Effects Of Rock Fragments Research Articles

Field experiments of aeolian dust accumulation on rock fragment substrata

ABSTRACTThe effect of rock fragments and rock fragment cover on the accumulation of airborne dust was examined in a long‐term field experiment in the Negev desert of Israel. Four parameters were studied: pebble eccentricity, pebble size, pebble flattening and cover density. The effect of these parameters on the accumulation of dust on the pebbles, on the accumulation of dust between and underneath the pebbles, and on total dust accumulation (pebbles+interpebble space) was measured separately. Accumulation on the pebbles increased as the pebbles became larger, less flattened and more elongated, and as cover density increased. Accumulation between and underneath the pebbles increased as the pebbles became smaller, more flattened and more elongated, and as cover density increased (although the accumulation area available became smaller). Total dust accumulation increased as the pebbles became smaller, more flattened and more elongated, and as cover density increased.Rock fragments act as a dust trap as dust accumulation on pebble‐covered surfaces is several tens of times larger than dust accumulation on similar but pebble‐free surfaces. However, less than 20% of the initially settled dust is protected from further erosion.In the Negev desert, dust erosion by wind is of the order of 10 times larger than dust erosion by water. The rate of long‐term dust accumulation in the Negev is of the order of 15–30 g.m−2.yr−1. This is considerably lower than the rates proposed elsewhere in the literature, which are based upon over optimistic dust retainment percentages.

The role of rock fragments in evaporation from cultivated soils under mediterranean climatic conditions

A large proportion of the soils in the Mediterranean contains considerable amounts of rock fragments (i.e. mineral particles larger than 2 mm). In contrast to the reduction of evaporation depth by a continuous gravel mulch, the effects of rock fragments within the soil profile and at the soil surface is largely unknown. This paper presents the results of laboratory experiments where evaporation rate from soils containing a range of rock fragment contents is compared to a non-stony control soil and a non-stony soil covered with a rock fragment mulch. Evaporative demand (E o) during the experiments varied between 7.7–9.2 mm day −1. Two contrasting conditions frequently found in the Mediterranean were tested: soils at field capacity (beginning of the growing season) and air-dry soils that received only 10 mm of rain (end of the growing season). The lower evaporation depth from stony soils is most evident in the initial phase (constant rate stage) when the soil is wet. During this stage rock fragments act as a partial mulch. However, when a limited amount of rain falls on an air-dry soil, the constant rate stage is absent and the falling rate stage dominates water losses. During this phase, dry stony soils loose their water more quickly, probably due to a concentration of soil moisture in the fine earth.

Effects of rock fragments on physical degradation of cultivated soils by rainfall

To understand better the role of rock fragments in soil and water conservation processes, the effects of rock fragments in maintaining a favourable soil structure and thus also in preventing physical degradation of tilled soils was studied. Laboratory experiments were conducted to investigate the effects of rock fragment content, rock fragment size, initial soil moisture content of the fine earth and surface rock fragment cover on soil subsidence by rainfall (i.e. change in bulk density by one or more cycles of wetting and drying). A total of 15 rainfall simulations (cumulative rainfall, 192.5 mm; mean intensity, 70 mm h −1) were carried out. Before and after each rainfall application the surface elevation of a 19-cm thick plough layer was measured with a laser microrelief meter. In all experiments, the bulk density of the fine earth increased with applied rainfall volume to reach a maximum value at about 200 mm of cumulative rainfall. From the experimental results it was concluded that the subsidence rate decreased sharply for soils containing more than 0.50 kg kg −1 rock fragments, irrespective of rock fragment size. Fine earth bulk densities were negatively related to rock fragment content beyond a threshold value of 0.30 kg kg −1 for small rock fragments (1.7–2.7 cm) and 0.50 kg kg −1 for large rock fragments (7.7 cm). Initial soil moisture content influenced subsidence only in the initial stage of the experiments, when some swelling occurred in the dry soils. Surface rock fragment cover had no significant effect on subsidence of the plough layer. Therefore, subsidence of the plough layer in these experiments appears to be mainly due to changing soil strength upon drainage rather than the result of direct transfer of kinetic energy from falling drops. The relative increase in porosity of the fine earth as well as the absolute increase in macroporosity with rock fragment content will cause deeper penetration of rainfall into the soil, resulting in water conservation. Therefore, crushing of large rock fragments into smaller ones is to be preferred over removal of rock fragments from the plough layer.

