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On reefless volcanic islands, insular shelves are thought to have formed essentially by the combined effects of wave erosion and glacio-eustatic sea-level oscillations. Subsidence, however, has also been recognized as having an important role in the development of these morphologies. Yet, few studies have quantified the relative contribution of subsidence to shelf generation and development, particularly to shelf width. A better understanding of this contribution, however, is key to understand the long-term evolution of coasts at volcanic islands, particularly given that subsidence may exacerbate the effects of marine erosion. In this study we assess quantitatively the role of subsidence in shelf development by comparing real cross-shore shelf profiles with modeled profiles using varied rates of subsidence. To achieve this, we used shelf bathymetric profiles from Faial and São Jorge islands in the Azores, which we compared with predictions of a numerical model of coastal erosion that has been calibrated previously against other field data. The first set of model runs was made to calibrate the model by determining the values that produced shelves with break depths, widths, and profile shapes that were similar to those observed. The second set of runs, which served to evaluate the contribution of subsidence to shelf widening, revealed that subsidence may have been responsible for increasing shelf widths by almost 2.5 times relative to shelves formed only by the combined effects of wave erosion and glacio-eustatic oscillations. Modeling shelf formation on the same islands but with increased subsidence rates up to 2.2 mm/yr resulted in shelf profiles up to 19 km wide, a value 3 to 6 times greater than observed on these islands. Our study therefore reinforces the idea that subsidence is a key contributor to the generation of broad insular shelves, given its role in enhancing coastal retreat. Our shelf evolution modeling also suggests that, notwithstanding the crucial role subsidence plays in increasing the width of shelves, on islands subjected to energetic wave regimes (as it happens in the Azores and Hawaii), modification by erosion is important enough to result in shelf morphologies that are not constructional in essence, but rather dominantly erosive surfaces. This study therefore contributes to a better understanding of how insular shelves and submarine terraces form and develop by proving important new quantitative insights of the role of subsidence on the generation of these morphologies. [ABSTRACT FROM AUTHOR]

Most work on cliff notches in the Tropics and Subtropics has been on calcareous substrates that experience chemical or biochemical corrosion and the erosional and protectional effects of boring and encrusting marine organisms. There are deep notches in coarse andesitic lahar deposits along the subtropical western coast of the La Paz Peninsula in Baja California, Mexico. Many notch-forming mechanisms cannot operate here. There are no visible marine organisms at the cliff foot, the waves are generally too weak to dislodge rock fragments, there is little loose material for abrasion, and the substratum is resistant to corrosion. To determine the processes of notch formation, and their rates of erosion due to weathering and debris removal, rock samples were alternatively immersed in water for 90 min and then dried in air for one or two weeks to simulate conditions in the upper high tidal zone, corresponding to the deepest part of the notches in the field. These experiments ran for 17 months and used either fresh water to determine the efficacy of wetting and drying, or synthetic sea water to include the additional effect of salt weathering. The results suggested that salt weathering is more effective in this environment than wetting and drying and, because one week of drying was more effective than two weeks, that optimum notch development is probably closer to the neap than to the spring high tidal levels. As the experimental rock breakdown could also result from chemical weathering, rock samples were analyzed using inductively coupled plasma optical emission spectroscopy and mass spectrometry. The results suggested that, despite some Mg and Ca leaching, chemical weathering is not important in this area. Although the dominant notch-forming mechanism is salt weathering, field evidence implied that its efficacy rapidly increases with exposure to wave action, probably because of more frequent and stronger spray and splash and possibly because of the premature removal of loosened, weathered grains. [ABSTRACT FROM AUTHOR]

Abstract: A mathematical wave-erosion model is used to simulate postglacial shoreline profiles along the rocky, high energy coast of the north of Ireland. The wave erosion model is driven by a suite of relative sea-level (RSL) curves for the last 16,000years produced from four glacial rebound models. Multiple runs are performed with different initial shore profiles and rock resistances to investigate shoreline evolution and the significance of inherited morphology on the resultant profile shape. The simulated profiles are then compared with mapped profiles from three areas of the north of Ireland with different lithological and hydrographic properties. Modelled profiles generally replicate the overall mean shoreline gradients observed across the region when rock resistance is relatively high and erosion rates correspondingly low. In these profiles, breaks in mean slope are observed at depths comparable to the RSL minima in several of the RSL scenarios (at c. −10m, −15m and −20m for North Antrim, Derry and Donegal respectively). At Portrush and Portballintrae (Derry), profiles may be influenced by structural controls relating to the underlying basalt surface and the removal of overlying glaciogenic sediments. All RSL scenarios replicate the observed eastward increase in cliff–platform junction height, reflecting the differential glacioisostatic rebound experienced along the coast. However, the precise elevation at which the simulated cliff base occurs is sensitive to the choice of RSL scenario, suggesting that this parameter may prove useful in evaluating glacial rebound model performance. Several of the RSL scenarios generate raised shore platforms or terraces in North Antrim and Derry at heights comparable to raised shoreline features reported in the literature. However, no single curve or combination of parameters is capable of generating the range of platform and terrace features observed in the bathymetric and topographic data. These misfits are consistent with the idea that many rock platform or terrace features are inherited from an earlier phase or phases of RSL. [Copyright &y& Elsevier]

