Trench) Abyssal Plain, 56, 60

Sohm Abyssal Plain, 55, 57, 71, 72, 74

Tagus Abyssal Plain, 56, 57

Weddell Sea, abyssal plain in, 55

African rift valleys, 103, 107

ALBATROSS III, Research Vessel, vii

Ampere Seamount, 59, 78

Andros well, 47, 48

Anegada Passage, 36

Antilles Outer Ridge, 32, 33, 36, 52, 53, 59, 60

Army Map Service, 5

Atlantis-Plato-Great Meteor Seamount Group, 98

ATLANTIS Research Vessel, vii, 7, 8, 79

Atlantis Seamount, 98

Axis of maximum depth, North Atlantic Ocean, 41, 63, 96

Azores Plateau, 97

Azores-Gibraltar Ridge, 98

Bahamas, 33-36, 47

Banquereau Bank, 24

Basalt, 75, 79, 99

"Basins and deeps", 11

Basins

Cape Verde, 12

Guiana, 12

Iberia, 12

Labrador, 12

Nares, 56

Newfoundland, 12, 56

North America, 12

North Canary, 12

South Canary, 12

West Europe, 12

Bay of Biscay, 17, 56, 60

Tertiary outcrops, 49

Beaches, ancient submerged, 41, 42

Bell Telephone Laboratories, vii

Benches, 1

Bahamas, 34, 35, 49

Bay of Biscay, 49

Blake Plateau, 33, 47-49

Cape Hatteras, 46-48

Georges Bank, 43, 46

Gibraltar, 37, 38

Puerto Rico, 36

Bermuda Apron, 75, 77, 81

Bermuda Pedestal, 75, 77

Bermuda Plateau, 75, 81

Bermuda Rise, 74-77

Bermuda Apron, 75, 77, 81

Bermuda Pedestal, 75, 77

Bermuda Plateau, 75, 81

Bermuda Scarp Zone, 76, 79

Crescent Peaks, 75

crustal structure, 80, 81, 102

Muir Seamount Group, 76

PDR records of, 79

sediments on, 76

Bermuda Scarp Zone, 76, 79

Bioclastic debris, 59

Biscay Abyssal Plain, 39, 56, 57, 60, 65

Blake-Bahama Abyssal Plain, 32, 33, 34, 56, 57, 58, 59

Blake-Bahama Basin, 33, 34

abyssal plain in, 59

sediments in, 59

Blake Escarpment, 17, 19, 47

benches on, 32, 47

outcrops on, 47

PDR record of, 34

seismic investigations, 47, 48

Blake Plateau, 17, 32, 47, 48, 59

description of, 32-33

PDR record of, 35, 38

Browns Bank, 24

Cable failures, submarine, 23, 67

Cable ships, 79

Campeche Escarpment, 33

Canary Abyssal Plain, 56

Cape Breton Submarine Canyon, 37

Cape Hatteras, 25, 46, 47, 48, 50

bottom currents, 50

Esso No. 1 Test, 46

geologic section at, 46, 48

Cape Verde Abyssal Plain, 57, 61

Cape Verde Basin, 12

Carte Générale Bathymétrique des Océans, 5, 11, 12

CARYN, Research Vessel, vii, 78

Caryn Seamount, 28, 72, 78

Cayman Trench (Abyssal) Plain, 56, 60

Classification of deep-sea relief

bathymetric system, 11, 12, 19

textural system, 11

Coast and Geodetic Survey, viii, 25, 43, 44, 45,48

Columbia University, vii, 3, 6, 8, 21

Compaction of sediments, 50, 106

Continent and ocean, 16, 107

Continental margin

benches and terraces, 38, 41-51, 107

categories of, 15, 17

definition, 17

magnetic anomalies, 51

past, present, and future, 53

photographs of, 39, 50

regional description of, 21-41

seismicity of, 51

Continental margin, development of, 53

youth, maturity, and old age, 53

geosynclines, 53, 54

Continental margin Europe and Africa, description of, 36-41

Anglo-French sector, 37

Gibraltar sector, 37-38

Iberian sector, 37-38

North African sector, 38-41

Continental margin North America, description of, 21-36

Anegada Passage, 36

Antilles Outer Ridge, 33

Bahamas sector, 33

Blake Escarpment, 33

Blake Plateau, 32

Flemish Cap, 21

Gulf of Maine, 24-25

Laurentian Channel, 23

Northeastern United States sector, 25

Northern Grand Banks sector, 21

Puerto Rico sector, 36

Scotian Shelf sector, 23

Southeast Newfoundland Ridge, 22

southern Grand Banks sector, 22

Continental margin, provinces