Effect of rock fragments on eolian deposition of atmospheric dust

The effect of rock fragments and rock fragment cover on the deposition of airborne dust was examined in a wind tunnel. Four parameters were studied: pebble size, pebble flattening, cover density and wind speed. The effect of these parameters on the deposition of dust on the pebbles, on the deposition of dust bewween and underneath the pebbles, and on total dust deposition (pebbles + interpebble space) was measured separately. Deposition on the pebbles increased with pebble size, pebble flattening and wind speed. Deposition between and underneath the pebbles increased with increasing wind speed and with decreasing pebble flattening, but there was no consistent relationship with pebble size. Total dust deposition was not influenced by pebble size, pebble flattening and cover density, but it increased linearly with wind speed. It was also demonstrated that the larger and the flatter the pebbles, and the higher cover density, the more the dust will tend to settle on the pebbles rather than between or underneath them. Air flow separation seems to play a primary role in the spatial distribution of dust deposition within a rock fragment field, as it determines the location and size of the low-sedimentation areas that are connected to the air flow separation bubbles. It should be noted that all data in this study refer to dust deposition (not accumulation), since only wind velocities below the dust deflation threshold were used.

Effects of rock fragments on soil erosion by water at different spatial scales: a review

This paper reviews the various effects of rock fragments on soil erosion by water. Since these effects are scale dependent, they are investigated at three different nested spatial scales: the microplot (4 × 10 −6–10 0 m 2), the mesoplot (10 −2–10 2 m 2) and the macroplot (10 1–10 4 m 2). For each scale the corresponding process mechanisms are discussed. Particular attention is paid to the effects of rock fragment cover on the intensity of soil erosion processes. At the mesoplot scale, i.e. on interrill areas, rock fragments at the soil surface can have negative as well as positive effects on sediment yield. These ambivalent effects are conditioned by the type of fine earth porosity, soil surface slope, vertical position and size of rock fragments and by the occurrence of horseshoe vortex erosion. At the microplot scale, i.e. the soil surface area which is covered by a single rock fragment, and at the macroplot scale, i.e. upland areas where both interrill and rill erosion takes place, rock fragments at the soil surface have a negative effect on sediment yield. In these two scales rock fragments can thus be considered as natural soil surface stabilizers. At the macroplot scale the mean decrease of relative interrill and rill sediment yield with rock fragment cover can be expressed by an exponential decay function. The scatter of the data indicates that a given rock fragment cover can have different efficiencies in reducing interrill and rill sediment yield depending on the varying intensities of the hydrological and erosion subprocesses. These findings have implications for erosion modelling and soil conservation.

The effect of rock fragments on wheat biomass production under highly variable moisture conditions in Mediterranean environments

The effects of soil properties that affect water availabilty to plants such as soil depth, rock fragments, parent material and degree of erosion on biomass production of rainfed wheat were studied on hilly soils under different weather conditions. Total above ground biomass production (TAGBP) of wheat was measured along catenas of marl and shale-sandstone formations in two successive growing periods. During the first growing period which was extremely dry (total amount of rainfall R = 95 mm versus 370 mm in an average year), the soils on marl, free of rock fragments, produced an average TAGBP of 1100 kg ha −1 despite their great soil depth, whereas on shoulders and footslopes of the stony soils on shale-sandstone formations TAGBP values of 4620 and 11,100 kg ha −1 were measured, respectively. The opposite occurred the following growing period which was exceptionally wet ( R = 663 mm). Soils on marls were more productive due to the higher fertility status than the soils on shale-sandstone formations. Rock fragments, more responsible for the lower exploitable soil volume and the generally lower biomass production under wet climatic conditions, play an important role on soil water conservation under dry climatic conditions.

Rock fragments in top soils: significance and processes

This introductory paper reviews various aspects of rock fragments in top soils. Such information is required for various reasons and in particular for predicting the impact of climatic or landuse changes on the response of these soils. Particular attention is paid to the definition and measurement of rock fragment content in top soils, the density of soils containing rock fragments, the spatial distribution and movement of rock fragments in top soils, the effects of rock fragments on some key hydrological processes, thermal properties of top soils, physical soil degradation, soil erosion and soil productivity.

Study of the effect of an explosion on mica crystals for the purpose of increasing their durability

1. Mica crystals which are in contact with a charge of any explosive receive a multitude of damage in the form of cracks in every possible direction and are practically transformed into scrap. 2. Around each explosive charge there exists a zone of damage to the mica crystals, in which they receive different amounts of damage—from single small cracks to numerous cracks crisscrossing the entire crystal. 3. The damage zone of the crystals has a variable size and is dependent upon the local action of the explosive charge. 4. Along with the damage to the mica crystals from shock-wave action, damage is noted from the dynamic effect of rock fragments, as well as warping and rupture of the crystals, caused by explosion of adjacent charges which occur at different times, i.e., formed as a result of the general action of the explosion. 5. In order to reduce loss of mica in the process of its explosive cutting, the following measures must be taken: use of low-brisance ammonia-saltpeter mixtures as the explosives; use of cutting methods which maximally reduce the dynamic interaction of the crystals with the country rock, such as drillhole cutting in a compressed medium during short-delay explosion.