Though Alfred Wegener’s Continental Drift was being rejected, Arthur Holmes elaborated one of the Wegner’s hypotheses that the mantle undergoes thermal convection.
- This idea is based on the fact that as a substance is heated its density decreases and rises to the surface until it is cooled and sinks again.
- This repeated heating and cooling results in a current which may be enough to cause continents to move.
- Initially not accepted, but Harry Hess (1962) published on the Mantle convection current or “sea floor spreading”. This idea was basically the same as that proposed by Holmes over 30 years earlier.
MECHANISM OF RISING CONVECTIVE CURRENT
Radioactive elements such as Uranium, Thorium etc. heat the asthenosphere.
The fluid asthenosphere circulates as convection currents underneath the solid lithosphere.
The divergent convective currents move the crustal blocks away in opposite directions and thus create seas and oceans. Examples- East Africa lakes system, Mid-Atlantic Ridges etc.
The convergent convective currents cause subsidence in the crustal zones giving birth to geosynclines and closing of sea which leads to formation of island arcs, Mountains etc.
Convection currents occur within the molten rock in the mantle and act like a conveyor belt for the plates.
These currents are closed in nature and the total change in the lithospheric al materials is constant.