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

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Figure 3.11: A constructive boundary,where new oceanic plate is being formed.Figure 3.12: <strong>The</strong> formation of magneticstripes at a constructive plate margin.pole is where the north magnetic pole is today. If the magnetic field was in the samedirection for a long time, a broad stripe of ocean floor formed before it ‘flipped’ again.But if it changed quickly, only a narrow stripe was formed. Since the same thing happenedon both sides of the ridge, the pattern on either side is the same, but reversed. So one sideis a mirror image of the other. <strong>The</strong> ‘magnetic stripes’ do not form straight lines, since themagnetism is recorded in the igneous rocks, and particularly the basalt lava flows at thesurface. How the stripes form, with irregular borders, is shown in Figures 3.12 and 3.13.<strong>The</strong> oceanic ridges and the pattern of their magnetic stripes show many places whereboth ridges and stripes are offset. <strong>The</strong>se are the transform faults, first recognised by J.Tuzo Wilson (see Figure 3.14). <strong>The</strong>se are very unusual faults since, bet<strong>we</strong>en the two offsetridges, the plates are moving in opposite directions, but beyond the ridges, the plates aremoving in the same direction, sometimes at slightly different rates. So, small earthquakesare common in the sections bet<strong>we</strong>en the ridges, but can also occur beyond the ridges too.One of the longest transform faults, joining two oceanic ridges that are now far apart,is the San Andreas Fault, which runs through California in the USA (see Figure 3.15).As the plate on the <strong>we</strong>stern side is moved north, relative to the south-moving plate onthe eastern side, there are frequent small earthquakes. <strong>The</strong>re is also the chance of a verylarge earthquake, like the one that destroyed most of San Francisco in 1906. Since platematerial is neither created nor destroyed at transform faults, but is conserved, these arecalled ‘conservative plate margins’.As the plates move away from the oceanic ridges they cool down, causing the oceanicridge to subside. Meanwhile, the blanket of deep sea sediment deposited on the surfacebecomes thicker. <strong>The</strong> further the plate has moved, the older the oldest sediment found onthe ocean floor. By drilling into these deep sea sediments, <strong>we</strong> can recover the fossils theycontain and date the rocks. This shows that the further away from a plate margin theocean floor is, the older it is, providing more evidence for the movement of the plates. Somaps of the age of ocean floor sediments, such as Figure 3.16, show how the plates havemoved over time, and confirm Harry Hess’s idea that the ocean floors are geologically81
Figure 3.13: <strong>The</strong> magnetic stripes south of Iceland.young, much younger than many of the rocks that form the continents.As they are moved across the ocean floors, the plates continue to cool, becoming steadilymore dense. Where two oceanic plates meet, the one that has travelled the further willnormally be the cooler and the more dense. Since they are moving towards one another,something has to happen, and the denser plate sinks.<strong>The</strong> sinking of a dense plate is called subduction. We know the angle of plate movementsince subduction isn’t smooth, but as the plate slides downwards, the friction bet<strong>we</strong>enthe rocks causes it to stick. When the pressure increases, it moves suddenly causing anearthquake. We can plot the positions and depths of these earthquakes and find that theyfollow a sloping zone into the mantle, called the Benioff Zone. As earthquakes occur toa depth of 700km, <strong>we</strong> know that the lithosphere stays solid and rigid to that depth too.Plates can be subducted into the mantle at different angles, but 45° is common.<strong>The</strong> plate of lithosphere which is subducted is made of the upper part of the mantle withmafic igneous crustal rock on top, covered by a blanket of sediment saturated in seawater.As this sinks it becomes heated up and partially melts. <strong>The</strong> silicon-rich minerals meltfirst, so that the newly-formed molten rock is richer in silicon, and poorer in iron, thanthe mafic rock that melts, so the new melt has an intermediate composition. This rises tothe surface above the sloping plate and erupts. Since intermediate melts are much thickerthan runny basalt melts, they often solidify in the vents of volcanoes resulting in violent82
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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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Figure 1.17: Sedimentary rocks show
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Figure 1.20: A close up view of a p
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Figure 1.26: Ancient wave ripple ma
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Figure 1.30: Coal seams in an openc
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Figure 1.32: Close up view of a pie
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Figure 1.35: A fossil colonial cora
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Figure 1.38: Fossil ammonites, indi
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Figure 1.39: ‘Massive’ igneous
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Figure 1.41: A close up view of a p
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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
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
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Page 67 and 68: Figure 1.68: A shelled cephalopod f
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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 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
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Page 129 and 130: Figure 5.6: An ash eruption rising
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Page 135 and 136: Figure 5.14: A GPS (global satellit
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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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