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

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Figure 3.25: Global temperature change over the last 500 million years (the Phanerozioctime period), obtained by measuring change in the oxygen isotope compositions of fossils.feedback. Fortunately for us, the human body has many negative feedback systems.Some of these affect our body temperature; if <strong>we</strong> get too hot, <strong>we</strong> s<strong>we</strong>at and this cools usdown again. Similarly, if <strong>we</strong> get too cold, <strong>we</strong> shiver and this warms us up. <strong>The</strong>se twonegative feedback systems stop our body overheating or becoming too cold, and so keepus a<strong>live</strong>. A human example of positive feedback is a baby breast-feeding; the sucking ofthe baby causes more milk to be produced so the baby can continue to feed. A secondhuman example occurs when <strong>we</strong> cut ourselves, the damaged skin releases chemicals thatcause the blood to clot and the clotting mechanism releases more chemicals causing moreclotting, until the bleeding stops.<strong>The</strong> Earth has several feedback mechanisms that affect its temperature and one of theseis caused by ice sheets. <strong>The</strong> white colour of ice sheets reflects sunlight much more thanother coloured surfaces; the more light that is reflected, the less there is to warm theEarth. So if the Earth cools, the ice sheets will expand and reflect more light, adding tothe cooling of the Earth; a positive feedback effect. Similarly, if the Earth warms, theice sheets will contract, reflecting less light and adding to the warming effect; positivefeedback in the opposite direction.Another feedback mechanism that affects Earth’s temperature involves the amount of carbondioxide (CO 2 ) in the atmosphere. Carbon dioxide is one of the so called ‘greenhousegases’ that cause the Earth to be warmer than it otherwise would be the ‘greenhouseeffect’. Visible light from the Sun is absorbed by the Earth’s surface and some of thisis re-radiated as infra-red radiation (heat). All this infra-red radiation would be lost to89
Figure 3.26: Global temperature change over the last 450 thousand years, obtained fromduterium isotope measurements. <strong>The</strong> ratios <strong>we</strong>re measured in the EPICA and Vostockice cores and have been converted to ice volume estimations in the lo<strong>we</strong>r graph.space if it <strong>we</strong>re not for the greenhouse gases. <strong>The</strong>se absorb some of the outgoing infra-redradiation causing a warming of the atmosphere and of the Earth. If it <strong>we</strong>re not for thewarming effect of the greenhouse gases, heat from solar radiation would be lost and theEarth would be frozen and could not support life. So the atmosphere has to contain somegreenhouse gases for the Earth to be warm enough for life to exist.Some of the carbon dioxide in the atmosphere dissolves in rain and is carried into thesea. <strong>The</strong>re it is used by organisms to make calcium carbonate shells and reefs. <strong>The</strong> morecarbon dioxide there is in the atmosphere, the more becomes dissolved in the sea andis then available to organisms. So this results in a negative feedback effect; the morecarbon dioxide there is in the atmosphere, the more is removed, reducing the amount inthe atmosphere.<strong>The</strong>se are two of the important feedback effects that affect the Earth, but there are manyothers, some <strong>we</strong>ll understood and some not <strong>we</strong>ll understood at all. <strong>The</strong> overall effect ofthese has been the change of Earth’s temperature over geological time, as shown in Figure3.25. This shows that the Earth has had glacial periods in the past, when it was partlycovered by ice sheets, and much warmer periods, when there <strong>we</strong>re no ice sheets. As <strong>we</strong>currently have ice sheets in polar areas, this shows that the Earth is undergoing one ofthe colder periods of Earth’s geological history.We can obtain information on how Earth’s temperature has changed in much more recentgeological time by measuring isotope ratios in cores drilled into ice sheets, as shown inFigure 3.26.This shows that even over the last few hundred thousand years, Earth’s temperature has90
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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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Figure 1.44: A basalt flow that coo
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Figure 1.47: Deposit of volcanic as
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Figure 1.49: An old slate quarry, s
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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 71 and 72: Chapter 2Reading landscapes: how la
Page 73 and 74: Figure 2.3: Rock fragments loosened
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Page 87 and 88: found evidence that the Earth was a
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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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