Rates Of Postglacial rock weathering on glacially scoured outcrops (Abisko–Riksgränsen area, 68°N)

Ice–polished quartz veins, feldspar phenocrysts and quartzite layers were used as reference surfaces to assess the impact of Postglacial rock weathering in Lapland (68°N). Over 3200 measurements were carried out on roches moutonées and glaciofluvially scoured outcrops distributed within three study areas covering 8 km². Inferred weathering rates demonstrate that 10,000 years of Holocene weathering did not significantly modify the geometry of Weichselian rock surfaces. However, rates of general surface lowering range from 1 to 25, depending on the rock type, with average values at 0.2 mm ka⁻¹ for homogeneous crystalline rocks (irrespective of their acidity and grain size), 1 mm ka⁻¹ for biotite–rich crystalline rocks, and 5 mm ka⁻¹ for carbonate sedimentary rocks. Accelerated rates were recorded in weathering pits and along joints with values up to ten times higher than on the rest of the rock surface. Comparisons with cold and temperate areas suggest that solution rates of carbonate rocks are highly dependent on climate conditions, whilst granular disintegration of crystalline rocks operates at the same rate whatever the environment. It probably means that microgelivation is not efficient on ice–polished crystalline outcrops even under harsh climate conditions, and that granular disintegration proceeds under various climates from the same ubiquitous combination of biochemical processes. Last, the weathering state of Late–Weichselian roches moutonées can be usefully compared to that of Preglacial tors of the nearby Kiruna area.     

Rates Of Postglacial rock weathering on glacially scoured outcrops (Abisko–Riksgränsen area, 68°N)

Ice–polished quartz veins, feldspar phenocrysts and quartzite layers were used as reference surfaces to assess the impact of Postglacial rock weathering in Lapland (68°N). Over 3200 measurements were carried out on roches moutonées and glaciofluvially scoured outcrops distributed within three study areas covering 8 km². Inferred weathering rates demonstrate that 10,000 years of Holocene weathering did not significantly modify the geometry of Weichselian rock surfaces. However, rates of general surface lowering range from 1 to 25, depending on the rock type, with average values at 0.2 mm ka⁻¹ for homogeneous crystalline rocks (irrespective of their acidity and grain size), 1 mm ka⁻¹ for biotite–rich crystalline rocks, and 5 mm ka⁻¹ for carbonate sedimentary rocks. Accelerated rates were recorded in weathering pits and along joints with values up to ten times higher than on the rest of the rock surface. Comparisons with cold and temperate areas suggest that solution rates of carbonate rocks are highly dependent on climate conditions, whilst granular disintegration of crystalline rocks operates at the same rate whatever the environment. It probably means that microgelivation is not efficient on ice–polished crystalline outcrops even under harsh climate conditions, and that granular disintegration proceeds under various climates from the same ubiquitous combination of biochemical processes. Last, the weathering state of Late–Weichselian roches moutonées can be usefully compared to that of Preglacial tors of the nearby Kiruna area.     

QUARTZ WEATHERING IN FREEZE–THAW CYCLES: EXPERIMENT AND APPLICATION TO THE EL'GYGYTGYN CRATER LAKE RECORD FOR TRACING SIBERIAN PERMAFROST HISTORY

The object of this study is to test the assumption that cryogenic weathering (here understood as in‐situ disintegration of rock under cold‐climate conditions including ice as a weathering agent) preferentially breaks up quartz grains. We apply the results of laboratory tests to a Quaternary sediment record. The combination of silt production, relative quartz enrichment in the silt fraction, and quartz grain micromorphology is traced in a multi‐100‐kyr lake sediment archive as indicator data for cryogenic weathering. Constant cryogenic weathering conditions are inferred for at least the last 220 000 years from a lake sediment core of El'gygytgyn Crater, northeast Russia. This is the longest continuous terrestrial archive currently known for the continental Arctic. Quartz enrichment in the fines evolves from seasonal freeze–thaw weathering as demonstrated in laboratory testing where over 100 freeze and thaw cycles crack quartz grains preferentially over feldspar. Microscopic grain features demonstrate that freeze–thaw cycling probably disrupts quartz grains along mineral impurities such as bubble trails, gas–liquid inclusions, or mineralogical sub‐grain boundaries. Single‐grain micromorphology (e.g. angular outlines, sharp edges, microcracks, brittle surfaces) illustrates how quartz becomes fragmented due to cryogenic cracking of the grains. The single‐grain features stemming from the weathering dynamics are preserved even after a grain is transported off site (i.e. in mobile slope material, in seasonal river run‐off, into a lake basin) and may serve as first‐order proxy data for permafrost conditions in Quaternary records.     

