The Sea Shapes CoastsGeology and Geography
Wherever seawater meets dry land, we speak of a coast. Because the coast is exposed to the force of the sea without interruption, it is constantly changing. How strongly the water gnaws at the mainland depends on the strength of the rock, the height of the waves, the ocean currents and the tides.
Gentle ocean waves that wash sand and gravel onto the flat land heap up sandbanks and create a beach. The water further crushes the debris, reshaping the beach again and again. If waves and wind shift the sand sideways, a hook of sand grows into the sea. When this hook reaches the opposite end of a bay, the hook becomes a spit. Enclosed by the spit, a lake remains from the seawater: the lagoon.
But the surf does not only work the fine sand. It can even erode hard rock when it thunders with force against the cliffs of a steep coast. If the water drags broken-off pieces of rock with it, it grinds the rock further at the height of the waves: Cavities form. If the overlying rock collapses, receding bays and capes remain, reaching into the sea like arms of land. Sometimes only individual towers of rock remain in the sea, which are further worked by the water and eventually also collapse. The power of the sea is particularly strong during storm surges. They can extremely change the shape and course of the coast.
An eternal to and fro of fine sand and clay prevails on shallow tidal coasts. The alternation of low and high tide ensures that the material is washed up and away again and again. The result is a mudflat coast. The mudflats were washed up and deposited by the water and are covered by the sea at high tide. At low tide, channels appear in the mudflats – the tideways. The seawater flows through them, similar to a river bed, at low tide and back towards the land at high tide.
Cut off from the mainland
For hours yesterday, two tourists were trapped on a rock in the roaring sea. One of the two arches of the rock sculpture “London Bridge” on Australia’s famous “Great Ocean Road” suddenly collapsed. This cut off the visitors’ way back. They had to be rescued by a helicopter.
The young couple had walked to the end of the second arch to enjoy the fantastic view of the sea and coast. Once there, they heard an ominous crunching sound. When they looked around, the arch had already collapsed, cutting the connection to the shore. Fortunately, no one had just stayed on the first arch, there were no other victims. After five hours of waiting, the couple was happily brought back to shore by a helicopter.
The double arch of “London Bridge” was one of the most famous rock formations on Australia’s south coast. Wind and waves are increasingly eroding this coastline, causing part of the tourist attraction to now collapse. After the collapse, the “London Bridge” was renamed without further ado: It is now called “London Arch”.
On the Great Ocean Road
The surf is wild on Australia’s south coast, along which the famous Great Ocean Road runs. The stormy sea has already claimed many victims here: Over a hundred ships have already been wrecked on the rocky coast. Wind and waves grind up everything in their path here. And that is above all the relatively soft limestone with its bizarre rock colossi: London Bridge was only one of them, the “Twelve Apostles” or the “Island Archway” are also world-famous. The collapse of “London Bridge” shows how fragile the coast is: the rock disintegrates in the raging sea almost like sugar in hot tea. Without pause, the forces of nature gnaw at the coast and reshape it. So if you still want to see the twelve apostles in all their glory, you should hurry.
Sand in sight
Germany’s most famous North Sea island is threatened with extinction. Every year Sylt gets a little smaller. The “Blanke Hans”, as the stormy North Sea is also called, gnaws ceaselessly at the island’s west coast. To stop the shrinking of Sylt, sand is now to be pumped from the seabed onto the beach.
A hundred years ago, the island’s inhabitants were already thinking about how they could protect their coast from the raging North Sea. Long rows of wooden piles, so-called groynes, were driven into the sea at right angles to the coastline. Intended as breakwaters, they unfortunately did not bring the desired success on Sylt. Beach walls and concrete monstrosities were also built in the sand to fortify the island. Unfortunately, they were not only ugly but also useless. The masses of sand continued to be washed away and partly stranded off the coast of Amrum.
Sylt is now hoping for a new coastal protection measure: sand flushing. Dredgers dig up sand off the west coast, which is pumped onto Sylt’s beach and distributed there. When the North Sea rages again, it will initially only take the flushed-up sand with it. The original coastline is to be preserved in this way. Presumably, the flushing will have to be repeated regularly to preserve the holiday paradise in the long run. One can only hope: “Trutz Blanke Hans!”
Ballad of Detlev von Liliencron (1844-1909)
The extent to which the North Sea raged and wiped out the settlement of Rungholt and other villages in the terrible Marcellus Flood of 1362 is described by the poet Detlev von Liliencron in his ballad “Trutz Blanke Hans”, first published in 1883, which roughly translates as “Defy the stormy North Sea!”
Friesland. In the night of 16 January, the waves crashed more than two metres high over the tops of the dikes. On the coast of North Friesland, 21 dikes broke. The devastating storm surge probably cost thousands of lives. Whole swathes of land and several villages in North and East Frisia were submerged in the sea.
The “grote Mandränke”, as the storm surge is called, wreaked terrible havoc on the North Sea coast. The low summer dikes could not withstand the destructive water masses. The floods swept away terps (inhabited hills) along with houses and people, leaving a landscape in chaos.
