Mid-ocean spreading ridges

Oceanic constructive margins are characterised by a central submarine spreading ridge where new oceanic crust is injected into the central rift. Oceanic constructive margins represent the next evolutionary stage on from continental rift systems such as the East African Rift Valley and Red Sea area.

The Mid-Atlantic Ridge

The opening of the Atlantic ocean was initiated about 180 million years ago in the Jurassic period during the fragmentation of the super-continent known as Pangaea, and the separation of the American plates from the African and Eurasian plates. The early phase of this plate separation was similar to the situation in the East African Rift Valley today. The gradual enlargement of the ocean basin since Jurassic times can be traced in the pattern of ages of ocean crust preserved in the ocean basins. New crust is continually being created at the spreading ridge, and gradually moves away from the ridge as spreading continues as if on a conveyor belt. This creates a pattern of symmetrical age bands on either side of the spreading ridge, as shown in the diagram below. A similar pattern is seen in the Pacific.

In the Atlantic ocean, the oldest ocean crust rocks are preserved at the continental margins adjacent to the continental shelves. However, in the Pacific, the oldest continental crust is being destroyed at the subduction zones at the margins of the ocean:

Magnetism in the oceanic crust

The age bands preserved in the oceanic crust can be matched with the patterns of magnetic striping. The Earth possesses a magnetic field, which acts rather like a bar magnet within the Earth. This magnetic field possesses north and south magnetic poles, which are located approximately at the geographic poles of rotation. When magma is injected along the mid ocean ridge and cools, the iron-bearing minerals within the basalt align themselves according to the Earth’s magnetic field, so they preserve a record of the orientation of the Earth’s magnetic field at the time of crystallisation. At present, the new oceanic crust formed at spreading ridges preserve the current orientation of the Earth’s magnetic field - this is called normal polarity. However, periodically, the Earth’s magnetic field switches orientation, so that the magnetic north pole changes to the position of the geographic south pole, and vice versa. This is termed reverse polarity. Rocks which form during periods of reverse polarity preserve this magnetic field orientation in the alignment of the iron-bearing minerals. Periodic magnetic reversals therefore give rise to a pattern of magnetic 'stripes' of normal and reverse polarity in the ocean crust which run parallel to the spreading line. These stripes can be correlated with the age bands of the oceanic crust, and the banding pattern is symmetrical either side of the spreading line. The irregularity in the width of the magnetic bands is due to the changes in rate of spreading. You should also note that the pattern of reversals is irregular in terms of the length of time between reversal events. The changing of the Earth’s magnetic field is thought to be due to changes in the convection patterns of material in the liquid outer core where the magnetic field is generated.

Mid-ocean ridge topography

The rate of seafloor spreading and the formation of new oceanic crust varies over time, giving rise to different widths of age bands and magnetic stripes. At times, an excess of magma is injected into the spreading line, and this overflows onto the seafloor to build up a large submarine ridge with a series of smaller, sub-parallel ridges. An example of this is the Mid Atlantic Ridge. The ridge forms a very prominent feature near the spreading line, where it develops on the crust domed up by the rising convection currents in the underlying mantle. As the newly-formed crust is carried away from the spreading line, the doming is reduced as the underlying mantle is cooler. In places, the magma outpouring and doming can be so extensive that volcanic islands can be built up, such as Iceland which is currently located directly on the Mid Atlantic Ridge. As these volcanic islands are carried away from the spreading line by plate movement they subside, become volcanically extinct and sometimes become submerged to form guyots. Numerous examples of these extinct volcanic islands and guyots can be seen in the Atlantic basin - e.g., the Azores, Madeira, Cape Verde and the Canary Islands. These islands were formed along the spreading line and were carried away by plate motion so that the age of the islands increases with distance from the spreading zone.

Above: Map showing the seafloor around Iceland. The Mid Atlantic Ridge passes through the middle of Iceland, which is a volcanic island built up from lava extruded through the spreading ridge. Near Iceland, the ridge is known as the Reykjanes Ridge.

Above: 3D image showing the seafloor off NW Africa. Madeira, the Canary Islands and numerous seamounts can be seen protruding from the seafloor. Click on image to enlarge.

Magma generation

The rocks making up the ocean floor, mid ocean ridge and volcanic islands are mainly basaltic in composition. The magma is produced by partial melting of the mantle peridotite through mantle decompression below the spreading ridge. The magma collects in a large chamber below the central spreading line, and is gradually injected as dykes into the spreading zone to form sheets of a rock known as dolerite (medium grained gabbro).
Above the layer of sheeted dolerite dykes, the lava cools very rapidly on contact with seawater and a layer of fine grained pillow lavas form. Pillow lavas are created by hot magma oozing out into cold seawater and cooling very quickly into globule-shaped rocks - imagine the blobs of oil in a hot lava lamp!! This layered sequence is characteristic of the ocean crust.

Above: Structure of the oceanic crust, as seen in cross-section.

Above: Formation of pillow lavas.
Hot lava spills out from the central ridge like globules of oil.
Contact with the cold sea water freezes the lava,
preserving the globule shape.

Find out more about plate tectonics:

Plate tectonics make the world go round: introduction
Constructive margins: Continental (rift valleys)
Oceanic (mid-ocean ridges)
Destructive margins: Continental collision
Ocean-continent destructive margins
Ocean-ocean destructive margins (island arcs)
Conservative margins
Continental drift
Plume and hotspots

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February 2007