Micropiping processes and biancana evolution in southeast Tuscany, Italy

Biancane badlands consisting of small domes dissected by rills and micropipes, with rough disordered microrehef, can be found along the Apennines in Italy. The dominant processes forming biancane differ from those of badlands formed on smectite-rich mudrocks, as micropipes associated with pseudokarstic enlargement of pores and cracks predominate and form the main routes for evacuation of eroded material.Biancana evolution is controlled by water infiltration into intact bedrock, producing an erodible weathering ‘rind’ which is more porous than intact rock. This rind is easily removed by rill or micropipe flow, and erosion is therefore ‘weathering-controlled’, depending on rind production by infiltrating water. Infiltration is initially slow and stepped, due to slow water movement through very small capillary pores in intact rock alternating with rapid filling of macropores and cracks. This occurs due to rapid matrix pore enlargement by dispersion and/or dissolution. The infiltration pattern is accurately reproduced by a model built on progressive development of weathering layers by moisture penetration. Model results are consistent with weathering rind depths and erosion observed in the field, and show that a pipe network can be generated on newly exposed rock by the rainfall of one year.Propagation of the pipe network diverts a progressively larger proportion of runoff into micropipes, expanding weathering rind production within the biancana as well as on the surface. Internal weathering and flow progressively dominate with few unweathered corestones, and the biancana gradually collapses into a penultimate “soufflé-like” form.     

福建长乐屏山风化坑与河流壶穴的成因及其证据

风化坑和壶穴是一种常见的岩石坑穴地形,其形成原因和形态完全不同,但容易被误解和混淆.结合对福建长乐三溪河地区这两种坑穴地形的实地考察,本文从地貌学,沉积学,岩石化学和矿物学等各个方面对两者的形成过程、影响因素进行了论证和对比.研究表明：风化坑形成于岩石面积水的风化作用,壶穴则是由河流的旋转水流对河床产生的侵蚀作用形成;山顶风化坑的发育与河流流水作用没有关系,河流壶穴的形成与河谷的发育过程有关,河谷中的风化坑只能在河流深切后,流水不再作用到的河床部位发育;只要条件合适,风化坑或壶穴随时可以生成;风化坑与河流壶穴的形态,坑内沉积物的磨圆度、粒度特征等反映出各自不同的形成过程.风化坑内碎屑与周边岩石的化学蚀变指数CIA 值的差异反映了风化坑的化学风化成因;风化坑内碎屑与周边岩石石英长石比例的差异说明风化坑是矿物差异风化的结果;用CIA 值和英长比均无法区分河流壶穴和风化坑中的碎屑颗粒,但两者化学元素迁移特征的差别反映了风化坑的风化作用和河流壶穴的流水搬运作用的成因差别;风化坑的风化程度达不到当地风化壳的风化程度,但不同气候带风化坑碎屑的CIA 值能反映不同气候带风化作用的强度差异.     

Newly Discovered Submicron-Scale Weathering in Quartz: Geographical Implications*

Quartz is regarded as one of the minerals most resistant to chemical weathering. Nevertheless, quartz does weather under certain conditions. Several geographic phenomena, including downstream sorting of fluvial sediments, the distribution of loess, and the absence of silica in tropical soils, may be directly or indirectly related to quartz weathering. Weathering may impact the use of quartz in geographic applications of geochronology and paleoenvironmental analysis. This paper defines the molecular-scale mechanisms of chemical weathering in quartz, and presents the first examination of quartz weathering in the terrestrial environment using high resolution transmission electron microscopy (HRTEM). HRTEM images illustrate crystalline disintegration, or amorphization, of exterior surfaces and internal fractures of quartz sand grains collected from tills of the Eastern Sierra Nevada, California. Of several alternative mechanisms, in situ hydration of the crystalline matrix is the most plausible for quartz amorphization. Scale and mineral structure appear to be prominent factors in quartz amorphization; microenvironment and weathering rates may also be significant, but data are insufficient to make more precise generalizations.     

Experimental simulation of rapid rock block disintegration by sodium chloride in a foggy coastal desert

An environmental chamber was used to simulate a 24-h cycle of rock surface temperature and relative humidity, a stone pavement with differing levels of sodium chloride content, and differing levels of fog precipitation to assess some of the controls of rock weathering in the vicinity of Swakopmund in the Namib Desert. The rock employed was Cretaceous Chalk. Some treatments were effective at causing disintegration after 76 temperature/humidity cycles and 10 fog cycles. Small amounts of fog moisture (0.5 mm per event) were associated with the greatest amounts of breakdown, whereas the amount of salt in the simulated pavements appeared to be of less significance. The experiment showed that sodium chloride can be an effective agent of weathering in a coastal foggy environment.     

