Spheroidal Weathering Research Articles

Migration of dissolution front in a fracture network − Implications for weathering of fractured bedrock systems and boulder formation

Fracture networks contribute to the differential weathering of fractured bedrock at multiple scales. The influence of competing fracture networks is discussed in light of hydrogeochemical conditions to predict the dissolution pattern resulting from spheroidal weathering. The investigation of the dimensionless Péclet (Pe) and Damköhler (Da) numbers on a simple case of a dual-scale fracture network and its application to three fracture network weathering reveals the influence of fluid flow and reaction rate conditions on the dissolution patterns.The conceptual model was numerically validated and compared with observations on lateritic nickel deposits in New Caledonia. A fracture network will contribute to boulder development if the associated fluid flow regime reaches a Pe threshold of around 10. On the other hand, predominant diffusion will inhibit the fracture set in favour of a higher permeability fracture network. The Da drives, for its part, the width of the dissolution halo. Owing to the dependency of the fluid flow regime on the fracture network characteristics, scaling laws were established to estimate the size of the boulders at different scales by predicting the fracture contribution as a function of their Pe.

Origin of spheroidal weathering within terra rossa over limestone

Origin of spheroidal weathering within terra rossa over limestone

Discovery of supergene REE-fluorocarbonate minerals in weathered spheres of Xiajialing regolith-hosted rare earth element deposit in Xiangshan, Jiangxi Province, South China

The Xiangshan ore field is developing not only a volcanic-related hydrothermal U deposit, but also a regolith-hosted rare earth elements (REE) deposit in the middle of the volcanic rock basin, where the spheroidal weathering layer has the highest REE content. Weathered spheres may play an important role in REE mineralization. To explore the enrichment and migration processes of REE in weathered spheres at different weathering stages, the mineralogy and geochemistry of the bedrock and weathered spheres with different weathering degrees of Xiajialing REE deposits were investigated. Results showed that the REE+Y concentration was 270–310 ppm in bedrock and 372–2377 ppm in weathered spheres, respectively. With an increase in weathering, the REE+Y content increased and then decreased, and the maximum value was observed in the core of the moderately weathered sphere sample. A large number of supergene REE-fluorocarbonate minerals were discovered in the weathered spheres (particularly in samples with a high REE content), and were mainly distributed in biotite and plagioclase. The REE2O3 (including Y2O3) content of the supergene REE minerals was 69.04–80.89 wt%. The supergene REE minerals were enriched in La2O3 (21.69–25.95 wt%), Nd2O3 (12.13–23.58 wt%), and Y2O3 (2.74–14.27 wt%), but lacked Ce2O3 (<1 wt%), and contained a certain amount of F (2.66–6.23 wt%). The Raman spectra of the supergene REE-fluorocarbonate showed several bands at 272, 741, 1103, and 3100–3700 cm−1. These results indicate that REE are adsorbed on clay minerals that were formed by weathering of biotite and plagioclase; thus, the concentrations of REE around the clays were extremely high and may have exceeded the solubility product (Ksp) of the REE-fluorocarbonate, which contributed to the supergene REE-fluorocarbonate deposits. The formation of supergene REE-fluorocarbonate minerals may be one of the most important causes of the extreme enrichment of REE in some weathered sphere samples. However, the supergene mineral is easily weathered and would dissolved with weathering proceeding, leading to a decrease in the REE content in the sphere samples that have experienced relatively strong weathering. The supergene REE-fluorocarbonate may be an important intermediate product in the formation of the Xijialing regolith-hosted REE deposit.

