Minor forms in granitic rocks: a record of their deformative history

[Resumen] Las formas menores sobre rocas graníticas pueden tener muy diferentes orígenes. En unos casos presentan una relación directa con la estructura de la roca sobre la que se desarrollan. Su generaci6n, ha tenido lugar en dos etapas: una endógena, después de la consolidación del magma, durante la que van a quedar impresos los rasgos esenciales de la forma, debidos principalmente a la deformación de la roca. Otra exógena, en que los procesos de alteración ponen de manifiesto, al atacar la roca, las estructuras deformativas existentes en esta. Las formas resultantes se denominan primarias y se definen como polifásicas y endógenas. Algunas veces, también los procesos geodinámicos externos, pueden producir fuerzas con magnitudes que superen la resistencia de esta, generando formas primarias, definidas como unifásicas y exógenas. La acción de la meteorización puede ocasionar la destrucción, total o parcial, de ambos tipos de formas. Las etapas intermedias en esa degradación han sido entendidas como un nuevo tipo de formas primarias, pero se trata de formas secundarias. Conociendo la génesis de las formas, se puede establecer la secuencia temporal de su formación, y relacionarlas, cualitativa y cuantitativamente, con los procesos de deformación sufridos por el macizo con el que se asocian en cada caso.[Abstract] Minar forms upon granitic rocks corresponds to different origins. In sorne cases they show a direct relationship too the estructure of the rack upon which they formo Their generation takes place in two stages: one endogenous, after the consolidation of the magma, during which the essential features of the form will be imprinted, due, principally, to the deformation of the rack; the other exogenous in which the alteration processes, on attacking the rack, demonstrate their structure. The resulting forms are known as primary forms, and they are defined as polyphase and endogenous. Sometimes, the external geodynamic processes, produce stresses which exceed the resistence of the rack, and can produced a1so primary forms, defined as uniphase and exogeous. The action of meteorization can cause the destruction (total or partial) of aH these primary forms. The intermediate stages of this desintegration have been interpreted as new types of primary forms, when, the fact, they are secondary forms. Once we are familiar with the genesis of the forms, we can establish the temporal sequence of their formation, and relate them, qualitatively and quantitatively, with the deformation processes suffered by the massif with which they are associated in each case

Kinds of plane fabric and their relation to the generation of granite forms

[Resumen] Los procesos de deformación tectónica que actúan sobre las rocas graníticas s. l., ttienen una gran influencia en la generación de formas mayores y sobre todo menores, durante la etapa epigénica de alteración del macizo rocoso. Se pueden relacionar algunas de las llamadas formas primarias con los distintos tipos de fabric habituales en las rocas graníticas deformadas, así como asignarles un tipo e intensidad de esfuerzos específicos para cada tipo de forma.[Abstract] The processes of tectonic deformation acting on granite rocks have great influence in the generation of major and chiefly minor forms during the epigenic weathering stage of rocky massif. We can relate sorne ofthe called primary forms with different kinds of tectonic fabric usual in deformed granite rocks, as well as to assing to thems the kind and the magnitude of specific stresses acting in each of cases described

Microformas granìticas tipo tafoni (cachola) y gnamma (pia): un micromodelado sin relación con el clima o la estacionalidad