category I, 1, 17, 53, 107

category II, 1, 18, 53, 107

category III, 1, 19, 53, 107

Continental rise, 1, 19, 20, 25, 26, 27, 37, 38, 40, 41

definition, 20

PDR records of, 32

tables of characteristics, 27, 41

Continental shelf, 15-28, 32-42, 107

definition, 18

structure of, 49, 51-54

submerged beaches, 42

Continental slope, 1, 15-48, 107

benches on, 41, 42-51

currents on, 50

definition, 18

geologic map of, 51

photographs of, 39

profiles of, 28, 29

Convection currents, 103

Coral, photograph of, 39

Corner Rise, 63, 64, 77

Corner Seamount, 77

Cretaceous outcrops

Bahamas, 47

Blake Escarpment, 47

Caryn Seamount, 78

Georges Bank, 43

Cruiser Seamount, 98

Crustal convection currents, 103

Crustal structure

continental margin, 52, 53, 102, 107

Mid-Atlantic Ridge, 100, 102, 107

ocean-basin floor, 80, 81, 102, 107

trans-Atlantic structure section, 102

Currents, deep-sea bottom

photographed evidence of, 39, 59, 79

scour, 50, 51, 59, 79

velocities of, 50

Deeps

Nares, 55

Sohm, 55

Deep-sea channels, 66

Deep-sea sands, 28, 35, 36, 53, 58, 59, 60, 61, 65, 74, 75, 80, 99,

106, 107

Diabase, 99

DISCOVERY II, Royal Research Ship, vii

Easter Island Ridge, 103

Echo sounders

early sounding machines, 8

invention of, 8

Lamont-Facsimile PDR, 8, 9

NMC, 7, 8, 22, 23, 24, 31, 68

UQN-1B, 7, 8, 69

Echo soundings

accuracy of, 3, 9

sources of, 3, 5, 6, 7

units employed, 12

"Echo-time" depth, nominal fathoms, 12

Emerald Bank, 24

Eocene outcrops, 46, 47, 48, 76

Epicontinental seas, 1, 15, 18

Erosion, submarine, 23, 39, 50, 51, 74, 79, 80, 99, 105, 106, 107

Esso Hatteras Light Test, 43, 46, 47

Eugeosyncline, 54

Exaggerated profiles, plotting of, 5

Exuma Sound, 35, 49

Faulting, 42, 51, 76, 103, 107

Fecal pellets, photographs of, 39, 79

Flemish Cap, 21

Foraminifera, displaced, 60

French Hydrographic Service, 38

Georges Bank, 25, 43-45

Georges Bank canyons

dredging, 44-45

identification of benches, 44

outcrops on, 43-44

Geosynclines, 53-54, 107

Gibraltar, Straits of, 38

GLACIER, U.S.S., 55

_Globigerina_ ooze, 33, 53, 106

Gradients, conversion tables, 12, 15

Gran Canary Island, profile near, 39

Grand Banks earthquake, 22

Grand Banks sector, 21

Gravel, 59, 79

Gravity anomalies, 2, 65, 100

Graywacke suite, 53

Great Meteor Bank, 98

Guiana Basin, 12

Gulf of Maine, 17, 24, 25

sub-bottom echoes, 105

Gulf of Mexico escarpments, 18, 33

Gulf of St. Lawrence, 17

"Gully," the, 24

Guyots, 78

_Halimeda_, 60

Hatteras Abyssal Plain, 56, 57, 58

Heat flow, measurement of, 103

High Fractured Plateau, 1, 15, 90, 91, 94, 95, 99

Hispaniola-Caicos Abyssal Plain, 56, 60

Hispaniola-Caicos Channel, 36

Holothurians, photographs of, 39

Horseshoe Abyssal Plain, 57, 61

Horseshoe Seamount Group, 59, 78

Hudson Submarine Canyon, 8, 27-28, 31, 58, 72

Hudson Submarine Channel, 8

Hudson Submarine Delta, 28, 78

Hydrographic Department, British Admiralty, vii, 3, 6

Hydrographer Canyon, 43

Iberia Abyssal Plain, 56, 60

Iberia Basin, 12

Intermontane basins, 95, 99

International Hydrographic Bureau (Monaco), vii, 3, 5, 11, 12

Josephine Seamount, 78

KEVIN MORAN, Tug, 70

Kelvin Seamount Group, 72, 77, 78

Labrador Basin, 12

Lahave Bank, 24

Lamont Geological Observatory, 3, 6, 8, 21

Landward slopes of trenches, 1, 19, 36

Laurentian Channel, 23

Lisbon Submarine Canyon, 37

Lower Continental Rise Hills, 56

Lower Step, Mid-Atlantic Ridge, 1, 15, 90, 91, 94, 95, 96