Anhydrite Weathering Zone with Hydration Caves at Dingwall (Nova Scotia, SE Canada) as a Potential Geosite and Geodiversity Site

The abandoned gypsum quarry at Dingwall, in Canada, is a unique place with a peculiar “living” landscape created by expansive hydration of anhydrite process actively operating in the weathering zone at the quarry bottom. It causes a rapid volume increase of the rocks and the formation of the unique domed and tepee-like hydration landforms (several meters in length and up to 2.09 m high) with internal hydration caves (swelling caves, German: Quellungshöhlen). The quarry also shows some rare evaporite minerals, sulphate karst, spheroidal weathering of anhydrite and the other phenomena. Several steps of geoconservation strategy are described to promote Dingwall quarry as a geosite and a geodiversity site: inventory, quantitative assessment, conservation, interpretation, promotion and monitoring. The performed inventory describes the elements of the geoheritage. The quantitative assessment of the site, made according to the method by Brilha (Geoheritage 8:119–134, 2016), was based on the four aspects: scientific value, potential educational use, potential tourist use and the rate of destruction. For showing the geological heritage, the plan of the site was prepared, with paths for visitors, attendant infrastructure, thematic interpretation panels and plates with QR codes. As a part of the promotion, application of the Internet with already operating sites (e.g. hydrationcave.com) is proposed. In turn, monitoring, with measurements of benchmarks and photographic and photogrammetric documentation, will ensure the proper functioning of the site after establishing it as a geosite and a geodiversity site.

Re-assessing the effect of differential weathering of minerals on strontium stable isotope behavior during granodiorite weathering

The shift in Sr stable isotope ratios (88Sr/86Sr, reported as δ88/86Sr values) in weathering systems provides insights into the transformation of the binding form of Sr between the dissolved and solid phases. However, the mechanism behind the fractionation of Sr stable isotopes during chemical weathering remains poorly constrained. We present mineral and elemental compositions along with the radiogenic Sr isotope ratios (87Sr/86Sr) and δ88/86Sr values from a ∼ 1 m deep horizontal spheroidal weathering profile of granodiorite in southeast China. Strontium was markedly lost during granodiorite weathering and a large amount of plagioclase and hornblende was decomposed in saprolite relative to the parent rock. The 87Sr/86Sr ratios substantially increased from 0.708704 to 0.712532 and δ88/86Sr values sharply decreased from 0.29‰ to −0.12‰ in the corestone-near saprolite as the weathering intensity increased. However, these values were rather constant in the corestone-distant saprolite, albeit with a large increase in the abundance of kaolinite. The Sr in the different chemical fractions indicates that the silicate components dominate the Sr isotopic composition in bulk rock and saprolite samples. Differential weathering of minerals is mainly responsible for the Sr stable isotopic composition of the corestone-near saprolite, while the lower δ88/86Sr values of the corestone-distant saprolite point to the possibility of preferential incorporation of lighter Sr isotopes into the structural sites of secondary clay minerals. In contrast, δ88/86Sr values of the exchangeable and reducible fractions (0.11‰–0.48‰) were more positive than those of the corresponding bulk samples because they were able to inherit the isotopic signatures of the weathered minerals. Further, an acid leaching experiment on the granodioritic parent rock reveals that mineral weathering reactions dominated the Sr isotope composition of the dissolved phase. The substantially higher δ88/86Sr values in the leachates relative to those in the residual solids (Δ88/86Srleachate-residue = 0.14‰–0.42‰) are consistent with the fact that isotopically heavier Sr in the dissolved load of rivers is mostly set by differential weathering of minerals (such as biotite and plagioclase). Combined with classic indices of chemical weathering (such as the chemical depletion fraction (CDF) and chemical index of alteration (CIA)), we suggest that Sr stable isotopes offer a promising proxy of the intensity of silicate weathering in saprolite studies in which isotope fractionation caused by biological cycling is negligible.

Weathering of Viamão granodiorite, South Brazil: Part 1 – Clay minerals formation and increase in total porosity

