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Chapter 4

Chapter 4.doc

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Department
Environmental Science
Course Code
EESA06H3
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S

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Chapter 4 The Earth’s Interior  the earth’s crust is a thin skin of rock  earth is divided into three major layers o the crust o the mantle o the core  seismic reflection and refraction suggest that the continents are largely igneous and metamorphic rock, such as granite and gneiss, overlain by a veneer of sedimentary rocks  geophysics – the application of physical laws and principles to a study of the earth  seismic reflection – the return of some of the energy of seismic waves to the earth’s surface after the waves bounce off a rock boundary o these reflected waves are recorded on a seismogram, which shows the amount of time the waves took to travel down to the boundary, reflect off it, and return to the surface  seismic refraction – the bending of seismic waves as they pass from one material to another o as a seismic wave strikes a rock boundary, much of the energy of the wave passes across the boundary o as the wave crosses from one rock layer to another, it changes direction  occurs only if the velocity of seismic waves is different in each layer What is Inside the Earth?  it was the study of seismic refraction and seismic reflection that enabled scientists to plot the three main zones of the earth  the crust o the outer layer of rock, which forms a thin skin on earth’s surface o it is thinner beneath the oceans than beneath the continents o seismic waves travel faster in oceanic crust than in continental crust o the two types of crust are made up of different kinds of rock o seismic P waves travel through oceanic crust at about 7 km per second, which is also the speed at which they travel through basalt and gabbro o the upper part of the oceanic crust is basalt, and the lower part is gabbro o seismic P waves travel more slowly through continental crust o continental crust is often called granitic o the continental crust is highly variable and complex, consisting of a crystalline basement composed of granite, other plutonic rocks, gneiss, and schists, all capped by a layer of sedimentary rocks o felsic (rocks high in feldspar and silicon) is used for continental crust o mafic (rocks high in magnesium and iron) is used for oceanic crust o seismic waves show that the crust is thickest under geologically young mountain ranges o the continental crust is also dense than the oceanic crust o the boundary that separates the crust from the mantle is called the Mohorovicic discontinuity o the mantle lies closer to the earth’s surface beneath the ocean than it does beneath continents  the mantle o made of solid rock o because P waves travel at about 8 km per second in the upper mantle, it appears that the mantle is a different type of rock from either oceanic crust or continental crust o the best hypothesis that geologists can make about the composition of the upper mantle is that it consists of ultramafic rocks such as peridotite  ultramafic rock is dense igneous rock made up of ferromagnesian minerals such as olivine and pyroxene  some ultramafic rocks contain garnet, and all of them lack feldspar o the crust and the upper mantle form the lithosphere  the outer shell of earth that is relatively strong and brittle  makes up the plates of plate tectonics theory  averages about 70km thick beneath oceans and may be 125-250 km thick beneath continents  seismic waves increase in velocity with depth as increasing pressure alters the properties of the rock • they travel more slowly than they do in shallow layers • so this zone is called the low velocity zone o extends to a depth of about 200 km o also called the asthenosphere o the rocks in this zone may be closer to their melting point than the rocks above or below the zone • if the rocks of the asthenosphere are close to their melting point, this zone may be important for two reasons: o it may represent a zone where magma is likely to be generated o the rocks here may have relatively little strength and therefore, are likely to flow • if mantle rocks in the asthenosphere are weaker than they are in the overlying lithosphere, then the asthenosphere can deform easily by plastic flow  because pressure increases with depth into the earth, the boundaries between mantle layers possibly represent depths at which pressure collapses the internal structure of certain minerals into denser minerals  the core o seismic wave data provide the primary evidence for the existence of the core of the earth o the region between 103 degrees and 143 degrees, which lacks P waves, is called the P-wave shadow zone  can be explained by the refraction of P waves when they encounter the core boundary deep within the Earth’s interior o P waves can travel through solids and fluids, An S waves can travel only through solids o An S-wave shadow zone is larger than P-wave shadow zone  Seems to indicate that S waves do not travel through the core at all  If this is true, it implies that the core of the Earth is liquid, or at least acts like a liquid o The way in which P waves are refracted within the earth’s core suggests that the core has two parts, a liquid outer core and a solid inner core o When evidence from astronomy and seismic-wave studies is combined with what we know about the properties of materials, it appears that the earth’s core is made of metal, not silicate rock, and that this metal is probably iron o Iron mixed with a small amount of a lighter element, such as oxygen, sulphur, or silicon, would have the required density o Therefore, many geologists think that such a mixture makes up the core o The choice of iron as the major component of the core comes fro
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