Our Changing Earth

Q34. How earthquakes are measured?

Ans. An earthquake is measured with a machine called a seismograph. The magnitude of the earthquake is measured on the Richter scale. An earthquake of 2.0 or less can be felt only a little. An earthquake over 5.0 can cause damage from things falling. A 6.0 or higher magnitude is considered very strong and 7.0 is classified as a major earthquake.

Q35. How a delta is formed?

Ans. As the river approaches the sea, the speed of the flowing water decreases and the river begins to break up into a number of streams called distributaries. The river becomes so slow that it begins to deposit its load. Each distributary forms its own mouth. The collection of sediments from all the mouths forms a delta.

Q36. Sea caves are turned into stacks. Give reason.

Ans. Sea waves continuously strike at the rocks. Cracks develop. Over time they become larger and wider. Thus, hollow like caves are formed on the rocks. They are called sea caves. As these cavities become bigger and bigger only the roof of the caves remain, thus forming sea arches. Further, erosion breaks the roof and only walls are left. These walls like features are called stacks.

Q37. Explain the work of ice.

Ans. Glaciers are “rivers” of ice which too erode the landscape by bulldozing soil and stones to expose the solid rock below. Glaciers carve out deep hollows. As the ice melts they get filled up with water and become beautiful lakes in the mountains. The material carried by the glacier such as rocks big and small, sand and silt gets deposited. These deposits form glacial moraines.

Q38. Why do buildings collapse due to earthquakes?

Ans. Buildings collapse due to earthquakes because

                      i.        Earthquakes move the ground side to side and up and down—simultaneously. The force behind this movement is powerful enough to turn soft soil instantly into quicksand, eliminating its ability to bear weight.

                     ii.        Most of the buildings are not safe enough to resist the vibrations of the earthquakes due to shallow foundation and lack of adequate steel in the interior design.

Q39. Write a short note on earthquake  preparedness.

Ans. Although earthquakes cannot be predicted, the impact can certainly be minimised if we keep the following points in mind.

Safe Spot – Take shelter during an earthquake under a kitchen counter, table or desk, against an inside corner or wall.

Stay Away from – Fire places, areas around chimneys, windows that shatter including mirrors and picture frames.

Be Prepared – Spread awareness amongst our friends and family members and face any disaster confidently.

Q40. Write a short note on work of wind.

Ans. Work of wind

                       i.        An active agent of erosion and deposition in the deserts is wind. In deserts we see rocks in the shape of a mushroom, commonly called mushroom rocks. Winds erode the lower section of the rock more than the upper part. Therefore, such rocks have narrower base and wider top.

                      ii.        When the wind blows, it lifts and transports sand from one place to another. When it stops blowing the sand falls and gets deposited in low hill – like structures. These are called sand dunes.

                     iii.        When the grains of sand are very fine and light, the wind can carry it over very long distances. When such sand is deposited in large areas, it is called loess. Large deposits of loess are found in China.

Q41. Give an account of the work of sea waves.

Ans. Work of sea waves

                         i.        The erosion and deposition of the sea waves gives rise to coastal landforms. Sea waves continuously strike at the rocks. Cracks develop. Over time they become larger and wider. Thus, hollow like caves are formed on the rocks. They are called sea caves.

                        ii.        As these cavities become bigger and bigger only the roof of the caves remain, thus forming sea arches.

                       iii.        Further, erosion breaks the roof and only walls are left. These walls like features are called stacks.

                       iv.        The steep rocky coast rising almost vertically above sea water is called sea cliff.

                        v.        The sea waves deposit sediments along the shores forming beaches.

Q42. Explain the work of a river.

Ans. Work of a river

                        i.        The running water in the river erodes the landscape. When the river tumbles at steep angle over very hard rocks or down a steep valley side it forms a waterfall.

                       ii.        As the river enters the plain it twists and turns forming large bends known as meanders.

                      iii.        Due to continuous erosion and deposition along the sides of the meander, the ends of the meander loop come closer and closer. In due course of time the meander loop cuts off from the river and forms a cut-off lake, also called an ox-bow lake.

                      iv.        At times the river overflows its banks. This leads to the flooding of the neighbouring areas. As it floods, it deposits layers of fine soil and other material called sediments along its banks. This leads to the formation of a flat fertile floodplain. The raised banks are called levees.

                       v.        As the river approaches the sea, the speed of the flowing water decreases and the river begins to break up into a number of streams called distributaries. The river becomes so slow that it begins to deposit its load. Each distributary forms its own mouth. The collection of sediments from all the mouths forms a delta.