阿尔泰山晚新生代以来的古风化壳及其形成时代

阿尔泰山古风化壳研究结果表明，晚新生以来，至少存在两个风化期，形成了不同类型的风化壳，本文阐述这两个风化期的特征及相应形成的风化壳的类型及特点。     

Rock control on microweathering of bedrock surfaces in a periglacial environment

Microweathering of ice-smoothed bedrock surfaces was investigated in the Røldal area of Hardanger Plateau (60°), southern Norway. Postglacial rates of weathering were determined from surface lowering using quartz veins as reference surfaces. Weathering processes are inferred from assessment of weathering rind formation, surface hardness, and the preservation of small-scale glacial erosional features.Surface lowering rates for a range of metamorphic rocks vary from 0.05 to 2.20 mm ka^− 1 and are broadly comparable with those obtained from crystalline rocks in other periglacial environments. The mean rate of surface lowering at 0.55 mm ka^− 1 is low and demonstrates the relatively small impact of microweathering on postglacial landscape evolution. Variations in bedrock microweathering can be explained by lithological variation. Amphibolite and mica-rich bedrock surfaces experience greater denudation and weakening, least weathering rind formation, and abundant preservation of glacial striae, despite greater surface lowering. Conversely, quartz-rich bedrock surfaces are most resistant to denudation and weakening, but have greater weathering rind formation and fewer preserved striae. Postglacial microweathering is achieved primarily through granular disintegration involving detachment and removal of mineral grains and weakening from increased porosity. Granular decomposition is manifest in the formation of weathering rinds. Analysis of interactions between weathering indices indicates that rind accumulation is limited by microerosion.A conceptual model is proposed that illustrates the temporal interrelationships between in situ and erosional facets of microweathering in two contrasting mineral assemblages. The model proposes that cyclic processes of in situ disintegration, decomposition, and erosion are at work. The relative balance between these processes varies with lithology so that in more resistant quartz-rich rocks the net effect is minimal surface lowering and accumulation of weathering rind. In weaker, amphibolitic and micaceous rocks, the net effect is greater surface lowering and minimal accumulation of weathering rind. The results of the research demonstrate the important influence of rock properties, notably mineral composition, in postglacial microweathering of crystalline bedrock in a periglacial environment.     

AN EVALUATION OF SURFACE HARDNESS OF NATURAL AND MODIFIED ROCKS USING SCHMIDT HAMMER: STUDY FROM NORTHWESTERN HIMALAYA, INDIA

Four rock types (quartz mica gneiss, schist, quartzite and calc-silicate) located in the Satluj and Alaknanda valleys were used to test whether a Schmidt hammer can be used to distinguish rock surfaces affected by various natural and man-induced processes like manual smoothing of rock surfaces by grindstone, surface weathering, deep weathering, fluvial polishing and blasting during road construction. Surfaces polished by fluvial process yielded the highest Schmidt hammer rebound (R-) values and the blast-affected surfaces yielded the lowest R-values for the same rock type. Variations in R-value also reflect the degree of weathering of the rock surfaces. It has been further observed that, for all the rock types, the strength of relationship between R-values for the treated surfaces (manual smoothing of rock surface by grindstone) and the unconfined compressive strength (UCS) is higher than for the fresh natural surfaces.     

The role of salt weathering in the origin of the Qattara Depression, Western Desert, Egypt

Field studies and petrographic examinations of core samples and of the bedrock of the floor of the Qattara Depression, Egypt, indicate that salt weathering predominates in its western part in marked contrast to its eastern part. The eastern part of the depression is covered with more than 120-cm-thick, moist sands with sporadic occurrence of halite and gypsum due to the low salinity of the groundwater table. At the western part of the depression, the strongly saline, sodium chloride nature of the groundwater table favors crystallization of halite (and sometimes gypsum) at or near the surface of the outcropping bedrock of the Moghra clastics and/or Dabaa shale. Crystallization of halite and/or gypsum generates increased pressure that leads to mechanical disintegration of the bedrock into fine-grained debris. Features related to disintegration include blistering of the rock surface, splitting, spalling and/or granular disintegration.Alternation of dry and wet cycles favor halite crystallization, mechanical disintegration of the outcropping bedrock and dissolution of the halite cement, which exposes fine-grained debris to wind deflation. Removal of the debris from the floor of the depression leads to the accumulation of lunettes and other dunes in the downwind direction.Therefore, salt weathering provides fine-grained debris that are easily removed by deflation, which accounts for the topographically lower level of the western part of the depression (134 m below sea level (b.s.l.)). In contrast, the presence of moist sediments at the eastern part of the depression inhibits deflation and encourages sedimentation by adhesion of wind-blown sand to the damp surface of the sabkha at an elevation of 45 m below sea level.The disintegration of the bedrock of the Qattara Depression by salt weathering has been in effect since the onset of aridity in northern Egypt in Quaternary time. Whereas the initial excavation of the depression started in Late Miocene or Pliocene time by fluvial erosion, karstic process, mass-wasting and wind deflation.     

