mid-ocean ridge

mid-ocean ridge A long, linear, elevated, volcanic structure often lying along the middle of the ocean floor. Such ridges tend to occupy central positions because the oceans have formed by the symmetrical spreading of two lithospheric plates from the ridge sites. Ocean ridges occur in all the Earth's oceans, but may be offset from a central position, e.g. the E. Pacific ridge, where one side of the oceanic crust is being consumed along a subduction zone. At mid-ocean ridges, ocean floor is being formed. At the centre there is a rift valley, formed as discrete segments, bordered by high mountains on both sides. At a fast-spreading ridge (opening at up to 15 cm a year) the crust is smoother, with flat lavas flowing from fissures, than at a slow-spreading ridge (about 2 cm a year), where the median valley contains a chain of small volcanoes linked by fissure eruptions.

mid-ocean ridges The longest linear, uplifted features of the Earth's surface are to be found in the oceans. They are giant submarine mountain chains with a total length of more than 60 000 km, are between 1000 and 4000 km wide, and have crests that rise 2–3 km above the surrounding ocean basins, which are 5 km deep. The average depth of water over their crests is thus about 2500 m.

These features are the mid-ocean ridges, famous now not only for their spectacular topography, but because it was with them, in the early 1960s, that the theory of ocean-floor spreading, the precursor of plate tectonic theory, began, and we now know that it is at these mid-ocean ridges that new lithosphere is created.

Similar ridges occur at the margins of oceans; the East Pacific Rise is an example. There are other spreading ridges behind the volcanic arcs of subduction zones. These are usually termed back-arc spreading centres.

The first ridge to be discovered, the Mid-Atlantic Ridge, was found during attempts to lay a submarine cable across the Atlantic in the mid-nineteenth century.

Although many of the earlier supporters of continental drift (Arthur Holmes, for example) had suggested that the mid-oceanic ‘swells’ and volcanic islands such as Iceland were the result of ascending and laterally spreading convection currents, it was not until the early 1960s that R. S. Dietz and H. H. Hess proposed the mechanism of ‘sea-floor spreading’ to explain continental drift. The theory suggested that continents moved in response to the growth of oceanic crust between them. Oceanic crust is thus created from the mantle at the crest of the mid-ocean ridge system, a volcanic submarine rise. In the Atlantic, this rise sits in the middle of the ocean between continents, which are symmetrically distributed on either side. The margins of these continents had long been thought by proponents of continental drift to have once been joined together.

Confirmation of the hypothesis of sea-floor spreading came with the discovery by F. J. Vine and D. H. Matthews that magnetic anomalies across the Mid-Atlantic Ridge were symmetrical on either side of the ridge axis. The only acceptable explanation for these magnetic anomalies was in terms of sea-floor spreading and the creation of new oceanic crust. When lava cools on the sea floor, magnetic grains in the rock acquire the direction of the Earth's magnetic field at the time of cooling. Studies of lava flows on land show that from time to time the Earth's magnetic field has reversed its direction (that is, the north and south magnetic poles have changed places). The upper oceanic crust thus shows alternate normal and reversed magnetization.

The anomalies found across the Mid-Atlantic Ridge could, moreover, be matched with similar anomalies that had been discovered in Iceland and other parts of the world where young volcanic rocks could be dated. Given that the distance of each anomaly from the present rifted ridge crest could be measured, and there was now a time for its creation, the rate of ocean floor spreading could be calculated. Over the next decade, as magnetic anomalies were detected in all the oceans and a magnetic stratigraphy was formulated, a picture of oceanic ages throughout the world was gradually built up.

The reason why the ridges are elevated above the ocean floor is that they consist of rock that is hotter and less dense than the older, colder plate. Hot mantle material wells up beneath the ridges to fill the gap created by the separating plates; as this material rises it is decompressed and undergoes partial melting. Geophysical data indicate that melting can occur at depths of more than 100 km and over a broad region perhaps several hundreds of kilometres across. The molten material from the mantle migrates upwards along boundaries between crystals or along channels to accumulate in magma chambers beneath the ridge. Magma will occasionally escape from these chambers to intrude the rocks above as dykes, and may erupt at the ocean floor to form volcanoes and lava flows. The settling of crystals in the magma chambers can result in the formation of gabbros and other cumulate rocks.

