What makes a coastline rocky

Waves that continuously pound on rocky coastlines are the most effective erosive agents, slowly wearing down the rock and, in some cases, quarrying away large boulders.

When waves carry sand and smaller rocks, these particles are thrown against the coastal rocks, causing significant abrasion and erosion. Abraded rock surfaces tend to be smooth, whereas those eroded by wave quarrying are irregular. In higher latitudes subject to the freeze-thaw cycle, water often gets trapped in cracks and joints in the rock and then freezes.

Biological processes also contribute to the erosion of rocks along rocky coasts. Microscopic blue-green algae burrow into limestone, using the calcium carbonate CaCo3 as a nutrition source and causing the limestone to be more easily weathered away, a fraction of an inch mm at a time. Rocks along the coast are also subject to chemical weathering , like other rocks in other environments. Limestones may be dissolved by acid rain, and feldspars in granites and other rocks may be converted by hydrolysis into soft, easily eroded clay.

The relative strength of these processes is determined by several factors, including rock type, climate, wave energy, rock structure, tidal range, and sea level. Highly fractured rocks tend to break and erode faster, especially in climates with a significant freeze-thaw cycle.

Headlands and pocket beaches of Channel Islands National Park in California are distinctively shown in aerial photographs. Waves are refracted around the headlands, increasing erosion at seaward positions on the islands in the park. The protected embayments, where wave action is subdued between headlands, are often transformed into sandy pocket beaches.

In lower-energy pocket beaches, sediment transport is not able to carry sediment downshore except during increased wave, wind, and storm activity. Pocket beaches may be ephemeral and change seasonally, or even disappear, on account of increased energy events.

The headlands on rocky coasts are exposed to intense wave, wind, and storm action. Eventually sea caves may form in less resistant, easily erodible bedrock located on promontories. These caves are distinctive environments that are particularly suited for bryozoans, sponges, barnacles, tubeworms, and some species of shade-tolerant red algae. Sea caves are popular with recreational boaters and divers. The constant erosion of rocky headlands may produce a variety of particular geomorphic structures, including sea arches and sea stacks.

These isolated remnants of the headland have been detached from the mainland. With prolonged erosion, sea arches may collapse to form sea stacks—steep pillars of rock a short distance from the mainland. Both sea stacks and sea arches are impermanent features that will eventually disappear with continued erosion. The reduction or removal of the fronting beach occasionally leads to a dramatic increase in sea-cliff erosion. This was caused by the attack of storm waves with extreme high runup levels, which reached the cliff base with less energy dissipation due to localized lowering and narrowing of the beach in front of the cliff, these corresponding to one of rip-current embayments of a cuspate shoreline formed by storm waves.

The role of the morphology and behavior of a fronting beach is further illustrated by erosion of sea cliffs cut into Pleistocene sandstones along the central Oregon coast. The coarser-grained reflective beaches are steeper sloped and respond more quickly to winter storms with larger changes in beach-profile levels than do the fine-grained, gently-sloping dissipative beaches.

As a result, the reflective beach is a weaker buffer against wave attack, and the sea cliff is more susceptible to erosion compared with areas where the cliff is fronted by a dissipative beach. A further decrease in buffer protection on the reflective beach is brought about by the more pronounced development of embayments eroded by rip currents, allowing for easy landward penetration of storm waves to the toe of the sea cliff.

The height of a fronting beach can be uniquely determined by the volume of beach material if a uniform beach slope is assumed.

Along the North Norfolk and Suffolk coasts, England, Lee 88 has examined on the year-by-year basis the recession rate of Pleistocene soft rock cliffs in connection with the volume above High Water Level of the fronting beach during a period of 11 years, and provided for each coast the relationship between recession rate and beach volume, plotted with highly scattered data.

An envelope of the data cluster shows that, as the sediment volume increases, the maximum recession rate for a given sediment volume tends to increase abruptly and decrease after the volume excesses some optimal value. A more marked trend is seen on the Suffolk coast. Let us examine here the effect of a fronting beach on cliff toe erosion considering the wave and cliff strength factors.

Bluffs composed of glacial deposits on the US shore of Lake Erie have suffered severe recession. The study has discussed erosion magnitude in connection with storm waves accompanying surges, water level storm surge plus lake level and beach width. Data acquired at Helen Drive, one of the five sites, will be used here for a quantitative examination of the effect of beach width on the cliff toe erosion by waves. The Helen Drive bluff 7 m high is composed of weak till having a mean compressive strength of 0.

In front of the bluff a narrow, sandy beach developed with a width varied from 0 to 9 m with time. Seven of them are selected here, which occurred under almost similar water-level conditions including surge height , 1.

The magnitude of storm surges at the seven events was in a range from 0. Carter and Guy 49 have observed that 1 wave breaking occurred offshore at a storm and broken waves acted on the cliff, 2 mean values for storm wave height and period in the surf zone were 1 m and 6 s, respectively, 3 the largest erosion events occurred when the storm durations exceeded 10 h, and 4 sand-laden waves abraded the cliff toe.

