CH- 4 THE STRUCTURE OF THE EARTH Class- 9th Part A A. FILL IN THE BLANK 1. The outer solid layer of the Earth is called Crust. 2. The average density of the Earth is about 5.5 per cubic cm. 3. The primary waves of the earthquake are also called Push waves P waves. 4. The boundary between crust and mantle is called the Mohorovicic discontinuity. B. EXPLAIN THE FOLLOWING TERMS: 1. BIOSPHERE- The narrow zone of contact between land, water and air , where all life forms exist 2. BARYSPHERE- The interior of the Earth beneath the lithosphere, including both the mantle and the core. 3. HYDROSPHERE- The part of the Earth covered with water. 4. LITHOSPHERE – The solid outer crust of the Earth. C. ANSWER THE FOLLOWING QUESTIONS VERY BRIEFLY. 1. Name the indirect sources of knowledge about the structure of the Earth. Ans. The indirect sources of knowledge about the structure of the Earth are: • Temperature inside the Earth • Pressure inside the Earth. • Density of different layers of the Earth. • Behaviour of earthquake waves. 2. What is the basis on which the Earth is divided into concentric layers? Ans. The Earth is divided into concentric layers on the basis of earthquake waves varying density and other physical and chemical properties of rocks. 3. Name the most abundant elements found in the Earth ‘s crust. Ans. The most abundant element found in the Earth’s crust are silica, magnesium and aluminium. 4.What is the main feature of the secondary waves? Ans. The main feature of the secondary wave is that these waves travel over the surface of the Earth and cause maximum destruction. C. DISTINGUISH BETWEEN THE FOLLOWING. 1. SIAL AND SIMA • SIAL • SIMA • SIAL forms the • SIMA forms the upper part of the lower part of continental crust. the oceanic crust. • The main The main elements elements of SIAL of SIMA are are silicates and silicates and aluminium. magnesium. • The average The average density of SIAL is density of SIMA is 5.5 g per cubic cm. 2g per cubic cm. 2. P WAVES AND S WAVES. P WAVES S WAVES • P waves are also • S waves are next known as Primary to arrive after the waves Primary waves. • These waves are • Their rate of the fastest waves movement is less and travel at a than that of speed of about 6 primary waves. Km. Per second. • They can pass • These waves through liquid and cannot pass gas. through liquids . 3. MOHOROVICIC DISCONTINUITY ANS GUTENBERG DISCONTINUITY Mohorovicic Gutenberg discontinuity Discontinuity • The boundary • The between the boundary crust and mantle between the is called Moho mantle and or the core is called Mohorovicic Gutenberg discontinuity discontinuity 4. CRUST AND THE CORE. THE CRUST THE CORE • The crust is the • The Core is the outermost solid innermost layer layer of the Earth. of the Earth . • The average • The core is about thickness of crust 2275 km thick. is about 60 km. • The average • The average density of the density of core crust is about 5.5 ranges between g per cubic cm. 5.0 per cubic cm and 13 .0 g per cubic cm. • The main • The core is made elements of the up of heavy crust are silica, metals such as aluminium and nickel and iron ( magnesium NIFE). .