Interpretation of Rock Varnish in Australia: case studies from the arid zone

Rock varnish has a long history of study in Australia. Although Australian varnishes have biological, chemical and morphological similarities to varnishes in other continents, rock varnishes in arid Australia tend to be less stable than in other drylands. Still, with careful sampling, radiocarbon and cation‐ratio dating of rock varnish has been used to constrain the age of Henbury Meteor Craters, Northern Territory, and rock engravings in South Australia and New South Wales, as well as to assess the geomorphic surface stability of Ayers Rock, Northern Territory, and the slopes of Pepuarta Bluff in the Olary Province of South Australia.     

INTERNAL WEATHERING IN QUARTZ GRAINS

While most research on quartz weathering has focused primarily on surface textures and morphologies, very little is known about the internal weathering of quartz. This study demonstrates that internal weathering is ubiquitous in quartz. Internal weathering is measured in terms of porosity, which represents mass loss from the quartz grain, hence silica lost through dissolution. Mass loss calculated from porosity suggests higher-than-expected rates of quartz dissolution in the terrestrial environment. Internal weathering occurs through various grain defects, and is classified into several forms (in decreasing order of frequency): fractures, enlarged grain boundaries, holes, and networks. These features may be distinguished from occasional artifact voids left by laboratory procedures. The most intensely weathered grains exhibit large fractures and extensive networks, and occasionally contain secondary weathering products within the void areas. The presence of internal weathering in quartz supports field and laboratory observations of particle comminution in sediment transport systems, and can account for at least part of the production of silt- and clay-sized quartz. Given the potentially large surface area afforded by these internal defects, internal weathering plays an important role in the generation of quartz particles and dissolved silica, and presents a new avenue of study for the generation of secondary porosity in detrital sediments. [Key words: geomorphology, quartz, silica, weathering.]     

丹霞山顺层洞穴风化特征的试验研究

以丹霞盆地丹霞组2段顺层洞穴为研究对象,探讨不同岩性红层抗风化能力的差异,重点关注软岩夹层的风化特征及其对顺层洞穴发育的影响。在野外考察的基础上,选择3个典型顺层洞穴为案例,采集了洞穴剖面不同岩性红层的岩样及夹层水样品,进行一系列的岩矿理化测试和水样分析实验。结果表明：洞穴剖面岩性的不均一性及软岩夹层的风化凹进是形成丹霞山顺层洞穴的主要原因。顺层洞穴发育的软岩夹层一般是粉砂质泥岩,粘土矿物含量高,以泥质胶结为主,吸水性较强,开型空隙率较大,抗压强度小,易于风化破碎;上下岩层主要是砂岩和砂砾岩,以钙质和铁质胶结为主,吸水性较弱,开型孔隙率较小,抗压强度较大,风化速度较慢。在洞穴岩层风化过程中,粘土矿物的胀缩、钙质胶结物的溶解, 及铁质胶结物的水化作用等是导致岩体结构破坏的重要因素,但促使不同岩性红层风化的主导因素有所差别。粉砂质泥岩的风化以粘土矿物的胀缩作用为主,砂岩和砂砾岩的风化则以钙质胶结物的溶解作用为主。此外,酸性雨水渗入岩体后与各种矿物进行的一系列离子交换作用也促进了顺层洞穴岩层的风化。     

黄河悬浮物组成的地理分异

根据对1980年汛期在黄河18个水文站采集样品的分析结果,本文讨论了黄河悬浮物的粒度、矿物和化学组成从上游到下游的地理分异。     

Laboratory study of the effect of temperature difference on the disintegration of redbed softrock

To explore the impact of temperature difference (TD) on the disintegration of redbed softrock, three types of redbed rock, collected from Nanxiong Basin, were analyzed under three different treatments: TD, wetting and drying (WD), and TDWD-temperature difference and WD. To better understand the influence of different ranges of TD on disintegration during WD cycles, pH (hydrogen ion concentration) values, electrical conductivity (EC) values, and concentration of cations in leachate released during treatment were measured. The results show that no significant change can be observed under single TD treatment but that TD can increase the disintegration rate by accelerating the water–rock interaction. The effect of TD is more significant for rock with weak resistance to disintegration.     

