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ISU ENVI 360 - Chapt06 THE EARTH

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Chapter 6Our Home, The EarthSize and Shape of the EarthSlide 4Composition of the EarthDensity of the EarthThe Earth’s InteriorA Sonogram of the Earth!Probing the Interior of the EarthInterior StructureInterior Structure of the EarthLayers of the EarthDifferentiationTemperature Inside the EarthSlide 15Age of the EarthSlide 17Motion in the Earth’s InteriorConvectionPlate TectonicsSubductionSlide 22Slide 23Continental DriftContinental PlatesThe Earth’s AtmosphereComposition of the Earth’s AtmosphereOrigin of the Earth’s AtmosphereThe Early AtmosphereThe Ozone LayerThe Greenhouse EffectStructure of the Earth’s AtmosphereThe Earth’s Magnetic FieldSlide 34Origin of the Earth’s Magnetic FieldMagnetic Effects in the Upper AtmosphereAuroraThe MagnetosphereMotions of the EarthAir and Ocean CirculationThe Coriolis EffectSlide 42PrecessionSlide 44Chapter 6The EarthCopyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Our Home, The Earth•Earth’s beauty is revealed from space through blue seas, green jungles, red deserts, and white clouds.•From our detailed knowledge of Earth, astronomers hope to understand what properties shape other worlds•Earth is a dynamic planet with its surface and atmosphere having changed over its lifetime.•Slow and violent motions of the Earth arise from heat generated within the planet•Volcanic gases accumulate over billions of years creating an atmosphere conducive to life, which in turn together with water affects the air’s compositionSize and Shape of the Earth•In simple terms, the Earth is a huge, rocky sphere spinning in space and moving around the Sun at a speed of about 100 miles every few seconds•Earth also has a blanket of air and a magnetic field that protects the surface from the hazards of interplanetary spaceSize and Shape of the Earth•The Earth is large enough for gravity to have shaped it into a sphere•More precisely, Earth’s spin makes its equator bulge into a shape referred to as an oblate spheroid – a result of inertiaComposition of the Earth•The most common elements of the Earth’s surface rocks are: –oxygen (45.5% by mass), –silicon (27.2%), –aluminum (8.3%), –iron (6.2%), –calcium (4.66%), and –magnesium (2.76%)•Silicon and oxygen usually occur together as silicates•Ordinary sand is the silicate mineral quartz and is nearly pure silicon dioxideDensity of the Earth–Density is a measure of how much material (mass) is packed into a given volume–Typical unit of density is grams per cubic centimeter–Water has a density of 1 g/cm3, ordinary surface rocks are 3 g/cm3, while iron is 8 g/cm3–For a spherical object of mass M and radius R, its average density is given by–For Earth, this density is found to be 5.5 g/cm3–Consequently, the Earth’s interior (core) probably is iron (which is abundant in nature and high in density)343MRpThe Earth’s Interior•Earthquakes generate seismic waves that move through the Earth with speeds depending on the properties of the material through which they travel•These speeds are determined by timing the arrival of the waves at remote points on the Earth’s surface•A seismic “picture” is then generated of the Earth’s interior along the path of the waveA Sonogram of the Earth!•This is the only way we have to probe the Earth’s interior!Probing the Interior of the Earth•Seismic waves are of two types: S and P–P waves compress material and travel easily through liquid or solid–S waves move material perpendicular to the wave direction of travel and only propagate through solidsInterior Structure•Observations show P waves but no S waves at detecting stations on the opposite side of the Earth from the origin of an Earthquake  the Earth has a liquid core!Interior Structure of the Earth•A solid, low-density and thin crust made mainly of silicates•A hot, thick, not-quite-liquid mantle with silicates•A liquid, outer core with a mixture of iron, nickel and perhaps sulfur•A solid, inner core of iron and nickelLayers of the Earth•The Earth is layered in such a fashion that the densest materials are at the center and the least dense at the surface – this is referred to as differentiation–Differentiation will occur in a mixture of heavy and light materials if these materials are liquid for a long enough time in a gravitational field–Consequently, the Earth must have been almost entirely liquid in the past•The Earth’s inner core is solid because it is under such high pressure (from overlying materials) that the temperature there is not high enough to liquefy it – this is not the case for the outer liquid coreDifferentiationTemperature Inside the Earth•Heating the Earth’s Core–The estimated temperature of the Earth’s core is 6500 K–This high temperature is probably due to at least the following two causes:•Heat generation from the impact of small bodies that eventually formed the Earth by their mutual gravitation• The radioactive decay of radioactive elements that occur naturally in the mix of materials that made up the EarthTemperature Inside the Earth•In either case, the thermal energy generated is trapped inside the Earth’s interior due to the long time it takes to move to the surface and escapeAge of the Earth•Radioactive decay used to determine the Earth’s age–Radioactive atoms decay into daughter atoms–The more daughter atoms there are relative to the original radioactive atoms, the older the rock isAge of the Earth•Radioactive potassium has a half-life of 1.28 billion years and decays into argon, which is a gas that is trapped in the rock unless it melts–Assume rock has no argon when originally formed–Measuring the ratio of argon atoms to potassium atoms gives the age of the rock–This method gives a minimum age of the Earth as 4 billion years–Other considerations put the age at 4.5 billion yearsMotion in the Earth’s Interior–Heat generated by radioactive decay in the Earth creates movement of rock–This movement of material is called convection–Convection occurs because hotter material will be less dense than its cooler surroundings and consequently will rise while cooler material sinksConvection•Convection in the Earth’s interior–The crust and mantle are solid rock, although when heated, rock may develop convective motions–These convective motions are slow, but are the cause of: earthquakes, volcanoes, the Earth’s


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ISU ENVI 360 - Chapt06 THE EARTH

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