Origin of landslides:
A description of the main factors that contribute to the formation of this process, follows:
A. Weather
The weather depending on its characteristics can favor the instability of the subsoil by contributing large quantities of water. This is due to the pressure that the liquid exerts on pores and fissures of the soil. Also, rains and the formation of currents of water on the surface (surface run-off) worsen the erosion processes.
The high precipitations in combination with the type of soil encourage the formation and acceleration of landslides. An argillaceous soil that is saturated by the amount of water it has received becomes very heavy and along with a grade or slope, it can slide.
B. Topography
Landslides occur most frequently in areas with pronounced slopes and a lack of vegetation.
C. Geology
Geology contributes a number of factors that are important to understand the of slopes.
a. Petrology
The types of rocks and the quality of soils determine in many cases the ease in which the surface is degraded by external factors (such as weathering).
b. Structures
They determine areas of weakness (faults, geological joints and folds, pliegue), and the positioning of materials in a manner favorable to the instability (layers).
c. Seismic activity
Vibrations caused by earthquakes can be sufficiently strong enough to produce landslides of diverse magnitude, affecting extensive areas. Examples are the earthquakes in Puriscal in 1990 and in Limon in 1991.
d. Volcanism
It is an element that triggers a phenomena of instability, as much from the volcanic activity (volcanic earthquakes and deformation of the volcano), as from the progressive accumulation of fragmentary materials (ash, boulders, etc.) that by their physical characteristics favor the instability of lands in areas bordering volcanoes.
D. Factors that are man made (man made activities)
All of the phenomena previously described comprise the natural geological balance which can be broken with the constructive and destructive activities of man. In this way, humans cause and accelerate these phenomena.
This happens, when human activity is carried out without suitable planning, particularly with regards to road works (highways and bridges), quarries, urban planning, poorly filled regions, cuts in the natural slope of hills, deforestation, and agricultural practices deficient in soil conservation, among others. All of this promotes processes of instability in soils that are already naturally vulnerable to a certain extent to this type of phenomena and that have serious consequences for the future.
Classification of landslides according to the movement
Depending on their speed, landslides can be classified as: slow and rapid.
Rapid:
They reach speeds of meters per second and they can originate in areas with very steep grades, where rocks and debris fall and accumulate forming a slope or they can produce the sliding of a huge mass in seconds or minutes. Among these are mud slides and flows.
Slow:
They travel at only centimeters or meters per year. They are characterized by transporting large amounts of material.
Evidence that demonstrate the presence of a slow slide is: the inclination of trees in favor of the slope, the inclination of fences, the cracking of houses, etc.
The most important effects of landslides are:
Blockage and generation of dams in fluvial channels with eventual mud and rock avalanches.
Bibliographical Source
National commission of Risk Prevention and Emergency Attention (Comisión Nacional de Prevención de Riesgos y Atención de Emergencias - CNE). Land Slides, San Jose, Costa Rica
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