No detailed data are provided on the height of the cliff-platform junction. From the surge heights described above, 0. A further assumption is made that the storm waves always had a breaker height of 1 m, a period of 6 s, and a duration of 10 h. The erosion rate is the mean value obtained by dividing the erosion distance during a single storm event by 10 h, the distance read from the diagram of Carter and Guy , Fig.

There is clearly an optimal value for beach width giving rise to the maximum erosion rate. In order to express mathematically the erosion-rate vs. The relationship between the cliff toe erosion rate and the width of a fronting beach at Helen Drive on the Ohio shore of Lake Erie. Based on data of Carter and Guy. The application of Eq. For this purpose, the following relation 93 will be used:. Broken waves with a height of 0. The rate of erosion caused by the waves is assumed to be 0. The line and curve in Fig.

Equation [ 11 ], thus determined Fig. The geometry and dimensions of the fronting platform may affect the wave intensity at the cliff base. Philpott 95 and Kamphuis 60 have considered that downcutting of the platform surface immediately in front of the cliff must have occurred prior to the cliff recession: increasing water depth at the cliff base will reduce energy dissipation of waves arriving at the cliff base to further cliff erosion.

Based on extensive studies on cliffs and shores composed of cohesive materials, Hutchinson 96 has also suggested that the lowering of shore platforms controls the cliff erosion.

The platform downwearing has been widely observed along the till shores on the Canadian side of the lower Great Lakes. The height of a cliff-platform junction tends to increase as the strength of coastal rocks increases, as conjectured from the result of laboratory experiments on notch initiation under the action of broken waves.

On the chalk coast of southeast England, shore platforms are of concave upward profiles with some having steeply inclined ramps at the cliff base, and the cliff-platform junction is located at about the high tide level, generally several meters above the mean platform level.

The lowering of chalk platforms involves various erosive processes: abrasion by waves armed with beach deposits residing on the platform, plucking of jointed blocks by hydraulic force and their removal, direct erosion of platform substrates due to grazing by marine organisms and deterioration of platform material due to frost and salt weathering especially in harsh winters.

Measurements of chalk platform downwearing have been carried out by use of Micro Erosion Meters — and laser scanners. Foote et al. Dornbusch and Robinson have attempted to investigate long-term chalk platform lowering on the basis of measurements of the block removal and step backwearing on the East Sussex coast, England, using air photographs taken in and recent field studies done in They converted the amount of step erosion into equivalent mean annual platform lowering rates during the period of 28 years.

The offshore bottom topography and its effect on wave refraction or wave-energy attenuation may control the assailing force of waves acting on a sea cliff. Robinson has examined the long-term variation in erosion rates at Dunwich soft cliffs in Suffolk, England. The average rates were 1. This drop can be attributed to the reduction in wave energy reaching the coast. Before the early 20th century, wave refraction over the offshore Sizewell Bank focused the wave energy on the shoreward Dunwich site, but the northward growth of the Bank during the last century produced a divergence in wave refraction and a reduction in the assailing force of the waves.

Lowering of shore platforms located offshore of rapidly receding till cliffs has been extensively studied along the Canadian shore of the lower Great Lakes, 60 , 97 — 99 , where till bottoms are covered with a sparse veneer of sand.

These studies have been conducted with a view that nearshore lowering will reduce the dissipation of wave energy reaching the cliff toe, which in turn will expedite cliff erosion. The presence of talus is a major controlling factor for toe erosion, because talus can protect the foot of the cliff from wave attack until it is removed by waves and currents. Knowledge on the residence time of the talus is necessary for predictive studies of cliff recession.

In the Lincoln City littoral cell on the mid-Oregon coast, USA, the life time of talus at the base of a cliff fronted by a dissipative beach is much longer than where a steeper reflective beach is found. On the chalk coasts in southeast England most cliff failures yield small amounts of debris, less than 1, m 3 in volume, which may be removed in a few weeks or months.

Lageat et al. Cliff undercutting by waves results in slope instabilities, eventually leading to intermittent mass movement. Such movement can be occasionally destructive to coastal properties. For example, the initial movement of the Miocene mudstone cliff in the Jump-Off Joe area on the Oregon coast resulted in the loss of a dozen homes during the s. Such a serious problem threatening coastal communities has been reported from many locations in California.

A variety of terms are used for mass movement phenomena. These types depend mainly on lithological factors such as geological structures, stratigraphic features, and geotechnical properties. Hybrid types exist between two or more of these failure modes. Falls denote movement of a rock mass that travels most of the distance through the air as a freely-falling body, but mass movement occurring along an almost vertical failure plane is also categorized into this mode, also referring to as a vertical failure type.

An example of deep-notch development responsible for the vertical failure can be taken from Eocene sandstone cliffs on the southern California coast. Topples differ from falls in that little free-fall movement takes place because rotation of a block around a fixed hinge dominates the motion. Topples are most common on precipitous, sometimes overhanging, cliffs composed of vertically-jointed hard rocks like those found on the Liassic coasts of South Wales.