Weathering transforms fresh un-weathered rock into saprolite and soils, porous materials that may hold available water for plants and nutrients. The conjoint characterization of mineral weathering and development of porosity is helpful in understanding rock weathering and the development of soil.The objective of this study was to investigate mineralogical transformations and the increase in total porosity during granodiorite weathering in South Brazil by combining petrographical observations, mineralogical analyses, and porosity measurements. The studied granodiorite profile presented a saprock of ∼2 m thickness with spheroidal weathering, a ∼15 m thick saprolite of and soil of ∼2 m thickness. The intensity of weathering was estimated using chemical indices (CIA and WIS) and by the production of fines particles of silt and clay sizes. As weathering increased from saprock to saprolite and soils total porosity increased from 1.73 % in the un-weathered rock, between 3.57 and 10.5 % in the saprock and 11.02 % in the saprolite. The chemical losses were limited in saprock and saprolite (CIA = 68, Δ4Si = 29.31 %) and increased in the topsoil (CIA = 85.73, Δ4Si = 56.83 %), indicating a moderate weathering intensity. Petrographic observations by optical microscope and SEM, chemical composition using SEM-EDS and X-ray diffraction results showed principal mineral weathering reactions were the transformations of biotite into vermiculite (with biotite/vermiculite mixed layers as intermediate weathering stages) and weathering of plagioclases and potassic felspars into kaolinite. The multi-mineral composition of the saprolite indicated a progressive transformation of primary minerals with increasing weathering intensity. The results show that the opening of the porosity is likely to control the development of the weathering and should deserve more detailed characterisation.

Geochemistry of a spheroidal weathering profile in a tropical mountainous landscape, Rio de Janeiro, Brazil

Boulders are an important material in debris flow and their source is coupled with spheroidal weathering profiles that produce corestones. The goal of this work was to establish the geochemical transformations that produced corestones and distinguished them from the surrounding grus in two tropical granite weathering profiles (P1 and P2). Sampling was not performed in a vertical profile; instead, we gathered 13 (P1) to 16 (P2) samples displaying different weathering degrees (corestone and saprolite) and spatial positions in the profiles. We conducted the geochemistry (EDXRF/EDX and INAA) and mineralogy (petrography and XRD) of the samples. The CIA values ranged from 46 (corestones) to 93 (saprolite). Granite spheroidal weathering under a tropical mountainous slope develops mostly due to feldspar weathering (foremost plagioclase) in the following sequence: porosity growth, kaolinite, and gibbsite crystallization. Zircon weathering stability and its probable mobility as grain along the weathering profile play an important role in REE concentration. Spheroidal weathering is mainly a lixiviation process, yet specific locations (below the individualized corestones) presented REE enrichment due to translocation. They are hosted mainly by clay minerals and, to some extent, by amorphous Fe oxyhydroxide. The evolution of spheroidal weathering results in a vertical patchy weathering profile.

Geomorphosites of Nui Chua National Park, Ninh Thuan Province, Vietnam

Nui Chua National Park, Ninh Thuan province, is a semi-arid climate while locating right next to a tropical forest climate, therefore, it has a rich biodiversity ecosystem, where the coastal dry forest ecosystem is rarely seen in Vietnam. This area contains many valuable geomorphological features in denuded – lithological – tectonic terrain, among them, three features can be considered as national treasures: 1) Nui Chua dome – block mountains; 2) Rai Cave tectonic – abrasive coastline and 3) Lang Choi Stone Park. Nui Chua dome – block mountains are the unique geomorphosite of Vietnam which has the right conditions for the dry ecosystem to develop. The tectonic – abrasive coastline of Rai Cave and Lang Choi Rock Park is home to unique and precious geoheritages such as two-stage development of boulders or balanced rocks, karren sculpturing, caves, notches on ancient coral reefs terrace or spheroidal weathering, tafoni on intrusive rocks, to name a few. To archive the goal of sustainable development, it is important to link the conservation activities of the biodiversity at Nui Chua National Park to the preservation efforts of geodiversity (geology, geomorphology, etc.) in the area. Providing scientific explanations at each geomorphosites together with safe sightseeing guides will help increase the quality of tourism, attract more visitors to explore and learn about this mysterious, wonderful, and diverse ecosystem.

The Kui boulder fields: Formation processes controlled by columnar joints of granodiorite

Boulder fields of granitic rocks have long been believed to be the result of subsurface spheroidal weathering and subsequent erosion of weathering products, such as saprolite, and the exhumation of corestones. However, the morphological characterization of boulder fields has not been sufficient to examine the details of their origin. We performed a 3-D morphological investigation of boulder fields using UAV and SfM for the Kui boulder field, Hiroshima, western Japan, and found that the boulder field consists of polygonal rock columns, as well as rounded corestones. Some of these rock columns had chamfering cracks making them primary corestones. An outcrop of the weathering profile near the boulder field showed that corestones covered by weathering rindlets were embedded in saprolite surrounded by a polygon that was inherited from the columnar joints. The saprolite was locally removed by subsurface erosion to leave openings, which eventually led to the collapse of the corestone framework. Thus, the columnar joints provided basic conditions for the boulder field formation.

