Micro-erosion Meter Research Articles

The Application of Information Theory to Interpret Shore Platform Erosion Rates

Advancements in information physics have recently introduced the application of information theory to investigate physical systems. The behaviour of erosion at the granular scale is to date still a complex system to unpack, and therefore geomorphology research requires novel approaches to better inform the interpretation of temporal and spatial erosion patterns at different scales. This paper applies information theory concepts to re-evaluate erosional data that were measured on limestone surfaces of two shore platforms in Malta with a traversing micro-erosion meter (TMEM). By representing erosion rates through their information content using a Box-Cox style transformation of the raw data (application of an inverse normal distribution function to fractionally ranked data), it is possible to identify points and measurement periods that contribute to a disproportionately large share of unexpected erosion rates that could provide more insight into the causes of erosion rates. Despite the variations in the information content from erosion rates at individual measurement points, most points consistently contribute to a similar amount of information. These findings illuminate the importance of considering the informational value of erosion data to further understand the underlying physical processes and potentially improve predictive models.

Tectonics and shore platform development: Rates and patterns of erosion on recently uplifted mudstone and limestone rocks at Kaikōura Peninsula, New Zealand

On tectonically active coasts, there is a dearth of erosion data documenting how rock coasts adjust (either fast or slow) in response to marine and subaerial processes immediately after coseismic uplift. Here we report erosion rates and evidence of reshaping of shore platform morphology at Kaikōura Peninsula, South Island, New Zealand, following the November 2016 Kaikōura 7.8 (Mw) earthquake, that uplifted platforms by ~1 m and extended their widths. High-resolution topographic data obtained from quarterly surveys over four years using a micro-erosion meter (MEM) and Structure-from-Motion Multi-View Stereo (SfM-MVS) surveys have provided rates of erosion and visual representation of surface morphologies. MEM data revealed variations in erosion, weathering and deposition rates across lithology, tidal positions, and platform elevation after the uplift. After four years, surface downwearing rates for all platforms were on average 2.25 mm/yr, a 104% increase from the pre-uplift rate of 1.10 mm/yr. Average lowering rates on limestone, hard, and soft mudstone platforms were 1.19 mm/yr, 2.31 mm/yr, and 3.21 mm/yr, respectively. Results show a change in patterns of erosion rates and faster rates on mudstone than limestone lithologies, but both faster when compared to pre-uplift rates. Previously reported seasonal trends in erosion rates have disappeared because post-uplift erosion rates are similar during the summer and winter. In contrast, statistically significant differences in lowering rates now exist across rock lithology and elevation. On one MEM site on the harder mudstone platform, a large increase from a pre-uplift erosion rate of 0.43 mm/yr to 19.23 mm/yr (1-year after uplift) and subsequent decline to 1.54 mm/yr after four years demonstrates the role of block detachment not previously seen in the MEM data from Kaikōura. For the first time, we complement MEM data with SfM-MVS-derived orthomosaics to provide evidence of changing rock morphology and processes such as granular disintegration, flaking and algal growth. Results from this study demonstrate how tectonism can fundamentally alter the way in which rock shore erosion rates are influenced by waves, tides, and weathering processes.