Geostatistical Modeling of Ore Grade Distribution from Geomorphic Characterization in a Laterite Nickel Deposit

Due to growing consumption of nickel (Ni) in a range of industries, the demand for Ni has increased rapidly around the world. This trend requires a more precise estimation of available Ni grade deposits and an identification of factors controlling the grade distribution. To achieve these requirements, this study applies geostatistical techniques to spatial modeling of the Ni grade in a laterite Ni deposit, with reference to geomorphic features such as slope gradient and the thickness of limonite and saprolite zones. The Sorowako area in Sulawesi Island, Indonesia, was chosen as a case study area because it has a representative laterite Ni deposit with large reserves. Chemical content data from drillhole cores at 294 points were used for the analysis. The slope gradient was found to have a remarkable correlation with the thickness of the limonite zone, but there was no correlation between the thickness of the limonite and the saprolite zones above the bedrock. One important feature was a general correlation between the thickness of the saprolite zone and the maximum Ni grade in this zone: the grade increases with the thickness of the zone. Co-kriging was adopted to incorporate this correlation into estimating the maximum Ni grade in the saprolite zone. As a result, the maximum Ni grade in the saprolite zone tends to be high mainly in areas of slight slope. The Ni accumulation at this topographic feature probably originates from deep weathering by groundwater infiltrating through well-developed rock fractures.     

Surface Moisture Availability and Rock Weathering in Cold Climates

The importance of moisture in determining which rock weathering processes may operate, as well as the rate at which rock weathering occurs, has long been recognised. However, little is known about how often or when moisture is available to the rock surface, or for how long the surface may be wetted. This paper gives some initial results from current research on granite rocks in Antarctica. Recordings of surface moisture, for two aspects and three locations in Victoria Land, show that moisture reaches the rock surface more frequently than might initially have been expected in this dry climate. The number of occasions the surface is wetted, and the quantity of moisture the surface received, varied with aspect and time of year. Rock surfaces could be wet for up to 40% of the time during the summer months. This has important implications for rock weathering where some processes are deemed not to operate, or to operate in very restricted locations, due to the apparent absence of moisture.     

Rock weathering processes and landform development in the Sør Rondane Mountains, Antarctica

Field observations of weathering processes and the related landforms, combined with laboratory analyses of weathering products, permit a synthetic evaluation of Late Cenozoic weathering environments in the Sør Rondane Mountains, Antarctica, an arid upland characterized by low temperatures and strong winds. Rates and character of weathering depend mainly on moisture availability and the bedrock geology. Under the humid weathering regime that occurs only locally around the margin of the present sheet, frequent diurnal freeze-thaw cycles in summer cause relatively rapid rock fragmentation. Most of the mountains are situated in the arid weathering regime, under which rock breakdown is very slow unless the rock contains plenty of salts. Salt weathering becomes more intensive and extensive with exposure age, as a result of salt accumulation in rock, eventually producing soils as small as fine-silt size. Lack of clay mineralization even in weathered rocks having been exposed above the ice sheet prior to 4 Ma ago indicates that hydrolysis or carbonation of rock minerals has been insignificant during the past 4 Ma. The final products of weathering are due mainly to salt action and reflect the parent lithology. Resistant fine-grained granite forms strongly oxidized tors carved with tafoni, or fields of mushroom-like boulders overlying the fractured bedrock. Less resistant rocks, like biotite gneiss and amphibolite, produce stone pavements underlain by saline, silty soils up to 30–40 cm thick, the thickness of which corresponds to the maximum thaw depth.     

滇中红层含盐层水文地质特征

红层沉积环境多样,易溶盐会在一些区段富集形成含盐层。含盐层岩石矿物成分复杂,可溶岩和可溶性矿物含量较高,更易形成溶隙、溶孔,地下水类型以溶蚀裂隙孔隙水为主,水质复杂,水化学成分、含量与易溶矿物成分、含量关系密切。云南红层地下水勘查示范成果表明,滇中红层含盐层浅部全－强风化带含盐层容易淋滤沉着形成自封闭带,富水性较差,地下水位以下的中等－微风化带岩层透水性好,富水性强;含盐层区亦有可饮用淡水分布,补给、排泄条件好的裸露型含盐层中水质较好,循环条件差的埋藏型、覆盖型含盐层中水质较差;浅层地下水水质随深度的变化不明显。