Rocky shore morphology
Rock coasts are highly diverse depending on rock type, geological evolution and hydrodynamic environment. An introduction is given to important common geomorphological characteristics and related physical processes. The article is intended to provide background understanding for the management of rocky shores. Related topics are dealt with in the articles Rocky shore habitat and Cliff stabilisation.
Contents
Morphology
Cliffs, steep rocky exposures at the land-sea interface, are the most prominent manifestation of rocky shores. Unlike sandy beaches, which can erode and subsequently rebuild, retreat of a rock cliff generally represents a permanent loss of land. Retreat is often episodic: the cliff may appear stable for years or decades and then recede abruptly during a rockfall, slide or slump. Cliffs exist in very different forms. This diversity depends, among other things, on mineralogy, lithology, tectonic history, climate, waves and tides. Some rocky coasts consist of steep plunging cliffs that descend directly into deep water without a conspicuous shore platform. They commonly occur where steep inherited topography has been drowned, uplifted or tectonically modified, and on resistant volcanic or glacially sculpted coasts. Cliffs which are degraded by subaerial weathering processes are more gently sloping. Such more rounded (usually vegetated) cliffs, are often termed “bluffs”(especially in the North American literature). A common coastal cliff morphology is the slope-over-wall profile: a rounded weathered top and an almost vertical wall below.
It is estimated that about half of the world's coastline is backed by a cliff[2]. A coastal geomorphic classification with satellite and geospatial data identified about 140 thousand km of rocky coasts globally between 56°S and 60°N (36.5% of the global coastline)[3].
In many cases cliffs are fronted by shore platforms, which can be tens to many hundreds (or even thousands) of meters wide. These shore platforms result from cliff retreat; the outer edge marks the cliff position in the past – often millennia ago, in the case of erosion-resistant rock[4].
Wave attenuation on the shore platform reduces cliff recession rates[5]. Cliff and shore platform both retreat by erosional processes; the rate of cliff retreat strongly depends on the width of the shore platform. However, shore platforms protruding into the sea also focus wave energy by wave refraction, which can partly offset the wave attenuation function. An equilibrium profile is reached when platform retreat and cliff retreat occur at the same pace.
Rocky coasts often have a crenulated planform. Some crenulated coasts were created by the drowning of river valleys by rising sea level during the Holocene. Other crenulated coasts are produced by differential erosion of material that is weaker in the bays than in the adjacent headlands. Coastal indentation is limited by feedbacks, such as wave attenuation in bays, wave refraction towards the bay headlands and accumulation of eroded material in bay-head beaches[6].
Cliff erosion
Wave-induced erosion
Cliff recession depends on many factors: rock structure (massive, bedding, faulted, granular), rock lithology (permeability, solubility, hardness, jointing), hydraulic action (exposure to waves more or less loaded with debris, tides, currents) and subaerial weathering (by surface and subsurface runoff, freeze-thaw, chemical processes, biota). According to data reported by Sunamura (1992[5]), hard rock cliffs retreat typically less than 1 cm/year, whereas soft cliffs (consisting of boulder clay, for example) retreat typically 1 m/year[5] and sometimes more. However, a compilation of more recent measurements indicates a less extreme spread in median cliff recession rates between hard and soft rocks[7].
Cliff retreat is often episodic; a single retreat event can double the average decadal retreat trend. Such events include rock-fall and toppling, after undercutting of the cliff base by wave quarrying, abrasion and weathering. Wave quarrying is the process of rock dissection by breaking waves, which produce shock pressures and air compression that widen existing joints and fissures[8]. If the front of the incident wave steepens and becomes more or less vertical just before it reaches the cliff face, the wave impulse impact can be very violent, either because of compression of the trapped air pocket or because of the so-called flip-through mechanism[9] (discussed in the article Wave loading on coastal structures). Incessant compression and shearing when waves hit the cliff face and tension when waves recede, lead to so-called 'fatigue' (reduction of rock strength), that facilitates the detachment of blocks[10]. This could be associated with microseismic vibrations caused by wave impacts that create strain within cliff bedrock during wave loading cycles, eventually leading to microcracking and lowering of the bedrock failure strength[11].
Weathered rock surfaces exhibiting discontinuities, holes and crevices, are much more susceptible to wave quarrying than smooth surfaces. Rock weathering is due to the mechanical action of alternate drying and wetting and salt crystallization in fissures, deterioration of cementing material between minerals and further due to chemical processes (dissolution) and biotic activity. Shales and mudstones are particularly sensitive to weathering by repeated wetting and drying. Chalk, limestone, and sandstones are broken down by repeated immersion in saline water, particularly where salt crystallisation or deterioration of mineral cements occurs.[6] Examples of biotic activity include grazing by limpets[12] and boring by sea urchins, in particular on limestone rock[8]. Plant roots can contribute to widening initial small cracks or crevices.
