Chapter 4

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Environmental Science
Jovan Stefanovic

CHAPTER 4: THE EARTH’S INTERIOR • Earth crust = thing skin of rock, less than 1% of earth’s total volume • Mantle rocks brought to the earth’s surface in basalt flows, diamond bearing kimberlite, the tectonic attachment of lower parts of the oceanic lithosphere to the continental crust • Meteorites give show possible composition of core of earth • Tools of geophysics- seismic waves, measurement of gravity, heat flow, magnetism Earth is divided into 3 major parts: crust on the surface; rocky mantle beneath the crust; metallic core at the center • Crust & uppermost mantle divided into: brittle lithosphere & plastic asthenosphere • Look of rocks from deeper levels seen where mantle rocks have been brought to the surface • Deep parts of earth studies indirectly through a branch of geology called geophysics: application if physical laws & principles to study earth; includes study of seismic waves & magnetic fields, gravity, heat WHAT CAN WE LEARN FROM THE STDUY OF SEISMIC WAVES? • Seismic waves from a large earthquake may pass through the entire earth; nuclear bomb explosion generates seismic waves; information about earth obtained after each earthquakes & bomb test Seismic reflection: way of learning about the earth’s interior; the return of some energy of seismic waves to the earth’s surface after the wave bounces off a rock boundary • Reflected waves recorded on seismogram ; shows amount of time the wave took to travel down to the boundary, reflect off it & return to the surface Seismic refraction: another method used to locate rock boundaries; the bending of seismic waves as they pass from one material to another ; wave strike a rock boundary, the energy passes across the boundary, as wave cross from one boundary to another it changes direction ( refraction) occurs only if the velocity of seismic waves is different ine ach layer ( rock layers must differ in density or strength • Even if refracted wave travels farther, it can arrive first if its path is in higher velocity • Velcovities od seismic waves within the layer can also be found Why do waves return to the surface?: Advancing waves give off energy in all directions, much of the energy continuous to travel horizontally • Sharp rock boundary is not necessary for the refraction of seismic waves; even in thick/uniform rock increasing pressure with debt tends to increase velocity of the waves WHAT IS INSDIE THE EARTH? • Study of seismic refraction/reflection allows the 3 main zones of earth to be shown 1) Crust: outer layer, forms thin skin on earths surface 2) Mantle: thick shell of rock that separates the crust above from the core below 3) Core: is the central zone of earth; metallic and the source of earth’s magnetic field THE CRUST: • Crust is thinner beneath the ocean than in continents • seismic waves travel faster in oceanic crust than continental; the 2 types of crusts made of diff types of rocks • upper part of oceanic crust is basalt; lower part is gabbro; thickness avg 7km or 5-8km • seismic P travels more slowly in continental crust • continental crust is often called “granitic” ; but most rocks exposed on land is not granite; consists of crystalline basement composed of granite, plutonic rock, gneisses, schosts silicon: rock high in feldspar& silicon ( term used for continental crust) mafic: high in magnesium & iron ( oceanic) • continental crust thicker than oceanic; thickest is under young mountain ranges; continental crust less dense than oceanic Mohorovivic discontinuity (moho): boundary that separates crust from the mantle • mantle lies closer to the earth’s surface beneath oceanic crust than continental THE MANTLE: • made of solid rock ; magma chambers of melted rock occur as isolated pockets of liquid in both crust/ upper mantle • mantle is diff type of rock from either oceanic or continental crust; composition of upper mantle: ultramafic (dnese igneous rock made up of ferromagneisn like olivine/pyroxene; all lack feldspar but can contain granite) rock such as peridoite • crust + upper mantle = lithosphere : outer shell of earth that is strong and brittle • lithosphere makes up the plates of plate tectonic theory • lithosphere about 70 km thick ( ocean) & 125-250 ( continental) • seismic waves increase in velocity with debts as increasing pressure alters the properties of rock • low velocity zone: zone extending to a debt of 200 km asthenosphere; rocks here closer to melting point; rocks are liquid slush? • Asthenospehre important for 1)may represent a zone where magma is generated 2) rocks here have lil strength, so they flow; asthenosphere can deform easily by plastic flow acts as a lubricating layer for lithosphere to move • Few ppl say there is no asthenosphere THE CORE: 1 • Seismic wave data- evidence for the existence of the core • Region b/w 103-14 degree which lacks P waves called the P-wave shadow zone • P-wave shadow zone can be explained by refraction • b/c path way of P wave can be calculated the size/ shape of earth’s core can be determined • earths core deflects P waves • P waves travel through solids/ fluids, S waves can travel only through skids S-wave shadow zone: larger than P-wave shadow zone, s waves do not travel through the core at all, implies core is not liquid • Core has two parts: a liquid outer core and solid inner core Composition of the core: • Earth’s core is made out of metal not silicate rock; that metal is iron along with small amounts of oxygen, silicon, sulphur or nickel • Density of the earth 5.5 gm/m3; core is very heavy • Choice of iron as major component of the core comes from looking at meteorites; 10% of meteorites are composed of iron mixed with lil nickel... • Earths magnetic field suggest the core is metallic The core-mantle Boundary: • Boundary b/w core and mantle marked by change in seismic velocity/ Density/Temperature; transition zone up to 200 km called the D layer base of the mantle where P-wave velocity decrase • ULVZ – ultras low velocity zone  border of the core-mantle boundary • Iron silicate & liquid iron in pores rise and collect in the uneven core-mantle boundary forms electrically conductive layer explains low velocity • Both mantle + core undergoing convection Convection: circulation pattern in which low-density material rises and high density material sinks • Light protion of the core may be incorporated into the mantle • Continent-sized blobs of liquid/ liquid-crystals accumulate at the ore-mantle boundary interferes with convection/ transfer of heat to surface also cause change in magnetic field HOW DOES THE ELEVATION OF CONRTINENTS CHANGE? Isotasy: is a balance or equilibrium of adjacent bocks of brittle crust “floating” on the upper mantle • Crust can be thought as floating on the denser mantle • Crustal rock can rise or sink gradually until they are balanced by the weight of displaced mantle rocks Isostatic adjustment: concept of vertical movement to reach equilibrium • Once crustal rock in isotatic balance, tall block e
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