Granite Landscape Research Articles

Landforms and landscape evolution in the Mylliem Granite Area, Meghalaya Plateau, Northeast India

The Mylliem granite is one of many igneous bodies within the basement complex of the Meghalaya Plateau, northeast India. Although relatively small in size at c. 90 km2, it is very diverse geomorphologically and shows a range of distinctive landscapes within its limits. Relict flat watershed ridges and topographic basins characterize the northern and eastern part of the pluton, whereas to the southwest the relief becomes higher, with steeper hillslopes and deeply incised valleys. Deep weathering and thick saprolites are abundant, as are residual landforms resulting from stripping of the saprolite: domes, tors and boulders. The major reason behind the diversity of granite landscape of the Mylliem pluton is the progress of headward erosion, initiated at the Dauki fault in the south of the Meghalaya. Headward erosion enhances local relief and hence, weathering systems. Multi‐concave morphology is gradually transformed into multi‐convex one, which is hypothesized to be the specific mode of plateau evolution and scarp retreat in granite bedrock.

Scottish Landform Examples: The Cairngorms – A Pre-glacial Upland Granite Landscape

Click to increase image sizeClick to decrease image sizeKey Words: granitepre-glacial glacial erosionlandforms Acknowledgements The authors thank Scottish Natural Heritage for commissioning interpretative work on the Cairngorms. A. M. Hall wishes also to acknowledge the support of The Carnegie Trust of the Universities of Scotland for fieldwork in the Cairngorms. M. R. Gillespie and C. W. Thomas publish with permission of the Executive Director, British Geological Survey, NERC. The paper benefitted from the careful reviews of John Gordon and Brice Rea.

In a granite landscape

In a granite landscape

Triggering threshold precipitation and soil hydrological characteristics of shallow landslides in granitic landscapes

Sierra de Gredos, a mountain range in Central Spain, is prone to the occurrence of shallow landslides, which can transform into sediment laden flows as demonstrated by geomorphologic evidence. The analyses of these complex processes require reliable design tools for predicting thresholds of triggering precipitation in order to assess the susceptibility characteristics. In the Sierra de Gredos there is a lack of knowledge concerning how weathering mantle properties vary spatially, and what effects these spatial patterns have on infiltration, surface runoff and shallow subsurface flow. In this context, this paper is focused on the hydrogeomorphic characterization of a shallow landslide that occurred on December 17, 1997. The Generalized Quasi-Dynamic Model (GQDM) was implemented to approach the critical rainfall intensity set for different durations and to establish the intensity–duration (ID) threshold. To this purpose, field and laboratory methods, such as: Seismic Refraction Tomography (SRT), Time Domain Reflectometry (TDR), and geotechnical tests were applied. The use of these field investigations and their combination with the GQDM have allowed us to provide a detailed description of the behavior of the catchment, and reproduce the pattern of instability generated by an intense storm which triggered the 1997 shallow landslide. Furthermore, this threshold can be used as a predictive tool for hazard prevention.

Landscape response to tipping points in granite weathering: The case of stepped topography in the Southern Sierra Critical Zone Observatory

The dynamics of granitic landscapes are modulated by bimodal weathering, which produces patchy granular soils and expanses of bare rock ranging from meter-scale boulders to mountain-scale domes. We used terrain analysis and with cosmogenic nuclide measurements of erosion rates to quantitatively explore Wahrhaftig’s decades-old hypothesis for the development of “stepped topography” by differential weathering of bare and soil-mantled granite. According to Wahrhaftig’s hypothesis, bare granite weathers slower than soil-mantled granite; thus random erosional exposure of bare rock leads to an alternating sequence of steep, slowly weathering bedrock “steps” and gently sloped, but rapidly weathering, soil-mantled “treads.” Our investigation focused on the terrain surrounding the Southern Sierra Critical Zone Observatory (CZO), which is underlain by granitic bedrock and lies outside the limits of recent glaciation, in the heart of the stepped topography described by Wahrhaftig. Our digital terrain analysis confirms that steep steps often grade into gentle treads, consistent with Wahrhaftig’s hypothesis. However, we observe a mix-and-match of soil and bare rock on treads and steps, contrary to one of the hypothesis’ major underpinnings – that bare rock should be much more common on steps than on treads. Moreover, the data show that bare rock is not as common as expected at step tops; Wahrhaftig’s hypothesis dictates that step tops should act as slowly eroding base levels for the treads above them. The data indicate that, within each landscape class (i.e., the steps and treads), bare rock erodes more slowly than surrounding soil. This suggests that the coupling between soil production and denudation in granitic landscapes harbors a tipping point wherein erosion rates decrease when soils are stripped to bedrock. Although broadly consistent with the differential weathering invoked by Wahrhaftig, the data also show that steps are eroding faster than treads, undermining Wahrhaftig’s explanation for the origins of the steps. The revised interpretation proposed here is that the landscape evolves by back-wearing of steps in addition to differential erosion due to differences in weathering of bare and soil-mantled granite.