Spatial analysis of eroding surface micro-topographies

Analysis of the spatial variability in erosion rates at the micro-scale has the potential to improve our understanding of how shore platforms erode. Comparing the erosion rate of a single measurement reading with the erosion rate of other increasingly distant readings would indicate whether average variation in erosion rate is homogeneous and at what spatial scale. Little variation in erosion rate from one measurement reading as distance increased would indicate that an area is eroding homogeneously and that the surface measured is responding as a single spatial unit. An increase or decrease in the variation in erosion rate difference with increasing distance from one reading would suggest that the area was not acting as a single spatial unit and that surface responses differ with scale. This study used a two-year dataset of traversing micro-erosion meter (TMEM) readings, collected from two limestone shore platforms on the north of Malta, at Ponta tal-Qammieħ and Blata l-Bajda, in order to explore the relationship between difference in erosion rate and distance from TMEM readings. A Microsoft Excel macro was developed and applied to calculate and analyse the average variation in erosion rate difference between all possible pairs of measurement readings over a set of fixed distances. The resultant analysis suggests that there are some consistent patterns between measurement periods and locations on a platform in terms of how erosion rate difference varies with distance between readings. These are not simple relationships to either characterise or explain but nevertheless, they suggest variations in how the same surface responds to erosional forces. These findings are significant for erosion research as they imply that spatial scales to erosion within even small areas may impact upon the representativeness of an average erosional loss for the platform site. It raises issues about how representative rates really are and contributes to the discussion about the wider understanding of erosion rates across spatial scale.

Spatial scaling in geomorphology: Extrapolating micro-erosion measurements to the landform scale on shore platforms using unoccupied aerial vehicles

Downwearing rates of shore platform have been quantified with a number of ground-based techniques (such as the micro-erosion meter (MEM)), providing knowledge of the evolution of coastal landforms. However, it remains problematic to extrapolate the micro erosion results to the landform scale. This is because only a very small proportion of the platform is monitored and usually biased to flat rock surfaces. In this study, we used the shore platforms in south-eastern Australia, which has the world's longest continual erosion records, as an example to address the fundamental question of how to extrapolate across spatial scales. Surveying using an unoccupied aerial vehicle (UAV) – post-processing kinematic (PPK) – structure-from-motion (SfM) workflow, indicates the shore platforms at Marengo and Jump Rock have similar morphology. The generated mm-scale dataset was used to contextualize 4 and 12 MEM sites into the orthomosaics and digital surface models (DSMs) at each location. By using two different methods, we assessed how these monitored surfaces were representative of the morphology of the entire platform. On average, the 4 MEM sites represented 52.2% of platform surfaces at Marengo and 73.2% with the 12 MEM sites at Jump Rock. To improve the representativeness of sampled surfaces, it was found that 20 randomly sampled MEM-size (4.33 × 10−3 m2) surfaces were able to represent (~90%) the landform morphology at the scale of 104 m2. The representativeness was only slightly improved by upscaling the surface size from original cm2, to dm2 (0.16 m2) and m2 (1 m2) scales, implying the importance of sample number over sample size. Using the cm-scale dataset covering a larger geographic region, a bias of MEM installation was found with all 43 MEM sites concentrated at the fast-eroding upper intertidal zone. An elevation-weighted erosion rate of 0.21 mm/yr was calculated, lower than the original mean of 0.25 mm/yr provided by the MEM network. Moreover, a spatial pattern of rock surface morphology was observed across the platform with higher slope and roughness at the seaward end, suggesting more intensive wave processes and varying erosion agents compared to those surfaces further inland.

Weathering processes and forms

Abstract This chapter considers the various processes of rock weathering, starting with physical weathering processes (frost, wetting and drying, heating and cooling or insolation weathering, and salt weathering (haloclasty)). It then turns to a consideration of chemical weathering processes and to the remarkable increase of interest in biological (organic) processes. This is followed by a discussion of rates of weathering and the techniques that have been developed to study them, including the analysis of sediment loads and the measurement of surface changes with instruments like the micro-erosion meter. The global carbon cycle and the links between geomorphology and soils are also considered. Progress in the study of weathering forms such as alveoli, pits and tafoni is described. There is also a discussion of materials deposited as a consequence of chemical weathering, such as various types of duricrust. Finally, weathering hazards are addressed together with an assessment of the ways in which humans have accelerated some weathering processes.