Characteristics and significance of intravolcanic saprolite paleoweathering and associate paleosurface in a silicic effusive volcano: The case study of Monte Amiata (middle Pleistocene, Tuscany, Italy)

A paleoweathering saprolite mantle and an associated paleosurface have been recognized into the stratigraphic volcanic succession of the silicic effusive (trachydacite) middle Pleistocene Monte Amiata volcano (Italy). This element has been used for applying UBSU (Unconformity Bounded Stratigraphic Units) subdivision to the volcano stratigraphy. All the lavas emplaced in the onset of the volcanic activity (Bágnore Synthem, BAS) have been indistinctly affected by a saprolite in-situ alteration. Saprolite facies varies from poor lithoid consistency but with conservation of the original texture, to fractured rock with spheroidal weathering, to incoherent rock consisting of a prevalent sandy fraction, up to accumulations of residual lava blocks (core-stones, mainly in coincidence with the arcuate ridges on top of the main lava flows). Unaltered lavas of the younger phase of volcanic activity (Monte Amiata Synthem, MAS) have preserved the paleosurface within its own saprolite. Where the MAS cover is absent, the saprolite mantle represents a relict paleosurface on the present landscape. Weathering of the silicic lavas was favored by the vitrophyric texture of the parent rock and by moderate topographic reliefs formed by extensive lava plateaus positioned at various elevations. The palaeowathering saprolite is indicative of warm and relatively humid temperate climatic conditions and relative tectonic and morphogenetic stability.

Interpreter's Corner

Basaltic reservoirs have produced hydrocarbon from Yurihara Field in Japan, Quiko Depression in China, and Padra Field of Cambay Basin and Western Offshore Basin in India. The availability of fractured, altered, and vesicular basalts contributes to reservoir development in this stratigraphic unit. This study is conducted in the Kutch-Saurashtra Basin, located at the western continental margin of the Indian subcontinent wherein, the Deccan basalt, with a thickness range of 200–2500 m, overlies Mesozoic sediments. The Jurassic and Cretaceous sediments constitute the main source rock in the area. Several wells have been drilled through the entire basalt section, and some are hydrocarbon bearing in basalt. The entire basalt section in the study area has been classified into four major units using gamma-ray logs. These units have been further subdivided into individual flows and correlated all over the basin. Analysis shows that the base of an individual basalt flow is massive, and the top is differentially altered. Crossplot analysis of P-impedance and VP/VS ratio carried out on logs delineates a zone of moderately weathered/altered basalt, which is due to spheroidal weathering and calibrated with sidewall cores. These moderately altered zones between two successive flows of basalt are the probable reservoir facies for hydrocarbon accumulation, provided that there is an overlying seal in the form of massive or completely altered basalt. Three-dimensional seismic data in the area show an alternating reflection pattern in the basalt section due to the alternation of massive and weathered basalt. The seismic signature of basalt in the area is similar to that of a sedimentary sequence in any given area. Continuity of the identified individual flows in seismic scale has been propagated over the entire seismic, and subsequent inversion has facilitated the deciphering of the probable hydrocarbon-bearing locales within basalt.

Considerations Relevant to the Stability of Granite Boulders

Granite boulders are characteristic geomorphological structures formed in granitic terrains. Due to their formation process associated with typical spheroidal weathering phenomena, they tend to show more or less ellipsoidal shapes prone to instability, and they often lie on small contact surfaces. Analyzing the stability of these boulders is not a straightforward task. First, these boulders may topple or slide. Additionally, their typically irregular geometry and uneven contact with the surface where they lie makes the analysis more complex. The authors have identified some critical issues that are relevant to characterize these boulders from a rock mechanics point of view, with the aim of estimating the stability of boulders. In particular, an accurate description of the geometry of the boulder is necessary to perform accurate toppling calculations. Additionally, the contact area and the features of the contact plane need to be known in detail. The study is intended to serve as a guideline to address the stability of these granite boulders in a rigorous way, since standard rock mechanics approaches (planar failure, toppling stability, standard rock joint strength criteria, etc.) may not be directly applicable to these particular cases.