Effects of fire on woody vegetation structure in African savanna

Despite the importance of fire in shaping savannas, it remains poorly understood how the frequency, seasonality, and intensity of fire interact to influence woody vegetation structure, which is a key determinant of savanna biodiversity. We provide a comprehensive analysis of vertical and horizontal woody vegetation structure across one of the oldest savanna fire experiments, using new airborne Light Detection and Ranging (LiDAR) technology. We developed and compared high-resolution woody vegetation height surfaces for a series of large experimental burn plots in the Kruger National Park, South Africa. These 7-ha plots (total area approximately 1500 ha) have been subjected to fire in different seasons and at different frequencies, as well as no-burn areas, for 54 years. Long-term exposure to fire caused a reduction in woody vegetation up to the 5.0-7.5 m height class, although most reduction was observed up to 4 m. Average fire intensity was positively correlated with changes in woody vegetation structure. More frequent fires reduced woody vegetation cover more than less frequent fires, and dry-season fires reduced woody vegetation more than wet-season fires. Spring fires from the late dry season reduced woody vegetation cover the most, and summer fires from the wet season reduced it the least. Fire had a large effect on structure in the densely wooded granitic landscapes as compared to the more open basaltic landscapes, although proportionally, the woody vegetation was more reduced in the drier than in the wetter landscapes. We show that fire frequency and fire season influence patterns of vegetation three-dimensional structure, which may have cascading consequences for biodiversity. Managers of savannas can therefore use fire frequency and season in concert to achieve specific vegetation structural objectives.

A geophysical analysis of hydro-geomorphic controls within a headwater wetland in a granitic landscape, through ERI and IP

Abstract. Wetlands are undergoing considerable degradation in South Africa. As interventions are often technical and costly, there is a requirement to develop conceptual process models for these wetland systems so that rehabilitation attempts will be successful. This paper presents an approach using the geophysical methods of Electrical Resistivity Imaging (ERI) and Induced Polarization (IP) to delineate sub-surface hydro-geomorphic controls that maintain equilibrium disconnectivity of wetland-catchment processes, which through gully erosion are increasing the catchments connectivity through loss of water and sediment. The findings presented here give insight into the geomorphic processes that maintain the wetland in an un-degraded state, this allows for the development of a conceptual model outlining the wetland forming processes. The analysis suggests that sub-surface clay-plugs, within an otherwise sandy substrate are created by illuviation of clays from the surrounding hillslopes particularly at zones of valley confinement.

Age and mineralogy of supergene uranium minerals — Tools to unravel geomorphological and palaeohydrological processes in granitic terrains (Bohemian Massif, SE Germany)

