A few years back, the bottom of the ocean was commonly believed to be an extensive flat, featureless plain dotted here and there by isolated volcanoes. But detailed oceanographic work has established the existence of somewhat a linear and comparatively narrow zone that stands at higher level than the ocean-bottoms.
These submarine relief features are elongated and fairly continuous. They generally remain submerged beneath the oceanic water and tend to occupy medial positions in each of the main ocean basins. These are commonly known as ‘Mid-oceanic Ridges’ and they provide evidences of global-tectonics.
By far the largest and the best known ridges is the Mid-Atlantic Ridge, which extends from near Iceland to a point in the South-Atlantic southwest of the cape of Good Hope. The oceanic ridge system stretches nearly 80,000 Kilometres around the world.
Apart from the Mid- Atlantic Ridge, the other well-known ridges are the Carlsberg Ridge in the Indian Ocean, the Lomonosov Ridge in the Arctic Ocean and the Pacific-Antarctic Ridge which lies between Antarctica and New Zealand and Australia.
In the Indian ocean the ridge is about halfway between Africa and India and extends upto the Red Sea; in the Pacific Ocean the ridge runs along the southern margin of the ocean basin to form a complex pattern off South America that runs northerly into the Gulf of California.
General Characteristics :
1. Mid-ocean ridges are typical submarine relief features, They are, in general, submerged beneath oceanic water, however, local crowning above the level of oceanic water gives rise to islands such as Iceland, Mauritius, Laccadive etc.
2. The ridges are fairly continuous with a sinuous pattern of distribution. They are centrally or latero-centrally placed within the oceans.
3. The width of the mid-oceanic ridges fluctuates within the range of 2000 to 3000 kms. The height above the adjacent basins is 2-3 kms and more.
Hydrographic work in the North- Atlantic (Tolstoy and Ewing, 1949, Tolstoy 1951) has thrown light upon the topography of the Mid-Atlantic Ridge. Their works indicated that the ridge consists of three rather distinct types of topography:
(i) There is a high central zone, the Main-Range, which consists of several parallel ridges extending in a general north east-southwest direction. This is also known as ‘Crestal Province’ and is characterised by block-faulting.
The topogra phy usually consists of a central-rift valley, lateral block-moun tains and steep scrap surfaces that bound the rift-valley and inwardly slope into it.
There is a deep valley along the axis of the mid-oceanic ridge, which is a narrow graben bounded by faults. The width of the graben is of the order of 15 kms and its depth below sea-level is around 2500 metres.
This valley dissects the ridge in a meridional direction for almost its whole length. Such valleys are called rifts and are considered’ characteristic elements of mid-oceanic ridges.
(ii) On the flanks of the Main Range, there is a series of flats, which were designated as the Terraced Zone. Individual terraces vary in width from 2 to 80 kms and collectively are 350 to 450 kms in width.
(ii) A third zone lies between the terraced zone and the seafioor plain. It is mountainous and rather distinct from the other two zones. It has individual peaks and is designated as the foot-hills of the Mid-Atlantic Ridge.
4. The mid-oceanic ridges are dissected into separate segments by the so-called transformed deep-fractures, perpendicular to the line of their strike. These faults have caused huge horizontal displacement of the crestal provinces along them.
Such faults are called transform-faults in the concept of global-plate tectonics, since surface area is neither created nor destroyed along such a fault.
5. Mid-oceanic ridges are characterised by an abnormally high value of terrestrial heat flows. In general the heat flow values are in conformity with the high thermal activity attested by volcanic eruptions at the crestal province.
Many active volcanoes are located along ridges. But, the heat flow pattern remains largely unknown.
6. Mid-oceanic ridges are the sites of active seismicity. The earthquakes that occur beneath ridges originate at shallow depth.
7. Mid-oceanic ridges are characterized by a high degree of per meability, expressed by intensive magmatism, which display abundant basaltic volcanism of a low-potassium, olivine- deficient type of lava.
Approximately two-thirds of the annual lava eruption on the earth’s surface is along the oceanic ridges.
8. So far as the lithologic composition of the mid-oceanic ridge is concerned, the rocks dredged, cored and exposed along the oceanic-ridges are basic igneous rocks and volcanic rocks most of which are basaltic in composition.
The samples include such rocks as olivine-gabbro, serpentine, basalt and diabase.
Local acidic volcanics represent the magmatic differentiates of basaltic magma.
Comparatively a thin sedimentary veneer may be found present on the crestal mounts and plateaus.
9. Cross-sectional pictures of the mid-oceanic ridges obtained through geo-physical study shows the presence of basaltic root, about 30 kms deep into the lower mantle for a height of the ridge of the order of 1.6 kms above the ocean botttom. This deep rooting ensures isostatic balance.
10. They are characterized by positive gravity anomaly. Also palaeomagnetic studies on the mid-oceanic ridges reveal that there exists a bilaterally symmetrical arrangement of the magnetic anomalies along the ridge.
In other words, the normally and reversely magnetized rocks on one side of the ridge were the mirror image of those on the other side.
11. According to the theories of Plate-tectonics, the mid-oceanic ridges mark the divergent-type of plate boundaries.
Several theories have been proposed to account for the mid-oceanic ridges. According to these theories-
(i) it is a horst;
(ii) it is an anticlinal fold due to lateral compression;
(iii) it is the bottom of the rift which opened as Gondwanaland began to fragment (continental-drift);
(iv) it is produced largely be extrusion of volcanic rocks along the linear openings on the sea-floor;
(v) it is believed to be an orogenic mountain belt similar to those on land;
(vi) it is an outcome of the convection current mechanism. Much convection with in the upper-mantle owing to differential temperature was believed by Holmes.
This hypothesis explains the existence of the ridge in two possible ways:
(a) The ridge may be pushed up from below by the rising mantle rock, or
(b) The expansion of the hot mantle-rock may cause the bulge of the ridge to form as a result of the increased volume of the mantle rock.
The high heat flow associated with the ridge crest is plausible, if hot mande rock is rising beneath it. The active vclcanism at the ridge- crest is due to the upward rising of very hot but still solid mantle rock beneath the mid-oceanic ridge, moving from a region of high pressure to a region of low pressure.
The drop in pressure lowers the melting point of the molten rock, so that some of the hot-mantle rock melts without the addition of any new heat. The melted mantle rock gives rise to magma near the ridge crest and the magma erupts as basaltic lava.
The rift-valley of the mid-oceanic ridge crest is formed when the rising mantle-rock splits and diverges sideways. As tensional crack opens, there occurs shallow-focus earthquakes. These assumptions very nicely fit into the structural frame-work of mid-oceanic ridges. But the geo-dynamic picture envisaged in this concept is entirely hypothetical.
All the theories advanced, as already explained, to account for the origin of the mid-oceanic ridges, have been discarded due to their failure in explaining the features associated with the ridges viz. form and alignment of ridges, central rift zone, offset fractures, rock- types comprising the ridges, seismicity of ridges, infra-structure etc. The concept of ocean-floor spreading seems to offer a more logical explanation regarding the formation of mid-oceanic ridges.