[Resumen] In granitic areas the weathering acting on a kind of rock with particular geotechnical properties produce similar or even identical forms in different climatic environments. We shall now refer only to gnarnrnas and tafoni understood as the result of exogene processes,(humectation-desecation, thermical micro-oscillations, solution and others). This great variety of causes to which these microforms can be attributed is interpreted as convergence of processes. Other explanations are based on a common factor to the environments where these forms have appeared; for the gnammas, the rainwater which periodically fills the concavity; for the tafoni a very marked dry season in the clirnate cycle of the area. Others consider them as relict forms born under a different climate and continued in the presente The fact is, however, that none of the aforementioned reasons is simultaneously valid for all the cases. In our opinion, these forms are failure surfaces originated in breaking superficial conditions. Their coincidence with structural surfaces(diaclases)proves that the genesis took place after the delimitation of these surfaces and consequently of the rocky blocks. Let us considera diaclase shaped block, laying on the surface of the ground and submited to its own weigth and supporting reactions, with a resistance to simple cpmpression of 1400 kg/cm2 (equivalent to a non weathered granitic rock). The block's deformation in this case would completely happen in the elastic domain. A change from elastic to failure conditions need an increase in the load acting on the block equivalent to a cover approximately 6 km. thick. However, such depth conditions in the earth crust(implies an increase of geothermic heat and a lithostatic distribution of stresses) inyalidates this model.More shallow conditions are necessary for the blocks to individualize and to allow the circulation of the water along their limiting surfaces. Thus a migration and concentration of charges takes place by means of the rock's differential weathering along the diaclase plains transforming the initially plain surface into an irregular one where the contactbetween the block and the surrounding massif is reduced to a few places. It's there where the load due to the block's weigth plus the cover will be concentrated. Thus, the failure load is much smaller than in our first assumption. requiring a cover thicknesses of only 200 meters. In these circumstances, finite desplacements are generated in certain places of the block which divide the rock initially coherent into units with intergranular or trans granular limits; located in superficial or internal areas of the block, the outter envolving surface of which is called gnarnma or tafoni respectively· Only when the weathering does not destroy totally the structure of the rock massif and if this is exhumated after the u impression " of these failure surface the exogene processes can evacuate the mineral grains which are already predisposed to this separation. This is how both gnarnma and tafoni can be observed as active forms under such different conditions as the detaching mechanisms which expose them. For these reason we cannot confuse the genesis of these forms with the moment in which are manifested regardless of climatic or genetic conditions present or fossil on the sur face of the Earth

Estudio teórico sobre el origen de las características morfológicas de Las Pías (Gnammas, Vasque)

[Resumen] Las pias(vasque,gnamma), son microformas graniticas a las que se atribuye comunmente un origen epigénico.Cuando una carga puntual,de magnitud adecuada actúa sobre una superficie rocosa,produce roturas según superficies similares a las de las pias(vasque,gnarnma).En este trabajo se resuelven dos supuestos de carga teóricos 7 para 2 y 10 Tm,deduciéndose para una roca granitica sana las formas y tamaños de las pias(gnamma,vasque) que se originarian según aquellas.[Abstract] Pias(vasque,gnamma) are granite microforms generally understood as originated by epigenic agents. When punctual charges,of adecuate magnitude acts over a rocky surface, are produced breaking planes with shapes similar to pias(vasque,gnamma).This paper determine for two theoretical cases of /punctual charges(2 and 10 tones)acting over a granite surface rock,the breaking surfaces produced,with shape and measures of each the

Las terrazas del río Miño en el tramo Chantada-As Neves (límite de Galicia-Portugal)

Geomorphologic mapping is specially difficult in areas with scarce deposits of uncertain chronology and with multiage and multiorigin forms. This is the case of Galicia. This paper is an attempt to reconstruct the geomorphologic history of Galicia during the Cenozoic in an area especially relevant because of its geodynamic situation. It has been made an exhaustive inventory of the erosive and accumulation terrace levels in a specific section of the Miño, the main river course of the region. Here it is presented the terrace levels list between Chantada and As Neves, their relation to the three etched surfaces R800, R600 and R400, wich are the most relevant because of their areal extent. It is also analyzed their relationship with the longitudinal profile of the river. Using the only existing criteria, i.e., topographic continuity and altitude, it is proposed an evolutionary model substantially determined by the river incision associated with the Paleogene collision among the Euro-Asian and the Iberian Plates. This process geomorphologically begun after the generation of R600 surface which is equivalent in Galicia to the Peninsular Fundamental Surface