Why is the North Sea coast susceptible to storm tides?
A storm tide is a natural phenomenon, and hence cannot be prevented. It happens when the normal water level 
raises high along the shore due to strong onshore winds and/or reduced atmospheric pressure as in the North Sea. It is not an open ocean, being blocked from the ocean by the British Isles. As a result, the water dams up very quickly. The waves whipped up by the wind destroy or flood the dikes and beach dunes. Parts of the coast are also washed away. The Halligen Islands in Germany, for instance, have emerged as a result of storm tide action. These are small islands on the coast of Schleswig-Holstein, which were earlier connected with the mainland. They are regularly inundated by flood waters.
When rain falls on the ground and seeps below it, it washes away substances from the soil and rocks. It carries these substances into the river and the sea. You can read the names of these substances on the label of a bottle of mineral water. River water contains only a little salt – about 1 g/I, whereas the salt content of the sea is much higher - about 35 g/I, on an average. The reason is that the salt remains in the sea. Although the rivers continue to deposit new salts, the seas do not become ‘over salty’ because animals and plants in the sea absorb the salts in their bodies. When they die, they sink to the seabed and decompose into sediments.
The seabed is that area of the Earth which has been researched the least. However, we know, through sound wave technology, that there are deep valleys, high mountain ranges, steep slopes, and flat regions on the seabed. We also know that the seabed undergoes continuous changes. Underwater volcanic mountain ridges form at place where two tectonic plates move away from each other, and these ridges grow every year by a few centimetres. Generally, the bed of the flat seas, such as the coral reefs is densely populated with plants and animals. In contrast, the deeper regions in the seabed are often covered by a layer of sand, dust, and dead organisms from the rivers, which is several hundred metres thick.
The word ‘oceans’ implies the large water bodies of the world, namely, the Pacific, Atlantic, Indian, and Arctic, as well as the Antarctic oceans where we find icebergs. Smaller seas are separated from the oceans by island chains or underwater ridges, which are called ‘bordering seas’. These include the North Sea, the Baltic Sea, the Mediterranean Sea, and the Caribbean Sea. The bordering seas and the oceans have some common features: both contain salty water and restless seabeds. But there are also differences in the variety of plants and animals living in them and the storm tides—the tides that lash the coast of the North Sea are hardly found along the Pacific coast.
The Japanese word ‘tsunami’ means ‘harbour wave’. It is a series of water waves caused by the displacement of a large volume of a body of water, usually an ocean, though it can occur in large lakes. Tsunamis are caused by undersea landslides or by ‘seaquakes’ that is an earthquake whose epicentre lies below the ocean. If the seabed is displaced up to several metres, the water layer of several kilometres in thickness above it also moves with it. The wave caused by this movement has a lot of energy—much more than a wave of the same height whipped up by the wind. This becomes apparent when the wave moves towards the coast.
Living in an earthquake-prone zone is often unavoidable. For instance, almost entire Japan lies in an earthquake-prone zone. For this reason, buildings here are primarily made ‘earthquake resistant’ so that the loss can be minimized. Buildings are built on foundations made of steel balls or from elastic materials such as bamboo. In skyscrapers, a pendulum weighing several tons is installed in the upper floors, which minimizes the vibrations that start in the upper storeys when an earthquake occurs. If an earthquake occurs, we should take refuge, at best, in the open or seek protection under a table.
A weak earthquake— up to an intensity of 4.0 on the Richter scale—hardly causes any damage. Earthquakes with stronger intensities can cause cracks in buildings. They move the foundations of buildings, cause houses to collapse and lead to landslides. Supply lines get disrupted and gas leakage causes fires. In large earthquake catastrophes, bridges, dams, and power plants get damaged—and often whole areas are devastated.
The moving tectonic plates, in which the uppermost layer of the Earth is divided, not only collide at many places but also often slide past each other. This causes friction and leads to problems once in a while. Stress builds up, which then gets discharged very quickly. There is a jerk—the Earth trembles. This gives rise to shock waves that can cause a lot of damage at places, depending on the intensity of the earthquake and where they occur. Luckily, to some extent we can protect ourselves from these waves. A tsunami occurs if the centre of the earthquake lies below the sea.
Farmers prefer to settle near volcanoes because the soil is very fertile due to the lava and volcanic ash deposits. When the volcano erupts, people leave the place for some time, and return to the risk zone after the eruption because the safer places are already inhabited. Moreover, for many people the area around the volcano is home, which they do not want to leave.
We can call a volcano dangerous only when it causes damage to human life. For instance, if a volcano erupts on the almost uninhabited East-Asian peninsula of Kamchatka, it is hardly reported in the news. But, if a volcano starts spewing hot gases and ash in a densely populated area, many lives are at risk. Explosive volcanic eruptions discharge huge quantities of dust and ash, often burying neighbouring areas. Thin, dark magma can flow at a fast speed of up to 1oo km/h making it impossible to escape. Volcanic eruptions can also have far-reaching consequences. When the Indonesian volcano Krakatau erupted in 1883, the volcanic ash reached up to a height of 8o km, which for some time darkened the sky worldwide.