The precision and accuracy of measuring micro-scale erosion on shore platforms

Shore platforms evolve over millennial scales but understanding precisely how instantaneous processes scale over these longer periods is a major scientific quandary. A reason for this, is that erosion often occurs on the granular scale and measurement of magnitude and frequency of this decay is difficult. Over the past 50 years, a number of techniques have been used to quantify the micro-scale (0.01–10 mm) erosion of coastal bedrock, from direct surface contact techniques to those which involve remote sensing. The micro-erosion meter (MEM) and traversing micro-erosion meter (TMEM) are the most common techniques, recording erosion rates from <0.1 to 5 mm/yr on rock surfaces of c. 50 cm2. The traversing erosion beam (TEB) is specifically designed to measure rapid erosion (>10 mm/yr) while the Swantesson laser scanner (SLS) and close-range Structure-from-motion (SfM) photogrammetry have been used for observation of larger rock surfaces of 0.2 m2. The rock tablet method is used to investigate the individual processes operating on downwearing. By comparing their advantages and disadvantages, the T/MEM is suggested to continue to play an important role in monitoring surface lowering in conditions where the total erosion is 20 mm over the monitoring period, with close-range SfM recommended as ideal for quantifying erosion of 2–50 mm. However, all the instruments are biased in the field conditions under which they can be operated, and it is important for researchers to contextualise their measured surfaces in regards to a whole platform. Future studies will benefit from the combination of the ground-based methods with new techniques that provide finer morphological features at the regional scale or interpret the evolution of shore platforms over long timescales.

High resolution mapping and analysis of shore platform morphology in Galicia, northwestern Spain

Most research on shore platforms has been at fairly large spatial scales over distances ranging from hundreds to thousands of metres. Other work at much smaller scales, ranging from one to several decimetres, often corresponds to the dimensions of micro-erosion meter stations. Few studies have been concerned with platform morphology in which the basic data are acquired at intermediate or meso-scales ranging up to a few metres. This is due, in part, to terrestrial surveying at meso-scales being time-consuming while aerial surveys using LiDAR are expensive. A meso-scale study was made on three shore platforms in western Galicia, northwestern Spain using data from an Unmanned Aerial Vehicle (UAV) to produce high resolution Digital Surface Models (DSMs) and to calculate local surface elevation, roughness, slope, and joint density at a 0.5 m pixel scale, and joint orientation and length. This was supplemented by Equotip field measurements of rock hardness. A granitic platform was higher and had rougher surfaces and steeper slopes than two platforms in schist. The relationship between platform morphology and rock hardness and joint density was complex, however, reflecting in part the role of schistosity in accounting for the formation of low, regular platforms in hard schists with low joint density. The study suggested that while tidal range, inheritance, and other environmental and evolutionary factors can be dominant in determining platform morphology at the macro- or regional scale, geological, and particularly structural, factors become increasingly important in Galicia as the scale diminishes, and they are generally dominant at the local or meso-scale.

Variation of micro-topographic and geotechnical limits on weathering progress on the constructional Triassic sandstone, Aachen city, Germany, case study

Rock's surface micro-topography and geotechnical properties' limits are expected to be altered on weathering progress to a new form(s) and limits, respectively. The quantification of weathering damage for a given rock is of value e.g. to compute weathering rate, weathering intensity and rock's durability to weathering processes and to take a decision regarding restoration urgency. The current study aims to examine the variation/constancy of micro-topography and geotechnical properties’ limits of the Triassic sandstone constituting well-aged wall, at Aachen city, on weathering progress over short duration (12 years of investigation from 2007 to 2019). The Equotip hardness tester and micro-erosion meter are tools used for micro-topographic and rock's surface hardness investigations on one hand, and the mercury intrusion porosimetry has been used for measuring rock's pore size distribution and salt susceptibility on the other hand. These tools are accurate, numerical, comprehensive, easily applicable and preferable particularly for ancient buildings and archaeological sites where sampling is not recommended or prohibited. The wall side under consideration has been selected as its constructional blocks present a wide spectrum of weathering forms as well as rock's surface micro-topography (over 12 years of investigation) through increasing the weathering forms' dimensions and/or creation of new weathering forms. The net result of the current study indicated a noticeable variation in stone's micro-topography on weathering progress, the rock's stone surface hardness highly controls rock's weathering susceptibility and no clear relation can be noted between stone's surface roughness and its susceptibility. The pore size distribution is the most controlling parameter for rock's weathering susceptibility.

