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Formas y Paisajes Graniticos
Abstract
This book provides an explanatory account of the landforms and landscapes developed on granitic rocks. Granite is exposed over about 15% of the continents and is thus of an areal significance comparable to the carbonate rocks. But whereas there are many texts concerned with karst landscapes few books bring together the collective wisdom of granitic terrains with granitic forms. Up to the present the normal approach is based on climatic control or is merely descriptive. The book not only provides a detailed consideration of the forms major and minor, well-known and not so familiar, developed over large areas of the continents, but also presents interpretations which are the wider or general significance in the analysis and understanding of landscapes. General theories are discussed in the context of granite landforms. The importance of structure, including crustal stresses, is emphasised as are the importance of etching or subsurface initiation, multi stages or two stages development, neotectonic forms, solution forms as well the antiquitiy of some forms and surfaces (inherited forms). Morphogenetic forms are placed in perspective and comparison drawn with similar forms in other rock types.
Supplementary resource (1)
... Presenta una forma alomada u ondulada, derivada de unos procesos erosivos protagonizados por la alteración, el lavado y la erosión fluvial bajo unas condiciones templado-húmedas. Se trata, además, de un ejemplo de conjunto multicíclico y escalonado, resultado de un descenso relativo del nivel de base, rejuvenecimiento de la red y reactivación del paisaje (Vidal y Twidale, 1998). Dentro de los límites del espacio natural protegido se han identificado seis niveles erosivos distribuidos suavemente escalonados hacia el Oeste como consecuencia del basculamiento de la Meseta hacia el Atlántico y, por tanto, con edades posteriores al Oligoceno (Sanz y Rubio, 2000). ...
Se efectúa una valoración del patrimonio geomorfológico del Parque Natural de Arribes del Duero, concretamente, en un sector de la comarca zamorana del Bajo Sayago que tiene una extensión de unas 6.000 ha, a partir de la aplicación de varios métodos relacionados con el análisis de Lugares de Interés Geomorfológico (LIGm) en espacios naturales protegidos. Como resultado se han identificado nueve geomorfositios de los cuales tres, esto es, Peña Gazón y valle de Peña la Galga, Las Barrancas y finalmente Cerro de San Miguel, alcanzan una valoración global sobresaliente. A partir de la evaluación de los valores científicos, culturales y de uso y gestión de cada LIGm se establecen propuestas orientadas a su gestión, que pasan mayoritariamente por la interpretación del patrimonio geomorfológico enfocado al aprovechamiento geoturístico y educativo. La buena accesibilidad, el excelente estado de conservación y el escaso riesgo de degradación convierten dichos Lugares de Interés Geomorfológico en recursos patrimoniales útiles para el desarrollo local sostenible.
... A alveolização, apresenta-se como um processo de compromisso entre a arenitização das rochas graníticas , resultante de uma importante alteração química associadas a um modelado de duas superfícies (fundamentalmente desenvolvido em períodos de caraterísticas tropicais húmidas) e a evacuação " violenta e espasmódica " desses mesmos materiais alterados (relacionada com chuvas concentradas, que A. Godard associava a climas subtropicais semiáridos), desenvolvendo-se as formas, associadas à existência de uma erosão diferencial nos sectores anteriormente aplanados, as quais no seu momento terminal se terão assumido como de etchplains (Cordeiro, 2004). O ataque químico motiva assim o aprofundamento da superfície basal de meteorização após o início da degradação da superfície de aplanamento anterior, vindo a ter maior efi cácia sempre que a temperatura e a humidade fossem elevadas simultaneamente, no mínimo durante uma época do ano (Büdel, 1970; Thomas, 1974; 1994; Godard, 1977; Twidale, 1982; 2002; Romani e Twidale, 1998). A distribuição espacial dos alvéolos pelas diferentes manchas graníticas do Centro-Norte de Portugal demonstra, de um modo imediato, o peso da componente litológica na distribuição espacial deste modelado (Figura 2 ). ...
