Physics for Geologists, Second edition

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82 Electricity and magnetism Table 8.1 Positions (decimal degrees) and distances (kilometers and nautical miles) to nearest pole Datelepoch N magnetic pole S magnetic pole N geomagnetic pole 1990.0 78.10°N 71.41°W 64.91"s 138.90°E 79.14"N 71.13"W 1380 750n.mi 2780 1500 1205 650 1995.0 78.89"N 105.09"W 64.70"s 138.68"E 79.30°N 71.41°W 1240 670 2810 1520 1190 640 2000.0 80.6S0N 109.12"W 64.63"s 138.36"E 79.54"N 71.57"W 1040 560 2819 1520 1207 650 Table 8.2 Distance (km) between positions from epoch 1990.0 to epoch 2000.0 Date/epoch N magnetic S magnetic N geomagnetic pole pole pole 79.54"N 71.57"W, 79.54"s 108.43"E. The geomagnetic poles are opposite each other, and they were 1200 km from the geographic poles. The positions of the magnetic poles and their distances from their respec- tive geographical poles is shown in Table 8.1, and the distances between their positions in Table 8.2. There is an approximate relationship between the magnetic dip, i, and the latitude relative to the magnetic poles: Latitudemagnetic = tan-' I tan i . (2 ) This is clearly only approximate because the Earth's magnetic field is neither geometrically perfect nor symmetrical. Furthermore, the indicated field, like the compass today, does not necessarily point at the north magnetic pole; it points along a field line that leads eventually to the pole. It is, however, a useful tool in palaeomagnetic studies for estimating polar positions and the latitudes of continents in past ages. It is because the magnetic poles change position slowly, and the field is not constant that maps and charts have a note such as 'Magnetic variation (1976) 13"42' E, decreasing 11' annually'. There are also small daily, monthly and annual changes, which are due to external influences (e.g. diurnal changes due to the Earth's rotation, and aurora australis and borealis - 'southern lights' and 'northern lights' - and magnetic storms). The scientific terminology is a little different from the practical. The angle between the direction of the compass north and the North Geographic Pole Copyright 2002 by Richard E. Chapman
Electricity and magnetism 83 is known as the declination in geophysics, variation2 in navigation and on maps. This angle varies with position - and in any one position, it varies with time. The compass needle does not usually point to the magnetic poles but is aligned, as mentioned earlier, with a field line that eventually reaches the poles. Why does the Earth have a magnetic field? Assuming that we are correct in saying that most of the interior of the Earth is above its Curie point, and that it cannot therefore be permanently magnetized, the source of the field must either lie in the shallower, low-temperature crust or be continuously generated in the core. It is clearly something to do with the Earth's rotation for the magnetic poles to be so close to the geographic poles. It is thought to be caused by convection currents circulating conducting metallic fluids in the outer core, a sort of self-exciting dynamo. The sudden changes of polarity that have occurred in the past are not easily explained (sudden, that is, in the context of geological time). There is a small component, as mentioned above, caused by external influences that give rise to small changes on a short time-scale, such as the magnetic storms associated with sunspot activity. These components are in the field caused by electric currents in the ionosphere and solar wind. Remanent magnetism Remanent magnetism is the magnetism that remains after the magnetizing force has been removed or changed. In rocks there are minerals that have magnetic properties, such as magnetite (naturally!), that may have been magnetized in the past by a terrestrial magnetic field with its poles in a different position or reversed. Today, such an object is in the present field, the strength and direction of which is accurately known. Using a technique for removing the magnetization due to the present terrestrial field, the remanent magnetism can be determined. When a ferrimagnetic material cools from above to below its Curie point, as lava tends to do, it acquires its magnetism from the local field. This is called thermoremanent magnetism. If new magnetic minerals grow dur- ing diagenesis or metamorphism, they become permanently magnetized to a measurable degree once they attain a certain size. If magnetized sedimen- tary particles settle from suspension in still water, or are agitated gently on the bottom before accumulating into the stratigraphic record, they may orientate themselves in the geomagnetic field and so preserve a record of it. All these are examples of remanent magnetism. Remanent magnetism is the basis of palaeomagnetism, the elucidation of past magnetic fields and of sea-floor spreading, continental drift and plate tectonics. 2 Deviation in ships is the difference between compass north and magnetic north due to the ship itself. Copyright 2002 by Richard E. Chapman
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Physics for Geologists Second editi
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Copyright 2002 by Richard E. Chapma
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... vlii Contents Polarization 47 L
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Figures Copyright 2002 by Richard E
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Tables Copyright 2002 by Richard E.