The multidecadal spatial pattern of erosion on sandstone shore platforms in south-eastern Australia

Knowledge of the spatial pattern of erosion on shore platforms is needed to better understand their evolution. The long-term spatial and temporal variation of downwearing remains difficult to interpret from erosion data commonly collected over 1–3 years of monitoring. We report erosion data recorded with a micro-erosion meter (MEM) over 40 years (1979–2019) on sandstone shore platforms in a micro-tidal environment along the Otway coast, Victoria, Australia. This well-preserved monitoring network (43 out of 44 original MEM sites) allowed a detailed investigation over a multidecadal timescale not previously considered. Over 40 years the mean erosion rate was 0.25 mm/yr. This value was statistically similar to erosion rates calculated over 2-year (0.32 mm/yr) and 32-year (0.26 mm/yr) periods. In the vertical plane, two significant but opposite relationships between elevation and erosion rates were displayed in the central intertidal zone. There was also a peak in the erosion rate near mean higher high water level. This spatial pattern was strongly associated with wetting and drying controlled by tides and waves. At elevations lower than mean higher low water, downwearing rates were one or two orders of magnitude lower than the average rate of the entire platform. The low erosion rates may be caused by the longer wetting condition of surfaces at lower elevation. Fast erosion rates were recorded in the upper intertidal zone where the bedrock was only inundated by spring high tides or storm waves. Salt weathering may be the primary mechanism but the effect of thermal weathering cannot be neglected at higher elevation. Based on the experience from the near complete MEM network in the study area, suggestions were provided to extend the longevity of monitoring network for future long-term erosion records on shore platforms.

Forty three years of micro-erosion meter monitoring of erosion rates on shore platforms at Kaikōura Peninsula, South Island, New Zealand

Using micro-erosion meters (MEM) and traversing micro-erosion meters (TMEM), surface lowering rates of shore platforms on Kaikōura Peninsula, South Island, New Zealand have been measured over a total of 43 years. This record is the longest monitored network of this type. Since 1973, erosion rates have been calculated over two, two year periods 1973–1975, (n = 31) and 1993–1996, (n = 55) and at decadal scales; 1973–1993 (n = 15), 1973–2004 (n = 12), 1993–2004 (n = 46), 1993–2008 (n = 34), 1993–2016, (n = 18), 1973–2016 (n = 6). After 43 years, surface lowering rates remain similar to previously published rates at an average of 0.525 to 1.181 mm. a−1. Statistical analysis shows that erosion rates over all measurement periods are derived from the same population. Thus, short term rates derived from two years of monitoring remain indicative of decadal rates of erosion. Variations between measurement periods are best explained by the loss of the more rapidly eroding bolt sites. These losses point to the difficulty of maintaining monitoring over longer time scales. A means of statistical manipulation (previously published) allows for these losses to be accounted for in determining long term rates of platform lowering. In November 2016 a Mw 7.8 earthquake raised the Kaikōura Peninsula approximately 0.8–1 m, elevating much of the shore platforms above the tide range. This earthquake has reset the MEM record ending this long running erosion monitoring site of shore platform surface lowering. However the MEM network is now being used to monitor post-earthquake response of the uplift shore platforms and new marine terraces.

