The sea-floor is geologically very young, not more than 160 million years old only 1/30th of the age of the earth. The hypothesis of ‘sea-floor spreading’ was first formulated by the Late Professor Harry Hess of Princeton University in 1960.
Hess postulated that mid-ocean ridges are situated over the rising limbs of convection currents in the Earth’s mantle and that the thin oceanic crust is nothing more than a surface expression of the mantle and continuously created by a process of lateral accretion or spreading away from ridge crests.
From consideration of the earlier ideas regarding the age of the initiation of drift in the Atlantic area, Hess suggested that the sea-floor might be spreading at a rate of approximately 1 cm per year per ridge flank.
Hess suggested that the trench systems in the pacific are the sites of the descending limbs of the mantle-wide convection currents. He postulated that in these areas oceanic crust is thrust down into and largely resorbed by the mantle.
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Thus Hess suggested that despite their great age and apparent permanency, continents have been, and are being passively drifted apart and together on the backs of mantle-wide convection currents. In contrast, the ocean-floors are young and ephemeral features of the Earth’s surface, constantly being regenerated at ridge crests and. destroyed in the trench systems.
Supporting evidences:
(i) The occurrence of earth-quakes along the crest of the mid-oceanic ridge system, the dearth of sediments at ridge crests and the active volcanic islands associated with the crest of the Mid-Atlantic ridge are all readily explained by Hess’ model.
(ii) Moreover the ocean basins as a whole contain a remarkable thin veneer of sediments and small number of sea-mounts, if recent rates of accumulation and formation are extrapolated over the whole of geologic time.
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(iii) In 1960, Hess calculated that South America and Africa had both moved 2500 km from the mid-Atlantic ridge during an interval which he thought was 250 million years. This gives the rate of separation as 10 mm/year. Rates like 10 mm/year are appreciable in human terms.
(iv) Hess was also able to state that no material greater than about 100 million years in age had ever been recorded from the deep ocean floor or truly oceanic islands.
(v) Hess argued that the anomalous high values of heat reflect the emplacement of hot mantle-derived material in the vicinity of ridge crest.
(vi) Marine-geophysical studies have also revealed that ridge- crests are characterised sometimes by anomalously low seismic wave velocities in the upper-mantle. This was attributed to thermal expansion and micro-fracturing associated with the upwelling mantle, both effects producing a reduction in the seismic wave velocities and density of the mantle-material.
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(vii) Palaeomagnetic studies have also revealed that new sea- floor forms at and spreads laterally away from ridge crest.
These evidences have been accumulated to show that the crust is spreading apart along the rift. As this sea-floor spreading occurs basaltic lava rises from beneath the rift, solidifying and forming new oceanic crust.