30 / Basics of Environmental Science
Laterization does not necessarily render a soil useless and many relatively laterized soils are cultivated,
although some soils resembling lateritic soils, for example in parts of the eastern United States, are
not truly laterized. Indeed, there are doubts about the extent to which laterization is occurring at
present. Lateritic soils in the West Indies, Indonesia, Australia, India, and China may well be of
ancient origin (HUNT, 1972, p. 193).
Living organisms contribute to weathering. By moving through soil they assist the penetration of air
and water, and the decomposition of organic material releases acids and carbon dioxide, some of
which dissolves into the soil water. Biological activity contributes greatly to the formation of soil.
Physical weathering is also important in soil formation, especially in its initial stages, but it can also
degrade soils through erosion. Thermal weathering, which is the expansion and contraction due to
repeated heating and cooling, causes rocks to flake, especially if water is held within small crevices.
Small particles detached from the rock may then be carried by the wind and if they strike other rocks
more particles may be chipped from them. Depending on their size, the particles may be carried well
clear of the ground or may roll and bounce along the surface; the process is called ‘saltation’. Most
serious erosion is due to water, however. All water flowing across the land surface carries soil particles
with it. This can lead to the formation of rills and gullies into which more particles are washed and
then transported, or where water flows as sheets whole surface layers can be removed. In addition to
this, all rivers erode their banks, and waves erode the shores of lakes and the sea (HUDSON, 1971,
pp. 33–46).
These processes are entirely natural and part of the cycles by which originally igneous rocks are
converted into sediments and landforms are made and age, but human activities can accelerate them.
The UN estimates that in the world as a whole, some 1.093 billion hectares (ha) of land have been
degraded by water erosion, 920 million ha by sheet and surface erosion and a further 173 million ha
by the development of rills and gullies. Of the total area subject to serious degradation by water, 43
per cent is attributed to the removal of natural vegetation and deforestation, 29 per cent to over-
grazing, 24 per cent to poor farming practices, such as the use of machinery that is too heavy for the
soil structure to support and the cultivation of steep slopes, and 4 per cent to the over-exploitation of
vegetation (TOLBA AND EL-KHODY, 1992, pp. 149–150). There is, however, some evidence that
modern farming techniques can reduce soil erosion substantially. A study of a site in Wisconsin
found that erosion in the period 1975–93 was only 6 per cent of the rate in the 1930s (TRIMBLE,
1999). This may be due to higher yields from the best land, combined with methods of tillage designed
to minimize erosion (AVERY, 1995).
Weathering is the general name given to a variety of natural processes by which rock is recycled and
soil and landscapes created. It creates and alters environments, but human activities can accelerate it
on vulnerable land, degrading natural habitats and reducing agricultural productivity.
9. The evolution of landforms
The weathering of exposed rocks and the erosion and transport of loose particles create the landscapes
we see and change them constantly. Change is usually slow, but not always. The 1952 Lynmouth
flood was very sudden (see box), but not far away there are landscapes which record conditions long
ago. During the most recent glaciation the ice sheets did not extend as far south as Devon, but on the
high granite batholith of Dartmoor the climate was severe, with permanently frozen ground
(permafrost), and to this day parts of Dartmoor are periglacial landscapes. Rock masses were shattered
by the repeated freezing and thawing of water that penetrated crevices.