At the ridge crest the lithosphere may consist only of oceanic crust, which is basaltic in composition. As it moves away from the spreading centre, it cools and contracts and the peridotitic (olivine-rich) mantle component grows rapidly. The contraction leads to an increase in water depth. A simple relationship thus exists between depth and age, and, at least for the first 60 Ma (60 × 10⁶ years), the mean depth of the oceanic crust is proportional to the square root of its age. After that time the ocean floor deepens less quickly, probably because the lithosphere begins to receive a significant amount of heat from below.

Over the past 30 years a more detailed picture has gradually been built up of the internal nature of mid-ocean ridges and their composition in terms of rock-types. This was achieved initially by comparison with the supposedly equivalent ophiolite complexes exposed on land, especially in the Troodos Mountains of Cyprus and in Oman (see ophiolite sequences). Later, the models were confirmed by a combination of deep-sea drilling on the ridges (down to about 2000 m) and by the physical examination by submersibles of exposed escarpment faces at fracture zones and collection of rock samples at 4500 m depth, where even peridotites are exposed at the surface.

Spreading rates (that is, half the rate of separation from the axis) are not the same throughout the mid-ocean ridge system but vary considerably from a few millimetres per year in the Gulf of Aden to 1 cm year⁻¹ in the North Atlantic near Iceland and 6 cm year⁻¹ for the East Pacific Rise. This variation in spreading rates appears to influence the ridge topography. Slow-spreading ridges, such as Mid-Atlantic Ridge, have a pronounced rift down the centre, producing an axial valley at the ridge crest and a high relief across the ridge. Fast-spreading ridges, such as the East Pacific Rise, lack the central rift and have a smooth topography. In addition, spreading rates have not remained constant through time. There appear to have been periods in the past when they were either faster (perhaps 18 cm year⁻¹) or slower. If this is so, there may have been at least two consequences. If the Earth did not expand, then, at times of rapid spreading, loss of lithosphere by subduction must also have increased; secondly, the increase or decrease in the total volume of the mid-ocean ridge system would have had a major effect on global sea level. For example, it appears that the increase in volume of ridges in the late Cretaceous resulting from an increase in spreading rates led to worldwide transgression at that time, followed by regression during the early Tertiary when rates of spreading were reduced.

The prime reason for the differences in spreading rates is that the slow-spreading ridges are fed by small and discontinuous magma chambers, thereby allowing for some magma differentiation and eruption of a comparatively wide range of basalt types. Thus the ridge crest consists of numerous very small hills. Fast-spreading ridges have large, continuous magma chambers that generate comparatively homogeneous magmas. Because of the higher rates of magma discharge, sheet lavas are more common.

The differences are also apparent in the nature of the sediment cover. The fractured topography of slow-spreading ridges yields highly localized and lenticular sediment patterns, and these rest on a variety of basic and ultrabasic rocks (Fig. 5). Talus breccias (accumulations of coarse angular material) abound, derived from the exposed fault scarps, a 50–85 m thick breccia having been found within an axial/ median rift, itself 2 km deep. Slightly further away, the still-unfilled rift valleys receive a variety of fine-grained sediments, including volcaniclastic sediments derived from the break up of volcanic rocks. Pelagic sediments form on the highs and these may be swept off them to accumulate in the lows as calcareous turbidites, partly filling the valleys with several hundreds of metres of sediment. Eventually, as the ridge sinks to the depths of a typical basin plain, if this is within the reach of land-derived terrigenous sediment, the rugged topography is progressively covered up and smoothed so that it becomes an abyssal plain (see ocean basins).

Fast-spreading ridges, with their subdued topography, lack the axial rift, and volcaniclastic sediments are rare. Instead they are covered by patches of brown sediments rich in iron, manganese, and a host of other metals. Passing away from the ridge crest, carbonate ooze at first accumulates, but as the ridge deepens and passes below the CCD (carbonate compensation depth) the pelagic sediments change to siliceous oozes and pelagic clay.