Experimental investigations into the influence of biofilms and environmental factors on short‐term microtopographic fluctuations of supratidal sandstone

AbstractIn this study laboratory experiments were used to explore the role of biofilms, formed by lithobiontic microorganism communities, in causing hourly surface changes of supratidal sandstone and the potential linkage to long‐term rock decay. To isolate the influence of individual environmental factors (temperature and humidity) on rock surface changes (expansion and contraction), a colonized (biofilm‐covered) and a non‐colonized sandstone block (biofilm‐free) underwent the same univariate microclimatic simulations closely controlled by an environmental chamber. Simulations were run under three different light conditions, with a natural light lamp on, on and off at 20‐min intervals and off, to investigate the impact of light on rock surface dynamics. Measured with a traversing micro‐erosion meter (TMEM), two‐hourly microtopographic fluctuations of these two sandstone blocks were compared in the same environment. Induced by microclimatic variations, surface movements of significantly higher magnitude (12–120% under varying tempeature and 121–154% under varying humidity) and different change patterns were observed on the colonized block, indicating the primary role of biofilm in driving microtopographic fluctuations of supratidal sandstone. However, thermally driven changes of similar magnitude and pattern were observed on both surfaces, suggesting other mechanisms also operating on the non‐colonized rock surface in this process. Due to the sensitivity of biofilm microorganism communities to light, the magnitude and pattern of surface changes was impacted by light condition. Because biofilms increased the magnitude and number of cycles of expansion and contraction of the experimental rock surface, we propose that lithobiontic biofilms facilitate the detachment of grains and granular disintegration on the rock surface, consequently contributing to rock decay and accelerating the rate of breakdown of supratidal rock. This short‐term episode therefore needs to be superimposed on longer term studies to fully understand the role of biofilms in rock surface change. © 2019 John Wiley &amp; Sons, Ltd.

A comparison of structure from motion photogrammetry and the traversing micro-erosion meter for measuring erosion on shore platforms

Abstract. For decades researchers have used the micro-erosion meter and its successor the traversing micro-erosion meter to measure micro-scale rates of vertical erosion (downwearing) on shore platforms. Difficulties with “upscaling” of micro-scale field data in order to explain long-term platform evolution have led to calls to introduce other methods which allow for the measurement of platform erosion at different scales. Structure from motion photogrammetry is fast emerging as a reliable, cost-effective tool for geomorphic change detection, providing a valuable means for detecting micro-scale to mesoscale geomorphic change over different terrain types. Here we present the results of an experiment in which we test the efficacy of structure from motion photogrammetry for measuring change on shore platforms due to different erosion processes (sweeping abrasion, scratching, and percussion). Key to this approach is the development of the coordinate reference system used to reference and scale the models, which can be easily deployed in the field. Experiments were carried out on three simulated platform surfaces with low to high relative rugosity to assess the influence of surface roughness. We find that structure from motion photogrammetry can be used to reliably detect micro-scale (sub-millimetre) and mesoscale (cm) erosion on shore platforms with a low rugosity index. As topographic complexity increases, the scale of detection is reduced. We also provide a detailed comparison of the two methods across a range of categories including cost, data collection, analysis, and output. We find that structure from motion offers several advantages over the micro-erosion meter, most notably the ability to detect and measure the erosion of shore platforms at different scales.

Shore platform downwearing in eastern Canada; A 9–14 year micro-erosion meter record

