The planet we live on: The beginnings of the Earth Sciences

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Figure 1.70: <strong>The</strong> change of ammonoid suture lines over time, from the simple sutures ofgoniatites, common around 350 million years ago in the left hand diagram, to the morecomplex sutures of ceratites, common around 240 million years ago in the centre to thevery complex sutures of ammonites, found in 190 million year old rocks in the right handdiagram. Ribs on the outsides of the fossils are shown as dashed lines.at fixed rates over time. So, if <strong>we</strong> can find a rock or mineral that contains radioactiveatoms, and <strong>we</strong> can find out what proportion of them has decayed, and <strong>we</strong> also know therate of decay, <strong>we</strong> can work out the age of the rock. For example, potassium (with thechemical symbol, K) has a radioactive component and is commonly found in igneous rockslike granite. When the granite first forms, the potassium-containing minerals contain onlypotassium. But the radioactive part of potassium decays to argon (Ar) over time. So, if<strong>we</strong> measure the amount of potassium that should have been in the rock, and the amountof argon it now contains, <strong>we</strong> can work out the age of the rock. Half the radioactivepotassium would decay to argon in 1260 million years, so an igneous rock with halfradioactive potassium/half argon must have crystallised 1260 million years ago.Unfortunately <strong>we</strong> can only apply this method to igneous rocks and some metamorphicrocks that contain potassium (and the unusual sedimentary rock, greensand). This is trueof other radiometric methods as <strong>we</strong>ll - <strong>we</strong> can only apply them to certain sorts of rocks.This means, for example, that <strong>we</strong> usually can’t find the absolute ages of sedimentary rocksand fossils directly. If a bed of sedimentary rock containing a useful correlation fossilhappened to have a lava flow below and above it, and <strong>we</strong> could work out the radiometricages of the lava flows, <strong>we</strong> would have a good idea of the absolute age of the rock and itsfossil. Ho<strong>we</strong>ver, this is very unusual and, even when it does happen, the ages of the lavaflows may be several million years apart, so that <strong>we</strong> can only find the approximate age ofthe rock and fossil.Nevertheless, geologists have been working on this problem for many years, and nowhave a good idea of the ages of key correlation fossils and the rocks in which they arefound. <strong>The</strong>se ages have been added to the international geological time scale, so as <strong>we</strong>llas knowing the sequence of global geological events, <strong>we</strong> also know when in the geologicalpast they actually happened (Figure 1.71).53
You can try the rock sequencing principles yourself through thehttp://www.earthlearningidea.com activity, ‘What is the geological history?’. Evenbetter, try them on a nearby rock exposure in a cliff or a quarry instead.54
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Basic Books in ScienceBook 6<strong
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BASIC BOOKS IN SCIENCE- a Series of
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BASIC BOOKS IN SCIENCE- a Series of
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Looking ahead - If you came across
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3.2 Plate tectonics (20th Century)
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1.30 Coal seams in an opencast coal
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3.4 Alfred Wegener, the polar explo
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5.14 A GPS remote volcano monitorin
Page 17 and 18: Figure 1.2:minerals.A sandstone roc
Page 19 and 20: Even diamond can have different col
Page 21 and 22: Figure 1.8: A red garnet crystal in
Page 23 and 24: Figure 1.12: Crystals of minerals i
Page 25 and 26: Hematite, Fe 2 O 3 - earthy red, me
Page 27 and 28: Figure 1.17: Sedimentary rocks show
Page 29 and 30: Figure 1.20: A close up view of a p
Page 31 and 32: Figure 1.26: Ancient wave ripple ma
Page 33 and 34: Figure 1.30: Coal seams in an openc
Page 35 and 36: Figure 1.32: Close up view of a pie
Page 37 and 38: Figure 1.35: A fossil colonial cora
Page 39 and 40: Figure 1.38: Fossil ammonites, indi
Page 41 and 42: Figure 1.39: ‘Massive’ igneous
Page 43 and 44: Figure 1.41: A close up view of a p
Page 45 and 46: Figure 1.44: A basalt flow that coo
Page 47 and 48: Figure 1.47: Deposit of volcanic as
Page 49 and 50: Figure 1.49: An old slate quarry, s
Page 51 and 52: Figure 1.51: Slate, a low-grade met
Page 53 and 54: Figure 1.53: Schist, a medium-grade
Page 55 and 56: Figure 1.56: Metaquartzite, produce
Page 57 and 58: Figure 1.58: A region of folded roc
Page 59 and 60: Figure 1.61: A normal fault. This s
Page 61 and 62: Figure 1.64: An unconformity. Older
Page 63 and 64: proper geological maps. Fossils hav
Page 65 and 66: Cephalopods are not extinct, but th
Page 67: Figure 1.68: A shelled cephalopod f
Page 71 and 72: Chapter 2Reading landscapes: how la
Page 73 and 74: Figure 2.3: Rock fragments loosened
Page 75 and 76: Figure 2.6: The jo
Page 77 and 78: Figure 2.10: The m
Page 79 and 80: are forming them. Similarly the sha
Page 81 and 82: Figure 2.19: This photo was taken f
Page 83 and 84: Figure 2.25: Dinosaur tracks conser
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Page 87 and 88: found evidence that the Earth was a
Page 89 and 90: Figure 3.3: James Hutton, the ‘Fo
Page 91 and 92: Figure 3.5: A page of Wegener’s 1
Page 93 and 94: Figure 3.7: The st
Page 95 and 96: Figure 3.10: The r
Page 97 and 98: Figure 3.13: The m
Page 99 and 100: Figure 3.15: The S
Page 101 and 102: Figure 3.19: An ocean versus contin
Page 103 and 104: Figure 3.23: Map of the major tecto
Page 105 and 106: Figure 3.26: Global temperature cha
Page 107 and 108: Figure 3.29: A computer generated p
Page 109 and 110: Figure 3.31: The
Page 111 and 112: planet extended th
Page 113 and 114: Figure 4.1: William Smith’s geolo
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Figure 4.9: The ch
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Figure 4.12: The 4
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Millionsof years Some Major Earth E
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Figure 5.2: Strike-slip movement (r
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Figure 5.4: The So
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Figure 5.6: An ash eruption rising
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Figure 5.10: A hazard zone map of t
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isk to humans. The
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Figure 5.14: A GPS (global satellit
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None of these methods has yet prove
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Figure 5.17: A windfarm in Ireland.
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Figure 5.19: A beautifully preserve
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Another ‘missing link’ find has
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Figure 5.22: A dinosaur reconstruct
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Figure 5.25: A working aggregate-pr
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Figure 5.26: The E
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Chapter 6Understanding what geologi
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Figure 6.2: A drilling rig used for
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When an oil/gas field has been foun
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Figure 6.6: Groundwater flowing out
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Dam disaster in Italy, when the wav
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Figure 6.10: A slab foundation, bui
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An example of this is investigation
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GlossaryAbsolute age The</s
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Carbon capture The
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Crustal shortening This results of
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Evaporite deposits (or evaporites)
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Geophysical survey Using the method
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“Integrated waste management” <
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Metamorphism The r
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Pore spaces (or pores) Gaps bet<str
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Saltation Sediment movement by flui
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Suspension Sediment movement by flu
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AcknowledgementsPermission to repri
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Figure 2.3 A scree slope. Photo ID:
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Figure 1.15a Hematite.Figure 1.15b
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Figure 1.28 Dune cross bedding in s
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Figure 3.18 An island arc volcano,
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Figure 5.21 Excavations at the dino
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