Resumo

ARIAS L, LUIS ALBERTO. FIRST ORDER CONCAVITLES: EXPRESSION OF THE ACTIVE MECHANISM OF MODELING IN THE HIGHLAND OF SANTA ROSA DE OSOS. Bol. cienc. tierra [online]. 2007, n.20, pp.9-38. ISSN 0120-3630.

To consider the relief of the terrestrial surface as a complex dynamic system with the quality of leaving testimonies of its last behaviors is the axiomatic principle that presents the challenge of historical reconstruction of landforms in different regions. A fundamental aspect of the complexity of geomorphic systems is the existence of diverse histories in different regions. In previous studies they have been delineated the general guidelines of the evolutionary process of the relief in the uplifted erosion surface of Santa Rosa de Osos “altiplano”; the relief generations associated to each phase have been previously identified and described (Arias et al, 2000, 2002, 2006). If the relief of a region is fundamentally a historical product, it has relevance the question about the relief types that are being actively modeled. In more specific terms, which the active mechanisms of modeling are? and which the resulting geomorphic structures in Santa Rosa de Osos highlands are? Trying to respond this query satisfactorily constitutes the central objective of this essay. Semicircular fitted concavities in the flanks of the hills of the ASRO constitute the active relief generation. A description of morphometric characteristics of its constituent elements (flanks, plane bottom and terminal closing) is presented initially. The geomorphic configuration of these structures expresses a process of convergent evolution; the diversity of them is interpreted as different states of morphic development. Geomorphic diversity operates in the embryonic and early phases of modeling of first order concavities and express a divergent evolution while in the advanced phases it becomes explicit a process of convergent evolution. Postulated qualitative hypotheses about the modeling of first order concavities use quantifiable parameters (slopes, longitude and profile type of the flanks; area, gradient and its forms of perimeter line of the plane bottom; degree of fit of the plane bottom). The qualitative hypotheses could be undergone quantitative analysis in the moment that digital models of elevation (MDE) available have the appropriate spatial resolution. The essay approaches the investigation of the mechanisms of modeling of these geomorphic structures. These mechanisms operate like a succession of positive feedback circuits, unstable for its nature, which when are broken (to cross its threshold of stability) generates a process group and transient landforms, such as: · The transformation of an intergranular internal flow in a concentrated flow. · The transformation of an efficient internal chemical erosion to evacuate ionic materials -especially sílica - in internal physical erosion that evacuates particle material (clays, silts and sands). · Piping process generates tunnels and chimneys into weathering profiles. · Collapses of recovering materials of tunnels and chimneys generate a systematic cluster of hollows along axial concavities. As a consequence, morphogenetic processes shift from underground to superficial conditions. Identified processes in this study involve dissolution of kaolinitic clays in weathering profiles derived from granitic rocks. The rates of kaolinitic dissolution and formation of tunnels and chimneys are ignored; however, to deepen in this topic is an urgent necessity for their negative consequences in the infrastructural projects (dams, highways) and agricultural activities.

Palavras-chave : Relief generation; first order concavity; piping; clay dissolution; erosion surface; geomorphic systems; divergent evolution; convergent evolution.

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