On the origin and demise of coasts
Special allureCoasts are a special habitat. They are the transition area between land and sea and are influenced by both realms. Rivers carry nutrients from the land to the coastal waters and thus represent the basis of the marine food chain. The seas transport sediments – washing them ashore, reworking them or carrying them away, all of which change the shape of the coast. No other marine environment is more productive. Coasts provide nourishment in the form of fish and other seafood. But they are also important transportation routes for shipping and are intensively exploited for the production of natural gas and oil. At the same time, the coasts are highly desirable recreation areas for millions of vacationers. Numerous cities have been built on the coasts, and industries and power plants take advantage of their often well developed infrastructures.
- 1.1 > Many cities developed in coastal areas. The Beyog˘lu district of Istanbul, for example, is thousands of years old. It lies on the Golden Horn, a fjord-like inlet that divides the European part of the city into southern and northern areas.
- In general, the coastal zones of the Earth are extremely variable in shape and form. They are of great importance for humans, animals and plants, as well as for the atmosphere and climate because:
- they comprise around 20 per cent of the Earth’s surface;
- they represent important transportation routes and sites for industry;
- they are attractive recreation and tourist areas;
- they are sources for mineral and fossil raw materials;
- they encompass key ecosystems with great species diversity;
- they act as important sediment traps that consolidate river sediments;
- in their role as a buffer between the land and sea, they affect many global parameters;
- 75 per cent of all megacities (populations greater than ten million) are located in coastal zones;
- 90 per cent of global fisheries operate in coastal waters.
The coast – where does it start, where does it end?As a rule, maps depict coasts as lines that separate the mainland from the water. The coast, however, is not a sharp line, but a zone of variable width between land and water. It is difficult to distinctly define the boundaries of this transition zone. In the 1950s, scientists suggested using a definition of coast as the area that is influenced by the surf. Landward, this includes the extent to which the airborne saltwater spray can reach, thus encompassing some vegetation on the land. Seaward, this would extend to the area where the surf makes itself noticeable, for example, where it contributes to shaping the sea floor.
Although efforts are being made to establish a theoretical and universally accepted definition for the term “coast”, in practice disparate conceptions come into play. Different aspects predominate in science, depending on the particular sub-discipline being applied. Biologists, for example, concentrate primarily on life in the sea or in wetland areas along the coasts or in estuaries. Coastal protection specialists, on the other hand, who make plans for dykes and other protective infrastructures, are also interested in the hinterland to the extent that it could be impacted by storm floods. Economists have an especially broad definition of the term “coast”. As a rule they consider not only harbours and industrial areas near the coast, but also the flow of goods over the sea or to inland regions.
Over the past several decades, geologists and oceanographers have also attempted to systematically delineate and catalogue the world’s coasts. Here there are also different approaches depending on the focus of the effort. Coastal types are differentiated based on whether they are characterized by “high-energy” formations such as rocky or sandy coasts that are directly bathed by the surf or, like the Wadden Sea, are characterized by relatively calm, “low-energy” areas that are protected by sand banks or offshore islands.
In spite of their differences, many coasts have one thing in common: their great importance for humans. Coasts have been the starting points for explorers and the targets for conquerors. Archaeologists and ethnologists believe that the coasts have played a great role in the settlement of new continents or islands for millennia. Before people penetrated deep into the inland areas they travelled along the coasts searching for suitable locations for settlements. The oldest known evidence of this kind of settlement history is found today in northern Australia, where the ancestors of the aborigines settled about 50,000 to 40,000 years ago, presumably arriving on boats from islands that today are part of Indonesia.
- 1.2 > Alfred Wegener (1880–1930) was a German meteorologist, polar researcher and geoscientist. He proposed the scientific principle of continental drift. His theory, however, was long considered a foolish idea. It was not generally accepted until the 1970s.
Dynamic habitatCoastal contours are often viewed as fixed and immovable. People try to maintain a fixed line, not least of all to protect cities and systems that have developed and are concentrated at the coasts. But generally there is hardly any other area that is so dynamic and undergoes so much constant change as do the coasts. Experts call it a transient habitat. Depending on the time span being considered, different kinds of change can be observed. The slowest, but at the same time most drastic changes that coastlines undergo are caused by the motions of the continents. Movement of the continents was first postulated by the German researcher Alfred Wegener, who published his theory of continental drift in 1912. In the subsequent decades this theory was constantly expanded and improved. Today it is called plate tectonics. It states that the Earth is comprised of multiple layers, the uppermost of which, the lithosphere, is slowly moving. The lithosphere is made up of numerous large plates that lie side by side and move relative to each other by as much as 10 centimetres per year. The lithosphere includes the continents, but carry also the large ocean basins. It has an average thickness of around 100 kilometres and glides along atop a second Earth layer called the asthenosphere.
