In our Previous article, we discussed a few major theories proposed at different times regarding the distribution of Oceans and Continents. Continuing the discussion we will learn about Seafloor Spreading and Mid-Oceanic Ridges.
Seafloor spreading
In 1960, American geophysicist Harry H. Hess proposed the seafloor spreading hypothesis.
According to this theory, the ocean floor is moving or spreading away from ridges, due to repetitive magma intrusion that split and spread the older sea floor, causing them to be moved away from the ridges in a nearly horizontal position.
Cold sea water cools the magma, by creating a new crust. The upward movement and eventual cooling of magma have created high ridges on the ocean floor over millions of years.
Seafloor spreading also indicates, that since the new floor forms constantly at the ridge crust and moves sideways, there is a progressive increase in age away from the ridge crest. Accordingly, there is a lack of pelagic sediments at the ridge crust while it is progressively thicker on the older sea floor as it moves.
Mid-Oceanic Ridges (MOR):
Mid Oceanic Ridges (MOR), are submarine relief features. Generally, they submerge beneath the ocean water, however, local crowning above the level of oceanic water gives rise to islands such as Iceland, Mauritius, Lakshadweep, etc.
MORs are divergent-type plate boundaries, the sites of active seismicity. Shallow-focus earthquakes occur at MOR, whereas deep-focus earthquakes are generally associated with island arcs/ subduction zones (convergent plant boundaries).
The rift valley of MOR crust is formed when the rising mantle rock/lava splits and diverges sideways, as a result, tensional cracks open, where shallow focus earthquakes occur, consequently the rift valley fills up with basaltic lava and creates new crust.
Intense magmatism of low potassium-olivine lava types is associated with MOR, which results in the formation of basic rocks such as olivine-gabbro, serpentine, basalt diabase, etc.
Geophysical studies on MOR show that, the basaltic roots are present about 30 km deep into the lower mantle for a height of the ridge of 1.6 km above the ocean bottom. This deep rooting ensures isostatic balance.
MOR is characterized by high gravity anomaly, whereas the paleomagnetic studies on MOR reveal that, the normally and reversely magnetized rocks on one side of the ridge were the mirror image of those on the other side (which is also called bilaterally symmetrical arrangement of magnetized rocks).
So far the best known MOR is the Mid-Atlantic Ridge; Carlsberg Ridge in the Indian Ocean; Lomonosov Ridge in the Arctic Ocean and the Pacific-Antarctic Ridge which lies between Antarctica and New Zealand and Australia.
Deep Sea Trenches
Deep Sea Trenches are long narrow basins that extend 8-11 km below sea level. Trenches develop adjacent to subduction zones, where the oceanic lithosphere slides back into the mantle.
Strong negative gravity anomalies over the deep-sea trenches are thought to be due to some active force that must be pulling down the bottom of the trenches. According to the hypothesis of Sea-floor spreading, this force is the subduction of the sea floor caused by the cooling and shrinking of mantle rock.
Relatively low heat flow values, found in the oceanic trenches indicate that the rocks beneath the trenches are cooler than normal.
The trenches are the places of earthquake foci and associated volcanic activity. Most of the world's greatest seismic activity occurs in the trenches and the adjacent island arcs.