The myriad faces of the coasts
A million kilometres of coastsThe coasts of the world are highly diverse. The northern coast of Brittany in France is characterized by granite cliffs interspersed with numerous bays. In Namibia the high dunes of the Namib Desert extend to the Atlantic shore, where the coast runs nearly parallel to the dunes. In Siberia, by contrast, the flat coastal region is dominated by permafrost, a metres-thick layer of frozen soil whose surface thaws out for a few weeks each year during the short Arctic summer, when it is especially susceptible to wave action. During storm-flood events, several metres of the saturated banks can break off, creating a constantly changing shore face.
- 1.19 > This satellite photo shows the Lena River delta in Siberia with all its fine structures, extending around 150 kilometres into the Laptev Sea. A large proportion of the sea ice that eventually drifts out into the Arctic Ocean originates in this marine region.
- What all of these coasts have in common is that they are narrow strips of land exposed to the forces of the sea. Depending on the context they can be classified in different ways. Coasts can be distinguished based on whether they are strongly or weakly washed by the surf and currents. They can, alternatively, be classified according to the materials they comprise or by the rate that the material is eroded away by the sea. Coasts can furthermore be characterized by their ability to capture sediments that are delivered by rivers or currents. The ultimate form exhibited by a coast also depends significantly on the interplay between the materials that make up the substrate or that rivers transport to the coast, and the physical forces of wind and wave action that impact those materials.
Geologists estimate the total global length of coastline to be around one million kilometres. This projection, of course, depends on how fine a scale is applied. When considering the entire globe, any differentiation of the coasts is only practical at a relatively coarse scale. For this categorization the continental margins can be traced in their present forms, which are in part a result of plate tectonics. Researchers created such a classification system in the 1970s, under which six different categories of coasts were distinguished.
- Coastal plain: an area where the land gently flattens toward the sea. An example is the coast of the West African country of Mauritania, where the land merges into the sea through a broad strip of coastal marshes and low dunes.
- Major delta: a large river mouth where sediments from the river are deposited because the ocean currents or tides are not strong enough to transport the material away. This is the case with the delta of the Lena River in Russia, which flows into the Laptev Sea in the Arctic Ocean.
- Tropical coral reef: a structure composed of carbonate produced by sessile corals (Cnidarians). It develops as a fringe along the coasts in near-surface waters penetrated by abundant light. Reef-building corals occur in tropical and subtropical waters at temperatures consistently greater than 20 degrees Celsius. A spectacular tropical coral reef is situated along the Central American Caribbean coast between Honduras and Belize. It is around 250 kilometres long and is among the most popular diving areas in the world.
- Rocky coast and fjord: a coast of solid rock. Fjords, like those found abundantly on the west coast of Norway, represent a special kind of rocky coast. They were formed during glacial periods, when the motion of the glaciers scoured deep valleys into the bedrock.
- Permafrost coast: a deeply frozen soil covering large areas of the Arctic land masses in the northern hemisphere since the last glacial period. Permafrost is found over many thousands of kilometres along the coasts of North America, Siberia and Scandinavia.
- Headland-bay coast: a coast where rocky headlands extend into the sea. The headlands act as barriers to obstruct the surf and currents. Slow eddy currents form in the sheltered areas between headlands, gradually eroding the shore and forming bays. An example of this is Half Moon Bay on the Pacific Coast of the United States near San Francisco. There, over thousands of years, a half-moon shaped bay has formed behind a prominent headland.
Wind and waves shape the coastsThe physical forces of the sea – the waves, currents and winds – have a substantial effect on the shape of the coasts. The intensity of these forces is used to distinguish between low-energy and high-energy coasts.
The kind of material that makes up the substrate of a coastal area is also a key factor influencing the formation of the coasts. Tidal flats comprising relatively fine sediments can be reworked fairly quickly because these materials are easily transported by the currents. Fine sands can also be easily transported, as illustrated by the East Frisian Islands off the German North Sea coast. Because the prevailing winds there blow from the west, wave action carries sand away from the northwest side of the islands and redeposits it on the east side. In the past this has caused the islands to slowly migrate eastward. To impede this motion, rock jetties and breakwaters were built as early as the 19th century to fortify the islands. This greatly helped in preventing further migration.
- 1.22 > The village of Porthleven in the English county of Cornwall is located on an extremely high-energy rocky coast. Accordingly, the shoreline fortifications, including massive walls, are very substantial. Under conditions of very high seas, however, they are hardly noticeable.
The intertidal zone
The intertidal zone is the area of the coast that is flooded and then exposed again by the rhythm of daily tidal cycles. The surface can be mudflats, sandy beaches or rocky cliffs. Rocky shores are exceptional because they occur on steep coastlines, while most other intertidal zones are found on flat coasts. Large-scale flat intertidal areas which include the salt marshes on the shore are called mudflats.
- While changes in the shape of sandy coasts are often visible with the naked eye, they can be more difficult to recognize when other material is involved. But even high-energy rocky coasts do change their appearance over time. The rate of change, however, depends largely on the properties of the rocks. Coasts composed of compacted but not yet lithified ash, generated over time by ash falls from volcanic eruptions, are especially easily eroded. Examples of this kind of coast are found in New Zealand. Up to ten metres of coast can be lost there within a single year at some locations. Chalk cliffs, like the White Cliffs of Dover in the extreme southeast of England are also relatively soft. When exposed to strong currents they can be eroded by several centimetres per year. By contrast, hard granitic rock is depleted at most by only a few millimetres in the same space of time. Harder still, black volcanic basalt is only destroyed by water each year by a maximum of a few hundred billionths of a metre.
- 1.23 > Depending on the material making up the coasts, they can be eroded slowly or more rapidly. Some can be depleted by several metres in a single year.
A question of particle sizeUnderstanding the nature of the substrate in coastal areas is especially crucial for coastal protection, coastal management, and the planning of waterways and port installations. In particular, the size and density of the particles that make up the material play an important role. These can be factors, for example, in determining whether the shore of a populated island is in danger of erosion or whether shipping channels might shift their positions causing ships to run aground. With respect to the size of particles, the following coastal types are differentiated:
- muddy coasts,
- sandy coasts,
- pebble coasts,
- cobble coasts,
- rocky or boulder coasts.
The filtering function of the coastsIn many areas the character of the coasts is strongly shaped by rivers – through both their current strength and the material loads that they transport. They carry many minerals and nutrients that are incorporated to some extent into the sediments. Coasts that are rich in such sediments are also highly productive. A good example are the Sundarbans in Bangladesh and India, which, with a total area of around 10,000 square kilometres, comprise the largest block of mangrove forest in the world. The Sundarbans formed in the estuarine areas of the Ganges and Brahmaputra Rivers, which deliver immense amounts of material into the Gulf of Bengal. The Sundarbans are a vital unspoiled natural region. They are home to abundant birds, fish, crocodiles, pythons, deer and wild boar. Furthermore, rare animals such as the axis deer and Bengal tiger may also find refuge here.
Depending on the ability of a particular coast to filter and store the material transported by rivers, it can be designated as having an active or inactive filtering function. >