They change constantly

Jan 28, 2008 13:33 GMT  ·  By

1. The shore seems to be the edge of the continent, but that's not true: the continents continue under the sea, on a strip edging each continent, and called continental shelf. How much of the continental shelf is covered by the sea depends on how much ice is stocked in the Polar Ice.

2. Around 10,000 years ago, the polar ice caps were much larger and the sea level was much lower, so that Britain was united to Europe and Alaska to Siberia. The melting of the ice at the end of the Ice Age raised the sea level by 7 mm annually. In 10,000 years, this meant 7 m (23 ft). The Dover Strait of the English Channel, 36 m (120 ft) deep, was covered by sea 8,000 years ago. Behring Strait between Alaska and Siberia is at least 45 m (150 ft) deep.

The shore line has continuously changed along the Earth's history. The line coast is also modified by the tectonic movements. The Pacific plate enters continuously under the South American one (process called subduction), through a deep sinkhole near the shore. Andes Mountains descend abruptly close to the shore, and in some places, there is no shore. These slow changes occur along millions of years.

3. Waves' action can be seen from one year to another. Waves are caused by winds and influenced by the tides. Tides are caused by the gravitational force of the Moon. The Moon attracts the water mass beneath; this way the high tide emerges. The low tide appears when Moon force does not act. There are differences between the levels of the tide between Full Moon and New Moon, when the Moon and the Sun are located on the same ax with the Earth, on one side or another. In this situation, the gravitational forces sum. This happens twice a month. The tides are minimal when the gravitational force of the Moon makes a right angle with the gravitational force of the Sun. This also happens twice a month, when the Moon is found in its first and last quarter.

4. Most shores experience two high tides daily. In some regions, like southeastern Asia, this happens just once a day. Tides are very small on lakes and seas surrounded by lands, like the Great Lakes of North America and the Mediterranean Sea. Baltic and Black Sea do not have tides (almost) at all.

5. Hurricanes and other devastating storms produce killer waves, but the most devastating are those produced by underwater earthquakes, called tsunamis. If the tsunami strikes a shallow shore, its height can reach 25 m (83 ft) and destroys everything on land.

6. Sea currents and waves use sand and gravel to grind the shore continuously, destroying its rock base. Soft rocks are destroyed faster. If the base of the rock coast is eroded by waves, the upper parts will crumble. The broken rocks are taken by the waves. The best transporting waves are the short and abrupt ones which emerge close to the shore during the winter storms. They can have a force of 10 kg per square meter. Summer storms produce longer, stretched waves away of the shore. These waves carry more sand and gravel to the shore than what they bring back at the return, so that they actually build a new shore, not destroy it. Some destruction and construction can occur on the same place, like in Cap Canaveral (Florida), where waves build a new gulf while nearby they form a peninsula.

The action angle of the waves affects the sedimentation and abrasion of the waves. When waves reach the shore under certain angles, they depose sediments. The sea takes from one place and deposes onto another. This way, the length of Cape Cod, Massachusetts, is growing.

The grinding of soft rocks creates caves, rock tunnels, and if their roof crumbles, then blowers appear. In some places, waves dig chambers in the sides of the peninsulas. In time, the chambers can join each other, forming an abrasive gait or a spectacular rock bridge.

7. In Netherlands, over 1900 km (1190 mi) of dikes and sand dunes protect fertile land against sea flooding. Amsterdam and Rotterdam are located on islands recovered from the sea, called polders. In 1927, Dutch people started to transform the huge golf Zuider Zee in fertile land. A 32 km (20 mi) long dam at his entrance turned into a freshwater lake: Ijsellmeer. Dikes marked inside the lake 5 large polders, water being pumped out of them. Soil was washed of salt, and now agriculture is practiced there.

8. Islands are of two main categories. Some separate from a continent, like Great Britain and Ireland. These islands are located on continental shelves and if sea level decreased, they would be connected to the mainland.

Other islands are volcanic: they are the top of volcanoes raising from the sea bottom. This is the case of Hawaii or Azores Archipelagos.

9. Coral islands form over submarine volcanoes, sometimes, edging the submarine volcanoes, raising very little above the water. They only form in warm waters, being produced by the calcareous skeleton of tiny animals called coral polyps. The polyps form colonies of millions of individuals to a maximum depths of 30 m (100 ft). After their death, their skeletons form the coral banks. If the bank forms at the edge of a crater, the island has a ring shape and is called atoll. The atoll surrounds a saltwater lagoon. The ring breaks in several places, so that the lagoon is directly connected to the sea. Broken coral transforms into calcium carbonate sand. This will form the base of the soil on coral islands. On the external part of the atoll, the shore is thin and the sea deepens abruptly. On the inner side, the shore is more gentle and most lagoons are shallow.

Coral barriers form at a certain distance from the shore and between them and the shore there is a long lagoon. The coral barrier, too, can be interrupted, especially close to river mouths (corals lack in these areas because the sediments discharged by the rivers kill them; they need clean waters for their photosynthetic symbiont algae). The most famous is the Australian Great Coral Barrier, next to Queensland (northeastern Australia).

10. Mangrove forests appear in tropical coasts devoid of powerful waves.