Uniformitarianism’ that <strong>the</strong> ‘present is <strong>the</strong> key to <strong>the</strong> past’. Most <strong>of</strong> <strong>the</strong> different types <strong>of</strong>sediment and volcanic rock <str<strong>on</strong>g>we</str<strong>on</strong>g> find in <strong>the</strong> rock record are being formed <strong>on</strong> <strong>Earth</strong> today,so <str<strong>on</strong>g>we</str<strong>on</strong>g> can investigate modern <strong>Earth</strong> processes to get good insights into how <strong>the</strong>y <str<strong>on</strong>g>we</str<strong>on</strong>g>reoriginally formed. Similarly, many fossils have modern living relatives today, so <str<strong>on</strong>g>we</str<strong>on</strong>g> canexamine <strong>the</strong>se to find out how <strong>the</strong> fossil organisms probably <str<strong>on</strong>g>live</str<strong>on</strong>g>d and died. It is moredifficult to determine how extinct organisms <str<strong>on</strong>g>live</str<strong>on</strong>g>d and how rock-forming processes workedthat d<strong>on</strong>’t seem to be happening <strong>on</strong> <strong>the</strong> <str<strong>on</strong>g>planet</str<strong>on</strong>g> today, but even for <strong>the</strong>se, <strong>the</strong> ‘Principle<strong>of</strong> Uniformitarianism’ can give us clues if <str<strong>on</strong>g>we</str<strong>on</strong>g> examine similar organisms and processes.<str<strong>on</strong>g>The</str<strong>on</strong>g> five principles below are called ‘Stratigraphic Principles’ since <strong>the</strong>y apply tostrata, or sedimentary and volcanic layers. Two <strong>of</strong> <strong>the</strong>se are <strong>the</strong> ‘Principle <strong>of</strong> OriginalHoriz<strong>on</strong>tality’ and <strong>the</strong> ‘Principle <strong>of</strong> Lateral C<strong>on</strong>tinuity’. ‘Original Horiz<strong>on</strong>tality’states that most sedimentary and volcanic rocks <str<strong>on</strong>g>we</str<strong>on</strong>g>re originally laid down in nearhoriz<strong>on</strong>tallayers. This means that if <str<strong>on</strong>g>we</str<strong>on</strong>g> find <strong>the</strong>m in layers that are not horiz<strong>on</strong>tal, <strong>the</strong>ymust have been tilted by tect<strong>on</strong>ic activity. <str<strong>on</strong>g>The</str<strong>on</strong>g>re are unusual instances when sedimentsand volcanic layers are not deposited flat, as in current bedding and dune bedding, or <strong>the</strong>layers in scree slopes or <strong>on</strong> <strong>the</strong> sides <strong>of</strong> volcanoes, which is why ‘Original Horiz<strong>on</strong>tality’is a principle and not a law. ‘Lateral C<strong>on</strong>tinuity’ states that sedimentary and volcanicrocks <strong>on</strong>ce formed laterally c<strong>on</strong>tinuous layers over wide areas. We know that <strong>the</strong>y cannothave been laterally c<strong>on</strong>tinuous over <strong>the</strong> whole <strong>Earth</strong>, since <strong>the</strong>y must ei<strong>the</strong>r have hit <strong>the</strong>edge <strong>of</strong> <strong>the</strong> area where <strong>the</strong>y <str<strong>on</strong>g>we</str<strong>on</strong>g>re deposited or died out laterally, so this too is a principleand not a law.<str<strong>on</strong>g>The</str<strong>on</strong>g> next principles are vital in sequencing rock events. <str<strong>on</strong>g>The</str<strong>on</strong>g> ‘Principle <strong>of</strong> Superpositi<strong>on</strong><strong>of</strong> Strata’ states that <strong>the</strong> rocks <strong>on</strong> top are <strong>the</strong> youngest. This is because rocksare deposited in layers that build up over time, so <strong>the</strong> oldest is at <strong>the</strong> bottom and <strong>the</strong>youngest at <strong>the</strong> top. Since it is possible for such sequences to be turned over by intensefolding or for older rocks to be thrust over younger <strong>on</strong>es by intense compressi<strong>on</strong> causingthrust faulting, ‘Superpositi<strong>on</strong> <strong>of</strong> Strata’ remains a principle and not a law. <str<strong>on</strong>g>The</str<strong>on</strong>g> ‘Law <strong>of</strong>Cross-Cutting Relati<strong>on</strong>ships’ states that anything that cuts across anything else mustbe younger. <str<strong>on</strong>g>The</str<strong>on</strong>g> ‘anything’ can include faults, joints, dykes, plut<strong>on</strong>s and unc<strong>on</strong>formitysurfaces. This law applies in all circumstances and to all rocks, since something cannotbe cut until it is first formed. Ho<str<strong>on</strong>g>we</str<strong>on</strong>g>ver, sometimes you have to examine <strong>the</strong> evidence verycarefully to be sure that something is indeed cutting something else, and it doesn’t justlook that way. <str<strong>on</strong>g>The</str<strong>on</strong>g> ‘Law <strong>of</strong> Included Fragments’ states that anything included in arock must be older than <strong>the</strong> rock that includes it. Included fragments are such things as<strong>the</strong> pebbles found in c<strong>on</strong>glomerates and <strong>the</strong> pieces <strong>of</strong> surrounding rock called xenoliths,sometimes included in plut<strong>on</strong>ic igneous rocks. This is a law since, for anything to beincluded, it must be older, but sometimes you have to examine <strong>the</strong> evidence very carefullyto be sure. See for yourself how <strong>the</strong>se stratigraphic principles work, by trying <strong>the</strong>http://www.earthlearningidea.com activity, ‘Laying down <strong>the</strong> principles’.<str<strong>on</strong>g>The</str<strong>on</strong>g> ‘Law <strong>of</strong> Faunal Successi<strong>on</strong>’ applies to fossils, and so is not a stratigraphic principle,in spite <strong>of</strong> <strong>the</strong> fact that it is used to sequence strata. It states that groups <strong>of</strong> fossil animalsfollow <strong>on</strong>e ano<strong>the</strong>r in time in a predictable sequence and <str<strong>on</strong>g>we</str<strong>on</strong>g> now know that plant fossilscan be used in <strong>the</strong> same way. It was by using <strong>the</strong> ‘Law <strong>of</strong> Faunal Successi<strong>on</strong>’ that geologists<str<strong>on</strong>g>we</str<strong>on</strong>g>re first able to sequence rocks, divide up <strong>the</strong> geological time scale and make <strong>the</strong> first47
proper geological maps. Fossils have now been sequenced all over <strong>the</strong> world to providevery detailed evidence for <strong>the</strong> relative ages <strong>of</strong> rocks. When fossils appear at <strong>the</strong> sametime in rocks across <strong>the</strong> world, <str<strong>on</strong>g>we</str<strong>on</strong>g> can use <strong>the</strong>se fossils to say that <strong>the</strong> rocks must havebeen formed at <strong>the</strong> same time too. This method <strong>of</strong> identifying rocks <strong>of</strong> <strong>the</strong> same age indifferent areas is called correlati<strong>on</strong> and rock correlati<strong>on</strong> using fossils has been essential inlinking toge<strong>the</strong>r <strong>the</strong> geological sequences <strong>of</strong> different areas, regi<strong>on</strong>s, c<strong>on</strong>tinents and across<strong>the</strong> world. We now know that <strong>the</strong> reas<strong>on</strong> for <strong>the</strong> ‘Law <strong>of</strong> Faunal Successi<strong>on</strong>’, that fossilsare always found in <strong>the</strong> same sequence across <strong>the</strong> world, is evoluti<strong>on</strong>.A huge range <strong>of</strong> fossils can be found in rocks, but <strong>on</strong>ly a few <strong>of</strong> <strong>the</strong>se can be used forcorrelati<strong>on</strong>. <str<strong>on</strong>g>The</str<strong>on</strong>g> best fossils for correlati<strong>on</strong> have <strong>the</strong>se key features:• <strong>the</strong>y <str<strong>on</strong>g>we</str<strong>on</strong>g>re comm<strong>on</strong>, so many <strong>of</strong> <strong>the</strong>m could be fossilised;• <strong>the</strong>y <str<strong>on</strong>g>we</str<strong>on</strong>g>re easily preserved, usually because <strong>the</strong>y had hard parts, and so are