Climate Change Effects: An Ocean at risk

Beneath the Ocean Floor

By: Staff Reporter
Magazine Articles Oceans

From the late 1930s, new techniques have opened up in submarine geology. Gravity measurements and geotectonic imagery has allowed accurate mapping of the sea surface and the bottom structure. The ocean floor is marked by huge mountain ranges – the mid-oceanic ridges that form part of a global network, extending for more than 80,000 kilometres (Fig. 1). In places such as Iceland, Ascension and the Galapagos Islands – the ridges rise above sea level. The ocean floor is also cut by deep trenches which marks subduction zones and are punctuated by isolated seamounts. One of the key pieces of information came from paleomagnetic studies along the Mid Atlantic Ridge. It was found that only half the rocks on each side of the ridge-axis near Iceland showed normal magnetic polarity; the remainder had a reversed polarity (a magnetic needle would point south). The pattern of normal and reversed polarity was manifested in a magnetic striping of the oceanic crust, mirrored on each side of the ridge crest. The alternating pattern of normal and reversed polarity rocks is produced as successive belts of lava are extruded at the site of a divergent plate margin. At the mid oceanic ridges and associated rift zones, new sea floor is generated which is then carried away from the ridge axis by lateral mantle motions. When individual stripes were dated, it was found that the rocks became older with increasing distance from the crest. In other words, the sea floor was spreading apart.

The colour red indicates emergent plate margins under the oceans Age of Oceanic Lithosphere (m.y.) Data source: Muller, R.D., M. Sdrolias, C. Gaina, W.R. Roest 2008. Age, spreading rates and spreading symmetry of the world’s ocean crust, Geochem. Geophys. Geosyst., 9, Q04006  doi:10.1029/2007GCoo1743

 

The discovery of what mid oceanic ridge systems represented, the sites of crust formation or constructive plate margins was a major breakthrough in earth sciences. Basaltic volcanism, upwelling of magma consisting mainly of basalt characterises oceanic ridges. Convective movements within the mantle force the overlying lithosphere to move apart, allowing hot magma to reach the sea floor. At ridge crests, a zone of rifting separates regions of sea floor which are moving apart at 2-15 centimetres per year. Because the oceanic crust cannot withstand sufficient stress to allow for variations in spreading rate and changes in convection pattern, oceanic ridges consist of straight sections offset by transform faults along which different sections of a plate slide past each other. This phenomenon is known to be caused by convection currents in the plastic, very weak upper mantle, or asthenosphere.

The molten magma comes out along the fractures along the ridge and moves away from both sides of the ridges. The sea floor (60-100 km thick lithosphere) moves at a slow speed, say at 2-5 cm/yr like a conveyor belt over the molten viscous asthenosphere. During this slow movement the oceanic lithosphere gets cooled over millions of years and eventually gets subducted along oceanic trenches at the continental margins. The cool oceanic lithosphere again gets melted as it goes down into the asthenosphere through the subduction process and comes out as molten magma through the volcanic (island) arc and through the mid-oceanic ridges/fractures under convection dynamics of the Earth’s interior. Such spreading characterises all oceanic ridges where lithospheric plate divergence occurs. During the past 80 million years, the Atlantic has spread at a rate of 2 centimeters per year.

Among several spreading ridges, the Mid Atlantic Ocean ridge, the east Pacific Ocean ridge and the mid Indian ocean ridge are very large and distinct on the sea floor map. These ridges are 20,000 – 40,000 km long, few thousand km wide and rise from a depth of about 5 km to 2.6 km. The crests of the ridges are offset across faults and fracture zones. All spreading ridges are identified with earthquakes due to dominant tensional stress by pull apart of the moving plates.  In the Indian Ocean, the Mid Indian Ridge connects the southwest and the southeast Indian Ridge, and the Carlsberg Ridge, found to the north, connects the Red Sea. The other spreading ridge in the Indian Ocean region is the Andaman Sea Ridge to the east. The spreading ridges, however, should not be confused with the aseismic ridges like that of the Ninety-east Ridge or Chagos Laccadive Ridge in the Indian Ocean as these are of different origin and are not spreading sites.

Seafloor Spreading

The first stage in plate separation is the initiation of a new pattern of convection within the Earth’s mantle (Fig. 2), which brings hot mantle material to high levels inside the Earth. The elevated temperature and buoyant effect of the rising plume arches up the oceanic crust, causing it to extend. As the plates continue to diverge, further fracturing of the thinned oceanic crust occurs.

The new oceanic crust cools and moves away on either side of the spreading axis. As it cools, it becomes denser and subsides, gradually generating the low lying ocean floor, which becomes imprinted with magnetic anomalies as polarity reversals take place. The seafloor lavas acquire a veneer of marine sedimentary rocks, produced by marine organisms. The margins of the ocean are marked by normally faulted continental edges, partly due to subsidence of the oceanic crust. Magnetic anomalies in ocean floor lavas reveal polarity reversals. Because the cooled crust is brittle, the convection pattern is accommodated by transform faults which offset the ridge axis.

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