Nothing on the North Sea coast is as it was before. Large parts of the islands and Halligen between Sylt in the north and Eiderstedt in the south, the so-called “Utlande”, sank into the North Sea. The thriving settlement of Rungholt, a major trading centre on the island of Strand, was simply washed away, along with seven other communities on the island. The land is lost forever.
The disaster mainly affects the marsh farmers of the area. Thousands of people drowned in the floods, most of their fertile arable and pasture land was swallowed up by the waters. In the meantime, the devastating flood has been interpreted as a sign from God. He is said to have punished the people for their easy living and extravagance with a storm tide.
Eyewitness account of a monk:
Wind and waves – these two forces of nature belong inseparably together. Unlike the tides, waves are created by the wind. The wind glides over the surface of the water and pushes the water along. How high the waves get depends on the wind strength and on the distances over which the wind whistles across the water.
If the waves run up onto the land on the coasts, they become higher. This is because as the water depth decreases, there is less and less space for the water to move upwards. In shallow water, the wave is also slowed down at the bottom. The crest of the wave, on the other hand, tilts forward without braking and “breaks”. The swirling of the water in the air creates white whitecaps, the spray.
When an extremely strong wind blows across the sea, a storm surge occurs. Storm surges are particularly frequent in spring and autumn. With their force, they can cause severe flooding and completely change the shape of the coast. The North Sea coast with the German Bight is particularly at risk from storm surges. Because the North Sea is very shallow, the water here can pile up very high during a storm.
In addition, there are also individual particularly steep waves that are much higher than the waves in their vicinity. For a long time, such monster waves or “cavent men” were thought to be “sailor’s yarn”, i.e. exaggerated stories of adventure told by seafarers. However, satellite images and precise measurements can now prove that such monster waves really do exist. They can reach heights of up to 40 metres, posing a serious danger even to large ships. It is not yet clear exactly how they form. Presumably, they are formed by the coincidence of slow and fast waves, combined with ocean currents.
Unlike waves and monster waves, tsunami waves develop after earthquakes or volcanic eruptions. Tsunami waves can be devastating: In Japan, a ten-metre-high tsunami rolled over the country’s northern coast after a violent earthquake in March 2011. Thousands of people fell victim to the disaster
From rock to grain of sand – Weathering
Today, northern Canada is a gently undulating landscape. Many millions of years ago, however, a mountain range stood here. In fact, over a very long time, even high mountains can turn into small hills.
The reason for this transformation: the rock on the earth’s surface is constantly exposed to wind and weather. If, for example, water penetrates into cracks in the rock and freezes, it blasts the stone apart. This process is called frost blasting. Changes in temperature between day and night and the force of water and wind also cause the rock to become friable. In other words, it weathers. This process can also be observed on buildings or stone figures. During weathering, the rock breaks down into smaller and smaller components down to fine grains of sand and dust. Different rocks weather at different rates: granite, for example, is much more resistant than the comparatively loose sandstone.
Some types of rock even dissolve completely when they come into contact with water, for example rock salt and lime. Rock salt is chemically the same as table salt – and that already dissolves in ordinary water. Lime is somewhat more resistant, but limestone also dissolves in acidic water. Acid is formed, for example, when rainwater in the air reacts with the gas carbon dioxide. This “acid rain” attacks the limestone and dissolves it over time. On the earth’s surface, weathering leaves behind fissured limestone landscapes, while caves form underground.
But it is not only solution weathering, but also heat and pressure that wear down and crumble rock beneath the earth’s surface. Where plants grow, roots dig in, blast the rock apart piece by piece and also ensure that it is eroded millimetre by millimetre.
In this way, weathering not only works on individual rocks, it gnaws away at entire mountain ranges. But it will take a few million years before the Black Forest is as flat as northern Canada.
Constant dripping wears the stone
Deep gorges in the mountains, wide sandy beaches by the sea and broad rivers meandering through meadows and fields – these are all landscapes we know well. Because they are so varied, we find them impressive and beautiful.
The sculptor of all these landscapes is the cycle of water. More than any other force, water sooner or later shapes the surface of the earth. It washes away soil after a downpour. It burrows into the subsoil and loosens parts of the rock. It carries soil and weathered rock debris down into the valley. Where the water flows away more slowly, it releases its load of silt, sand and debris. At high tide, it floods the shallow areas of a valley, the river floodplains. Here, too, it deposits fine silt. When the water finally flows into the sea, it works the coasts and forms very different landscapes, for example cliffs or long sandy beaches.
Water also shapes the landscape in the form of ice. When water freezes in rock cracks, it blasts the stone. As a glacier, it planes notch-shaped river valleys into round trough valleys. And the moraine landscape in the foothills of the Alps with its scree hills and boulders is also the result of glaciers that shaped the subsoil long ago.
- Compaction in the Rock Cycle: Understanding the Process Behind Sedimentary Rock Formation
- Crystallization in the Water Cycle: A Fundamental Process in Water Distribution and Purification
- Understanding Crystallization in the Rock Cycle: A Fundamental Process in Rock Formation
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