Abstract: Two transverse micro-erosion meter (TMEM) stations were installed in each of four rock slabs, a slate/shale, basalt, phyllite/schist, and sandstone. One station was sprayed each day with fresh water and the other with a synthetic sea water solution (salt water). To record changes in surface elevation (usually downwearing but with some swelling), 100 measurements (the pilot survey), the maximum for the TMEM used in this study, were made at each station in February 2010, and then at two-monthly intervals until February 2011. The data were normalized using Box-Cox transformations and analyzed to determine the minimum number of measurements needed to obtain station means that fall within a range of confidence limits of the population means, and the means of the pilot survey. The effect on the confidence limits of reducing an already small number of measurements (say 15 or less) is much greater than that of reducing a much larger number of measurements (say more than 50) by the same amount. There was a tendency for the number of measurements, for the same confidence limits, to increase with the rate of downwearing, although it was also dependent on whether the surface was treated with fresh or salt water. About 10 measurements often provided fairly reasonable estimates of rates of surface change but with fairly high percentage confidence intervals in slowly eroding rocks; however, many more measurements were generally needed to derive means within 10% of the population means. The results were tabulated and graphed to provide an indication of the approximate number of measurements required for given confidence limits, and the confidence limits that might be attained for a given number of measurements. [Copyright &y& Elsevier]

Abstract: Laboratory and field work was conducted to determine rates of surface downwearing on shore platforms at two sites in the Bay of Fundy, Canada; these data are needed to assess and model the effect of downwearing mechanisms on platform evolution. Sandstone and basalt samples (900) were exposed to semidiurnal tidal cycles over 3years, using de-ionized water or artificial sea water. They were immersed for 1, 6, or 11h over each 12h tidal cycle, representing the lowest high (LHT), mid-, and highest low (HLT) tidal levels, respectively. A further 600 samples were immersed in de-ionized water or artificial sea water for 90min every 1, 2, or 3weeks, representing increasingly higher elevations above the LHT level, or exposed for 90min every 1, 2, or 3weeks, representing increasingly lower elevations below the HLT level; these experiments ran for 12months. In the field, surface downwearing was measured at 108 transverse micro-erosion meter (TMEM) stations over 2 to 6year periods. In the laboratory, mean downwearing rates between the LHT and HLT levels were 0.61–1.80mmyr−1 in sandstones and 0.01–0.18mmyr−1 in basalts; rates were highest at the LHT level. Rates decreased with elevation above the LHT level and were uniformly low below the HLT level. Salt weathering was dominant above the LHT level. Salt weathering and wetting and drying were important in sandstones between the LHT and HLT levels, but salts inhibited rock breakdown in basalts. In the field, the mean downwearing rate was 1.254mmyr−1 in sandstones, which was similar to the experimental data, and 0.722mmyr−1 in basalts, which was much higher than in the experiments with artificial sea water but similar to the experiments with de-ionized water. There was no relationship in the field between downwearing rate and rock hardness or TMEM station elevation. [Copyright &y& Elsevier]

Abstract: Work has been conducted, in the laboratory and field, on a micro-tidal, subhorizontal shore platform in argillaceous rocks at Mont Louis, in Gaspé, Québec. Surface downwearing rates for 450 rock samples were measured over 3years in the laboratory at the neap low, mid-, and high tidal levels, under semi-diurnal tidal conditions. De-ionized water was used to determine the role of wetting and drying, and artificial sea water for the additional effect of salt and possibly chemical weathering. Another 300 samples were immersed in de-ionized water or artificial sea water for 90min every 1, 2, or 3weeks, representing conditions between the neap and spring high tidal levels, and exposed in air for 90min every 1, 2, or 3weeks to represent conditions between the neap and spring low tidal levels. Surface downwearing was recorded in the field at 34 transverse micro-erosion meter (TMEM) stations, most of which were installed along 4 shore-normal profiles in 2004 and 2005. Mean downwearing rates in the laboratory ranged from 1.25mmyr−1 at the neap high tidal level to 0.63mmyr−1 at the neap low tidal level. Downwearing was much slower between the neap and spring tidal levels and it was uniformly slow below the neap low tidal level. The presence of salts inhibited downwearing within the neap tidal range, where wetting and drying was dominant, and promoted it between the neap and spring high and low tidal levels. Surface swelling was fairly common in the field, and the mean downwearing rate (0.242mmyr−1) was much lower than in the laboratory, possibly because the TMEM stations were installed in harder rocks. Modeling supports the contention that the Mont Louis platform was cut initially by waves at a higher elevation, and then lowered by weathering in the last few thousand years as relative sea level fell to its present elevation. [Copyright &y& Elsevier]