Although mid-ocean ridges appear at first sight to be continuous features within the oceans, on closer inspection this is clearly not so. They are all broken into segments by transverse fractures that displace the ridges by tens or even hundreds of kilometres. Fractures are narrow, linear features that are marked by near-vertical fault planes. Along these exposed fault scarps, 500 m or more high, not only can the nature of oceanic crust be examined, but complex patterns of fault breccias, talus, and lavas can be discerned. In addition, because the fault lines are subject to complex stress patterns, suffering both elements of extension and compression, they have many of the features of transtension and transpression that can be seen in strike-slip zones on land. Small, very deep, elongate basins also develop.

These transverse fractures show a spectrum from primary global lineaments that form plate boundaries, along which ocean-floor spreading patterns were initiated, to secondary transform faults that were an inevitable consequence of ocean-floor spreading.

Although the term ‘transform’ is frequently used for what should be called fracture zones or strike-slip faults (e. g. continental transforms), in its true, limited sense, as defined by J. Tuzo Wilson in 1965, it refers to those portions of the fracture zone that are active and lie between the offset ridge crests. Away from the ridge crests, on either side, there is no movement between the blocks.

Undoubtedly the most spectacular discovery in recent years has been the presence of extremely hot springs resulting from extensive hydrothermal activity within the ridges, and the realization that these quite extreme environments can yield unique ecosystems consisting of prolific indigenous faunas of crabs, giant clams, and tube worms. These ecosystems are not only quite unlike any other systems known in the world, but each is unique, all clearly evolving independently.

That the waters and sediments atop ridges are hot and sulphurous has been known for a long time. Temperatures of hydrothermal mineral-rich solutions are as high as 350 °C and pH is as low as 4 (acid) on the East Pacific Rise. The solutions are precipitated as a range of columnar structures, called chimneys, built of various sulphide and sulphate minerals. Two basic types of chimney exist: the high-temperature ‘black smokers’ growing typically at about 8 cm a day, and belching clouds of finely disseminated pyrrhotite, sphalerite, and pyrite as hydrothermal plumes, which can be detected many kilometres from the vent. The cooler vents (up to 300 °C) are the ‘white smokers’, which emit particulate amorphous silica, barite, and pyrite.

Black smokers have a characteristic concentric zonation. White smokers are chimneys in which hydrothermal precipitation has sealed the orifices and decreased the permeability of the chimney walls, thus allowing more intra-chimney cooling by sea-water entrainment and heat conduction. These less active chimneys sustain the most prolific growth of tube worms, which may become so densely packed that they form a spherical mass of white tubes called ‘snowballs’. Ultimately chimneys become mechanically unstable and collapse to form a mound of chimney talus on which chimney growth begins again. Sea-floor sulphides are chemically unstable in modern sea water, and if hydrothermal activity ceases they oxidize rapidly to form ochreous deposits dominated by hydrated iron oxides, some reacting with silica to form iron-rich smectites.

The hydrothermal fluids that emanate from mid-ocean ridges are sea water that has circulated through the oceanic crust within a convection cell and leached metals from the rocks along its flow path. The metal content of the fluids thus reflects the trace-element composition of the subjacent source rocks which, in mid-ocean ridges, are basaltic. The metals are copper-rich, with zinc as well, and contrast with spreading sites in back-arc basins with their bimodal volcanism of both acid and basic material which are relatively lead-rich (see island arcs).

Harold G. Reading and and N. C. Mitchell

Bibliography

Nicholas, A. (1995) The mid-oceanic ridges: mountains below sea level. Springer-Verlag, Berlin.
Kearey, P. and and Vine, F. J. (1996) Global tectonics. Blackwell Scientific Publications, Oxford.
Kennett, J. P. (1982) Marine geology. Prentice-Hall, Englewood Cliffs, N. J.

Mid-Atlantic Ridge The oceanic ridge which separates the North and South American Plates from the African and Eurasian Plates. It is a slow-spreading ridge, with rugged topography and a well-developed median valley.