Uranyl phosphates (torbernite, autunite, uranocircite, saleeite) and hydrated uranyl silicates (normal and beta-uranophane) found in various erosion levels and structures in the Late Variscan granites at the western edge of the Bohemian Massif, Germany, were the target of mineralogical investigations and age dating, using conventional and more advanced techniques such as Laser-Ablation-Inductive-Coupled-Plasma Mass Spectrometry (LA-ICP-MS). Supergene U minerals have an edge over other rock-forming minerals for such studies, because of their inherent ‘clock’ and their swift response to chemical and physical environmental changes on different scales. Uraniferous phoscretes and silcretes, can be used to characterize the alkalinity/acidity of meteoric/per descensum fluids and to constrain the redox conditions during geomorphic processes. This study aims to decipher the geomorphological and palaeohydrological regime that granitic rocks of the Central European Variscides (Moldanubian and Saxothuringian zones) went through during the Neogene and Quaternary in the foreland of the rising Alpine mobile fold belt. The study provides an amendment to the current sub-division of the regolith by introducing the term “hydraulith”, made up of percolation and infiltration zones, for the supergene alteration zone in granitic terrains. It undercuts the regolith at the brink of the phreatic to vadose hydrological zones. Based upon the present geomorphological and mineralogical studies a four-stage model is proposed for the evolution of the landscape in a granitic terrain which might also be applicable to other regions of the European Variscides, considering the hydrological facies changes along with paleocurrent and paleoslope in the basement and the development of the fluvial drainage system in the foreland.Stage I (U mineralization in the infiltration zone) is a mirror image of the relic granitic landscape with high-altitude divides and alluvial–fluvial terraces. Its characteristic features are preserved in the uplifted hinterland of a peneplain which in this case is tilted towards a lacustrine basin. Stage II (U mineralization in the infiltration zone, regolith and saprock) includes two sub process, planation and exposure, resultant in the exposure of inselbergs and quartz ridges in front of the hinterland (stage I). Stages III and IV (U mineralization in percolation zone and saprock) are controlled by the base level lowering in the foreland. Rapid incision caused pinnacle-like tors and large granitic land forms to form, whereas a slow-down of fluvial incision favored its destruction and the development of weathering pits of different kinds. A full blown cycle of planation and incision lasted for approx. 10Ma, a stage which covers planation and exposure, resulting in the formation of domal structures which lasted for as much as 2Ma. Climate is an important factor but the most important factors for the geomorphological processes shaping the granitic landscape in the study area are uplift and erosion. The study area is located within the stress field of an ancient Variscan craton (Mesoeurope) and a highly mobile Alpine fold belt (Neoeurope). The rate of vertical displacement in the mobile parts of the crust had a long-distance effect also on the granitic terrains in the rigid parts of the crust.

Are any granite landscapes distinctive of the humid tropics? Reconsidering multiconvex topographies

Geomorphologists have long debated over whether certain geomorphic types of landscapes may evolve only in certain climatic conditions, with granite terrains often chosen to illustrate such climate–relief relationships. The view deeming inselberg landscapes in particular to be distinctively tropical is difficult to sustain in the light of research demonstrating both the wide global distribution of inselbergs and evident rock control on their origins and development. However, another geomorphic type of granite landscapes has emerged as possibly specific to low latitude terrains, characterized by efficient deep weathering. In the context of the ongoing debate, this paper considers the case of multiconvex topography, consisting of low hills weathered throughout, which is widely distributed in South America, Africa and Southeast Asia but not yet reported from middle or high latitudes. Multiconvex landscapes require that weathering operates efficiently in all topographic settings and that dissection proceeds to maintain relative relief. Hence they tend to be associated with areas subject to moderate uplift. There is little chance for this type of relief to survive major environmental changes towards aridity or cooling because stripping of the weathering mantle will not be compensated by saprolite renewal.

Influence of climate and tectonic movements on granite landforms in China

Present granite landform characteristics and distribution are the integrated result of climate, tectonics and lithology. Various types of granite landforms in China signify climate zonality and differential vertical movement of earth surface, while published research results on Chinese granite landforms are very rare, especially in international journals. Based on the process analysis of chemical weathering and physical disintegration, four granite landform regions in China are classified according to the present climate regime. On the Tibetan Pla- teau, the cold and freezing climate induced periglacial landscapes; the northeast region is characterized by physical disintegration and low round mounds are widespread; in the northwest region controlled by arid climate, wind-carved minor landscapes are extremely prominent. The most spectacular granite landscapes in China are presented in southeast as a result of longtime chemical weathering under humid and warm conditions, as well as the dif- ferential uplift after Neogene. Correlating the weathering crust in southern China, Tibetan Plateau and India, a possible unified planation surface in Neogene is proposed. With corestones as indicators of original weathering front, the differential uplift extent of dissected planation surfaces can be estimated. At least three landforms implying uplift can be identified in southeastern China, with elevations of 300-400 m, 2000 m and 3600 m above the sea level respectively.

