The Earth Contents

4. Rock Cycle

Insoluble materials are left behind after soluble materials are transported away

The removal of soluble species leaves behind residual, partially solubilised, rock minerals which are now relatively enriched in the insoluble elements, especially silicon and aluminium. Changes in chemical composition of basalt rock as it progressively weathers to form soil illustrates this dramatically. The residual solid phases quickly lose their alkali and alkaline Earth elements leaving behind silicon and aluminium (reformed into new alumino-silicate clay minerals) and iron (found as an oxide). This is as predicted by the ionic potentials of the elements concerned.

  Si Al Fe Ca Mg K Na
Surface soil 4.2 12.9 25.0 0.20 0.18 0.16 0.0
Subsoil 4.6 15.3 24.8 0.10 0.12 0.08 0.0
Mantle 15.3 12.7 14.7 2.7 1.4 0.16 0.37
Basalt 22.9 7.2 9.2 5.1 8.1 0.24 1.20

Compositional changes during soil formation

What materials are typically left behind after soluble materials are transported away?