Our Changing Earth (PDF)

3 Our Changing Earth Activity The lithosphere is broken into a number of plates known ed as the Lithospheric plates. You will be surprised to know Take a small coloured that...

3 Our Changing Earth Activity The lithosphere is broken into a number of plates known ed as the Lithospheric plates. You will be surprised to know Take a small coloured that these plates move around very slowly – just a few paper pellet and put it millimetres each year. This is because of the movement in a beaker of the molten magma inside the earth. The molten magma h half filled inside the earth moves in a circular manner as shown in pu T with water. is Place the the activity. The movement of these plates causes changes on the beaker on a tripod stand re ER surface of the earth. The earth movements are divided bl and heat it. As the on the basis of the forces which cause them. The forces water warms up, you which act in the interior of the earth are called as will observe that the Endogenic forces and the forces that work on the surface be C paper pellet is moving of the earth are called as Exogenic forces (Fig. 3.1). upward along with the warm layers of water o N and then Earth movements sinks back along with Endogenic forces Exogenic forces © the cooler layers of water. The Sudden forces Diastrophic forces Erosional and Depositional molten magma inside the earth moves in a similar manner. Earthquake Building mountains River Volcano Wind Glossary tt Landslides Sea-waves Lithospheric plates: The earth’s crust Glaciers no consists of several large and some small, Fig. 3.1: Evolution of Landforms rigid, irregularly- shaped plates (slabs) Endogenic forces sometimes produce sudden movements which carry and at the other times produce slow movements. Sudden continents and the movements like earthquakes and volcanoes cause mass ocean floor. destruction over the surface of the earth.  A volcano is a vent (opening) in the Word Origin earth’s crust through which molten material erupts suddenly (Fig. 3.2). Endo Endogenic (inside) + genic Gases and ash Exo (origin) Crater (outside) Exogenic Vent Lava Activity ed Lava tube Take a container, fill it with water and Crust close it with a lid. Put Magma the water to boil. Now Mantle h Chamber put some peas, spoon pu T Fig. 3.2: A Volcano and beads on top on is re ER the lid. What do you notice? As the water Similarly, when the Lithospheric plates move, the boils the lid begins to bl surface of the earth vibrates. The vibrations can travel shake. The things all round the earth. These vibrations are called which you have put earthquakes (Fig. 3.3). The place in the crust where the on the lid also be C movement starts is called the focus. The place on the vibrate. The beads roll surface above the focus is called the epicentre. down and the spoon Vibrations travel outwards from the epicentre as waves. vibrates to make a o N Greatest damage is usually closest to the epicentre and sound. In the same manner, the earth the strength of the earthquake decreases away from vibrates when an the centre. © earthquake occurs. Plate boundary Epicentre Seismic waves radiate out in all directions Do you know? There are three types of earthquake waves: tt 1. P waves or longitudinal waves 2. S waves or no transverse waves 3. L waves or surface waves Focus – this is where the earthquake occurs. It is the Try to find out the origin of the seismic energy. properties of these waves from an Fig. 3.3: Origin of an Earthquake encyclopedia. OUR CHANGING EARTH 13  Although earthquakes cannot be predicted, the impact can certainly be minimised if we are prepared before-hand. Some common earthquake prediction methods adopted locally by people include studying animal behaviour; fish in the ponds get agitated, snakes come to the surface. Earthquake – A Case Study Fig. 3.3a: Destruction ed caused by an Earthquake 1 5 Fire in the city in Gujarat EARTHQUAKE HITS BHUJ Hundreds of fires A massive earthquake measuring 6.9 on Richter started as charcoal, scale hit Bhuj Town on 26th January 2001. cookers overturned. h 2 3 BHUJ RELIEF EFFORT School worst 6 Emergency declared BLIGHTED.. pu T affected Three days after the and medical supplies in quake zone is Atleast 971 students quake, concern rose not reaching everyone. The President declares and 31 teachers are about food, blankets a state of emergency. re ER feared to have lost 4 Destruction of Bhuj 7 CM’S APPEAL TO THE CENTRE bl their lives following Phone lines, water Gujarat appeals for an appeal for the Do you know? the collapse of pipelines and power financial help. The Centre to deal with stations transmission Chief Minister of the disaster. school buildings. Gujarat has launched lines were knocked out. An earthquake is measured with a be C machine called a seismograph. The o N magnitude of the earthquake is Activity measured on the 1. Read the ‘Earthquake – A case study’ given in the Richter scale. An form of headlines that appeared in the newspapers © earthquake of 2.0 or after the quake. Arrange the events in the right less can be felt only a sequence of their happening. little. An earthquake over 5.0 can cause 2. Imagine if a quake suddenly shook in the middle of damage from things the school day, where would you go for safety? falling. A 6.0 or higher magnitude is considered very EARTHQUAKE PREPAREDNESS tt strong and 7.0 is Where to take shelter during an earthquake — classified as a major Safe Spot – Under a kitchen counter, table or desk, earthquake. no 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 your friends and family members and face any disaster confidently. A Seismograph 14 OUR ENVIRONMENT MAJOR LAND FORMS Do you know? The landscape is being continuously worn away by two There are processes – weathering and erosion. Weathering is the thousands of small breaking up of the rocks on the earth’s