As bacias de meteorização química de escala média, ou “alvéolos graníticos” quando entendidos no sentido que Alain Godard lhe conferiu, constituem-se como um dos elementos fundamentais na interpretação da morfologia granítica portuguesa, em particular na compreensão da degradação das superfícies aplanadas que são um dos principais traços morfológicos do Portugal Central. Em termos morfológicos, assumem-se como bacias arredondadas, relativamente circunscritas, revestidas por um manto de alteração com espessura variável e com uma dimensão que vai desde as centenas de metros até à dimensão quilométrica.A alveolização é o resultado de um compromisso morfogenético entre fases de alteração química em clima tropical húmido e fases de ablação em clima subtropical seco, pelo que a sua génese se encontra associada a fatores morfoestruturais, enquanto a sua diferenciação se deve, no essencial, a fatores morfoclimáticos. A similaridade das condições de base estruturais nos diferentes sectores do Maciço Hespérico aponta para uma justificação em termos da eficácia dos processos morfogenéticos, em particular da meteorização química, no quadro das diferenças observadas em torno das condições climáticas (atuais e passadas), para a compreensão da manifesta diferença observada entre as formas alveolares do litoral e do interior. A existência de vários tipos de alvéolos – elementares, complexos, corredores de erosão e mistos – mostra que outros fatores, tais como a proximidade do nível de base ou as dimensões das bacias hidrográficas, têm de ser tomados em consideração na compreensão do processo de alveolização no Portugal Central no cenozoico e início do Pleistocénico.
Flutings are channels scored in steep slopes. They are found on granite bornhardts, blocks and boulders, and occur in a wide range of climatic conditions. Rivulets and seepages charged with chemicals and biota are capable of attacking and degrading granite, and this can occur below the surface, at the weathering front and on outcrops. Many, perhaps most, flutings are of subsurface provenance, but on exposed surfaces, algal coatings develop and so protect the beds. Abandonment and inversion of such flutings—the conversion of channels protected by algae into ribs – are the result of the comparative weakness of the adjacent unprotected laminated rock exposed in the erstwhile, commonly flared, weathering front. At some sites, newer, lateral channels are eroded through the laminated rock to expose fresh rock in 'button-holes'. Elsewhere, however, deep localised erosion has taken place on ribs on which the algal veneer has deteriorated.
Two A-tents are described from a large radius granite dome near Mt Magnet in the Yilgarn region of Western Australia. Their fractured crest lines are oriented normal to one another. This could be interpreted as implying two separate periods of compressive stress from different directions, but it is suggested that both could have formed in response to a single shear acting on a pre-existing orthogonal fracture set. -Authors
In the Mdzimba Hills of NW Swaziland huge exposed domes of coarse-grained granite exhibit a variety of micro-relief features which are attributed to the structural, textural and mineralogical fracturing of exfoliated sheets produces the rectilinear patterns. Pans also develop on the flatter, upper surfaces where water collects in hollows, enhancing the weathering of the granite. Karren are confined to exposures of microgranite on the flanks of the domes; their formation appears to be related to accelerated weathering in zones where the rock has more grain boundary cracks. Crazed and honeycombed surfaces are also weathering features; the former are associated with indurated veneers on granite boulders, and the latter with zones of granite containing more readily weathered epidote-bearing xenoliths. -from Authors
Gully gravure, essentially involving shifts in the loci of linear erosion and local relief inversions, is active on steep slopes in certain structural and climatic environments. Examples are described from weak sediments, where introduced quartzitic detritus protects channel floors and eventually causes them to become elongate ridges of 'false flatirons'; and from steep, even overhanging, granite slopes where desiccated algal slime protects the floors of Rillen sufficiently to permit the faster weathering and erosion of adjacent surfaces, leaving the former channel floors in positive relief, as blackened ribs or ridges. -Authors
Pedestal rocks have been attributed to aeolian sand blasting, to abrasion by rivers and to preferential weathering and erosion of the shaft and the cap under epigene conditions. Some of these processes may have local significance, but the field evidence suggests that most pedestal rocks are two-stage forms. Preferential weathering by soil moisture produces the shaft and takes place beneath the land surface. The forms are later fully exposed by erosion of the regolith, commonly by wash and streams, but in some instances by the action of wind or waves or soil creep. -from Authors
The forms and origin of micro-valleys and rimmed gutters on Domboshava, a granitic dome some 15 km north of Harare, Zimbabwe, are discussed. Micro-valleys, up to 10 m wide and typically 1-2 m deep, are characterised by localised slope flaring, flat floors, stepped long profiles and persistently diverging and converging forms. These characteristics are explained in terms of subsurface weathering rather than subaerial fluvial action. The micro-valleys are regarded as predominantly fossil features formed through long-term, possibly episodic, exhumation of the dome. -from Authors