Madeira Abyssal Plain, 56, 58, 61

Madeira Island, profile near, 28, 29

Madeira Rise, 58, 61

Magnetic anomalies

continental margin, 51, 52

Mid-Atlantic Ridge, 51, 100, 101

ocean-basin floor, 51, 65, 72

seamounts, 51

Magnetic surveys

airborne, 51

ship-towed, 51

Manganese nodules, 39, 78

Mantle, 80, 102

Marginal basin-outer ridge complex, 20, 36

Marginal escarpments

Bay of Biscay, 19, 37

Blake, 19, 32, 35

Gulf of Mexico, 19, 33

New Zealand, 19

Marginal plateaus, 18, 32, 37

Blake Plateau, 17, 32, 47, 48, 59

Western Europe, 37

Marginal trenches, 20, 36, 53, 107

filled, 52-54

Pacific, 19, 60

Puerto Rico Trench, 17, 36, 52, 56, 60

Marginal trench-outer ridge complex, 1, 20, 36, 53

Matawan (Cretaceous) formation, 44, 45, 46

Mercator projection, 3

METEOR, Research Vessel, 83, 98

Mid-Atlantic Ridge

axial profile, 96

crest provinces, 1, 84

flank provinces, 1, 95

heat flow, 103

High Fractured Plateau, 1, 90, 94, 95

limits of, 83

Lower Step, 1, 90, 94, 95

Middle Step, 1, 90, 94, 95

ocean-bottom photographs, 78, 99

origin of, 103

rocks of, 99

Rift Mountains, 1, 90, 92, 94

Rift Valley, 1, 84, 90, 91, 92, 93, 94

sediments on, 99

seismicity of, 98, 99

structure of, 81, 100, 102, 107

trilobites from, 99

type profile, 90

Upper Step, 1, 90, 94, 95

Mid-Atlantic Ridge Expeditions (1947-48), 99

Middle Step, 1, 15, 94, 95, 96

Mid-ocean canyons

Bay of Bengal, 66

Bay of Biscay, 60

deep-sea channels, 66

definition, 66

distribution of, 66-72

equatorial Atlantic, 66

Gulf of Alaska, 66

Mid-Ocean Canyon No. 2., 71-72

Northwest Atlantic Mid-Ocean Canyon, 22, 66-72

Mid-Oceanic Ridge, 1, 15, 83-104, 107

crest provinces, 1, 15, 83

definition, 83

flank provinces, 1, 15, 95

Milne Seamount, 79

Morpho-tectonic provinces, 2

MUIR, U.S.S., 76

Nares Abyssal Plain, 56, 57, 59, 74, 81

National Geographic Society, vii, 99

Navarro (Cretaceous) formation, 44, 45, 46

Navidad Bank, 34, 36

Navigational control, accuracy of, 5

Nazaré Submarine Canyon, 37

Newfoundland Abyssal Plain, 56, 58

Newfoundland Basin, 12, 56, 67, 68, 69, 70, 71

Nomenclature of deep-sea relief 11, 12, 15

North America Basin, 12

North Atlantic Ocean

major basins of, 12

map of earthquakes in, facing page, 122

map of sediment distribution, facing page, 122

soundings, 5

structure section, 102

North Canary Basin, 12

Northeast Providence Channel, 35, 36

Northwest Atlantic Mid-Ocean Canyon, 22, 67, 68, 69, 70, 71

Ocean-basin floor, 1, 15, 25, 55-82, 102, 107

abyssal floor, 1, 15, 55-74

crustal structure, 80-81, 102

oceanic rises, 1, 15, 55, 74-78

seamounts of, 78-79

seismicity of, 80

Oceanic crust, 80-81, 100, 102

Oceanic rises

Bermuda Rise, 74-77

Corner Rise, 64, 77

definition, 74

distribution, 74

Rio Grande Rise, 74

Rockall Rise, 78

Oceanographer Canyon, 43

Old Bahama Channel, 36

Olivine gabbro, 99

Orthogeosyncline, 54

Orthoquartzite, 53

Outer ridges

Antilles Outer Ridge, 32, 33, 36, 52, 53, 59, 60

definition, 20

sediments on, 53

Southeast Newfoundland Ridge, 22, 63, 67, 70, 71, 72

Paleozoic outcrops, 99

Pelagic deposits, 53, 81, 99, 107

Photographs, sea floor, 39, 50, 59, 79

Physiographic diagram, preparation of, 3, 4, 5

Physiographic province chart, Atlantic Ocean, 3, 16

Plant debris, 60

Plato Seamount, 98

Pleistocene beaches, submerged, 42

Portuguese submarine canyons

Lisbon, 37

Nazaré, 37

Setúbal, 37

Precision Depth Recorder (PDR)