surface. Erosion waterfalls in the is the wearing away of the landscape by different agents world. The highest like water, wind and ice. The eroded material is carried waterfall is Angel away or transported by water, wind, etc. and eventually Falls of Venezuela deposited. This process of erosion and deposition create in South America. different landforms on the surface of the earth. The other waterfalls are Niagara falls Work of a River located on the ed The running water in the river erodes the landscape. When border between the river tumbles at steep angle over very hard rocks or Canada and USA in North America and down a steep valley side it forms a waterfall (Fig. 3.4). Victoria Falls on h the borders of pu T Zambia and is Zimbabwe in Africa. re ER Hard rock bl Softer rock undercut by the power of the water be C The Niagra falls o N Fig. 3.4: Waterfall As the river enters the plain it twists and turns forming © large bends known as meanders. 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. At times the river overflows its banks. tt This leads to the flooding of the neighbouring areas. As it floods, it deposits layers of fine soil and other no material called sediments along its banks. This leads to the formation of a flat fertile floodplain. The raised banks are called levees. As the river approaches the sea, the speed of the flowing water decreases and the Fig. 3.5: Features made by a river in a flood plain OUR CHANGING EARTH 15  Let’s do river begins to break up Main river Distributary into a number of streams Find out the names of called distributaries. The a few rivers of the river becomes so slow that world that form a it begins to deposit its load. delta. Each distributary forms its own mouth. The collection of sediments from all the Fig. 3.6: A Delta mouths forms a delta. Work of Sea Waves ed The erosion and deposition of the sea waves gives rise to Stack coastal landforms. Seawaves continuously strike at the h rocks. Cracks develop. Over pu T time they become larger and is wider. Thus, hollow like caves re ER are formed on the rocks. They bl Sea Arch are called sea caves. As these Sea Cave cavities become bigger and bigger only the roof of the be C caves remain, thus forming sea Fig. 3.7: Features made by sea waves arches. Further, erosion breaks the roof and only walls o N are left. These wall like features are called stacks. The steep rocky coast rising almost vertically above sea water is called sea cliff. The sea waves deposit © sediments along the shores forming beaches. Work of Ice Glaciers are “rivers” of ice which too erode the landscape by bulldozing soil and stones to expose the solid rock below. tt Glaciers carve out deep hollows. As the ice melts they get filled up with water and become no 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 Fig. 3.8: A Glacier glacial moraines. 16 OUR ENVIRONMENT  Work of wind Have you ever visited a desert? Try to collect some pictures of sand dunes. An active agent of erosion and deposition in the deserts is wind. In deserts you can 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. 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. ed These are called sand dunes (Fig. 3.9). 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 h large areas, it is called loess. Large deposits of loess is pu T Fig. 3.9: Sand Dunes found in China. is re ER bl be C Exercises o N 1. Answer the following questions. (i) Why do the plates move? (ii) What are exogenic and endogenic forces? © (iii) What is erosion? (iv) How are flood plains formed? (v) What are sand dunes? (vi) How are beaches formed? (vii) What are ox bow lakes? 2. Tick the correct answer. (i) Which is not an erosional feature of sea waves? tt (a) Cliff (b) Beach (c) Sea cave (ii) The depositional feature of a glacier is: (a) Flood plain (b) Beach (c) Moraine no (iii) Which is caused by the sudden movements of the earth? (a) Volcano (b) Folding (c) Flood plain (iv) Mushroom rocks are found in: (a) Deserts (b) River valleys (c) Glaciers (v) Ox bow lakes are found in: (a) Glaciers (b) River valleys (c) Deserts OUR CHANGING EARTH 17  3. Match the following. (i) Glacier (a) Sea shore (ii) Meanders (b) Mushroom rock (iii) Beach (c) River of ice (iv) Sand dunes (d) Rivers (v) Waterfall (e) Vibrations of earth (vi) Earthquake (f) Sea cliff (g) Hard bed rock (h) Deserts 4.Give reasons. (i) Some rocks have a shape of a mushroom. ed (ii) Flood plains are very fertile. (iii) Sea caves are turned into stacks. (iv) Buildings collapse due to earthquakes. h 5. Activity. pu T Observe the photographs given below. These are various features made by a is river. Identify them and also tell whether they are erosional or depositional or re ER landforms formed by both. bl Photograph Name of the Type Feature (Erosional or Depositional or Both) be C o N © tt no 18 OUR ENVIRONMENT 6. For fun. Solve the crossword puzzle with the help of given clues. h ed pu T is re ER bl be C o N © Across Down 2. Loop like the bend of a river 1. Rise and fall of water caused by friction 4. Solid form of water of wind on water surface 7. Moving mass of ice 3. Flow of water in a channel 9. Sudden descent of water in the bed of a river 5. Steep perpendicular face of a rock along a sea coast 11. Natural cavity on weak rocks formed by tt action of waves 6. Debris of boulder and coarse material carried by glacier 12. Embankment on a river that keeps it in its channel 8. Crescent shaped lake formed by a no meandering river 13. Large body of sea water 10. Fine sand deposited by the action of the wind 14. Dry area where sand dunes are found 13. Isolated mass of rising steep rock near a 15. Small hill of sand caused by the action coastline of the wind 14. Alluvial tracts of land formed by the river 16. Flat plain formed by river deposits during deposits at the mouth of a river time of flood OUR CHANGING EARTH 19