- 1.3 > Crustal material is created and destroyed at time scales of millions of years. In a continuous cycle, the individual continental plates collide, drift, and change their position relative to one another. It is possible to break the cycle down into individual stages, some of which are named after a present-day region that represents that stage. John Tuzo Wilson, a Canadian geoscientist, was the first to describe these cycles.
- In some places one lithospheric plate is thrust over another, causing upward folding of the rocks over millions of years and forming high mountains like the Himalayas. In other regions the plates slide along beside each other or drift apart. Coastal regions and the shallow marine areas called shelves are especially affected by these movements because they are situated on the margins of the continental parts of the plates, and are thus strongly deformed by the drifting of the continents.
Today, the vestiges of coastal seas such as fossilized bivalves, snails and other organisms of the shallow coastal waters can be found in many mountain ranges worldwide, including the Alps.
Continental drift also changes the shape of coasts by another mechanism. When a mountain range is crea-ted on land by uplift and folding, that is, when part of a continental plate is thrust over another and rises out of the water, one result is a drop in sea level. However, sea level can also rise to the extent that magma gushes in at the mid-ocean ridges, displacing large volumes of water.
Breakup of the supercontinentThroughout the Earth’s history many alternating tectonic phases have occurred. There have been times when the continents were connected to form a single supercontinent or a few large continents. These were followed by phases when the giant and large continents drifted apart again. These repeating sequences are named the Wilson cycle, after the Canadian geologist John Tuzo Wilson who first described this principle in a journal article in the 1960s. The most recent cycle began about 300 million years ago when the continental plates collided to form the supercontinent Pangaea. Around 230 million years ago Pangaea began to break apart again, separating first into a northern (Laurasia) and a southern part (Gondwana). In the second phase, beginning about 140 million years ago, Gondwana split into the land masses that eventually developed into present-day Africa, South America, India and Australia.
- 1.4 > Continental plates carry both the land masses and the oceans. They move at speeds of up to several centimetres per year. At some places the continental plates move away from each other, for example, at mid-ocean ridges. At other places plates are thrust over or under one another. The Indian plate is being subducted below the Eurasian Plate, causing continued growth of the Himalayas.
- 1.5 >Millions of years ago the continental plates formed a largely contiguous land mass, the supercontinent Pangaea. At that time the Atlantic Ocean did not exist.
- The breakup of Laurasia began around 65 million years ago with the separation of the North American and Eurasian land masses. This opened the North Atlantic, and India drifted more than 6000 kilometres to the northeast to collide with the Eurasian Plate about 40 million years ago. Over time the Himalayas were thrust and folded upward. India is still drifting northward today, causing the Himalayas to grow about one centimetre higher each year. Evolutionary biologists believe that the early phase of the breakup of Pangaea, associated with the formation of new coasts, favoured the origin of new species.
Life goes ashoreAs a result of the drifting of continents, coasts were not only created and destroyed, but also moved laterally. Entire coastal regions drifted into different climate zones, resulting in adaptation by existing organisms and the emergence of new life forms.
- 1.6 > Evidence for movement of the continental plates can be seen in Iceland. The island lies partly on the Eurasian and partly on the North American Plate. These two plates are drifting apart by a few centimetres every year. The fissure that cuts across the island is called the Silfra rift.
- An interesting aspect of these developments is assessing the role of coasts in the transition of life from the sea onto land. Today it is generally accepted that the first life forms developed in the sea and expansion to the land occurred at multiple locations at different times and at different rates. This took place within different groups of organisms completely independently of one another. It is thought that the arthropods, a group with jointed appendages that includes crustaceans, insects and spiders, settled on land independently of vertebrates. Genetic analyses have shown that the ancestors of present-day insects made the transition from an aquatic to terrestrial life habit around 480 million years ago.
- 1.8 > Whales evolved from land mammals. Their terrestrial provenance can be recognized by the fact that they move their tail fins vertically, using the same up-and-down motion that large predatory cats employ. By contrast, fish move their tail fins horizontally back and forth.
- It is assumed that the move to land for vertebrates began around 415 million years ago and lasted until about 360 million years ago. The first land vertebrates presumably evolved from the bony fishes. The first amphibian creatures may have been animals of the genus Kenichthys. Remains of this small animal, whose skull is only a few centimetres long, were found in China and have been age-dated at about 395 million years. It is possible that they preyed on insects at first. They might also have settled in near-coastal wetlands, river estuaries, wet river banks and brackish water areas where river water mixed with sea water. Among amphibians today there is still an animal group that lives both in the water and on land. Toads need water to reproduce. The development of their larvae takes place in water. For the adult animals, on the other hand, land is the predominant habitat, where they mate and hunt for prey. >