accuracy of, 8

development of, 8

operation of, 9

positions of PDR sounding tracks, 6

Precision Depth Recorder (PDR) records, 28, 32, 34, 36, 38, 58, 64,

73, 78, 91, 92

Puerto Rico Trench, 36

Puerto Rico (Trench) Abyssal Plain, 17, 36, 52, 56, 60

Recent sediments, erosion of, 50

Red clay, 33, 76, 106

Reflections, sub-bottom, 25, 32, 41, 105-106

Research vessels

ALBATROSS III, vii

ATLANTIS, vii, 7, 8, 79

CARYN, vii, 78

DISCOVERY II, vii

KEVIN MORAN, 70

METEOR, 83, 98

TALISMAN, 99

THETA, vii, 2, 7

VEMA, vii, 6, 72

Rift Mountains, 1, 15, 84, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99,

100, 107

Rift Valley, 1, 15, 84, 90, 91, 93, 94, 95, 96, 98, 99, 100, 101, 102,

103, 107

Rio Grande Rise, 74

Ripple Marks, 39, 50, 51, 59, 79

Rock dredging, 39, 42, 43, 45, 46, 47, 76

Rockall Rise, 78

Sable Island, 24

Sable Island Bank, 24

Sand, deep-sea, 28, 60, 61, 74

Scarps, 61

Abyssal Plain, 61

Bermuda Rise, 76

Mid-Atlantic Ridge, 83

Scotian Shelf, 23, 24

Scour marks, photographs of, 39, 59

Seamounts

Ampere, 78

Atlantis, 98

Caryn, 78

Cruiser, 98

Great Meteor, 98

Josephine, 78

Kelvin, 77, 78

Milne, 79

Muir, 76

Plato, 98

Seamount Groups

Atlantis-Plato-Cruiser-Great Meteor Seamount Group, 98

Horseshoe Seamount Group, 59, 78

Kelvin Seamount Group, 76, 77, 78

Muir Seamount Group, 76

Seamounts, sedimentary processes on, 59, 77

Sediment distribution

abyssal plain, 74

Bermuda Rise, 76

continental margin, 53

correlation with topography, 107

map of North Atlantic, facing page, 122

Mid-Atlantic Ridge, 99

Seismicity

continental margin, 51

epicenter map, North Atlantic, facing page, 122

Mid-Atlantic Ridge, 98

ocean-basin floor, 80

Seismic-reflection measurements, 80

Seismic-refraction measurements

continental margin, 52, 81, 102

Mid-Atlantic Ridge, 81, 100, 102

ocean-basin floor, 65, 80, 81, 102

Serpentine, 99

Serpentization, 103

Setúbal Submarine Canyon, 37

Sharks tooth, photograph of, facing page, 39

Shelf break, 17, 50, 51

Sierra Leone Rise, 61

Slumping, 50, 51, 99, 105

Sohm Abyssal Plain, 55, 56, 57, 71, 72, 74

Sound velocity corrections, 12, 13

Sounding corrections

slope, 15

velocity, 12, 13

Sounding tracks

nonprecision, 7

PDR, 6

Sounding velocity, 12

Soundings

echo, 8, 9

hemp line, 8

wire, 8

South Atlantic, Physiographic Diagram, v

South Canary Basin, 12

Southeast Newfoundland Ridge, 22, 63, 67, 70, 71, 72

Sub-bottom echoes, 25, 32, 41, 105-106

Submarine canyons, 20

Cape Breton, 37

Fosse de Cayar, 41

Gilbert, 44

Grand Banks, near, 23

Hudson, 8, 27, 28, 30, 31, 58, 72

Hydrographer, 45, 46

Lisbon, 37

Lydonia, 44

Nazaré, 37

Northeast Providence, 35

Oceanographer, 44, 45, 46

Setúbal, 37

Starfish, photograph of, facing page, 39

Structural benches, 43, 48, 49

Submarine topography, outline of, 15

Sunken islands, 78

Tagus Abyssal Plain, 56, 57, 61

TALISMAN, Research Vessel, 99

Tectonic map, Atlantic Ocean, 2

Tectonic uplift, 77, 78, 98

"Terraced zone", 95

Terraces, continental shelf, 42

Tertiary ocean-bottom temperatures, 106

Tertiary outcrops, 46, 47, 48, 76, 79, 107

THETA, M/V, (sealer), vii, 72

Theta Gap, 66, 72, 73, 74

Times Facsimile Company

Precision Depth Recorder, 8, 9

Tracks and trails, photographs of, facing pages, 39, 79

Trench (Abyssal) Plains

Cayman, 56, 60

Kurile-Kamchatka, 60