Abstract: A mathematical wave and weathering model was used to study the effect of Holocene changes in relative sea level (RSL) on rock coast development. Model runs were made for fast- and slow-eroding coasts in macro- and mesotidal environments using Holocene RSL records from Sweden, northeastern USA, southwestern Britain, central Japan, western Africa, and southern Australia. A further series of otherwise identical runs were made with constant sea level. Changes in RSL determined the amount of time that marine processes have operated within the modern intertidal zone, and rates of wave attenuation and erosion continue to be influenced by subtidal morphology that developed in most areas when RSL was rising to its present level. The model suggested that although tidal range is also important, Holocene RSL changes promoted subhorizontal platform development in Australasia and over much of the Southern Hemisphere, and sloping platforms over much of the Northern Hemisphere. Shore platforms may have been partly inherited from earlier periods in the Holocene, when RSL was similar to today''s. Nevertheless, most platforms continue to be eroded at all tidal levels. Where geological and wave conditions are less favourable, erosion in the lower intertidal zone has essentially terminated in areas where RSL rose to its present position, whereas erosion only occurs today in the lower intertidal zone in places where RSL rose to above its present level in the middle Holocene. [Copyright &y& Elsevier]

Abstract: A mathematical, wave-erosional model was modified to study the additional effect of weathering by wetting and drying and salt weathering on the development of shore platforms in macro- to mesotidal environments. Model rates of downwearing by these processes, at different tidal elevations, were based on data obtained from a series of laboratory experiments on sandstones from eastern Canada. Backwearing by mechanical wave erosion was calculated using basic wave equations. There were several types of run which were designed to determine the effect of: weathering and the production of fine-grained sediment; the periodic accumulation of debris on weathering in the upper intertidal zone; and weathering in reducing rock resistance and facilitating wave quarrying. The results implied that, compared to mechanical wave erosion, the direct effect of weathering and fine-grained sediment production makes only a small contribution to the long-term development of shore platforms. The relationship between cliff-foot debris occurrence and platform development and morphology was inconsistent because of the negative feedback relationship between erosion rates, surface gradients, and rates of wave attenuation. The model suggested that weathering can play an important, indirect role in assisting wave quarrying of joint blocks and other rock fragments. [Copyright &y& Elsevier]

Abstract: Laboratory experiments have been conducted to measure rock expansion and contraction induced by wetting and drying on intertidal shore platforms in eastern Canada. Almost 800 rock cores and cubes were subjected to between 800 and 1000 tidal cycles. The rocks were put into tidal simulators and inundated in de-ionized water for 1, 6, or 11 h of each 12 h cycle, replicating conditions at the high, mid-, and low tidal levels, respectively. Micro-erosion meter (MEM) stations were also installed in slabs of argillite, sandstone, and basalt. The slabs were subjected to conditions at the high, mid-, and low tidal levels, and a traversing MEM was used to measure rock expansion and contraction as the slabs dried in air with variable temperature and humidity during low tide. The slabs were also exposed for long periods to air with variable humidity. Cores and cubes often gained in weight when they absorbed or adsorbed water and retained it for periods ranging from 1 month to 1 year or more, although this did not appear to be a prelude to rock breakdown. Tidal wetting and drying and exposure to humid air caused the argillite and basalt slabs to expand and contract by up to 0.14 and 0.04 mm, respectively. Wetting and drying did not induce expansion or contraction of the sandstone, but the sandstone did respond slightly to changes in air humidity. The greatest amount of expansion and contraction was at the high tidal level in the argillite and at the mid-tidal level in the basalt, which is consistent with previously measured rates of downwearing. [Copyright &y& Elsevier]