Flows move with increasing velocity towards the surface of the moving body; no block movement is present due to differential shearing within the body. Flows occurring on some coasts cut into soft clayey materials with fluidized potential are called mudflows, and are frequently initiated by mudslides. The sliding mass may disintegrate during its movement — the flow component increases, resulting in the generation of mudflows in the latter phase of the event.

However, the distinction between mudslides and mudflows is not easy in the field. These two failure modes contribute significantly to the recession of cliffs in soft materials such as clay, mudstone and till. Slides are shearing displacements occurring on a distinct slip surface, and the sliding mass exhibits block movement. The former has an almost linear sliding surface, whereas the latter is along a circular plane.

Planar sliding with a high-angle failure plane may be found on till cliffs, chalk cliffs , and Pliocene mudstone cliffs. Shallow-seated slides are observed on till cliffs of the Holderness coast in England, 12 , near Kilkeel in Northern Ireland, along the Lake Michigan and along the Canadian shore of the lower Great Lakes.

Aside from the wave factor, the most common controlling factors for the occurrence of mass movement are rainfall and groundwater. Rainfall results in gully erosion, , slumping and vertical failure. Most mass movement phenomena have occurred during the action of storm waves which facilitates cliff undercutting, or during and immediately after a long spell of wet weather. Other cliff failures, however, have occurred suddenly and unexpectedly during normal weather conditions, as seen from an event at Scarborough on the east coast of England.

The sliding lasted for several days, leading to the gradual collapse of a four-star hotel. Whatever the cause, mass movement acts to render the cliff face profile more stable — a more gently sloping profile develops.

Simultaneously, debris masses are supplied to the base of the cliff. Waves remove the debris, undercutting the base so that the overall cliff profile once again becomes steep and unstable, and mass movement ensues. As seen in this recurrence, a cyclic change between steep and gentle profiles occurs during the cliff-recession process. This indicates that similar cliff profiles will recur with a certain time interval.

The objective of this paper has been to review the status of our understanding of the recession processes of soft rock coasts, and to present two new findings: the temporal change in cliffline recession mode Fig. It should be emphasized that the driving force of the system is wave action that will remove the debris at the cliff base and then resume cliff erosion. Without cliff erosion by waves, no cyclic recession will occur. I thank Professor Kiyoshi Horikawa, a member of the Japan Academy, for recommending me to write this review article.

Sunamura received M. He started his research career as a research associate at the Coastal Engineering Laboratory, University of Tokyo in and moved to the Institute of Geoscience, University of Tsukuba in where he worked as an associate professor — and as a professor — His major research topic is morphodynamics of beaches and coasts.

National Center for Biotechnology Information , U. Author information Article notes Copyright and License information Disclaimer. Sunamura, Namiki, Tsukuba , Japan e-mail: pj. Received Jun 1; Accepted Oct 1. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article has been cited by other articles in PMC. Abstract Substantial progress in research on the recession of coastal cliffs composed of soft materials has been made in recent years and data with higher accuracy have been accumulated. Keywords: coastal cliff recession, recession rates, wave erosion, soft rocks, rock strength, beach sediment. Introduction Rocky coast landforms, usually characterized by steep sea cliffs, are categorized into two: shore platforms and plunging cliffs Fig.

Open in a separate window. Figure 1. Coastal changes can take hundreds of years. The way coasts are formed depends a lot on what kind of material is in the land and water. The harder the material in the land, the harder it is to erode. Coastlines of granite , a hard rock, stay pretty stable for centuries. Sugarloaf Mountain, on the coast of Rio de Janeiro, Brazil, is made mostly of granite and quartz.

It has been a landmark for centuries. The famous White Cliffs of Dover, in England, are made of calcium carbonate. This is a soft material and erodes easily. However, it exists in such great quantities that years of erosion have not made a visible impact on the coastline. The other coast is French. The sandy coastlines of islands, on the other hand, change almost daily. The island of Mont Saint Michel is only an island when the tide is in.

It is part of the coast of France during low tide. Islands are also the site of Earth's newest coastlines, like a Tongan island created in March by the eruption of the volcano Hunga Tonga-Hunga Haapai. The "Big Island" of Hawaii, created by five volcanoes, sometimes expands its coastline when one of its active volcanoes, Mauna Loa or Kilauea, erupts.

If lava flows reach the ocean, the lava cools and forms new coastline along the Pacific Ocean. Tides, the rise and fall of the ocean, affect where sediment and other objects are deposited on the coast. The water slowly rises up over the shore and then slowly falls back again, leaving material behind. In places with a large tidal range the area between high tide and low tide, waves deposit material such as shells and hermit crabs farther inland.

Areas with a low tidal range have smaller waves that leave material closer to shore. Waves that are really big carry a lot of energy. The larger the wave , the more energy it has, and the more sediment, or particles of rock, it will move. Coastlines with big beaches have more room for waves to spread their energy and deposits.

Coastlines with small, narrow beaches have less room for waves to spread out. All the waves' energy is focused in a small place. This gives the small beaches a tattered, weathered look.