Rock hardness and its resistance to wave impacts can be qualitatively inferred from a Schmidt hammer test, that measures the rebound of a spring-loaded mass impacting against the surface of a rock sample. The test yields an approximate indication of the rock compression strength, but is not a direct measurement of resistance to wave erosion.
The persistent effectiveness of wave impact for cliff erosion depends on the removal of debris that protect the cliff base after the occurrence of mass wasting events. When debris removal leaves only a narrow beach, wave abrasion power is reinforced, due to scraping and shattering by remaining rock fragments moved back and forth by wave-orbital motion. Wide beaches, in contrast, can provide effective protection of the cliff toe from erosion. Observed erosion rates of a soft cliff were much higher at an intermediate beach width of a few meters than when the beach width was nil or more than five meters[13]. Field studies into cliff erosion further show that the wave impact on cliff erosion is significantly greater at high water levels than at low water levels[14]. Sea-level rise will allow waves to attack higher parts of the cliff and platform, and is therefore likely to accelerate retreat at many rocky coasts.
Suction: Negative pore pressure (below atmospheric pressure) that increases the stability of unsaturated soil (due to capillary, adhesive, and osmotic forces that sucks water into the soil skeleton)
Tension cracks: Cracks that appear when tensile stresses tear the soil skeleton apart (when drying of poorly drained soil induces matric suction)
Sliding: Mass wasting along an approximately planar shear failure surface or geological discontinuity when the available shear strength is insufficient to resist the downslope gravitational force, often due to an over-steepened or overhanging cliff
Slumping: Mass wasting along a curved failure surface where the effective stress and shear strength is reduced below a critical value due to elevated pore-water pressure.
Mass wasting: Downslope mass movement of cliff material
Soft cliff erosion
Soft rock cliffs are not only susceptible to wave erosion; they can also become unstable due to surface water running down the cliff face and by groundwater pressure and exfiltration that washes material downslope. Groundwater increases the weight of the cliff and raises pore-water pressure in soils and discontinuities, thereby reducing effective stress and shear resistance. Seepage and exfiltration can also erode fine material from the cliff face. In poorly drained soft rock (high clay content) tension cracks (also called tensile cracks) can form due to desiccation. These cracks run to the cliff surface, allowing the infiltration of rainwater. Infiltration of rainwater in soft rock causes suction loss that reduces the cohesion of rock material[15]. This may finally lead to the collapse of these cliffs by slumping (rotational slide) or sliding (planar slide) events. Such events are often preceded by soil creep. Debris fans (also called talus) at the cliff base after a slumping or sliding event are subsequently reworked by wave action; they are washed away or become part of the shore platform.
Cycles of wetting and drying can have a significant impact on the stability of soft rock cliffs. Changes in rainfall, drought duration and storm sequencing can alter the formation of desiccation cracks, groundwater recharge and pore-pressure peaks. Future cliff retreat therefore cannot be inferred from sea level rise and wave-climate change alone.
Limestone coasts are affected by both mechanical erosion and chemical dissolution. Dissolution is promoted particularly by rainwater, groundwater, fresh–salt water mixing and biologically modified water entering pores, joints and fissures. Salt crystallisation and bioerosion by boring and grazing organisms can further weaken the rock[8]. These processes can produce coastal karst features such as solution pits, grooves, pinnacles and dissolution-enlarged cavities.
Shore platforms [17]
Shore platforms can be either approximately horizontal, with a steep seaward edge, or gently seaward sloping. Tides modulate the frequency and intensity of wave attack at different levels of the shore platform. Sloping platforms (typically 1 degree to approximately 5 degrees) are commonly associated with macrotidal conditions and strong wave attack, whereas horizontal platforms are more common on micro-tidal coasts. Sloping platforms are also more common on soft rock coasts[10]. The width of near-horizontal platforms varies between only a few meters to hundreds of meters. Platform elevation also varies considerably, with some platforms formed around low-tide level, others at a range of intertidal elevations, and yet others at a range of supratidal elevations up to several meters above high tide[6]. High-tidal-level platforms are typical for hard-rock coasts and low waves, low-tidal-level platforms for limestone rocks. In the latter case, erosion resistance of the platform surface can be enhanced by carbonate precipitation and other cementing agents (silica, iron). Soft cliffs often have a beach at the base, with an intertidal veneer of sand and gravel. The seaward edge of shore platforms is much less subjected to eroding processes than the cliff base; the seaward edge is generally considered stationary[5]. However, fast retreat of the cliff base generates a wide shore platform, increased platform down-wearing and reduced wave attack at the cliff base. Trenhaile (2020[18]) therefore suggested that the shore platform may adopt an equilibrium slope, consistent with results of cosmogenic isotope analysis (see Appendix) of the sandstone shore platform in North Yorkshire, UK[19].
