Rock Fragments (rock + fragment)

Distribution by Scientific Domains


Selected Abstracts


The dynamics of organic matter in rock fragments in soil investigated by 14C dating and measurements of 13C

EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2002
A. Agnelli
Summary Rock fragments in soil can contain significant amounts of organic carbon. We investigated the nature and dynamics of organic matter in rock fragments in the upper horizons of a forest soil derived from sandstone and compared them with the fine earth fraction (<2 mm). The organic C content and its distribution among humic, humin and non-humic fractions, as well as the isotopic signatures (,14C and ,13C) of organic carbon and of CO2 produced during incubation of samples, all show that altered rock fragments contain a dynamic component of the carbon cycle. Rock fragments, especially the highly altered ones, contributed 4.5% to the total organic C content in the soil. The bulk organic matter in both fine earth and highly altered rock fragments in the A1 horizon contained significant amounts of recent C (bomb 14C), indicating that most of this C is cycled quickly in both fractions. In the A horizons, the mean residence times of humic substances from highly altered rock fragments were shorter than those of the humic substances isolated in the fine earth. Values of ,14C of the CO2 produced during basal respiration confirmed the heterogeneity, complexity and dynamic nature of the organic matter of these rock fragments. The weak 14C signatures of humic substances from the slightly altered rock fragments confirmed the importance of weathering in establishing and improving the interactions between rock fragments and surrounding soil. The progressive enrichment in 13C from components with high- 14C (more recent) to low- 14C (older) indicated that biological activity occurred in both the fine and the coarse fractions. Hence the microflora utilizes energy sources contained in all the soil compartments, and rock fragments are chemically and biologically active in soil, where they form a continuum with the fine earth. [source]


The first Raman spectroscopic study of San rock art in the Ukhahlamba Drakensberg Park, South Africa

JOURNAL OF RAMAN SPECTROSCOPY, Issue 5 2008
Linda C. Prinsloo
Abstract San rock art sites are found throughout southern Africa; unfortunately this unique heritage is rapidly being lost through natural weathering processes, which have been the focus of various studies conducted in the uKhahlamba Drakensberg Park since 1992. It has recently been shown that the ability of Raman spectroscopy to identify salts on rock faces on a micro, as well as nano scale, can make a contribution to these projects. In order to test the feasibility of undertaking on-site analyses, a small rock fragment with red and white pigments still attached, which had weathered off the rock face, was analysed with Raman spectroscopy under laboratory conditions, using a Dilor XY Raman instrument and a DeltaNu Inspector Raman portable instrument. A small sample of black pigment (<1 mm2), collected from a badly deteriorated painting and a few relevant samples collected on site, were analysed as well. It was possible to identify most of the inorganic pigments and minerals detected with previous XRD and EDX measurements including whewellite and weddellite coatings, which could be a tool for carbon dating purposes. Two carotenoid pigments were detected for the first time in San rock art pigments. Animal fat was also observed for the first time on both red and white pigments, on the rock face adjacent to the paintings and in highest concentrations on the back of the rock fragment. The spectra quality makes successful on-site measurements a good prospect. Copyright 2008 John Wiley & Sons, Ltd. [source]


Laboratory simulation of the salt weathering of schist: II.

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 5 2007
Fragmentation of fine schist particles
Abstract Recent developments in long term landform evolution modelling have created a new demand for quantitative salt weathering data, and in particular data describing the size distribution of the weathered rock fragments. To enable future development of rock breakdown models for use in landscape evolution and soil production models, laboratory work was undertaken to extend existing schist/salt weathering fragmentation studies to include an examination of the breakdown of sub-millimetre quartz chlorite schist particles in a seasonally wet tropical climate. Laser particle sizing was used to assess the impact of different experimental procedures on the resulting particle size distribution. The results reveal that salt weathering under a range of realistic simulated tropical wet season conditions produces a significant degree of schist particle breakdown. The fragmentation of the schist is characterized by splitting of the larger fragments into mid-sized product with finer material produced, possibly from the breakdown of mid-sized fragments when weathering is more advanced. Salinity, the salt addition method and temperature were all found to affect weathering rates. Subtle differences in mineralogy also produce variations in weathering patterns and rates. It is also shown that an increase in drying temperature leads to accelerated weathering rates, however, the geometry of the fracture process is not significantly affected. Copyright 2006 John Wiley & Sons, Ltd. [source]


Development of partial rock veneers by root throw in a subalpine setting

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 1 2006
W. R. Osterkamp
Abstract Rock veneers stabilize hillslope surfaces, occur especially in areas of immature soil, and form through a variety of process sets that includes root throw. Near Westcliffe, Colorado, USA, data were collected from a 20 500 m transect on the east slope of the Sangre de Cristo Mountains. Ages of pit/mound complexes with rock fragments exposed at the surface by root throw ranged from recent (freshly toppled tree) to unknown (complete tree decay). Calculations based on dimensions of the pit/mound complexes, estimated time of tree toppling, sizes of exposed rock fragments, and percentage rock covers at pit/mound complexes, as well as within the transect area, indicate that recent rates of root throw have resulted in only partial rock veneering since late Pleistocene deglaciation. Weathering of rock fragments prevents development of an extensive rock veneer and causes a balance, achieved within an estimated 700 years, between the rates of rock-fragment exposure by root throw and clast disintegration by chemical reduction. The estimated rate of rock-fragment reduction accounts for part of the fluvial sediment yields observed for forested subalpine areas of western North America. Copyright 2005 John Wiley & Sons, Ltd. [source]


