Category The Earth, Earth Science, Planet Earth

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.