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-Ocean Ridges

The mid-ocean ridge is an interconnected system of undersea volcanoes that meander over the Earth like the raised seams on a baseball. It is a continuous 40,000-mile (60,000-kilometer) seam that encircles Earth and bisects its oceans. The mid-ocean ridge represents an area where, in accordance with plate tectonic theory, lithospheric plates (also called tectonic plates) move apart and new crust is created by magma (molten rock) pushing up from the mantle . The mid-ocean ridge system is an example of a divergent (rather than a convergent or transform) plate boundary.

The mid-ocean ridge system has been understood only since the development and acceptance of plate tectonic theory in the 1960s. Four major scientific developments spurred the formation of the theory: (1) demonstration of the young age of the ocean floor; (2) confirmation of repeated reversals of Earth's magnetic field in the geologic past; (3) emergence of the seafloor-spreading hypothesis and associated recycling of the oceanic crust; and (4) precise documentation that Earth's earthquake and volcanic activity is concentrated along subduction zones and mid-ocean ridges.

Ridge Characteristics

Mid-ocean ridges have different shapes (morphology) depending on how fast they are spreading, how active they are magmatically and volcanically, and how much tectonic stretching and faulting is taking place. Scientists believe that the most likely reason for the different morphologies is due to the strength of the ocean crust at these different sites, and how cold and brittle the upper part of the tectonic plate is.

Ridge Types.

There are two types of mid-ocean ridges: fast-spreading and slow-spreading. Fast-spreading ridges like the northern and southern East Pacific Rise have smoother topography at the ridge crest, and look somewhat like domes. They have relief of 100 to 200 meters (328 to 656 feet). The East Pacific Rise moves at an average of 15 centimeters (5.9 inches) per year.

Slow-spreading ridges like the Mid-Atlantic Ridge have large, wide, rift valleys, sometimes as wide as 10 to 20 kilometers (6 to 12 miles) and very rugged terrain at the ridge crest that can have relief of up to 3.2 kilometers (2 miles). The Mid-Atlantic Ridge moves at an average of 2.5 centimeters (1 inch) per year.

Fast-spreading ridges are "hotter," meaning that more magma is present beneath the ridge axis, and that more volcanic eruptions occur. Because the plate under the ridge crest is hotter, scientists think that the plate responds to the divergent spreading process more fluidly, and that the ridge behaves like hot taffy being pulled apart. In this scenario, the ridge crest does not have a chance to subside (sink or settle).

At slower spreading ridges, the seafloor behaves more like a cold chocolate bar—when pulled, it cracks and breaks to form ridges and valleys. As the sheets of oceanic crust move away from the mid-ocean ridge, the rock is cooled and thus becomes heavier. After about 200 million years, the cooled lithospheric plate has become heavier than the asthenosphere that it rides over, and it sinks, thereby producing a subduction zone.

Fracture Zones.

Mid-ocean ridges do not form straight lines but are instead offset in many places by fracture zones, or transform faults. Fracture zones are thought to occur due to zones of weakness in the pre-existing continent before it was rifted apart. Most mid-ocean ridges are divided into hundreds of segments by fracture zones. Along the Mid-Atlantic Ridge, fracture zones occur at an average interval of 55 kilometers (34 miles). As the Mid-Atlantic Ridge is some 16,000 kilometers (10,000 miles) long, it is divided by fracture zones into about 300 distinct segments. The ridge crest and its associated faults are the locus of nearly all shallow earthquakes occurring in mid-ocean areas.

Water and Minerals.

Ocean water is constantly percolating through fissures (cracks) at the mid-ocean ridge. Downward-convecting cold ocean water meets the hot new crust far below the surface, and many types of metals such as sulfur, copper, zinc, gold, and iron are transferred to the water. This hot, mineral-laden water gushes back up through the cracks, forming hydrothermal vents. As the hot water, which can reach temperatures of 371°C (700°F), escapes from the vents and comes in contact with the near-freezing water of the ocean bottom, the metals quickly precipitate out of solution. The results are surging black clouds of particle-rich water called black smokers, which often erupt out of tall chimneys of previously deposited solidified mineral.^* Because so much metal is spewed out, hydrothermal vents have been responsible for many of the world's richest ore deposits. These unique features also are found to harbor a diverse array of deep-ocean life.

see also Hot Springs on the Ocean Floor; Life in Extreme Water Environments; Mineral Resources from the Ocean; Ocean-Floor Bathymetry; Plate Tectonics; Volcanoes, Submarine.

Larry Gilman

and K. Lee Lerner

Bibliography

Coulomb, J. Seafloor Spreading and Continental Drift. Dordrecht, Netherlands: D. Reidel Publishing Co., 1972.

Nicolas, A. The Mid-Oceanic Ridges. Berlin, Germany: Springer Verlag, 1995.

Thurman, Harold, and Elizabeth Burton. Introductory Oceanography, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

^* See "Hot Springs on the Ocean Floor" for a photograph of a black smoker.

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 ridge see plate tectonics .