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

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Figure 1.65: Graptolites. <strong>The</strong> largest fossil here had four stipes and the small thecae orliving chambers can be seen along each stipe.Figure 1.66: How graptolites evolved, from the four-stiped form common around 480million years ago on the left to the single-stiped form common around 420 million yearsago on the right.49
Cephalopods are not extinct, but the sorts with coiled shells that are useful in correlation,nearly became extinct and only one type can be found a<strong>live</strong> today (it is shown inFigure 1.67). Nevertheless, <strong>we</strong> know from these living animals that they <strong>live</strong>d in the sea,floating in the water, and could move around by squirting jets of water backwards. <strong>The</strong>animal living in the coiled shell had tentacles, like an octopus, that could grab passingsmall animals for food. When they <strong>we</strong>re attacked they could withdraw into their shells forprotection. Inside the coiled shells there <strong>we</strong>re chambers of gas that helped the animal tofloat in the water. Where the chamber walls joined the outside of the shell they formed aline, called a suture line. You can see these if you peel away the outer shell, exposing thechambers and chamber walls beneath (as has been done in Figure 1.68). In the earliestshelled cephalopods, the chamber walls <strong>we</strong>re straight lines, but as the animals evolved,the lines became more and more complex, and it is these complex suture lines that help usto identify the different types. <strong>The</strong> cephalopods with the most complex suture lines, that<strong>live</strong>d bet<strong>we</strong>en about 200 and 65 million years ago, are called ammonites (Figure 1.69).Shelled-cephalopods have all the properties that make them excellent fossils for correlation:• like graptolites, they also <strong>we</strong>re very common in ancient seas;• their hard shells <strong>we</strong>re easily fossilised;• their suture lines evolved quickly, and three of the main shapes are shown in Figure1.70;• they <strong>live</strong>d in all seas and oceans;• since they floated in the sea, and continued floating even after they died, their shellscan be found in all sorts in environments and sediments;• experts can easily identify them from their suture lines.With all these properties, it is not surprising that cephalopods have been key fossils inhelping us to unravel the geological history of the Earth.A final rule that helps us to work out geological histories of rock sequences is the fact thatrocks cannot be deformed or metamorphosed until they have first been formed. So <strong>we</strong> nowhave all the principles and laws needed for working out how rocks <strong>we</strong>re first formed andthe sequence of changes that happened to them after formation. When all the changeshave been put into order, the geological history of the rock sequence has been explained.<strong>The</strong>se methods <strong>we</strong>re used to work out the geological time sequence of all rocks on Earth,as shown in Figure 1.71. This is the international geological time scale. <strong>The</strong> principlesabove only help us to put things into order, to work out the relative ages of the events,but they do not tell us how old the rocks and events are. To do this, <strong>we</strong> need anothertechnique that will give us the absolute ages of the rocks.Geologists have been able to work out the relative ages of rocks and events for more than200 years, but an absolute dating technique has only been widely available for about 50years. This is radiometric dating and is based on the fact that radioactive atoms decay50
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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
Page 13 and 14: 3.4 Alfred Wegener, the polar explo
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Page 17 and 18: Figure 1.2:minerals.A sandstone roc
Page 19 and 20: Even diamond can have different col
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Page 25 and 26: Hematite, Fe 2 O 3 - earthy red, me
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Page 37 and 38: Figure 1.35: A fossil colonial cora
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Page 49 and 50: Figure 1.49: An old slate quarry, s
Page 51 and 52: Figure 1.51: Slate, a low-grade met
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Page 55 and 56: Figure 1.56: Metaquartzite, produce
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Page 61 and 62: Figure 1.64: An unconformity. Older
Page 63: proper geological maps. Fossils hav
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Page 71 and 72: Chapter 2Reading landscapes: how la
Page 73 and 74: Figure 2.3: Rock fragments loosened
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Page 89 and 90: Figure 3.3: James Hutton, the ‘Fo
Page 91 and 92: Figure 3.5: A page of Wegener’s 1
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Millionsof yearsago (Ma)01000Some M
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ocks whilst the first definite plan
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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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