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Landslides are of interest to geographers for three main reasons. First, by eroding, transporting and depositing soil and rock, they represent one of the important geomorphic processes involved in shaping the surface of the Earth. In unstable areas, they may displace up to 2000 m3/km2/year (Crozier 1989), severely depleting the soil resource and threatening the sustainability of primary production (Sidle et al. 1985). Although they are particularly common in tectonically active mountainous areas, and along river banks and coasts, they may also occur in other areas that have weak material or a susceptible geological structure.

The second reason for geographical interest is that landslides are sensitive indicators of environmental change. As a geomorphic process, a landslide represents a short-term adjustment to disturbance of the natural system. As they take place, they rapidly convert unstable slopes to a more stable condition, allowing other slow-acting processes to assume the role of denudation. In terms of landform evolution, this means that most slopes are stable for most of the time. Thus when landslides occur they are generally responding to some significant change within the natural system. Initiating factors may include tectonic activity, climate change, and natural or human-induced disturbance to the vegetation cover, slope hydrology or slope form. Knowledge of both past and present landslide activity can therefore provide useful information on environmental change. Indeed, there has been a major international research effort aimed at reconstructing past climates and climatic change in Europe, based on landslide evidence preserved in the landscape (Crozier 1997).

The third reason landslides are often studied by geographers is that landslides can present a serious natural hazard (Varnes 1984; Crozier 1996). A full appreciation of hazard requires knowledge not only of the physical process but also of the nature of the threatened society. In a sense, hazards are an aspect of human ecology. They involve interrelationships between physical, social and economic systems; as such, they constitute a field of study in which geographers are able to make a valuable contribution.

In dealing with landslides, it is important to use a classification that distinguishes between characteristics that are relevant to the intended end-use of the study. The classification should use clearly defined and internationally understood terms. The working party on the World Landslide Inventory (1990) has made an attempt to standardize terminology and defines a landslide simply as: "the movement of a mass of rock, earth, or debris down a slope." A more comprehensive definition, which helps to distinguish landslides from the other geomorphological processes, is: "the downward or outward movement of a mass of slope-forming material under the influence of gravity, occurring on discrete boundaries and taking place initially without the aid of water as a transportational agent." As this second definition indicates, landslides are more than just a simple downslope movement of material. The three most widely used classifications involving landslides (Sharpe 1938; Varnes 1958; 1978; Hutchinson 1988) separate 'mass movements' (Fairbridge 1968) into two categories: 'subsidence' (which is the vertical sinking of material) and those movements that occur on slopes. These 'slope movements' are then usually divided first into 'landslides', as defined above, and second into the slower, more widespread and ill-defined movements such as 'creep', 'sagging' and 'rebound'. Of all the types of slope movement, it is landslides that have the potential to undergo rapid movement, making them a potentially dangerous form of natural hazard.

In order to choose the most appropriate method of reducing risk from the landslides (whether prevention, control, avoidance or compensation for loss) it is important to know something about the range of factors that lead to slope failure and how they operate. For example, it may be found that risk can be reduced more cheaply and more effectively by draining groundwater from a slope than by zoning it as unsuitable for use. Comprehensive lists of causative factors are available (Varnes 1958; Cooke and Doornkamp 1990; Crozier 1995), but it is useful to simplify these into categories. One way of doing this is to consider the function that various factors have in changing the conditions of slope stability.

The question of where a landslide may occur may not always be a major factor in landslide hazard analysis. This is because the site of concern may have already been identified by the potential risk or by the fact that a landslide already exists and presents a serious threat. Such situations can occur along highways or reservoirs, or near population centers or valuable assets.

The assessment of hazard at an important site usually takes the form of stability analysis. In the first instance, this may be done by qualitative observations. For example, if landslide features are present, an attempt may be made to determine whether the slide is still active, or when the last movement took place. There are many features that can be used to determine the state of activity (Crozier 1984). If the site has no evidence of previous movement, then stability may be determined by the 'precedence' approach. This involves comparing both stable and unstable slopes in the same terrain in order to identify the threshold conditions (e.g. slope angle and height) that have been associated with landsliding in the past. The site of concern is then compared with these conditions, and if it is found to have similarities to failed slopes, a more detailed quantitative analysis may be required. . .

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