The dynamics of organic matter in rock fragments in soil investigated by 14C dating and measurements of 13C

EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2002
A. Agnelli
Summary Rock fragments in soil can contain significant amounts of organic carbon. We investigated the nature and dynamics of organic matter in rock fragments in the upper horizons of a forest soil derived from sandstone and compared them with the fine earth fraction (<2 mm). The organic C content and its distribution among humic, humin and non-humic fractions, as well as the isotopic signatures (,14C and ,13C) of organic carbon and of CO2 produced during incubation of samples, all show that altered rock fragments contain a dynamic component of the carbon cycle. Rock fragments, especially the highly altered ones, contributed 4.5% to the total organic C content in the soil. The bulk organic matter in both fine earth and highly altered rock fragments in the A1 horizon contained significant amounts of recent C (bomb 14C), indicating that most of this C is cycled quickly in both fractions. In the A horizons, the mean residence times of humic substances from highly altered rock fragments were shorter than those of the humic substances isolated in the fine earth. Values of ,14C of the CO2 produced during basal respiration confirmed the heterogeneity, complexity and dynamic nature of the organic matter of these rock fragments. The weak 14C signatures of humic substances from the slightly altered rock fragments confirmed the importance of weathering in establishing and improving the interactions between rock fragments and surrounding soil. The progressive enrichment in 13C from components with high- 14C (more recent) to low- 14C (older) indicated that biological activity occurred in both the fine and the coarse fractions. Hence the microflora utilizes energy sources contained in all the soil compartments, and rock fragments are chemically and biologically active in soil, where they form a continuum with the fine earth. [source]


Flow-retarding effects of vegetation and geotextiles on soil detachment during concentrated flow

HYDROLOGICAL PROCESSES, Issue 17 2009
A. Knapen
Abstract Roughness elements at the soil surface (e.g. crop residues, rock fragments, vegetation, geotextiles) strongly reduce the erosivity of overland flow (both interrill and concentrated overland flow) and hence soil detachment rates. Common methods for shear stress partitioning that allow the calculation of effective flow shear stress in the presence of roughness elements originate from river hydraulics but seem invalid for overland flow. An alternative method to estimate the effective flow shear stress in the presence of a soil surface cover has been presented and tested for soil detachment by concentrated runoff on soil surfaces covered by crop residues by Knapen et al., (2008). In this method, the estimation of effective flow shear stress is based on the recalculation of the hydraulic radius for covered soil surfaces using flow hydraulics on uncovered surfaces. However, the applicability of this method for roughness elements different from crop residues and under field conditions needs to be tested to confirm its validity. Therefore, runoff data from three experimental studies (conducted on geotextile and grassed-covered soil surfaces) are analysed in this study. The results show that effective flow shear stress, calculated using this method, is not only a good soil detachment predictor for soil surfaces covered with crop residues but also for the tested soil surfaces with a vegetation or geotextile cover. However, the geotextile experiments point to one of the shortcomings of the method. At high flow shear stress levels, vortex erosion due to flow turbulence is reported for the thickest geotextiles. These flow turbulences are not accounted for since the method is based on average flow characteristics. Copyright 2009 John Wiley & Sons, Ltd. [source]


Applying discontinuous deformation analysis to assess the constrained area of the unstable Chiu-fen-erh-shan landslide slope

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 5 2007
Jian-Hong Wu
Abstract Chiu-fen-erh-shan landslide is a remarkable slope failure occurred during the Chi-Chi earthquake in 1999. In November of 2002, abnormal geomorphologic features, including buckling and ground subsidence, were observed on the lower slope of the Chiu-fen-erh-shan landslide. This study attempts to assess the constrained area of a future collapsing on the slope using a dynamic discrete numerical analysis method, discontinuous deformation analysis (DDA). The simulation results show that the depression in front of the toe of the slope provides a space for arresting the whole sliding rocks when only the unstable lower slope fails. However, as the whole slope slides, the rock fragments move farther into the memorial park and can impact other facilities resulting in the enlarging of constrained area. The authority should prohibit people from entrancing the constrained area in the rainy season. Copyright 2006 John Wiley & Sons, Ltd. [source]


Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central Argentina

JOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2005
S. J. WHITMEYER
Abstract The Tres Arboles ductile fault zone in the Eastern Sierras Pampeanas, central Argentina, experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes. Much of the fault zone is characterized by a (D2) ultramylonite, composed of fine-grained biotite + plagioclase, that lacks a well-defined preferred orientation. The D2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep. These ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase, sillimanite and garnet inherited from the gneissic and migmatitic protolith (D1) of the hangingwall. The assemblage of garnet + sillimanite + biotite suggests that D1-related fabrics developed under upper amphibolite facies conditions, and the persistence of biotite + garnet + sillimanite + plagioclase suggests that the ultramylonite of D2 developed under middle amphibolite facies conditions. Greenschist facies, mylonitic shear bands (D3) locally overprint D2 ultramylonites. Fine-grained folia of muscovite + chlorite biotite truncate earlier biotite + plagioclase textures, and coarser-grained muscovite partially replaces relic sillimanite grains. Anorthite content of shear band (D3) plagioclase is c. An30, distinct from D1 and D2 plagioclase (c. An35). The anorthite content of D3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite. Biotite is partially replaced by muscovite and/or chlorite, particularly in areas of inferred high strain. Quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric. All D3 reactions require the introduction of H+ and/or H2O, indicating an open system, and typically result in a volume decrease. Syntectonic D3 muscovite + quartz + chlorite preferentially grew in an orientation favourable for strain localization, which produced a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of aqueous fluids, on a scale of hundreds of micrometre. Local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids. However, the predominant greenschist facies deformation (D3) along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system. [source]


Australasian microtektites and associated impact ejecta in the South China Sea and the Middle Pleistocene supereruption of Toba

METEORITICS & PLANETARY SCIENCE, Issue 2 2006
Billy P. Glass
Unmelted ejecta were found associated with the microtektites at this site and with Australasian microtektites in Core SO95,17957,2 and ODP Hole 1144A from the central and northern part of the South China Sea, respectively. A few opaque, irregular, rounded, partly melted particles containing highly fractured mineral inclusions (generally quartz and some K feldspar) and some partially melted mineral grains, in a glassy matrix were also found in the microtektite layer. The unmelted ejecta at all three sites include abundant white, opaque grains consisting of mixtures of quartz, coesite, and stishovite, and abundant rock fragments which also contain coesite and, rarely, stishovite. This is the first time that shock-metamorphosed rock fragments have been found in the Australasian microtektite layer. The rock fragments have major and trace element contents similar to the Australasian microtektites and tektites, except for higher volatile element contents. Assuming that the Australasian tektites and microtektites were formed from the same target material as the rock fragments, the parent material for the Australasian tektites and microtektites appears to have been a fine-grained sedimentary deposit. Hole 1144A has the highest abundance of microtektites (number/cm2) of any known Australasian microtektite-bearing site and may be closer to the source crater than any previously identified Australasian microtektite-bearing site. A source crater in the vicinity of 22 N and 104 E seems to explain geographic variations in abundance of both the microtektites and the unmelted ejecta the best; however, a region extending NW into southern China and SE into the Gulf of Tonkin explains the geographic variation in abundance of microtektites and unmelted ejecta almost as well. The size of the source crater is estimated to be 43 9 km based on estimated thickness of the ejecta layer at each site and distance from the proposed source. A volcanic ash layer occurs just above the Australasian microtektite layer, which some authors suggest is from a supereruption of the Toba caldera complex. We estimate that deposition of the ash occurred ,800 ka ago and that it is spread over an area of at least 3.7 times 107 km2. [source]


A fossilized Opal A to Opal C/T transformation on the northeast Atlantic margin: support for a significantly elevated Palaeogeothermal gradient during the Neogene?

BASIN RESEARCH, Issue 4 2002
R. J. Davies
ABSTRACT Rock samples , collected from a recent deep-water exploration well drilled in the Faeroe-Shetland Channel, northwest of the UK , confirm that a distinctive high-amplitude seismic reflector that cross-cuts the Upper Palaeogene and Neogene succession and covers an area of 10 000 km2 is an example of a fossilized Opal A to Opal C/T (Cristobalite/Tridymite) transition. Analysis of these rock fragments tied to an extensive two-dimensional and three-dimensional seismic database constrains the time at which the boundary was fossilized and in addition reveals the unusual geometrical characteristics of a relict bottom-simulating reflector. The diagenetic transformation of biogenic silica (Opal A) to Opal C/T is predominantly temperature-controlled and requires sediments that contain biogenic silica. The reflector (termed as Horizon E) probably initially represented a biosiliceous ooze or a siltstone that contained a component of biogenic silica that underwent transformation as the diagenetic front migrated upsection during burial. The parallelism it shows with a shallower early Pliocene reflector and its apparent upsection migration during a compressional episode in the basin indicate that it was active during the middle and late Miocene and ceased activity during the early Pliocene when there was between 200 and 400 m of overburden. The present-day burial depth of the boundary is ca. 700 m and the temperature at the inactive diagenetic front at the well location is 24 C. Given these temperature and depth constraints, we hypothesize that even if this is an example of a relatively low-temperature Opal A to Opal C/T transformation, a temporarily elevated geothermal gradient of ca. 60 C km,1 would have been required to initiate and arrest upsection migration of the boundary during the middle and late Miocene. Factors such as climatic deterioration and the onset of cold deep-water circulation are likely to only have had a contributory role in arresting the upward migration of the boundary. [source]