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.
Figure 36.—Index to natural-scale Mid-Atlantic Ridge profiles reproduced in Figures 37-41