Weathering phases recorded by gnammas developed since last glaciation at Serra da Estrela, Portugal

The morphometrical analysis of gnammas (weathering pits) in granite landscapes has been used to establish the relative chronology of recent erosive surfaces and to provide the weathering history in a region. To test the validity of gnammas as relative chronometer indicators, and the reliability of the obtained weathering record, two sites have been studied in Serra da Estrela, Portugal. The first site is within the limits of the glacier that existed in these mountains during the last glaciation, whereas the second site is located in an unglaciated sector of the mountains, which preserves a longer record of weathering in the bedrock surface. The number of gnamma weathering phases recorded in the latter site (8) is larger than those from the former (6). Correlation between both measurement stations based on morphometrical criteria is excellent for the younger six weathering phases (1 to 6). Consequently, the parameter used for relative chronology ( δ-value) has been verified to be age dependent, although absolute values are modulated by microclimate due to altitude variations. The weathering record was essentially duplicated once the surfaces at both sites were exposed, demonstrating the reliability of gnamma evolution as a post-glacial environmental indicator for the region.

Assessment of the instability hazard of a granite boulder

This paper aims to assess the instability hazard of a granite boulder. The procedure was first to consider the geological setting and geomorphologic features of the boulder in relation to typical granite landscape forms. Climatic and seismic data were next obtained from different sources, and geomechanical parameters were measured in situ and in the laboratory, with terrestrial laser scanning techniques used to measure shape and volume and to conduct a detailed survey of the boulder. Different analytical approaches were then applied to the calculation of boulder safety factors against sliding and toppling. Since the boulder was considered to be unstable in the worst possible scenario, a particle code approach was used to determine its fall trajectory, calculate the final run out of the block, and assess the risk for houses located at the foot of the slope. Finally, conclusions were drawn regarding the instability hazard, and recommendations were made in regard to definitively stabilizing the granite boulder.

Quantifying the climatic and tectonic controls on hillslope steepness and erosion rate

Hillslopes in humid regions are typically convex to concave in profile and have a relatively thick, continuous regolith cover. Conversely, hillslopes in arid regions are typically cliff-dominated and have a relatively thin, discontinuous regolith cover. The difference between these two end-member slope forms is classically attributed to climate, but climate, tectonics, and lithology all play a role. In this paper, we describe a mathematical model for hillslope gradient and regolith thickness using basic climatic and tectonic input data for a given rock type. The model first solves for the regolith thickness on a planar slope segment in topographic steady state using the soil production function and a prescribed uplift/incision rate. The climatic and lithologic controls on soil production rates are quantified using an empirical energy-based model for the physical weathering of bedrock. The slope gradient is then computed by balancing uplift/incision rates with sediment fluxes calculated using a nonlinear depth- and slope-dependent sediment transport model. The model quantifies the ways in which, as aridity and uplift/incision rates increase, regolith thicknesses decrease and hillslope gradients increase nonlinearly until a threshold condition is reached, beyond which bare, cliff-dominated slopes form. The model can also be used to estimate long-term erosion rates and soil residence times using basic input data for climate and regolith thickness. Model predictions for erosion rates closely match cosmogenically derived erosion rates in granitic landscapes. This approach provides a better quantitative understanding of the climatic and tectonic controls on slope form, and it provides a simple, widely applicable method for estimating long-term erosion rates and the thickness of regolith cover on hillslopes.

Granite Landscapes of the World - by Piotr Migo?

Granite Landscapes of the World - by Piotr Migo?

Multi‐phase evolution of gnammas (weathering pits) in a Holocene deglacial granite landscape, Minnesota (USA)

AbstractThe morphometry of 85 gnammas (weathering pits) from Big Stone County in western Minnesota allows the assessment of the relative ages of the gnamma population. The ratio between maximum and minimum depths is independent of the initial size of the cavity and only depends on the weathering evolution. Therefore, the distribution of depth ratios can be used to assess the gnamma population age and the history of weathering. The asymmetrical distribution of depth ratios measured in Big Stone County forms three distinct populations. When these sets are analyzed independently, the correlation (r2) between maximum and minimum depths is greater than 0·95. Each single population has a normal distribution of depth ratios and the average depth ratios (δ‐value) for each population are δ1 = 1·60 ± 0·05, δ2 = 2·09 ± 0·04 and δ3 = 2·42 ± 0·08.The initiation of gnamma formation followed the exhumation of the granite in the region. This granite was till and saprolite covered upon retreat of the ice from the Last Glacial Maximum. Nearby outcrops are striated, but the study site remained buried until it was exhumed by paleofloods issuing from a proglacial lake.These Holocene‐aged gnammas in western Minnesota were compared with gnammas of other ages from around the world. Our new results are in accordance with the hypothesis that δ‐values represent the evolution of gnammas with time under temperate‐ to cold‐climate dynamics. Phases of the formation of new gnammas may result from changes in weathering processes related to climate changes. Copyright © 2007 John Wiley & Sons, Ltd.

