Biological Weathering

Weathering effects which are small in themselves but noticeable in the aggregate can be attributed to plants and animals {biotic weathering). Plants retain moisture and any rock surface on which they grow is kept damp, thus promoting the solvent action of the water. The chemical decay of rock is also aided by the formation of vegetable humus, i.e. organic products derived from plants, and this is helped by the action of bacteria and fungi. Organic acids are thereby added to percolating rain-water and increase its solvent power. Bacteria species may live in the aerobic and anaerobic pore space of the weathering zone, and mobilize C, N, Fe, S and O, so assisting the process of weathering and sometimes attacking concrete and steel. Their mineral by-products can accumulate and cause expansion of the ground if not washed away by percollating water. The mechanical break-up of rocks is hastened when the roots of plants penetrate into cracks and wedge apart the walls of the crack. Global trends Present distribution of the weathering described is illustrated in Fig. 3.8. Different distributions existed during the Quaternary (p. 28) because much of the continental area between the tropics was drier than at present and to the north and south lay cooler and colder conditions peripheral to the extended glaciers: the former position of permafrost is shown in the figure. Weathering profiles produced under these earlier conditions are frequently preserved beneath more recent drift and the present weathering profiles. Excavations and foundations of moderate depth may therefore expose rock and soil weathered under a previous, and perhaps more severe, weathering regime.