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In this article we will discuss about the origin of ocean basins based on the plate tectonic theory.
The rigid lithospheric slabs or rigid and solid crustal layers are technically called ‘plates’. The whole mechanism of the evolution, nature and motion of plates and resultant reactions is called ‘plate tectonics’. In other words, the whole process of plate motions is referred to as plate tectonics.
‘Moving over the weak asthenosphere, individual lithospheric plates glide slowly over the surface of the globe; much as a pack of ice of the Arctic Ocean drifts under the dragging force of currents and winds’.
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Plate tectonic theory, a great scientific achievement of the decade of 1960s, is based on two major scientific concepts e.g.:
(i) The concept of continental drift, and
(ii) The concept of sea-floor spreading.
Lithosphere is internally made of rigid plates (fig. 5.7). Six major and 20 minor plates have been identified so far (Eurasian plate, Indian-Australian plate, American plate, Pacific plate, African plate and Antarctic plate).
It may be mentioned that the term ‘plate’ was first used by Canadian geophysicist J.T. Wilson in 1965. Mckenzie and Parker discussed in detail the mechanism of pate motions on the basis of Euler’s geometrical theorem in 1967. They postulated ‘a paving stone’ hypothesis wherein the oceanic crust was considered to be newly formed at mid-oceanic ridges and destroyed at the trenches.
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Isacks and Sykes confirmed the ‘paving stone hypothesis’ in 1967. W.J. Morgan and Le Pichon elaborated the various aspects of plate tectonics in 1968. Now the continental drift and displacement are considered a reality on the basis of plate tectonics.
It may be highlighted that tectonically plate boundaries or plate margins are most important because all tectonic activities occur along the plate margins e.g. seismic events, vulcanicity, mountain building, faulting etc. Thus, the detailed study of plate margins is not only desirable but is also necessary.
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Plate margins are generally divided into three groups, as follows:
(1) Constructive plate margins are also called as ‘divergent plate margins’ or ‘accreting plate margins’. Constructive plate margins (boundaries) represent zones of divergence where there is continuous upwelling of molten material (lava) and thus new oceanic crust is continuously formed. In fact, oceanic plates split apart along the mid-oceanic ridges and move in opposite directions (fig. 5.9).
(2) Destructive plate margins are also called as ‘consuming plate margins’ or ‘convergent plate margins’ because two plates move towards each other or two plates converge along a line and leading edge of one plate overrides the other plate and the overridden plate is sub-ducted or thrust into the mantle and thus part of crust (plate) is lost in the mantle (fig. 5.9).
(3) Conservative plate margins are also called as shear plate margins. Here two plates pass or slide past one another along transform faults and thus crust is neither created nor destroyed.
H. Hess postulated the concept of ‘plate tectonics’ in 1960 in support of continental drift. The continents and oceans move with the movement of these plates. The present shape and arrangement of the continents and ocean basins could be attained because of continuous relative movement of different plates of the second Pangaea since carboniferous period.
Plate tectonic theory is bases on the evidences of:
(i) Sea- floor spreading, and
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(ii) Palaeomagnetism.
The concept of sea floor spreading was first propounded by professor Hary Hess of the Princeton University in the year 1960. His concept was based on the research findings of numerous marine geologists, geochemists and geophysicists. Mason of the Scripps Institute of Oceanography obtained significant information about the magnetism of the rocks of sea-floor of the Pacific Ocean with the help of magnetometer.
Later on he surveyed a long stretch of the sea-floor of the Pacific Ocean from Mexico to British Columbia along the western coast of North America. When the data of magnetic anomalies obtained during the aforesaid survey were displayed on a chart, there emerged well defined patterns of stripes (fig. 5.9).
Based on these information Hary Hess propounded that the mid- oceanic ridges were situated on the rising thermal convection currents coming up form the mantle (fig. 5.10). The oceanic crust moves in opposite directions from mid-oceanic ridges and thus there is continuous upwelling of new molten materials (lavas) along the mid-oceanic ridges.
These molten lavas cool down and solidify to form new crust along the trailing ends of divergent plates (oceanic crust). Thus, there is continuous creation of new crust along the mid-oceanic ridges. This, according to Hess, proves the fact that sea- floor spreads along the mid-oceanic ridges and the expanding crusts (plates) are destroyed along the oceanic trenches. These facts prove that the continents and ocean basins are in constant motion.
