Why is sand of different types?



Sand is not a rough mixture of various rock particles, but a granular material. The size of a sand particle ranges from approximately 0.02 mm to 2 mm. The appearance of the sand particles depends on the rock from which they were formed by erosion. Black sand is made from volcanic rock, and oatmeal sand from quartz and feldspar grains. Red sand contains a lot of iron-rich minerals, which are frequently found in sandstones, and white sand is mostly composed of small, grated seashells, snails or the chalk from coral reefs. If a rock containing gold erodes, the gold particles are freed and get stored in rivers. Such findings led to the so-called gold rush in 1848 in California. 


How are gorges formed?

Gorges, also called ‘ravines’, ‘flumes’ or ‘canyons’, are deep, narrow valleys with sharply steep cliffs. They are formed mostly due to the action of flowing water when a stream or a river digs into the surrounding rock. The most famous gorge is the Grand Canyon in the USA, Arizona; it is almost 1800 m deep in places. Over a period of 600 million years, the Colorado River carved the Grand Canyon by eroding the surface of the Colorado Plateaus, which are to a large extent made up of sandstone and argillite. 





 

Why is the landscape so varied?

Different types of landscapes such as plains, wide valleys, gorges, dunes, rounded hills, and rugged ridges are formed largely by the forces of water, wind, and moving ice, which nibble, sharpen, and polish the surface of the Earth. Experts call this process ‘erosion’. These forces are much stronger than they appear. For instance, flowing water can move whole blocks of rock. The largest bits of rock thus eroded later become sand, which may be fine or coarse in texture and red, white, or grey in colour. Human beings also contribute to the constant changes taking place on the face of landscapes. 




How are waves formed?

Waves are formed as a result of the constant friction between water and wind. If a wind is blowing over the water surface, it carries away particles of water with it. The water starts moving in this way. Therefore, the distance between the ‘waves crest’ is determined by the wind. A constant wind gives rise to long, soft waves, which are also known as ‘groundswell’; a high wind, on the other hand, causes small and strong waves, the typical ‘motion of the sea’. Unlike the tsunamis, these waves occur only at the surface of the water. At a depth of a few metres, the waves do not create any disturbance. 





 

How are icebergs made?

The icebergs in the North Atlantic are large pieces of ice that have broken off from the glaciers of Greenland. Since frozen water is lighter than liquid water, the broken pieces float as icebergs on the sea. They are pointy and often interspersed with debris. In contrast, the icebergs in the South Atlantic are mostly flat. They are broken pieces of the flat sea ice, which surrounds the Antarctic. Icebergs can float for thousands of kilometres before they melt. Only one-ninth of an iceberg stays above the water. If the part above the water is 100 m high, then the part below is 800 m deep. 

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. 


How is salt formed in the sea?

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.

What does the seabed look like?

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. 

What is the difference between oceans and seas?

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. 

How does a tsunami occur?

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.

 


How do we save ourselves from earthquakes?

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. 

What damages can earthquakes cause?

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. 

How do we measure the intensity of earthquakes?

The intensity of an earthquake is measured by recording the tremors of the Earth’s surface with a ‘seismograph’. These tremors appear when the earthquake waves reach the surface of the Earth. Such a seismograph consists of a large, inert weight, which stays at rest even during earthquakes. A pen is attached to this pendulum. When the Earth trembles, the surface of the Earth, together with the paper placed there, moves to and fro below the ‘writing pendulum’ which does not move. We can then detect the different types of waves on the paper, which move at different speeds. The intensity of the earthquake can be calculated from these wave patterns. The intensity is measured on a scale, called ‘Richter scale’, named after its inventor. Earthquakes with an intensity of 2.5 can be felt, but ones with an intensity of 8.0 are very destructive.





 

Where do earthquakes occur?

Earthquakes occur mainly at the borders of the plates, where two plates slide past each other. Most earthquakes occur in the Himalayan region. The actual seismic centre, the ‘hypocenter’, lies deep below the surface of the Earth. The point on the surface of the Earth lying immediately above this is called the ‘epicentre’. Scientists can determine this point. They measure the time it takes the shock waves to reach the seismic stations and draw circles around the epicentre and two other seismic stations. The radius of these circles corresponds to the measured time. The point of intersection is the epicentre. Right below the epicentre, beneath the Earth’s surface, lies the hypocenter.





 

Why does the Earth tremble sometimes?

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.