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

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Figure 1.57: Folded rocks, with a downfolded syncline to the left and an upfolded anticlineto the right. <strong>The</strong> axial plane surfaces of these folds go into and out of the cliff face andthe compressional stresses that deformed them came from the left and right. <strong>The</strong>se aretight folds and you can see their scale from the street light in front of them.axial plane surface is vertical and runs through the join bet<strong>we</strong>en your hands. Each handforms the limb of the fold and dips towards the centre. <strong>The</strong> strike direction is parallel toyour fingers and to the fold axial plane surface.If rocks in a cliff face are dipping towards the left and you follo<strong>we</strong>d them to the leftfar enough, you would find them bending up again. <strong>The</strong> rocks are therefore forming adownfold, this is called a syncline. Similarly, if you follo<strong>we</strong>d the dipping rocks to theright far enough, you would eventually find them bending down again. Upfolds like thisare called anticlines. So dipping rocks are just the visible parts of much larger folds thatbecome synclines in one direction and anticlines in the other.You can use these principles on any dipping rocks to work out where the synclinal andanticlinal axial planes are. For example, in Figure 1.57, the synclinal axial plane surfacemust be to the left and the anticline to the right. You can also work out the directionsof the stresses that caused this tilting; here, they must also have come from the left andright. So, you should now be able to use these principles on any tilted or folded rocksyou find, to work out the directions of the stresses that caused them. Since folds arethree dimensional, you can not only see them in vertical cliff faces but also on horizontalsurfaces too, as in the aerial view in Figure 1.58. <strong>The</strong> same principles are used to workout the stress directions wherever you see folded or tilted rocks. Folding results in crustalshortening, where the crustal rocks have been compressed and take up less space thanthey used to.When rocks are compressed and fracture, compressional faulting occurs as one slab ofrock slides up over the other, again resulting in crustal shortening. <strong>The</strong> sliding surfaceis called the fault plane and fault planes in compressional faults usually dip at around41
Figure 1.58: A region of folded rocks seen from the air. An anticline makes the ridge inthe distance and there is a syncline in the left foreground. <strong>The</strong> fold axial plane surfacesrun up and down the photo and the compressional stresses came from left and right.45°(bet<strong>we</strong>en 30 and 60°). <strong>The</strong>se faults, shown in Figure 1.59, are called reverse faults(because they have moved in an opposite direction to the more common ‘normal’ faults,described below). When compressional forces are very great, slabs of rock can be forcedgreat distances over the rocks beneath, when the sliding surface usually has a much lo<strong>we</strong>rslope of 10°or less. <strong>The</strong>se types of reverse faults are called thrust faults (Figure 1.60),and sometimes rock can be moved tens of kilometres along them. You can make yourown folds and compressive faults using sand and flour in a small plastic box using the‘Himalayas in 30 seconds!’ activity on the http://www.earthlearningidea.com <strong>we</strong>bsite.When rocks are pulled apart by tensional stresses, they can fracture to form normalfaults. <strong>The</strong>y are called normal faults because they are the most commonly seen typesof faulting (Figure 1.61). <strong>The</strong> rocks fracture, usually along a steep fault plane of 60°ormore, and the rocks on one side slide down relative to the rocks on the other. <strong>The</strong> tensionwas at right angles to the fault plane and the result is crustal extension, with the rockstaking up more space than they did originally. Try making tensional faults in a box usingthe http://http://www.earthlearningidea.com, ‘A valley in 30 seconds’ activity.Shear stresses cause slabs of rock to move sideways across the Earth in relation to therocks on the other side. <strong>The</strong>se are called strike-slip faults because rocks striking acrossthe land surface have slipped sideways relative to the rocks on the other side (Figure 1.62).<strong>The</strong> fault planes of strike-slip faults are usually vertical. <strong>The</strong>se result in neither crustalshortening nor crustal extension and are most easily seen by viewing rock sequences fromabove.Where you can see layers on each side of a fault that used to match up, but are nowbroken, it is easy to tell there is a fault there and to work out which type of fault it is andhow far the rocks have been moved (the fault displacement). Ho<strong>we</strong>ver, if you can’t see42
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Basic Books in ScienceBook 6<strong
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BASIC BOOKS IN SCIENCE- a Series of
Page 5 and 6: BASIC BOOKS IN SCIENCE- a Series of
Page 7 and 8: Looking ahead - If you came across
Page 9 and 10: 3.2 Plate tectonics (20th Century)
Page 11 and 12: 1.30 Coal seams in an opencast coal
Page 13 and 14: 3.4 Alfred Wegener, the polar explo
Page 15 and 16: 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: Figure 1.56: Metaquartzite, produce
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 79 and 80: are forming them. Similarly the sha
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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 101 and 102: Figure 3.19: An ocean versus contin
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Page 105 and 106: Figure 3.26: Global temperature cha
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Figure 3.29: A computer generated p
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Figure 3.31: The
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planet extended th
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Figure 4.1: William Smith’s geolo
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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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