Downwearing rates (erosion in the vertical plane) were measured with a micro-erosion meter (MEM) in eastern Canada, on an argillacious, sub-horizontal shore platform at Mont Louis in eastern Québec, and on two sloping, basaltic and sandstone platforms at, respectively, Scots Bay and Burntcoat Head in the Bay of Fundy, Nova Scotia. The original data covered a period from 2002 to 2009. This dataset was extended by measurements repeated at surviving MEM stations in 2017, producing records ranging over 9–14 years, depending on when each station was installed. Because of rapid surface downwearing, many of the original MEM stations were inoperable in 2017, especially at Burntcoat Head. Nevertheless, data were obtained from 19 stations at Burntcoat (35% of the 2009 original), 25 at Mont Louis (83% of the original), and 38 at Scots Bay (75% of the original). For the stations at Mont Louis and Scots Bay that were still functioning in 2017, there were no significant differences in rates of downwearing over the shorter (from station installation up to 2009) and extended periods (from installation to 2017). Mean rates of downwearing calculated from all the stations in each area declined through time, however, due to the loss of the more rapidly eroding stations. A simple procedure, which was proposed to compensate for this decrease, produced mean downwearing rates that were broadly similar to those reported over the original measurement period. There were significant relationships between downwearing rates and elevation (R2 = 0.32) and downwearing rates and rock hardness (R2 = 0.41) in the extended record at Scots Bay, and a small but significant relationship between downwearing rates and rock hardness at Mont Louis (R2 = 0.17). Differences in downwearing rates across the platforms suggest that salt weathering and wetting and drying are dominant weathering mechanisms at Scots Bay and Mont Louis. Chemical weathering of the sandstone cementing agent and the premature removal of weathered grains by wave-generated bottom currents may, however, be more important at Burntcoat Head.

Cryo-conditioned rocky coast systems: A case study from Wilczekodden, Svalbard

This paper presents the results of an investigation into the processes controlling development of a cryo-conditioned rock coast system in Hornsund, Svalbard. A suite of nested geomorphological and geophysical methods have been applied to characterise the functioning of rock cliffs and shore platforms influenced by lithological control and geomorphic processes driven by polar coast environments. Electrical resistivity tomography (ERT) surveys have been used to investigate permafrost control on rock coast dynamics and reveal the strong interaction with marine processes in High Arctic coastal settings. Schmidt hammer rock tests, demonstrated strong spatial control on the degree of rock weathering (rock strength) along High Arctic rock coasts. Elevation controlled geomorphic zones are identified and linked to distinct processes and mechanisms, transitioning from peak hardness values at the ice foot through the wave and storm dominated scour zones to the lowest values on the cliff tops, where the effects of periglacial weathering dominate. Observations of rock surface change using a traversing micro-erosion meter (TMEM) indicate that significant changes in erosion rates occur at the junction between the shore platform and the cliff toe, where rock erosion is facilitated by frequent wetting and drying and operation of nivation and sea ice processes (formation and melting of snow patches and icefoot complexes). The results are synthesised to propose a new conceptual model of High Arctic rock coast systems, with the aim of contributing towards a unifying concept of cold region landscape evolution and providing direction for future research regarding the state of polar rock coasts.

Erosion study of limestone on the Cat Ba islands in north east Vietnam by transverse micro-erosion meter

Erosion rate at the Cat Ba island stations ranged from 0.196 to 0.282 mm/year shows that the average value in comparison with previous study results ranges from ±0.2 to ±0.5 mm with strong eroded level. Relative erosion rate value is the lowest at measuring stations X2 on April 8th, 2015 and the highest is at measuring stations Z7 on November 9th 2015, the average value in comparison with previous study results, ranges from ±0.05 to ±0.2 mm with average eroded level and from ±0.2 to ±0.5 mm with strong eroded level. The results of relative erosion rate on April 8th, 2015 are lower than on November 9th, 2015. Total rainfall values counted from the beginning of the study to the first measuring date (April 8th, 2015) and the second measuring date (November 9th, 2015) are 980.2 mm and 2280 mm, respectively. Therefore, the erosion rate of limestone is strongly affected by changes of total rainfall on the Cat Ba Island. Average relative erosion rates in this study are quite high at some submerged zone stations (about over 0.2 mm/year, on average) and quite low with some stations on land in comparison with some previous studies all over the world. Therefore, it needs more assessment in longer period to study more accurately some other impacts on weathering and erosion processes.