frequentlyfound;• <strong>the</strong> fossil group evolved quickly over time, meaning that <strong>the</strong> fossils in different bedsare slightly different;• <strong>the</strong>y <str<strong>on</strong>g>we</str<strong>on</strong>g>re widespread, so are found in many rocks across <strong>the</strong> world;• <strong>the</strong>y are found in many different rock types, such as sandst<strong>on</strong>es, limest<strong>on</strong>es andshales;• and <strong>the</strong>y can be easily identified, at least by experts.Two types <strong>of</strong> fossil that fit <strong>the</strong>se requirements are graptolites and cephalopods.Graptolites are now extinct, so it is difficult for us to know how <strong>the</strong>y <str<strong>on</strong>g>live</str<strong>on</strong>g>d. Ho<str<strong>on</strong>g>we</str<strong>on</strong>g>ver,this doesn’t matter if <str<strong>on</strong>g>we</str<strong>on</strong>g> are just using <strong>the</strong>m for correlati<strong>on</strong>. <str<strong>on</strong>g>The</str<strong>on</strong>g>y <str<strong>on</strong>g>we</str<strong>on</strong>g>re small col<strong>on</strong>ies <strong>of</strong>animals that <str<strong>on</strong>g>we</str<strong>on</strong>g>re strung toge<strong>the</strong>r in saw-blade-like shapes. Each animal <str<strong>on</strong>g>live</str<strong>on</strong>g>d in a smallliving chamber called a <strong>the</strong>ca and <strong>the</strong> string <strong>of</strong> animals is called a stipe (see Figure 1.65).<str<strong>on</strong>g>The</str<strong>on</strong>g>ir important properties are:• <strong>the</strong>y <str<strong>on</strong>g>we</str<strong>on</strong>g>re very comm<strong>on</strong> in ancient seas;• <strong>the</strong>y <str<strong>on</strong>g>we</str<strong>on</strong>g>re made <strong>of</strong> a hard organic material that readily fossilised;• <strong>the</strong>y evolved quickly over time, so have many different forms, in particular <strong>the</strong>shapes <strong>of</strong> <strong>the</strong> stipes changed, as you can see in Figure 1.66, and <strong>the</strong> shapes <strong>of</strong> <strong>the</strong><strong>the</strong>cae <str<strong>on</strong>g>we</str<strong>on</strong>g>re also very varied;• <strong>the</strong>y <str<strong>on</strong>g>live</str<strong>on</strong>g>d right across <strong>the</strong> world’s oceans;• as <strong>the</strong>y floated in <strong>the</strong> sea, <strong>the</strong>y could be found in sandy, muddy and limest<strong>on</strong>eenvir<strong>on</strong>ments, and so are preserved in different rock types;• experts can easily identify <strong>the</strong>m from <strong>the</strong>ir stipe orientati<strong>on</strong>s and <strong>the</strong> shapes <strong>of</strong> <strong>the</strong>ir<strong>the</strong>cae.48
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Basic Books in ScienceBook 6<strong
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Looking ahead - If you came across
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3.2 Plate tectonics (20th Century)
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Figure 4.1: William Smith’s geolo
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Millionsof yearsago (Ma)01000Some M
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Figure 4.9: The ch
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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 5.6: An ash eruption rising
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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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Another ‘missing link’ find has
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Figure 5.22: A dinosaur reconstruct
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Chapter 6Understanding what geologi
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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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Geophysical survey Using the method
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Metamorphism The r
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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