Several processes contribute to erosion of shore platforms. The rate of platform down-wearing is strongly variable from site to site; rates compiled from field studies are commonly of the order of hundredths to more than one millimetre per year[6]. Attack by waves loaded with rock fragments is an effective erosion mechanism, which polishes and wears down the platform surface. However, large amounts of abraded material and large rock clasts may have a protecting function instead. Part of the abraded material can form a sandy veneer at the cliff base. The finest fraction is washed away by waves and currents. A laboratory study on wave-cut platform formation by Sunamura (1975[20]) suggests that impulsive pressure (proportional to wave height) is predominant in the physical action of breaking waves, while abrasion by shearing force (proportional to wave height squared) is dominant in broken waves. Cliff base erosion observed in a multiyear field experiment was consistent with wave impact proportional to wave power (approximately wave height squared)[21].
Platform surfaces usually present many irregularities, with gullies, ponds and potholes scoured in faults, joints and in excavations of less resistant rock around hard outcrops. Chemical and physical weathering plays an important role, through the effects of alternate wetting and drying, salt crystallization and frost. Marine organisms (see Rocky shore habitat) accelerate platform erosion by grazing activity (surface scraping by gastropods) and by boring into the rock (e.g. by cyanobacteria, sponges, bivalves, sea urchins). Plant roots can widen crevices and dislodge rock fragments. However, biota can also provide protection against erosion. Seaweeds (e.g. kelp), barnacles and mussels contribute to limiting the impacts of physical weathering agents on shore platforms via moisture retention and buffering effects on temperature variations, wetting-drying, and salt crystallization[22].
A beach of sand or pebbles is often present at the cliff base and on the highest part of the shore platform. This sedimentary deposit is produced by local cliff erosion, as coastal transport is usually (but not always) small due to the presence of headlands and rocky outcrops. Cliff beaches are narrow, but wide pocket beaches can occur in sheltered zones between headlands. Sediments are usually coarse and poorly sorted, and can consist of sand and wave-rounded clasts, as well as angular debris, ranging to large boulders in many regions[23].
Appendix: Cosmogenic exposure dating
Long-term cliff erosion and platform-down-wearing rates can be inferred by measuring the concentration of rare isotopes (radionuclides) in the rock surface due to cosmic ray bombardment. In the simplest case, radionuclide concentration increases with exposure time, but erosion and shielding continually modify the accumulated concentration. From the analysis of isotope concentrations across an active shore platform, estimates can be derived of erosion rates over timescales ranging, depending on the isotope, from a few hundred to millions of years. The radionuclide [math]^{10}Be[/math] is most commonly used for cosmogenic exposure dating[24].
The interpretation of [math]^{10}Be[/math] concentrations requires assumptions regarding the exposure history of the coastal platform. These assumptions include: history of relative sea level change (including land level change); shielding of the platform by temporary beach cover; water shielding related to tidal distribution; temporal variability in the incoming cosmic ray flux; modes of cliff retreat (retreat rate, shape); modes and rates of platform-down-wearing. Model scenarios are compared with the measured [math]^{10}Be[/math] concentration pattern to identify the exposure histories and erosion rates that are most consistent with the observations. More than one scenario may provide an acceptable fit. These scenarios provide estimates of cliff erosion and platform abrasion rates[19][25].
Related articles
References
- ↑ Carter, R.W.G. 1988. Coastal Environments. An Introduction to the Physical, Ecological and Cultural Systems of Coastlines. Academic Press, London.
