What is the Formulate Hypothesis of Sea-floor Spreading of Prof. Harry Hess

Hess postulated that the major structures of the sea-floor are the surface expression of the convection processes in the mantle.

He states that the mid-oceanic ridges are situated over the ascending limbs of convection currents in the earth’s mantle, and the oceanic trenches over the descending limbs of the convection current.

According to this concept, the sea-floor moves like a conveyor belt away from the crest of the mid-oceanic ridges across the deep ocean basin and then by plunging beneath a continent or island arc get disappeared, since these are the areas where the oceanic crust is largely resorbed by the mantle.

Hess also suggested that the rate of movement of the sea-floor is approxi­mately 1-2 cm per year per ridge flank and that the continents, despite their great age and permanency, have been and are being passively carried on the convecting mantle.

Hess has attempted to explain his ideas about the sea-floor spread­ing through a number of geological facts as mentioned below-

1. The Mid-Oceanic Ridge

The Mid-Atlantic ridge has thoroughly been studied for this purpose. Along the axial part of the ridge a large depression is stretched. It is bound by deep fractures called a rift valley.

The mid-oceanic ridges are connected with intensive earthquakes, volcanism, tectonic movements of various nature and a high heat flow value.

The formation of the ridge has been thought to be due to convection-either by the rising mantle rock or by expansion of the hot mantle.

(a) The occurrence of shallow-focus earthquakes along the crest of the mid-oceanic ridge system is readily explained by Hess’s model.

The rift-valley at the crest of the mid-oceanic ridge is formed as a large tensional crack, when the rising mantle-rock splits and moves in opposite direction on either side of the ridge crest.

As the tensional crack opens, earthquakes occur and the rift-valley tends to be filled up by the basaltic lava erupted at the ridge crest. The eruption of basaltic lavas create new oceanic crust.

(b) Hess argued that the anomalously high heat values, associated with the ridge crest is possible because of the rising of the hot mantle-derived material beneath it.

(c) The occurence of active volcanism at the ridge crest has also been explained by Hess. According to him, there is a drop in the pressure on the hot, solid mantle rock while they move upward beneath the mid-oceanic ridge.

The decrease in the pressure lowers the melting point of the mantle rock and thus some of the hot mantle-rock undergo melting without any new addition of heat. Thus basaltic magma is formed near the ridge crest from the melted mantle rock and erupts causing volcanism.

(d) The sea-floor has turned out to be geologically very young, not more than 160 million years old which is only 1/30th of the age of the earth; whereas the ancient rock exposed on the land are sometimes three billion years old.

This was thought to be due to the generation of new sea-floor at the mid-oceanic ridges that displaces the older sea-floor which finally disappears by subduction down into the trench i.e. at the sites of converging convection cells.

(e) If the recent rates of accumulation and formation of pelagic sediments are extrapolated over the whole of geologic time, it will be seen that the ocean basin as a whole contain a very thin veneer of sediments and quite small number of sea-mounts.

Sea-floor spreading also indicates, that since new sea-floor forms constantly at the ridge crest and moves sideways, there is progressive increase in age away from the ridge crest.

Ac­cordingly, there is lack of palagic sediments at the ridge crest while it is progressively thicker on the older sea-floor as it moves.

(f) 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 movements as 10 mm per year (i.e. 1 cm/year). Rates like 1 cm per year are appreciable in human terms.

2. Trenches

Sea-floor spreading concept also helps in explaining the occurrence of trenches around the margins of the Pacific Ocean and are interpreted as the lines along which the sea-floor spreading towards the continents plunges beneath lower density continental or ocean-floor rocks at the subduction zone to a depth where the rock materials are fused re­sulting in volcanic and earthquake activity.

Since the rocks of the sea- floor are denser than the rocks of the continents, the sea-floor always tend to slide under continents when the two come together.

(a) The strong negative gravity anomalies over the deep-sea trenches, where the isostatic equilibrium does not operate, is thought to be due to some active force which 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 cooling and skinking of mantle rock.

(b) Relatively low heat flow values, found in oceanic trenches indicate that the rocks beneath the trenches are cooler than normal.

(c) The trenches are the locales of the earthquake foci and associ­ated volcanic activity. The world’s belts of greatest seismic activity occur in the trenches and adjacent island arcs.

If the positions of the earthquake epicentres and their depths are plotted it will be found that the deepest quakes (700 Km depth) are on the continental side and the more shallow quakes are in the vicinity of the bottom of trenches.

The hypothesis of the sea-floor spreading indicates that the sea- floor material is subducted beneath a continent along what is known as a Benioff Zone. The bending of the sea-floor and the friction resulting from the subduction gives rise to earthquake shocks and generates heat which leads to the fusion and melting of upper mantle material com­bined with ocean-floor lavas and sediments.

Localized melting during subduction may create andesitic volcanoes. Andesite was thought to be formed due to the partial melting of the basaltic oceanic crust or of the mantle which undergoes magmatic differentiation and mixes with the rocks through which it passes.

This could create a more silica-rich andesitic magma and the composition of the andesite varies with the increase in the depth of the subduction zone and the distance from the trench.

3. Aseismic Ridges

These ridges are always earthquake free. These are island chains which often have an active volcano at one end and a line of dead and increasingly eroded volcanoes extending from the active one.

Away from the active volcano, the island chain gradually does not appear above sea-level, but there is usually a line of submarine sea- months. This gives the sequence of volcanoes being activated, aban­doned and eroded.

Hess termed the eroded, flat-topped sea-mounts as Guyots. The extinct volcanoes are progressively increasing in age away from active volcano. An aseismic ridge seems to acquire its alignment as the sea-floor moves over a centre of eruption (i.e. the fixed point of a la source).

If the sea-floor moves over an eruption centre, the volcano would gradually be carried away from it and because of cooling the sink as they are moved.

The above explanations although support the hypothesis of ocean floor spreading, the study of marine magnetic anomalies and deep-sea drilling supports the concept and most of the geologists accept this concept as a theory.