Abstract: Rocks were subjected to wetting and drying cycles with de-ionized water under real time conditions. Using tidal simulators, one-third of the samples experienced 11 h of exposure and 1 h of inundation over a 12-h tidal cycle (representing high tidal conditions), one-third experienced 6 h of exposure and 6 h of inundation (representing mid-tidal conditions), and one-third experienced 1 h of exposure and 11 h of inundation (representing low tidal conditions). We obtained 324 cores from igneous, metamorphic, and sedimentary rocks from Ontario, which were exposed to more than 930 wetting and drying cycles over 16 months. We obtained 675 cores and cubes from basalts, sandstones, and argillites from shore platforms in eastern Canada, which were exposed to about 700 cycles over 12 months. Many of the cores and cubes experienced very little to no breakdown, particularly those from the igneous and metamorphic rocks of Ontario. Sandstones from a sloping platform in the macrotidal Bay of Fundy and argillites from a horizontal, mesotidal platform in Gaspé, Québec, were the most susceptible rock types, with equivalent surface downwearing rates ranging from 0 to 4 mm year− 1. The results suggest that downwearing rates decrease with elevation within the intertidal zone, probably because rocks require much longer to desorb than to absorb water. The experiments demonstrated that wetting and drying can be an important process on shore platforms in some types of rock, helping to lower the height and reduce the gradient of sloping macrotidal platforms and to reduce the height of horizontal micro- and mesotidal platforms. [Copyright &y& Elsevier]

Rock coasts are one of the most common elements of the world''s littoral zone, and they are often important sources of sediment for estuaries and beaches. Economic development, growing populations, and the potential effects of rising sea level provide a practical need to understand the dynamics of these coasts. Although there have been significant advances in our understanding of rocky coastal systems, further progress has been hindered by a lack of researchers and by the often imperceptible changes that generally occur within human time scales. This paper reviews rock coast processes and landforms in a variety of morphogenic environments. One of our most fundamental challenges is to determine the degree to which rock coasts are contemporary rather than inherited features from the Quaternary, when changes in sea level and climate were responsible for marked variations in the nature, intensity, and elevational distribution of the marine and subaerial processes that sculpture rock coasts. [Copyright &y& Elsevier]

Abstract Historical air photographs, LiDAR, and an unmanned aerial vehicle (UAV) were used to record the movement, from 1956 to 2018, of a clay and clast earthflow in a coastal valley in northwestern Spain. Two procedures were employed. The first tracked changes, in a GIS environment, in the location of eight, easily identified objects on the surface of the deposit (large boulders, topographic lobes, and the foundations of an old hut). The second used DEMs of Difference (DoDs) based on Digital Elevation Models from a 2011 LiDAR flight and two UAV flights in 2016 and 2018 obtained by Structure from Motion-Photogrammetry techniques. While the first procedure provided estimates for earthflow movement over a 62-year period, the second produced more precise data for periods of up to 8-years. The first procedure indicated that the mean rate of movement was 0.48 m·yr−1, increasing from only 0.14 m·yr−1 from 1956 to 1983 to between 0.50 and 0.83 m·yr−1 from 1983 to 2018. Despite some temporal and spatial changes in direction, rates of surface movement were quite uniform on the deposit. The increase in earthflow movement after 1983 may be related to an increase in rainfall, although human activities associated with the removal of a wrecked ship from the nearshore may have been a contributing factor. The role of debuttressing due to the wave-induced removal of lateral support from the toe of the deposit is less clear. While there was no clear relationship between wave erosion and rates of movement, coastal retreat may have triggered changes in the direction and sediment flux in the toe of the deposit. This effect could have been tempered by negative feedback, however, whereby coastal erosion and increased flow activity were countered by the protection afforded by the accumulation of large, dislodged boulders on the beach. Because of this feedback, it is difficult to predict the impact of sea level rise and other elements of climate change along this coast. Highlights • Remote sensing monitoring of Spanish coastal-valley earthflow over 62 years • Movement was 0.14 m·yr−1 from 1956 to 1983 and 0.50–0.83 m·yr−1 from 1983 to 2018. • Faster movement due to heavier rainfall and possibly human activities • Coastal erosion may periodically influence flow rate and direction. • Boulder induced negative feedback may temper the effect of wave erosion. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes a new method to distinguish river terrace levels based on clast hardness and degree of weathering measured with an Equotip hardness tester. The technique was applied to a series of terraces on the Miño River in the northwestern Iberian Peninsula, where the lack of suitable material, high degree of weathering, and intense iron cementation precluded routine dating. Detailed mapping demonstrated that terrace sediments occupy a range of altitudes that make assignment to a specific terrace, and/or correlation between levels, difficult. Statistical analysis of the Equotip hardness data from quartzite clasts using k-means clustering allowed four probable terrace levels to be identified; a series of t-tests generally supported these groupings. A fifth, lowermost terrace level, was not included in the analysis because of limited exposure above a reservoir. Clast hardness and degree of terrace weathering were generally consistent with progressive river downcutting. The occurrence of faulted sediments, however, suggested that terrace elevations were modified locally by post-depositional tectonic movements, which may explain why probable younger terraces in some sectors of the Miño River are at higher elevations than older terraces in adjacent sectors. The Equotip tester helped to resolve stratigraphic uncertainties and to assign deposits to specific terrace levels and was found to be a useful tool to distinguish and correlate river terraces. [Copyright &y& Elsevier]