Piotr Migoń, Granite Landscapes of the World

Piotr Migoń, Granite Landscapes of the World

Granite Landscapes of the World ‐ by Piotr Migon

Granite Landscapes of the World ‐ by Piotr Migon

Lychee tree parameters for water balance modeling

Lychee (Litchi chinensis Sonn.) is widely grown under irrigation in the tropical northern Thailand highlands. Water efficient irrigation requires sound irrigation scheduling for which the requisite soil moisture information can be obtained from water balance modeling. A prerequisite for water balance predictions are plant parameters which describe interception, root distribution evaporation and transpiration. These parameters depend on climate, soil, as well as plant physiology, variety and age. This study investigated the plant parameters for 7-year-old lychee trees in tropical granite landscapes, as they are as yet unavailable. Interception could be satisfactorily predicted with the modified Gash model. The Gash parameters (canopy capacity per canopy cover area (Sc), canopy cover factor (c)) were determined to be 21.8 mm and 0.22, respectively. The spatial distribution of lychee tree roots depended on slope inclination. On the level plateau position, root length density (RLD) distribution was trunk-symmetrical and could be predicted with an empirical function. On the slope, the asymmetrical and irregular root development was not predictable. The suitability of the dual crop coefficient approach predicting daily potential evaporation (Epot) and daily potential transpiration (Tpot) rates for water balance modeling was limited due to the weak correlations of Epot and Tpot with the potential reference evapotranspiration (ETo). As a result, no universal values for the potential evaporation coefficient (Ke,max) and the potential transpiration coefficient (Kcb) could be determined. Hence, Epot and Tpot measurements are mandatory if accurate Epot and Tpot data are necessary. In the case of missing measurements, Ke,max values of 0.6 and 1.6 are recommended for rough Epot estimates underneath and in-between the lychee tree canopies. For Tpot predictions in irrigation scheduling, a relatively high Kcb of 0.8 is recommended in order to ensure a water stress free fruit development within the irrigation season.

Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate regimes

We used cosmogenic nuclide and geochemical mass balance methods to measure long-term rates of chemical weathering and total denudation in granitic landscapes in diverse climatic regimes. Our 42 study sites encompass widely varying climatic and erosional regimes, with mean annual temperatures ranging from 2 to 25 °C, average precipitation ranging from 22 to 420 cm·year −1, and denudation rates ranging from 23 to 755 t·km −2·year −1. Long-term chemical weathering rates range from 0 to 173 t·km −2 year −1, in several cases exceeding the highest granitic weathering rates on record from previous work. Chemical weathering rates are highest at the sites with rapid denudation rates, consistent with strong coupling between rates of chemical weathering and mineral supply from breakdown of rock. A simple empirical relationship based on temperature, precipitation and long-term denudation rates explains 89–95% of the variation in long-term weathering rates across our network of sites. Our analysis shows that, for a given precipitation and temperature, chemical weathering rates increase proportionally with fresh-material supply rates. We refer to this as “supply-limited” weathering, in which fresh material is chemically depleted to roughly the same degree, regardless of its rate of supply from breakdown of rock. The temperature sensitivity of chemical weathering rates is two to four times smaller than what one would expect from laboratory measurements of activation energies for feldspar weathering and previous inter-comparisons of catchment mass-balance data from the field. Our results suggest that climate change feedbacks between temperature and silicate weathering rates may be weaker than previously thought, at least in actively eroding, unglaciated terrain similar to our study sites. To the extent that chemical weathering rates are supply-limited in mountainous landscapes, factors that regulate rates of mineral supply from erosion, such as tectonic uplift, may lead to significant fluctuations in global climate over the long term.

Geomorphological surfaces of different age and origin in granite landscapes: an evaluation of the Schmidt hammer test

AbstractGranite landforms in Sierra Nevada, California, and the southern part of Sweden were used to test whether a Schmidt hammer can be used to distinguish surfaces of different age and origin. Based on 3260 readings, statistically significant differences were obtained from surfaces formed and/or affected by different geomorphological processes. Aeolian, fluvial or glacially polished surfaces yield higher values than surfaces exposed to surface weathering, which are distinguishable from surfaces at a weathering front caused by deep weathering. Copyright © 2004 John Wiley & Sons, Ltd.