‘Convection’ as we know, is a process of heat-transfer which particularly takes place in fluids and liquids. According to this hypothesis, it is believed that the mantle (intervening layer between the crust and core) is made up of molten silicates and therefore in this medium heat is transmitted by convectional process. The convection cells are thought to be thousands of miles across.

This hypothesis suggests that the convection currents flow hori­zontally under the crust and exert a powerful drag on it. Two adjacent convection cells throw the crust into tension and pull the crust apart where they diverge and in to compression where they converge. Accordingly, orogenic belts are believed to form where the approach­ing currents turn down as shown in the following diagram:

Demerits of Hypothesis

This hypothesis fails to explain why mountain-belt patterns are not symmetrical, while the convection cells in a relatively homogeneous earth are believed to be symmetrical. Besides, the materials deep beneath the earth are not quite suitable for the operation of convection currents in them.