W.G. Vine and Mattheus conducted the magnetic survey of the central part of Carlsberg Ridge in Indian Ocean in 1963 and computed the magnetic profiles on the basis of general magnetism. When he compared the computed magnetic profiles with the profiles of magnetic anomalies plotted on the basis of actual data obtained during the survey, he found sizeable difference between the two profiles.
When he plotted the magnetic profiles on the basis of alternate bands of normal and reverse magnetism in separate stripes of 20 km width on either side of the ridge, he found complete parallelism between the computed profiles and observed profits.
Vine and Mattheus have opined on the basis of the evidences of temporal reversal in the geomagnetic filed and the concept of sea-floor spreading as propounded by Deitz and Hess that when molten hot lavas come up with the rising thermal convection current along the mid-oceanic ridges and get cooled and solidified, these (lavas) also get magnetized, at the same time, in accordance with the then geomagnetic field and thus alternate bands or stripes of magnetic anomalies are formed on either side of the mid-oceanic ridge.
In other words, when molten lavas are upwelled along the mid-oceanic ridges, these divide the earlier basaltic layer into two equal halves and these basaltic layers slide horizontally on either side of the mid-oceanic ridges.
The findings of Cox, Doell and Dalrympal (1964), Opdyke (1966) and Heritzler (1966) have validated the following facts:
(i) There is reversal in the main magnetic field of the earth (known as geocentric dipole magnetic field),
(ii) Normal and reverse magnetic amomalies are found in alternate manner on either side of the mid-oceanic ridges,
(iii) There is complete parallelism in the magnetic anomalies on either side of the mid-oceanic ridges and
(iv) There is parallelism in the time sequence of palaeomagnetic epochs and events calculated for 4.5 million years on the basis of magnetism of basaltic rocks or sedimentary rocks.
Fig. 5.11 depicts the position of magnetic stripes on either side of the mid-oceanic ridge along with the time-scale of their formation.
It may be concluded, on the basis of above discussion, that there is continuous spreading of sea- floor. New basaltic crust is continuously formed along the mid-oceanic ridges. The newly formed basaltic layer is divided into two equal halves and is thus displaced away from the mid-oceanic ridge. Alternate stripes of positive and negative magnetic anomalies are found on either side of the mid-oceanic ridges.
Such magnetic anomalies (positive and negative) are formed because of temporal reversal in the geomagnetic field. The rocks formed during normal magnetic field contain positive magnetic anomalies while the rocks formed during reverse polarity (reversed geomagnetic field) denote negative magnetic anomalies.
The age of magnetic stripes, the rate of sea floor spreading and the time of drifting of different continents are calculated on the basis of above facts. The dating of the magnetic stripes formed up to 4.5 million years before present has been completed on the basis of information obtained from the survey of palaeomagnetism of the sea floors of different oceans.
The rate of sea floor spreading is calculated on two bases e.g.:
(i) On the basis of the age of isochrons (isochrons are those lines which join the points of equal dates of the magnetic stripes plotted on the map) and
(ii) On the basis of distance between two isochrons.
Thus, the rates of spreading (drifting) of different oceans have been determined on the basis of above principles. The Atlantic and Indian Oceans are spreading (expanding) very sluggishly i.e. at the rate of 1.0 to 1.5 cm per year while the Pacific Ocean is expanding at the rate of 6.0 cm per year.
It may be pointed out that the rate of seafloor spreading always means the rate of expansion only on one side of the mid-oceanic ridge. For example, if the rate of sea-floor spreading is reported to be 1.0 cm per year, the total spreading of the concerned ocean would be 1 + 1=2 cm per year.
The recent studies have shown that:
(i) The maximum spreading of the Pacific Ocean is 0 to 9 cm per year (total expansion 12 to 18 cm/year) along the eastern Pacific ridge between equator and 30°S latitude,
(ii) The southern Atlantic ocean is spreading along the southern Atlantic ridge at the rate of 2 cm per year (total expansion 4 cm/year) and
(iii) The Indian Ocean is expanding at the rate of 1.5 to 3 cm per year (total expansion being 3 to 6 cm/year).