Surface hardness as a proxy for weathering behaviour of limestone heritage: a case study on dated headstones on the Isle of Portland, UK

This study estimates stone weathering rates using in situ surface hardness testing. Surface hardness changes are precursors to erosion and may be utilized to describe stone weathering behaviour. The method proposed here complements previous approaches to determining stone weathering rates by surface loss/change. A time series covering 1–248 years of exposure is investigated using a sample of 12 headstones in two nearby cemeteries. Using an Equotip D surface hardness tester, rates of change in surface hardness for top and bottom sections of the headstones were determined and the data evaluated using robust, nonparametric statistical methods. When considering all headstones as one time series, nonlinear behaviour is observed with rates of decline in surface hardness slowing over time. However, breakpoint analysis shows a breakpoint at c. 100 years, with higher rates of surface hardness decline (as measured by QC 50—the regression coefficient for 0.50 quantile regression) up to that point and lower rates thereafter. Up to c. 100 years, surface hardness declines more rapidly in the top versus bottom sections. Possible explanations for the differing rates in surface hardness changes are: (a) inherent natural stone variability and/or different weathering-stress history; (b) the use of two different Portland limestone varieties; (c) synergistic effects of microclimates and lichen cover. In order to gain a deeper insight into stone weathering behaviour, future studies could combine surface hardness measurements with surface change methods such as micro-erosion meter and lead plug index over short- and long-term time series on architectural heritage under real-world conditions.

Micro-topographic and Geotechnical Investigations of sandstone Wall on Weathering Progress, Aachen City, Germany, case study

Rock’s surface micro-topography and geotechnical properties’ limits are almost altered, on weathering progress, to a new form(s) and limits, respectively. The quantification of weathering damage for a given rock is of value, e.g., to compute weathering rate, weathering intensity, and rock’s durability to weathering processes and to take a decision regarding restoration urgency. The current study aims to examine the variation/constancy of micro-topography and geotechnical properties’ limits of the sandstone constituting well-aged wall, at Aachen City, on weathering progress over short duration (7 years of investigation from 2007 to 2014). The micro-erosion meter and Equotip hardness tester are tools used for micro-topographic and rock’s surface hardness investigations on one hand, and the mercury intrusion porosimetry has been used for pore size distribution and salt susceptibility investigations on the other hand. These tools are accurate, numerical, comprehensive, easily applicable, and preferable particularly for ancient buildings where sampling is not recommended or prohibited. The wall side under consideration has been selected as its constructional blocks present a wide spectrum of weathering forms as well as rock’s surface micro-topography (over 7 years of investigation) through increasing the weathering forms’ dimensions and/or creation of new weathering forms. The net result of the current study indicated a noticeable variation in stone’s micro-topography on weathering progress particularly for stones’ surface with scaling, exfoliation, and a decrease in stone’s surface hardness. The critical pore size distribution that has increased rock’s susceptibility to weathering particularly by salts has been defined.

Shore platform and cliff notch transitions along the La Paz Peninsula, southern Baja, Mexico

Increasing exposure to wave action produces a northerly transition from high tidal notches to shore platforms inthe andesitic lahar deposits of the La Paz Peninsula, southern Baja, Mexico. Twenty-four notches were surveyedand wear pins were cemented into the apex of each notch. Thirty-six transverse micro-erosion meter (TMEM)stations were installed on three surveyed platforms. Field measurements were made over a 2.5 year period.The wear pins suggested notch backwearing (horizontal erosion) is <2mm yr -1 . The shore platforms were fairlynarrow (a few tens of metres) and steeper (1o) than most platforms in similar microtidal environments, reflectinga weak wave environment and resistant rocks. Mean TMEM downwearing (vertical erosion) rates for each of thethree platforms ranged from 0.14mm yr -1 to 0.42mm yr -1 . There was a good relationship between notch height(difference in elevation between the floor and the roof at the front of the notch) and exposure to wave action, butnotch depth is time-dependent and the relationship with exposure was not statistically significant. Notch heightwas also related to the orientation and wave fetch of the site. Field evidence suggested that the notches were notproduced by bioerosion or chemical weathering but by alternate wetting and drying or salt weathering from hightidal immersion and wave-generated splash and spray. Coastal morphology is fairly well adjusted to present sealevel although notch occurrence in the upper portion of the high tidal zone suggests that there is slow tectonic uplift in this region.

