The principal erosive forces are weathering, glacial action and water erosion, but plants and animals also have a considerable effect on the breakup of the rocks of the Earth's crust. The force of gravity, too, causes broken rock or loose soil to move from its original site.
Biological Erosion
Small pockets of soil in exposed rocks may be large enough to allow a tree seed that lands there to germinate. As the seed grows, the roots of the young tree force their way down through the pocket and enlarge it by pushing its sides apart. The material broken down by plant roots in this way forms the basic inorganic component of new soil.
Once the rock is broken down into soil the presence of vegetation tends to keep it stable. Decaying vegetable matter produces the humus content of soil, and the interlocking network of roots and stems holds the individual particles together. This interdependence is dramatically demonstrated when natural vegetation is removed to make room for cultivated crops; the soil breaks down and is eroded by wind and water so that in a few years a dustbowl results.
To prevent the removal of coastal sand dunes by the wind, marram grass (Ammophila armaria) or pine trees are often planted on them, the roots of which bind the sand particles together.
Animals, too, have an influence on the landscape, although the breakdown of rocks by animal activity is not as extensive as that by plants. Certain marine mollusks, such as the piddock (family Pholadidae), burrow into solid coastal rocks and so accelerate the rate at which they are eroded.
The destruction of shrubs and overgrazing of grasslands by increased populations of herbivorous animals can also lead to rapid erosion of the soil. Constant grazing by the herds of the Bedouin nomads, for example, has caused the destruction of large areas of grassland on the margins of the Sahara and the Arabian desert, enabling the deserts to extend their boundaries.
Slow Earth Movements
Loose rock material that results from weathering and erosion always moves downhill due to gravity, and it may do so slowly or rapidly. The slower process is called soil creep.
During soil creep the soil surface tends to hold together and the movement - often less than a centimeter per year - is perceptible only from its effects on the things growing in or placed upon it. Fences and telephone poles that were originally erected upright may lean downhill, for example. Tree trunks curve outwards and upwards as the progressive lean caused by the creep is overcome by the natural upright growth of the tree.
Any steeply-dipping rock structure under the creeping soil can be seen, in cross-section, to curve over near the surface as the downslope movement pulls it away.
Soil creep may be caused by expansion and contraction in the soil produced by moisture or frost. During expansion, soil particles rise perpendicular to the surface, whereas when the soil contracts, the particles settle vertically downwards. As a result, every time the soil expands and contracts on a slope the particles move a little farther downhill.
The soil creep process is particularly marked in the periglacial areas of arctic and subarctic regions, where the spring thaws may cause many square meters of topsoil to move downhill in the form of a lobe or a tongue. This process is known as solifluction and occurs only in the spring, producing a movement of about 10cm per year down slopes that are as gentle as only 2 degrees.
Rapid Earth Movements
Slumps and landslides are similar phenomena and are more rapid downslope movements than solifuction and soil creep.
A slump is characterized by the splitting up of the slope into concave slices that rotate over each other with a downward movement. Slumping can begin slowly, the first stage being the appearance of a series of concentric cracks at the edge of the slope or cliff, outlining the area that is about to slump. As the slump continues, the slices break up and become a jumbled earthflow that takes the form of a tongue reaching out from the area of slip.
A landslide usually occurs with a mass or rock that is lubricated by a bed of slippery material such as shale or clay, which may break down under wet conditions. If the support at the lower end of the bed is removed, a rapid landslide may take place.
Other landslides occur because of the undercutting of a rock surface by the sea or river currents. The undercutting eventually becomes so great that the weight of the rocks above causes a collapse. Natural joints or bedding planes, or other lines of weakness in the rock, determine the shape of the fracture.
Another form of downslope movement is mudflow. This usually occurs where dry, loose material in an arid area is suddenly saturated by a seasonal downpour and flows down a dry canyon and across flatter areas beyond. Mudflows sometimes take place in volcanic regions during eruptions. Loose volcanic dust that has been ejected over the landscape soaks up the water from the torrential rains that usually accompany eruptions, and causes destructive flows called lahars.
Slopes and Landscapes
The effect of such soil flows is to reduce the slopes of the ground. Steep faces, for example, are carved away and the eroded material is spread out over hollows. In addition, hills are flattened and plains are built up.
Incipient slopes can be divided into several parts consisting of a free slope, which is the outcrop of bare rock, surmounted by a gentle, upper wash slope that is formed by the material above the outcrop. Below the free slope is the debris slope, which is the pile of material that has been worn from above. This slope lies at a constant angle of repose which tends to be the same however eroded the rest of the slope is on which the spread of material washed away from the area collects.
Over a long period of time, slopes tend to retreat under constant weathering until they are softened and worn down to a series of gentle convex and concave curves.
A mature landscape has all its surface features worn away, theoretically to sea level, and is known as a peneplain.