Look at the cross-section of the Earth and give a definition to the following terms: EARTH, CORE, MANTLE, CRUST, ATMOSPHERE.
Fig. 8 . Structure of the Earth.
Inner core; 2 – convection currents; 3 – outer core; 4 – lower mantle; 5 – upper
Mantle; 6 – asthenosphere; 7 – lithosphere.
READING COMPREHENSION
INFORMATIVE READING - The Earth (R.P – 3.1, 3.2)
Read the text and for questions 1-10, choose the best answer: A, B, C or
D. Then explain the words in bold.
The Earth’s structure is a reflection of its original accretion growth and subsequently physical and compositional evolution. The Earth consists of a series of concentric shells of different physical and chemical properties. The globe is surrounded by a gaseous envelope consisting chiefly of nitrogen (78%) and oxygen (21%). The atmosphere is densest at sea level and becomes progressively upward. The lower zone, approximately 0-10km is the troposphere with the less dense stratosphere overlying.
The ocean basins make up approximately two-thirds of the surface of the solid Earth: they are great depressions lying average 5 km below sea level, while continents and islands, constitute one-third of the solid Earth’s surface.
The Earth’s surface consists mainly of ocean water, covering slightly more than 70% of the exposed area; land, some of which is lake-or ice-covered, constitutes the remainder. The outer rind of the solid Earth, or crust, is two distinctly different types. Continental crust, rich in silica, alkalis, volatiles and radioactive elements, is characterized by granitic rocks and is approximately 20-60 km thick. Ocean crust is almost exclusively basaltic in composition and is approximately 5km. thick. The crust is a very thin skin, representing a little less than 1% by volume of the whole Earth and only approximately 0.4% by mass.
The more or less horizontal boundary between the crust and the underlying mantle-both beneath the continents and ocean basins is known as the Mohorovicic Discontinuity (Moho / M). This discontinuity is a zone of finite thickness in which lower crust and upper mantle are interlayered. The mantle represents the largest proportion of the Earth, some 83% by volume and 68% by mass. It is rich in magnesium and less rich in iron. It consists of a number of distinct but intergradational layers. The uppermost layer is relatively rigid and with the overlying rigid crust is called the lithosphere (or rocky surface). The lithosphere may be several hundred kilometers thick. The lithosphere (both mantle and crust) covers the entire planet and is divided into seven gigantic plates and numerous smaller ones. Beneath the lithosphere lies the more ductile mantle layer known as the asthenosphere (glassy or weak zone). The so-called upper mantle consists of lithosphere plus asthenosphere plus more rigid mantle material down to depths of approximately 300-400 km. The transition zone of the upper mantle is situated between the depths of 400-670 km. In this region the
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physical properties of the upper mantle change fairly and continuously with increasing depth. The gradual emergence of nearly constant physical characteristics below the transition zone signals passage into the lower mantle. This innermost and most voluminous mantle shell appears to be relatively homogeneous.
The core-mantle boundary and a thin basalt mantle (termed “D”) represent a layer of profound change in the constitution of the materials that makes up the Earth. The overlying mantle shells consist of magnesium and silicon oxide phases. The inner core appears to be a solid iron-nickel alloy. The outer core (molten) makes up approximately 15% of the Earth volume. The inner core constitutes the remaining 1% by volume of the planet. Because of its great density, the core represents more than 31% of the Earth’s mass. (Barret E., Hunt A. And Milner B.” Earth and Atmosphere”, 1993, Longman)
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