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Figure 2.5 Temperature distribution and general shape of a section through the Pacific Plate (after Parsons and Sclater, 1977).

than 0.25° and is, therefore, barely perceptible. Only near the crest of the spreading-ridge is the average dip of the ocean floor likely to exceed a few degrees.

A generalised section across any spreading-ridge will show the simple form of Figure 2.6. However, in a limited and detailed profile, which extends for only 200-500 km across the ridge, the features are seen to be of two types. Either they form a reasonably well-defined peak, comparable with that shown in Figure 2.6a, or else define a central graben (Figure 2.6b and c). The profiles shown in these figures greatly exaggerate the relief of the features.

The East Pacific Rise is typical of the single-peak type of section, while the Mid-Atlantic Ridge is representative of the central, or axial, graben type. By inspection of a suitable map showing more detailed section of these ridges and their spreading rate, in various parts of the world, it can be inferred that those ridges with a central graben are associated with a relatively slow spreading rate (<5.0 cm a-1), while the single-peak ridges occur where the spreading rate is relatively fast (i.e. >5.0 cm a-1).

The morphology of the simple section, exhibited by the East Pacific Ridge, extends throughout the length of this spreading-ridge. The Atlantic-type ridge is much less uniform. Although grabens are a common feature, they are not of uniform size. Along the Mid-Atlantic Ridge, for example, the floor of the graben is as much as 1.5 km below its rim; it is often between 10-40 km in width and devoid of sediments. Such dimensions are comparable with some of the major terrestrial grabens.

The detail of the morphology of the submarine graben floor is similar to parts of Iceland, with minor fault scarps which run approximately parallel to the trend of the ridge, though in Iceland, because of the infilling by sediments, the depth of the grabens is small compared with those in the Atlantic Ridge proper. Some of the fractures are the loci of fissure eruptions, or may determine the location of a range of small volcanic cones.

Sections through the Atlantic and the Carlsbad Ridges reveal that, unlike the Rhine Graben, the floor of the graben is usually inclined rather than flat. However, the flat floor of the Rhine Graben can be attributed to erosional and depositional effects of the Rhine. Such an influence is, of course, absent in oceanic spreading-ridges.

In addition, oceanic lithosphere is cut by a large number of vertical planes, or zones of shear displacement, which sometimes off-set spreading-ridges by tens or hundreds of kilometres and which usually trend at high angles to these ridges, and, indeed, tend to trend in the direction of plate motion. These features which are

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