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Transcript
Chapter 17 Earth’s interior Earth’s interior structure • Earth is composed of three shells; –Crust –Mantle –Core Earths interior and seismic waves • The study of earth’s interior can be done by; –Direct sampling –Indirect methods – Indirect methods • Meteorites • Seismic waves (P and S) • P-waves can pass through all materials (solids and fluids) • S-waves can only pass through solids. The crust • Outermost layer of the earth forming a thin skin. Divided into 2 parts; –Continental crust –Oceanic crust • Mantle: –upper mantle and –lower mantle • Upper mantle and crust constitute Lithosphere where the rocks are in rigid form. • Lower mantle is also called Asthenosphere where the rocks are in plastic state. • The boundary between the crust and mantle is called the Moho. The Core • Is there really a core? –Combined Density of the crust and mantle is less than that of the whole earth. –S-waves shadow zone The Core • Core is divided into 2 parts; –The outer core is about 2,250 km thick, composed of molten iron and nickel. –The inner core is probably solid iron and nickel at about 5000° C. What is isostasy? • It’s a balance or equilibrium of the earth’s crust floating on the dense mantle. • Can be compared with wooden blocks floating on water. The higher the wood block appears, the deeper its root. • Erosion of continental crust shows isostatic equilibrium. • Erosion of glacial ice shows isostatic equilibrium in the form of crustal rebound. Physical properties of Earth • Gravity • Magnetism • Heat flow • Gravity: gravitational attraction exists among all bodies in the universe • Gravity anomalies: abnormal gravity readings. –Positive anomaly: gravity reading higher than the regional gravity readings –Negative anomaly: gravity readings lower than the regional gravity readings • Magnetism: the earth acts like a giant dipolar magnet. • Magnetic poles: oriented 11.5° from the geographic poles. –North pole –South pole • Magnetometer is used to measure magnetic strength. Source of magnetism • the convection currents in outer liquid core produce electric currents and any electric field is associated with a magnetic field. • Normal and reversed polarity • Paleomagnetism: the study of ancient magnetism preserved in rocks. • Magnetic anomalies –Positive –Negative • Heat flow: the rate at which temperature increases within the earth is called Geothermal gradient. • 25° C/km at shallow depths (crust) but drops to 1°C/km within the mantle. • Sources of heat inside the earth: –Radioactivity –Rising magma bodies –Gravitational energy from accretion.