Mainzer Geographische Studien, Heft 23

Abele, Gerhard: Trockene Massenbewegungen, Schlammströme und rasche Abflüsse. Dominante morphologische Vorgänge in den chilenischen Anden.



Dry mass-movements, mudstreams and rapid discharge, dominant morphological processes in the Chilean Andes.

The quaternary forms of the Chilean Andes and of the adjoining longitudinal depression are strongly characterized by cataclysmic accumulations. They can be classified according to the way in which water contributes to the movement, i. e. into accumulations caused by dry mass-movements, where water is of no or minor importance, mudstreams which flow down as a mixture of solid material and water, and rapid discharge, where water is the transporting medium.

It is not only the "normal" landslides and rockfalls which belong to the dry mass-movements but also the masses gliding down from the flancs of a volcano after being set into motion by an initial volcanic impulse (mass-movements of Mt. St. Helens-type: at the Rio Claro of Los QueZes, No. 18 [The numbers refer to the localities indicated in Fig. 1.] and Antuco, No. 21).

Many valleys in the Chilean Andes are interrupted by barriers accumulated by big prehistoric landslides, which have up to now been regarded as pleistocene moraines. Examples are:
Embalse la Laguna (No. 6), Salto del Soldado (No. 8), Guardia Vieja (No. 9), Portillo (No. 10), Embalse el Yeso or Laguna Negra (No. 13), Termas del Flaco (No. 16), Rio Claro of Los QueZes and Antuco. (Moraines of the last glacial period can be found for instance south of the Rio Juncal - Rio Aconcagua, along the Rio Blanco and Rio de los Leones: Glacial stages of El Saladillo and Los Leones).

The mudflows are divided into those flowing down due to present or past volcanic activity (lahars) and those due to precipitation. Among the lahars two groups can be distinguished according to the form and sequence of accumulation: on the one hand the monogeneous lahars, which are very thick and have developed conspicuous characteristic forms, and on the other hand the polygeneous lahars, which consist of many rather thin but large banks piled up in lahar-heaps. Here it is not the single lahar-bank which brings forth the characteristic form, but the lahar-pile as a whole.

If the polygeneons lahars proceeded from a limited area they were accumulated like cone sectors. By shifting from one side to the other the single lahar courses formed a mudflow-fan with piled up or interlocking banks. This can be seen at the big lahar-fan of Victoria (No. 22), which spread in the longitudinal depression, covering about 2.300 to 2.800 km2.

The pyroclastic fan at the Rio Claro of Molina (No. 19), which covers about 1.250 km2, does not only consist of lahars, but also of pyroclastic flows of different origin. There the lahars cannot be traced back to a limited volcanic area; the mudflow courses rather descended from valleys without any volcano. Therefore the lahars were set into motion, when the ash layers on the slopes were mobilized.

Many instances of big mudflows and rapid discharge by precipitation (avenidas) occur at the fringe of the Chilean arid zone. There the avenidas run down from steep slopes with no or sparse vegetation yet exposed to rare but heavy rainfall (area between Rio Elqui and Rio Copiapó and in the zone of medium altitude in the Norte Grande).