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

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Figure 3.21: A continent versus continentdestructive margin.Figure 3.22: <strong>The</strong> Hawaiian island chain,formed as the Pacific plate was movedacross a hot spot in the mantle.Our modern understanding of plate tectonics has given much more evidence of how itworks. J. Tuzo Wilson was able to use his plate tectonic ideas to explain the Hawaiianchain of volcanoes in the Pacific Ocean. <strong>The</strong> chain trends from south east to north <strong>we</strong>stand only the south eastern-most island still has an active volcano. <strong>The</strong> islands to thenorth are extinct volcanoes and there is a continuing chain of extinct volcanoes underwater further to the north <strong>we</strong>st. Wilson saw that there must be a rising plume of hotrocks in the mantle beneath, which partially melted to form mafic magmas. <strong>The</strong>se eruptedto build up the basaltic volcanoes that form the Hawaiian chain. As the Pacific plate wasmoved across this plume, older volcanoes became extinct as new ones formed, rather likea piece of paper might burn a line if you moved it across the top of a candle flame. Thissho<strong>we</strong>d not only how the chain of Hawaiian volcanoes formed, but also the direction ofmovement and the speed of the plate (Figure 3.22).Today <strong>we</strong> have the technology to measure the rate of movement of plates, using suchsystems as satellite laser ranging and Global Positioning Systems (GPS), used to makeregular measurements. <strong>The</strong>se show that plates move from a few millimetres a year to upto 30 millimetres a year. This is about as fast as your fingernails grow.We have now been able to map the plate boundaries in great detail (Figure 3.23), based onthe geographical features found there (oceanic ridges, trenches, island arcs and mountainchains), the tectonic features active there, and the directions of plate movement. Basalticvolcanoes are found at constructive plate margins and at hot spots, andesitic volcanoesat ocean-ocean destructive margins and andesitic and silicic volcanoes at ocean-continentplate margins. Small shallow focus earthquakes occur at constructive plate margins andconservative margins (transform faults) whilst shallow, medium depth and deep earthquakes,that can sometime be large and destructive, are found at destructive plate margins.We can now use maps of the plate boundaries to predict where future tectonicactivity is likely, and to understand deep Earth processes across the Earth.87
Figure 3.23: Map of the major tectonicplates on Earth.Figure 3.24: Stromatolite fossils in ancientrocks.Thus the theory of plate tectonics explains the locations and character of islands andcontinents, mountain chains and volcanoes, oceanic ridges and trenches. It is the causeof the internal rock cycle processes, like metamorphism and melting, igneous activity anduplift, and it explains why <strong>we</strong> find desert rocks in tropical regions and tropical rocks inpolar regions, as <strong>we</strong>ll as the distribution of many species on Earth. Like all good theories,it has provided po<strong>we</strong>rful explanations for many of the Earth’s features and processes, butnot all. So <strong>we</strong> need to keep investigating the Earth to find more about how plate tectonicsaffects our <strong>planet</strong> and more about the other processes that must be at work when whenplate tectonic explanations don’t seem to fit.3.3 Global temperature/sea level change (21st Century)We know that life has existed on Earth for nearly 3.5 billion years (3.5 thousand, millionyears), since colonies of stromatolite fossils have been found in rocks that old.Since life like this can only exist in liquid water, <strong>we</strong> know that the temperature of Earth forall that time cannot have been below the freezing point of water (0 ◦ C) or above the boilingpoint of water (100 ◦ C). Indeed geological research indicates that the mean temperature ofthe Earth has been bet<strong>we</strong>en 10 ◦ C and 30 ◦ C throughout the past 3.5 billion years. This isdespite the fact that the Sun has been getting steadily hotter during this time so that theEarth now receives more than 30% more solar energy than it did when it formed. Thisshows that there must be some natural mechanism or mechanisms that have kept theEarth’s temperature bet<strong>we</strong>en 10 ◦ and 30 ◦ as the solar energy input has increased. <strong>The</strong>semechanisms must result in negative feedback.Feedback affects many different types of systems. When something causes a changein a system, it will have effects. If these effects work against the change, this is callednegative feedback, whereas if the effects cause the change to increase, it is positive88
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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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Page 71 and 72: Chapter 2Reading landscapes: how la
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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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