A number of objections have been raised to the theory of plate-tectonics. Some of them are as follows:

(i) There are weak evidences of thermal convection in the mande, which is usually considered as the driving force for plate motion.

The existence of horizontal seismic separation boundaries in the mantle is the evidence of changes in the physical composi­tion of its different layers and consequently of the absence of radial convection currents.

It is being assumed at present that the driving force in the dis­placement of lithospheric plates is not the thermal but the thermo gravitational convection.

ADVERTISEMENTS:

(ii) The assumption of pushing down of solid lithosphere into the mantle to a depth as much as 700 kms is not adequately substantiated.

(iii) The continental drift hypothesis has not so far produced any tenable interpretation concerning the planetary net-work of long-living deep faults common to continents and oceans.

(iv) The finding of Precambrian and Cambrian rocks near the crest of the Mid-Atlantic ridge contradicts the theory of plate-tecto­nics. According to the plate tectonic theory the rocks of the sea- floor cannot be more than five or ten million years old.

However, the plate tectonic theory explains a number of geologi­cal phenomena quite convincingly and it is considered to be a revolu­tion in earth science.