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

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Figure 3.9: <strong>The</strong> Mid-Atlantic Ridge.is solid and rigid and so transmits all forms of earthquake waves. Beneath the lithosphereis a narrow zone of the mantle called the asthenosphere (‘astheno’ is Greek for ‘<strong>we</strong>ak’)which is about 150km thick. We know about the asthenosphere because earthquake wavesslow down there. It is so hot that it has begun to melt, containing bet<strong>we</strong>en 1 and 5%molten rock as coatings around the solid crystals. This means it is able to flow veryslowly, about as fast as your fingernails grow. Beneath that is the rest of the mantle,which is solid until it reaches the outer core, at a depth of around 3000km. Because ofthe enormous temperatures and pressures there, this deep mantle also able to flow, eventhough it is completely solid.Where the mantle is particularly hot, the rock there has lo<strong>we</strong>r density than the surroundingrock and rises. As it reaches the underside of the lithosphere, it flows out sidewaysand cools. Eventually, it becomes cool enough to sink again. Currents driven by heatand density differences like this are called convection currents, and it is the convectioncurrents in the largely solid mantle that carry the plates of lithosphere across the Earth’ssurface.Where convection currents in the mantle rise, the plates of lithosphere above are carriedapart. If this happens beneath oceans, this is where Hess’s ‘sea floor spreading’ occurs.New material is added to the plate and as the newly-formed lithosphere is so warm, its<strong>we</strong>lls up to form an oceanic ridge. Oceanic ridges are the longest, widest and highest (frombottom to top) mountain ranges on Earth and link together to form an interconnectedchain beneath all oceans. In the centres of the ridges, the movement apart of the platescauses tension. This results in normal faults that move through small earthquakes, causingthe central piece of newly-formed lithosphere to slide down in a long narrow rift valleyat the centre of the ridge. You could visit this rift valley yourself, since the Mid-AtlanticRidge is above sea level on Iceland, and the rift valley there is a tourist attraction.79
Figure 3.10: <strong>The</strong> rift valley in the centre of the Mid-Atlantic Ridge on Iceland.In the centre of the rift valley, on the ocean floor, new plate material is made. <strong>The</strong>ultramafic mantle material beneath has partially melted to form a mafic melt. This risesand some cools down slowly below the surface to form coarse-grained gabbro. Some risesthrough fractures as sheets of magma that cool more quickly to form dykes of mediumgrainedmafic rock called dolerite. Some reaches the surface of the sea floor, and eruptsas basalt pillow lavas. <strong>The</strong> newly formed oceanic plate is moved sideways over time whilstsea floor sediments settle onto the surface. This means that the new oceanic crust thatforms the top part of the new oceanic lithosphere, has four layers. Three of the layersare of mafic rock, coarse-grained gabbro, with medium-grained dolerite dykes above andpillow basalt flows above that. <strong>The</strong> top layer is fine-grained deep sea sediment. <strong>The</strong> rockis new oceanic plate material and so the margin is called a ‘constructive plate margin’.<strong>The</strong> newly-formed plates are carried sideways, allowing more new plate material to formin the centre, and giving these margins another name, ‘divergent plate margins’.Iceland is one place on Earth where oceanic plates are growing as new plate materialis formed. In Figure 3.10 <strong>we</strong> can see the central rift valley, where fissures opoen fromfrom time to time, to erupt mafic basalt lava. This runny lava flows quickly and forlong distances and can build up over time into great thicknesses of individual lava sheets.<strong>The</strong>se sheets give parts of Iceland a stepped plateau surface, as the flat-lying sheetsbecome eroded back at different rates.We can map this production line of new plate material through the ‘magnetic stripes’.Mafic rocks contain magnetic minerals that record the direction of the Earth’s magneticfield when the minerals crystallised. As the Earth’s magnetic field has ‘flipped’ in thegeological past, there <strong>we</strong>re times of ‘normal’ magnetism, when the magnetic field wasthe same as it is today, and ‘reversed’ magnetism’ when the Earth’s south magnetic80
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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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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 65 and 66: Cephalopods are not extinct, but th
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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 83 and 84: Figure 2.25: Dinosaur tracks conser
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Page 87 and 88: found evidence that the Earth was a
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Page 91 and 92: Figure 3.5: A page of Wegener’s 1
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Page 139 and 140: Figure 5.17: A windfarm in Ireland.
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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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The planet we live on: The beginnings of the Earth Sciences

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