Spheroidal weathering of basalt from Gebel Qatrani, Fayum Depression, Egypt

BackgroundThree stages of basalt weathering are known: early or incipient weathering, intermediate weathering and advanced weathering. The Late Oligocene basalt of Gebel Qatrani in Fayum Depression, Egypt, shows signs of early weathering, particularly exhibited in basalt spheroids found at the top of the basalt flow. The present paper gives the results of detailed petrographical, mineralogical and geochemical study of the weathering of these basalt spheroids.ResultsThe core-stones of the basalt spheroids are composed of phenocrystals of plagioclase feldspars and clinopyroxenes set in a groundmass of tiny feldspars and pyroxenes, relatively altered olivine and opaque minerals. The basalt is subalkali (tholeiitic). The outer weathered shells surrounding the core-stones are composed of partly altered feldspars and pyroxenes. The calculated weathering indices show that there is marked weathering trend from the core-stones of the spheroids to the outer shells. The chemical mobility of the elements shows marked depletion of Mg, Ca, Na and K from the core-stones to the outer shells due to the weathering of olivine, pyroxene and feldspars. The trace elements Rb, Sr, Ni, V, Cr and Zn are also depleted.ConclusionsThe weathering of basalt spheroids from Gebel Qatrani, Fayum Depression, Egypt, is of the incipient type. The degree of weathering from the core-stones of the basalt spheroids to the corresponding weathered shells indicates that the weathering occurred under predominantly semiarid to arid conditions.

Stability analysis of block-flexural toppling of rock blocks with round edges

One of the most conventional toppling instabilities is the block-flexural toppling failure that occurs in civil and mining engineering projects. In this kind of failure, some rock columns are broken due to tensile bending stresses, and the others are overturned due to their weights, and finally, all of the blocks topple together. A specific feature of spheroidal weathering is the rounding of the rock column edges. In the mode of flexural toppling failure, rounding of edges happens only at the upper corners of the block but in the block toppling failure mode, due to the presence of cross-joints at the base of the block, rounding of edges also occurs at the base of the block. In this work, a theoretical model is offered to block-flexural toppling failure regarding the erosion phenomenon. The suggested methodology is evaluated through a typical example and a case study. The results of this research work illustrate that in the stable slopes with rectangular prismatic blocks, where the safety factor value is close to one, the slope is subjected to failure due to erosion. Also the results obtained show that the recommended approach is conservative in analyzing the block-flexural toppling failure, and this approach can be applied to evaluate this failure.

Evaluating weathering of palaeosols in Cameroon (Central Africa) as a tool for paleoenvironmental reconstruction

Palaeosols formed from different igneous rocks have been identified and characterized in the humid savannas (1400 mm mean annual rainfall, 22 °C mean annual temperature) of the Adamaoua region in north-central Cameroon by means of macro- and micromorphology, geochemical analyses and bulk mineralogy. The studied sequence at Mangoli is about 11 m deep and is divided into three sections, in terms of weathering products: (1) Precambrian granite, located in the base; (2) vesicular porphyritic basalt, in the middle section; (3) aphanitic basalt in the uppermost layer. The products of weathering from the Precambrian granite consist of the following horizons, from bottom to top: a saprolite with granite structure, a yellowish to whitish fine saprolite, and Btg and Bt horizons. The saprolite horizon is dominated by quartz, feldspars (plagioclases and microcline), and muscovite. Primary minerals, except quartz, show weathering, while clay fills the pores and fractures and kaolinite dominates the clay mineralogy. The granite weathering products are slightly acidic (pH ranging from 5.9 to 6.3) and display low values of Fe extracted with dithionite-soluble Fe (Fed) and high Chemical Index of Alteration (CIA) and Mineral Index of Alteration (MIA) values. The middle section of the sequence consists of different areas of the basalt’s spheroidal weathering zone, from the more advanced to the less weathered phase. The clay mineralogy is dominated by kaolinite. The zones with the most alteration are the fractures, where thick, reddish clay coatings are observable. In these areas, the minerals are well crystallized. The kaolinite crystals share areas with Fe oxides. The upper section of the sequence constitutes the modern soil from the aphanitic basalt and consists of a spheroidal weathering zone divided into three horizons. The lowest is a C horizon, overlaid by Bt horizons. The C horizon shows a high degree of weathering in some areas, but it is still possible to identify the primary minerals. Their products of weathering are slightly acidic (pH = 6) and have high Fed, CIA and MIA values, which are indicative of intense weathering. The results suggest warm and humid climates during the formation of all of the palaeosols, similar to modern environmental conditions. We conclude that Palaeosol 1, formed from the granite, represents a soil formed during the Mid-Miocene Climatic Optimum, when the temperatures are assumed to have been 3–6 °C warmer than today. Features discovered during field investigations reveal that the former valley at the surface of the landscape has been filled by lava flows and today occupies the top of the plateau. Meanwhile, the former top of the plateau is located on the slopes and lower altitudes; the burial of soils by volcanic materials at Mangoli increased the altitudes of the former plateau and resulted in landscape inversion (slopes). These palaeosols and their top layers that induced the slope inversion have not yet been described and appear as a unique example worldwide.