- ↑ Young, A.P. and Carilli, J.E. 2019. Global distribution of coastal cliffs. Earth Surface Processes and Landforms 44: 1309–1316
- ↑ Mao, Y., Harris, D.L., Xie, Z. and Phinn, S. 2022. Global coastal geomorphology – integrating earth observation and geospatial data. Remote Sens. Environ. 278, 113082
- ↑ Trenhaile, A.S. 2011. Cliffs and Rock Coasts. In: Treatise on Estuarine and Coastal Science Vol. 3, eds. Flemming, B.W. and Hansom, J.D., Elsevier, p. 171-192
- ↑ 5.0 5.1 5.2 5.3 Sunamura, T. 1992. Geomorphology of Rocky Coasts. Wiley, Chichester
- ↑ 6.0 6.1 6.2 6.3 Stephenson, W.J., Dickson, M.E. and Trenhaile. A.S. 2022. Rock coasts. Chapter 8.19, Treatise on Geomorphology 2nd edition. Elsevier
- ↑ Prémaillon, M., Regard, V., Dewez, T.J.B. and Auda, Y. 2018. GlobR2C2 (Global Recession Rates of Coastal Cliffs): a global relational database to investigate coastal rocky cliff erosion rate variations. Earth Surf. Dynam. 6: 651–668
- ↑ 8.0 8.1 8.2 Trenhaile, A.S., 1987. The Geomorphology of Rock Coasts. Oxford University Press, 384 pp.
- ↑ Shen, X., Whittaker, C.N., Thompson, CV.F., Raby, A.C., Young, A.P. and Dickson, M.E. 2025. Wave impacts on vertical cliffs: Insights from laboratory experiments and field observations. Coastal Engineering 201, 104794
- ↑ 10.0 10.1 Sunamura, T. 2015. Rocky coast processes: with special reference to the recession of soft rock cliffs. Proc. Japan. Acad., Ser. B 91: 481-500
- ↑ Thompson, C.F., Dickson, M.E. and Young, A.P. 2022. Seismic signatures of individual wave impacts on a coastal cliff. Earth Surf. Process. Landf. 47: 2833–2845
- ↑ Andrews, C. and Williams, R.B.G. 2000. Limpet erosion of chalk shore platforms in southeast England. Earth Surface Processes and Landforms 25: 1371–1381
- ↑ Carter, C.H. and Guy, D.E. Jr. 1988. Coastal erosion: processes, timing and magnitudes at the bluff toe. Marine Geology 84: 1–17
- ↑ Hackney, C., Darby, S. and Leyland, J. 2013. Modelling the response of soft cliffs to climate change: A statistical, process-response model using accumulated excess energy. Geomorphology 187: 108–121
- ↑ Brooks, S.M., Spencer, T. and Boreham, S. 2012. Deriving mechanisms and thresholds for cliff retreat in soft-rock cliffs under changing climates: rapidly retreating cliffs of the Suffolk coast, UK. Geomorphology 153: 48–60
- ↑ Trenhaile, A.S. 1999. The width of shore platforms in Britain, Canada, and Japan. Journal of Coastal Research 15: 355–364
- ↑ Bird, E.C.F. 2008. Coastal Geomorphology: An Introduction, Second Edition. JohnWiley and Sons, Ltd, pp. 411
- ↑ Trenhaile, A.S. 2020. Modelling the development of dynamic equilibrium on shore platforms. Marine Geology 427, 106227
- ↑ 19.0 19.1 Swirad, Z.M., Rosser, N.J., Brain, M.J., Rood, D.H., Hurst, M.D., Wilcken, K.M., Barlow, J., 2020. Cosmogenic exposure dating reveals limited long-term variability in erosion of a rocky coastline. Nature Communications 11, 3804
- ↑ Sunamura, T. 1975. A Laboratory Study of Wave-Cut Platform Formation. Journal of Geology 83: 389–397
- ↑ Young, A.P., Guza, R.T., Matsumoto, H., Merrifield, M.A., O'Reilly, W.C. and Swirad, Z.M. 2021. Three years of weekly observations of coastal cliff erosion by waves and rainfall. Geomorphology 375, 107545
- ↑ Gonzalez, J.A., Coombes, M.A., Palomo, M.G., Isla, FI., Soria, S.A .and Gutiérrez, J.L. 2021. Enhanced Weathering and Erosion of a Cohesive Shore Platform Following the Experimental Removal of Mussels. Front. Mar. Sci. 8:756016
- ↑ Trenhaile, A. 2016. Rocky coasts ― their role as depositional environments. Earth-Science Reviews 159: 1-13
- ↑ Trenhaile, A. S. 2018. Shore platform erosion and evolution: Implications for cosmogenic nuclide analysis. Mar. Geol 403: 80–92
- ↑ Jeong, A., Seong, Y.B., Choi, K.H., Swirad, Z.,M., Lee, C.-H. and Yu, B.Y. 2024. Shore platform erosion and cliff retreat in the Eastern Korea: A quantified assessment using 10Be concentrations and numerical modeling. Marine Geology 472, 107291
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