Middle America, 60

Peru-Chile, 60

Puerto Rico, 17, 36, 52, 56, 60

Trilobites, Mid-Atlantic Ridge, 99

Turbidity currents, 23, 50, 51, 74, 79, 80, 99, 106, 107

Unconformities, 50

U. S. Coast Survey, 8

U. S. Navy

Bureau of Ships, vii

Hydrographic Office, 5

Office of Naval Research, vii

Units: depth, slope, distance, 12

Upper Step, 15, 78, 90, 91, 94, 95, 96

Vema Gap, 58, 60, 66, 72, 74

VEMA, Research Vessel, vii, 6, 72

Virgin Islands Bank, 36

Volcanic cones, buried, 51

Volcanic detritus, 53

Volcanoes, 39, 77, 78

West Europe Basin, 12

West Florida Escarpment, 33

Woods Hole Oceanographic Institution, vii, viii, 3, 7

Yorktown (Miocene) formation, 44, 45, 46

[Illustration: Heezen _et al._, PL. 20

PHYSIOGRAPHIC PROVINCES, ATLANTIC OCEAN</a>]

[Illustration: Heezen _et al._, PL. 21

CONTROL CHART

All continuously recorded echo-sounding profiles used in preparation

of Plates 1 and 20 are shown. Solid lines indicate Lamont Geological

Observatory and Woods Hole Oceanographic Institution cruises. Dashed

lines indicate soundings supplied by British Admiralty. Lines shown

by alternate long dash and two short dashes indicate closely spaced

discrete soundings of METEOR.]

[Illustration: Heezen _et al._, PL. 22

SIX TRANS-ATLANTIC TOPOGRAPHIC PROFILES

Soundings in fathoms continuously recorded by an NMC echo sounder on

the R. V. ATLANTIS. The letters a&mdash;q indicate where soundings from

different cruises were joined.]

[Illustration: Heezen _et al._, PL. 23

INDEX CHART SHOWING LOCATIONS OF PROFILES OF PLATES 24, 25, 26 AND

FIGURE 45]

[Illustration: Heezen _et al._, PL. 24

THIRTY-FOUR PROFILES OF THE CONTINENTAL MARGIN: WESTERN NORTH ATLANTIC

Profile locations shown on Plate 23. Note small topographic details.

Soundings spaced approximately 1 mile apart.]

[Illustration: Heezen _et al._, PL. 25

TWENTY-THREE PROFILES OF THE CONTINENTAL MARGIN OF EUROPE AND AFRICA]

[Illustration: Heezen _et al._, PL. 26

CRUSTAL STRUCTURE AND CONTINENTAL-MARGIN PROVINCES FOUR REPRESENTATIVE

PROFILES, POSITIONS OF PROFILES SHOWN ON PLATE 23.

Data for W-6 from Bentley and Worzel (1956), W-10 and W-11 from Katz

and Ewing (1955), and W-33 from Ewing and Worzel (1954) and Officer _et

al._ (1957). Seismic data projected to continuously recorded sounding

profiles. Vertical exaggeration 40:1.]

[Illustration: Heezen _et al._, PL. 27

FIVE TOPOGRAPHIC PROFILES, WESTERN NORTH ATLANTIC

Soundings continuously recorded on NMC sounder.]

[Illustration: Heezen _et al._, PL. 28

DISTRIBUTION OF DEEP-SEA SANDS IN RELATION TO PHYSIOGRAPHIC PROVINCES

Data from Ericson _et al._ (1952; 1955) based on piston cores taken by

Lamont Geological Observatory expeditions.]

[Illustration: Heezen _et al._, PL. 29

EARTHQUAKE EPICENTERS, NORTH ATLANTIC

Date from Gutenberg and Richter (1954) and the epicenter cards of the

U.S. Coast and Geodetic Survey through 1956.]

[Illustration: Heezen _et al._, PL. 30

LOCATION OF PDR RECORDS AND BOTTOM PHOTOGRAPHS REPRODUCED AS

ILLUSTRATIONS.]

Transcriber's Notes:

The large diagram for plate 1 mentioned in the note on p. v is

missing. It was not physically attached to the book per note.

Silently corrected simple spelling, grammar, and typographical

errors.

Retained anachronistic and non-standard spellings as printed.

Enclosed italics markup in _underscores_.