Landslides associated with spheroidally weathered mantle of granite porphyry induced by 2011 Typhoon Talas in the Kii Peninsula, Japan

Typhoon Talas brought heavy rain to the Kii Peninsula, Japan, during 2–5 September 2011, causing hundreds of debris avalanches and debris flows in the southeastern part of the peninsula. We undertook field investigations to understand the geological and geomorphological context of the landslides and found that the most important geological factor is spheroidal weathering of granite porphyry. This granite porphyry is deeply weathered with corestones in higher elevations and forms low-relief surfaces, of which marginal areas were the major sources of debris avalanches. Corestones that failed were embedded in grus in-situ or on nearby slopes of mudstone in which the granite porphyry intruded. The corestones are made by spheroidal weathering from well-developed columnar joints and sheeting joints. Sheeting joints in the source granite porphyry acted as sliding surfaces both before and during the occurrence of the 2011 debris avalanches.

THE EFFECT OF SPHEROIDAL WEATHERING ON THE PYSICO-MECHANICAL PROPERTIES OF TROPICAL GRANITE BOULDER

The aim of this research is to investigate the effect of spheroidal weathering on the physico-mechanical properties of in situ granite boulder in tropical environment. This research was conducted to gain more understanding due to lack of study related to this topic especially on tropical boulder in completely weathered zone. A number of 34 in situ granite boulders that naturally formed in completely weathered zone were selected and investigated from two quarries located in Southern Johor, Malaysia. Classification of weathering grade was conducted on the material surrounding boulder based on discolouration, degree of friability and texture characteristics as suggested by ISRM(1). The colour of the weathered material were classified based on Rock Colour Chart (2). Petrographic analysis and scanning electron microscopy (SEM) were carried out to analyze the mineralogical and microstructure alterations, respectively due to weathering effect. The changes of physico-mechanical properties at the surrounding of boulder due to weathering were also analyzed including dry density, porosity, point load strength, uniaxial compressive strength and permeability. The dry density and porosity were conducted based on ASTM (3) and Brown (4), respectively. The point load uniaxial compressive strength were carried out according to ISRM (1) while permeability test was undertaken based on Head (5). The tropical granite boulder possessing rindlets with significant different of colours, degree of friability and texture. This differences were classified into four primary zones: corestone (C), inner rindlets (IR), outer rindlets (OR), and saprolites (S) with weathering grade of II, III/IV, IV/V and V, respectively. Due to spheroidal weathering, three to six concentric sheets of rindlets with whitish grey to light brown colour were formed at the surrounding of corestone's boulder. Each sheets of rindlets has 2.0 cm to 3.0 cm thick with total thickness up to 80 cm. The microcracks aperture of IR and OR is ranging from 150 to 200 µm and 200 µm to 1 mm, respectively. Analysis revealed that the higher the weathering grade, the higher and wider the fracture opening formed in the rindlets up to 2 mm width. Analysis exibited that the increase of weathering grade from Grade II to IV along C to S were significantly reduced the plagioclase, K-feldspar, and biotite with reduction of 74%, 74.2% and 87.5%, respectively. The alteration of minerals and microstructure due to the weathering were significantly reduced the dry density, point load strength, uniaxial compressive strength and permeability with 32%, 99.5%, 98.6% and 84.8%, respectively.The reaction of spheroidal weathering is significantly affecting the physico-mechanical properties of the tropical granite boulders by alteration of its minerals and microstructure characteristics at the surrounding of the boulder.

