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

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Figure 2.11: A V-shaped valley, cut by ariver in an upland area.Figure 2.12: A U-shaped valley, cutthrough this mountainous area by a glacierduring the ice age.Figure 2.13: A meandering river channel,cut across broad flat valley floor sedimentsin a lowland area.Figure 2.14: This plateau is formed oftough, flat-lying lava flows. <strong>The</strong> rocks ofthe lowland below <strong>we</strong>re much less resistantto erosion.originally covered by another set of rocks, and the river developed its pattern on theseoverlying rocks. Now that it has cut down through the overlying rocks and removed them,the pattern has become superimposed on the geology beneath, with rivers cutting acrossridges and other upland areas for no clear geological reason.<strong>The</strong> valleys seen in lowland areas are rather different. Here the main process is not erosion,but deposition, and rivers deposit sediment, filling up old valleys rather than eroding newones. So lowland valleys tend to be broad with flat floors. Sediments are deposited acrossthese flat-bottomed valleys during floods, so that the valley floors gradually build up overtime. Bet<strong>we</strong>en floods, the rivers cut their channels across the flat valley-floor sediments,often in winding meandering patterns (Figure 2.13). So a meandering river pattern is asign that deposition is the main process active there.So, the shapes of valleys can give clues to the underlying geology and the processes that63
are forming them. Similarly the shapes of hills can often tell us about the underlyinggeological pattern as <strong>we</strong>ll.2.3 Landforms often reflect underlying geological structureWhen sedimentary sequences have tough layers with <strong>we</strong>aker rocks bet<strong>we</strong>en them, severaltypical landscape features appear, depending on how steeply the rocks dip. Tough rocksstand out as higher land and <strong>we</strong>aker rocks have been eroded away to make valleys andlowlands.If the rock sequence is horizontal, the tough rocks form horizontal upland areas calledplateaus. Tough sandstones and limestones often form plateaus, but plateaus are alsocommonly formed by widespread lava flows of basalt, since the basalt lavas are usuallymore resistant to erosion than the surrounding rocks (Figure 2.14).If the sequence of alternating tough rocks and <strong>we</strong>aker rocks is dipping at a small angle(usually less than 10°), ridges develop that slope gently downwards in the direction ofdip, but are steeper in the other direction. <strong>The</strong>se asymmetrical ridges are called cuestas(Figure 2.15). Where there are several tough beds in a sequence, a series of these ridgesoften forms, called scarp and vale topography. <strong>The</strong> topography, or land surface, hasscarps, or cuestas, divided by valleys or vales.When the sequence dips more steeply than 10°, ridges of the harder rocks usually form,with steep slopes on either side (Figure 2.16).Where large faults cut across the landscape, with different rocks on either side, thenerosion tends to remove one type of rock at a faster rate than the other and a steep slopedevelops bet<strong>we</strong>en the two rock types (Figure 2.17). This is called a fault scarp and, sincesuch large faults are usually straight, fault scarps tend to cut straight across the country.If the main rock underlying an area is granite, a tough erosion-resistant rock, it usuallyforms undulating upland areas with poor soils. Sometimes the granite is so resistant thatit is exposed on the hilltops as rounded domes of jointed rock called tors (figure 2.18).<strong>The</strong>se features mean that you can stand on a hill top and look across at the shapes ofthe hills to see how the geology is lying. You can do the same in coastal areas, where thetough rocks form headlands and the <strong>we</strong>aker rocks form the bays bet<strong>we</strong>en them (Figure2.19). So a series of alternating tough and <strong>we</strong>aker rocks at the coast can form a series ofheadlands and bays. You can find out which of the rocks in your local area are likely toform hills and headlands, and which will form valleys and bays by doing the ‘Rock, rattleand roll’ test on the http://www.earthlearningidea.com <strong>we</strong>bsite.Try looking at the view from your window. Any hills or upland areas you can see aremade of tougher rocks, while <strong>we</strong>aker rocks form any valleys or lowlands. Can you tellfrom the shapes of the hills what the underlying geology is like? You might be able totell from the shape of any valleys you can see how they <strong>we</strong>re formed as <strong>we</strong>ll.64
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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
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Figure 1.2:minerals.A sandstone roc
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Even diamond can have different col
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Figure 1.8: A red garnet crystal in
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Figure 1.12: Crystals of minerals i
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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 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 and 68: Figure 1.68: A shelled cephalopod f
Page 69 and 70: You can try the rock sequencing pri
Page 71 and 72: Chapter 2Reading landscapes: how la
Page 73 and 74: Figure 2.3: Rock fragments loosened
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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
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Page 97 and 98: Figure 3.13: The m
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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
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Page 113 and 114: Figure 4.1: William Smith’s geolo
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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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