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xiv Preface waves diminishes with d
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xvi Preface provides. This book is
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xviii Acknowledgements to K. Whaler
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xx Symbols mass refractive index bu
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2 Basic concepts Table 1.1 Dimensio
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4 Basic concepts Table 1.2 Basic SI
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6 Basic concepts pascal (Pa) and si
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8 Basic concepts all the relevant d
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10 Basic concepts relating them by
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2 Force The dimensions of force are
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14 Force The ratio of the Moon's ac
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16 Force Figure 2.1 The sum of the
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18 Force When a motorcycle takes a
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20 Force Figure 2.3 Torque is the p
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22 Force How does momentum differ f
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24 Force the weights - that is, the
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26 Force Figure 2.6 Wall-sticking i
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3 Gravity Universal gravity When di
Page 47 and 48: 30 Gravity are significant to geolo
Page 49 and 50: 32 Gravity Figure 3.2 The tractive,
Page 51 and 52: 34 Gravity Mean sea level It might
Page 53 and 54: 36 Gravity The benefits of space fl
Page 55 and 56: 38 Gravity Figure 3.6 Profile of th
Page 57 and 58: 40 Gravity Figure 3.8 The siphon at
Page 59 and 60: 4 Optics Light is an electromagneti
Page 61 and 62: 44 Optics Figure 4.2 Refraction. (a
Page 63 and 64: 46 Optics The coeficient of reflect
Page 65 and 66: 48 Optics only the light oscillatin
Page 67 and 68: 50 Optics called Iceland Spar. Two
Page 69 and 70: 52 Optics Diffraction If you shine
Page 71 and 72: 54 Optics Figure 4.7 Geometrical op
Page 73 and 74: 56 Optics Figure 4.9 The mirrors in
Page 75 and 76: 5 Atomic structure and age-dating P
Page 77 and 78: 60 Atomic structure and age-dating
Page 83 and 84: 66 Electromagnetic radiation radiat
Page 85 and 86: 68 Electromagnetic radiation Source
Page 87 and 88: Heat and heat flow The noun heat ha
Page 89 and 90: 72 Heat and heat flow Second Law: A
Page 91 and 92: 8 Electricity and magnetism Electri
Page 93 and 94: Electricity and magnetism 77 Figure
Page 95 and 96: Electricity and magnetism 79 rivett
Page 97: Electricity and magnetism 8 1 defin
Page 101 and 102: 9 Stress and strain Pressure, p, is
Page 103 and 104: Stress and strain 87 than some natu
Page 105 and 106: These three elastic constants are r
Page 107 and 108: Stress and strain 91 Figure 9.1 New
Page 109 and 110: Stress and strain 93 Figure 9.2 Com
Page 111 and 112: Stress and strain 95 Figure 9.3 Ide
Page 113 and 114: Fracture Stress and strain 97 We fo
Page 115 and 116: Stress and strain 99 from which we
Page 117 and 118: 10 Sea waves The nature of water wa
Page 119 and 120: Sea waves 103 Figure 10.1 Wave moti
Page 121 and 122: Sea waves 105 refraction by islands
Page 123 and 124: Acoustics: sound and other waves 10
Page 125 and 126: Acoustics: sound and other waves 10
Page 127 and 128: 12 Fluids and fluid flow Introducti
Page 129 and 130: Fluids and fluid flow 1 13 Figure 1
Page 131 and 132: Fluids and fluid flow 115 Figure 22
Page 133 and 134: Fluids and fluid flow 117 and the f
Page 135 and 136: water above a vertical thickness h
Page 137 and 138: Fluids and fluid flow 121 where V i
Page 139 and 140: Fluids and fluid flow 123 ardentes,
Page 141 and 142: Fluids and fluid flow 125 Figure 12
Page 143 and 144: - 14 $ 12 E 10 K .- 3 8 r cll + 6 0
Page 145 and 146: Fluids and fluid flow 129 the liqui
Page 147 and 148: Some dangers of mathematical statis
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Some dangers of mathematical statis
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Appendix 139 What would happen if y
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Notes 141 and since x is very large
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References Allum, J. A. E. (1966) P
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References 145 -(1950) 'Mechanism o
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Further reading 147 Trigg, G. L. an
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Physics for Geologists, Second edition

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