Spheroidal weathering of columnar-jointed basalt of the Ogura lava in the Takurayama Volcano, western Japan

Spheroidal weathering is an important form of rock weathering and has been attributed to chemical weathering from joint surfaces. However, the effects of the original rock structures on the processes of spheroidal weathering have not been well addressed. A columnar-jointed basalt contains primary structures that resulted from cooling, which exerted strong controls on the development of weathering with hard crusts. The primary structures were small-pore-size layers beneath the surfaces of columnar joints and some cross joints. Weathering of plagioclase was less intense in the small-pore-size layers, resulting in the exfoliation of surface parts of the layers and internal cracks.

Understanding fracture distribution and its relation to knickpoint evolution in the Rio Icacos watershed (Luquillo Critical Zone Observatory, Puerto Rico) using landscape‐scale hydrogeophysics

AbstractThe Rio Icacos watershed in the Luquillo Mountains (Puerto Rico) is unique due to its extremely rapid weathering rates. The watershed is incised into a quartz diorite that has developed a large knickzone defining the river profile. Regolith thickness within the watershed generally decreases from 20 to 30 m at the ridges to several meters in the quartz diorite‐dominated valley to tens of centimeters near the major river knickpoint, as determined from previous studies. Above the knickzone, we observe spheroidal corestones, but below this weathering is much less apparent. Measured erosion rates from previous studies are also high in the knickzone compared with upper elevations within the river profile. A suite of near‐surface geophysical methods (i.e. ground penetrating radar and terrain conductivity) capable of fast data acquisition in rugged landscapes, was deployed at kilometer scales to characterize critical zone structure. Concentrations of chaotic ground penetrating radar (GPR) reflections and diffraction hyperbolas with low electrical conductivity were observed in vertical zones that outcrop at the land surface as areas of intense fracturing and spheroidally weathered corestones. The width of these fractured and weathered zones showed an increase with proximity to the knickpoint, and was attributed to dilation of these sub‐vertical fractures near the knickpoint, as postulated theoretically by a stress model calculated for the topographic variability across the knickzone in the Rio Icacos, and that shows a release of compressive stress near the knickpoint. We hypothesize that erosion rates increase in the knickzone because of this inferred dilation of fractures. Specifically, opened fractures could enhance access of water and in turn promote spalling, erosion, and spheroidal weathering. This study shows that ground‐based hydrogeophysical methods used at the landscape‐scale (traditionally applied at smaller scales) can be used to explore critical zone architecture at the scales needed to explain the extreme variability in erosion rates across river profiles. © 2018 John Wiley &amp; Sons, Ltd.

Block toppling stability in the case of rock blocks with rounded edges

A characteristic feature of spheroidal weathering is the rounding of rock block edges, which affects the mechanical stability of slender rock blocks and, consequently, that of slopes prone to toppling. We analysed the influence of this erosion phenomenon on block toppling stability, first discussing the geological environments that produce this kind of phenomena and then reviewing classic limit equilibrium equations for block toppling that account for the role played by rounded edges. On the basis of this approach, it is clear that rounded edges do not greatly affect stability against sliding. However, since the equation to compute stability against toppling tends to overestimate the stability of slopes with round-edged slender blocks, we propose a modification that results in a more accurate estimation.In physical model testing in the laboratory, we compared results for sharp-edge block models and artificially weathered rounded-edge blocks, confirming our formulated hypothesis and enabling us to explain the failure of sets of a small number of not-so-slender blocks. Fieldwork case studies confirm that rounded edges play a role in decreasing stability against toppling. We suggest that our proposed approach can be an appropriate tool to take this effect into account.