ABSTRACT
Tensional margins are pedologically less significant for they generally coincide with the mid-oceanic ridge, which is usually submerged beneath the deep ocean. It is only in exceptional circumstances, such as when a tensional margin crosses a continent, that basic rocks appear on the Earth's land surface to be affected by pedogenesis. Of much less significance in terms of area are the volcanic oceanic islands that occur where localized hotspots have broken through the lithospheric plate and the resulting sea-floor volcano has been built up at such a rate that it reaches the ocean surface. When such a phenomenon is combined with lateral plate movement, an island chain develops, of which the most famous. Epimorphism of basalt under conditions of good drainage produces a red, brown or yellow clay, consisting of kaolin physils with oxides and hydroxides of iron and aluminium, which generally possesses a certain degree of subplasticity.