Towards an improved understanding of erosion rates and tidal notch development on limestone coasts in the Tropics: 10 years of micro‐erosion meter measurements, Phang Nga Bay, Thailand

AbstractKnowledge and understanding of shore platform erosion and tidal notch development in the tropics and subtropics relies mainly on short‐term studies conducted on recently deposited carbonate rocks, predominantly Holocene and Quaternary reef limestones and aeolianites. This paper presents erosion rates, measured over a 10 year period on notches and platforms developed on the Permian, Ratburi limestone at Phang Nga Bay, Thailand. In so doing it contributes to informing a particular knowledge gap in our understanding of the erosion dynamics of shore platform and tidal notch development in the tropics and subtropics – notch erosion rates on relatively hard, ancient limestones measured directly on the rock surface using a micro‐erosion meter (MEM) over time periods of a decade or more.The average intertidal erosion rate of 0.231 mm/yr is lower than erosion rates measured over 2–3 years on recent, weaker carbonate rocks. Average erosion rates at Phang Nga vary according to location and site and are, in rank order from highest to lowest: Mid‐platform (0.324 mm/yr) &gt; Notch floor (0.289 mm/yr) &gt; Rear notch wall (0.228 mm/yr) &gt; Lower platform (0.140 mm/yr) &gt; Notch roof (0.107 mm/yr) and Supratidal (0.095 mm/yr). The micro‐relief of the eroding rock surfaces in each of these positions exhibits marked differences that are seemingly associated with differences in dominant physical and bio‐erosion processes. The results begin to help inform knowledge of longer term shore platform erosion dynamics, models of marine notch development and have implications for the use of marine notches as indicators of changes in sea level and the duration of past sea levels. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Long-term erosion rate measurements in gypsum caves of Sorbas (SE Spain) by the Micro-Erosion Meter method

The present work deals with the results of long-term micro-erosion measurements in the most important gypsum cave of Spain, the Cueva del Agua (Sorbas, Almeria, SE Spain). Nineteen MEM stations were positioned in 1992 in a wide range of morphological and environmental settings (gypsum floors and walls, carbonate speleothems, dry conduits and vadose passages) inside and outside the cave, on gypsum and carbonate bedrocks and exposed to variable degree of humidity, different air flow and hydrodynamic conditions. Four different sets of stations have been investigated: (1) the main cave entrance (Las Viñicas spring); (2) the main river passage; (3) the abandoned Laboratory tunnel; and (4) the external gypsum surface. Data over a period of about 18years are available. The average lowering rates vary from 0.014 to 0.016mmyr−1 near the main entrance and in the Laboratory tunnel, to 0.022mm−1 on gypsum floors and 0.028mmyr−1 on carbonate flowstones. The denudation data from the external gypsum stations are quite regular with a rate of 0.170mmyr−1. The observations allowed the collecting of important information concerning the feeding of the karst aquifer not only by infiltrating rainwater, but under present climate conditions also by water condensation of moist air flow. This contribution to the overall karst processes in the Cueva del Agua basin represents over 20% of the total chemical dissolution of the karst area and more than 50% of the speleogenetically removed gypsum in the cave system, thus representing all but a secondary role in speleogenesis. Condensation–corrosion is most active along the medium walls, being slower at the roof and almost absent close to the floor. This creates typical corrosion morphologies such as cupola, while gypsum flowers develop where evaporation dominates. This approach also shows quantitatively the morphological implications of condensation–corrosion processes in gypsum karst systems in arid zones, responsible for an average surface lowering of 0.047mmyr−1, while mechanical erosion produces a lowering of 0.123mmyr−1.