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

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1.2.3 EvaporitesWhen salt water evaporates as lakes or arms of the sea dry out, salts crystallise out asdeposits that cover the old lake or sea bed (Figure 1.10). <strong>The</strong>se are called evaporitedeposits and the most common of these is the compound of sodium and chlorine, halite.<strong>The</strong> sodium chloride (NaCl) molecules are <strong>we</strong>akly bonded, so halite is a mineral with avery low hardness. <strong>The</strong> sodium chloride bonds are broken in water so that halite is solubleand easily dissolves again. Because of this solubility and its salty taste, a refined form ofhalite is used as table salt in many of the foods you eat. If you want to crystallise yourown salt crystals, see the ‘Salt of the Earth: Who can make the biggest salt crystal?’activity on the http://www.earthlearningidea.com <strong>we</strong>bsite.1.2.4 Sedimentary rocksSedimentary rocks are made of sediments which are fragments of other rocks or fossils,although sometimes sediments can also be laid down by the evaporation of water. Oftenthe grains of sediment are single crystals which have been broken and worn down. <strong>The</strong>sehave often become rounded during transportation and sorted into grains of similar sizes.After they have been deposited, water flows in the pore spaces bet<strong>we</strong>en the grains. Thisslow-flowing water dissolves minerals from some parts of the deposit and they recrystallisebet<strong>we</strong>en the grains in other parts of the deposit, ‘gluing’ the grains together. This natural‘glue’ is called cement and changes soft sediments into harder sedimentary rocks (Figure1.11).<strong>The</strong> most common cementing minerals in sedimentary rocks are quartz and calcite. Asthe pore spaces become filled with cement, the tougher the rocks become. Try the ‘Makeyour own rock’ activity on the http://www.earthlearningidea.com <strong>we</strong>bsite to find outhow this cementing process works.1.2.5 Veins and oresSedimentary rocks are cemented by cool fluids. Ho<strong>we</strong>ver, magmas can produce hot wateryfluids, whilst in other areas where rocks are heated, the water in the rocks can dissolveminerals. This watery hot liquid containing dissolved minerals is a hydrothermal fluidthat, being less dense than the surroundings, rises through pore spaces and cracks in therock. As it rises it cools and minerals crystallise out of solution in the pore spaces or ascoatings on the sides of the cracks. Mineral-coated cracks are called mineral veins. <strong>The</strong>secan be thin sheets that fill a fracture such as a joint in a rock, or more complex sheetsfilling faults or other fractures and spaces.Quartz and calcite commonly crystallise in mineral veins, but sometimes more unusualminerals can form (Figure 1.12). If mineral deposits contain metal minerals and thedeposits are rich enough to be worth mining, the deposits are called metal ores. Oreminerals include the red iron oxide (Fe 2 O 3 ) mineral hematite, and the shiny grey, leadsulfide (PbS) mineral galena. In deposits of ore minerals, the uneconomic minerals thathave to be thrown away, like quartz and calcite, are called gangue minerals.7
Figure 1.12: Crystals of minerals in a mineralvein.Figure 1.13: <strong>The</strong> red ore mineral hematitewith the colourless gangue mineral, calcite<strong>The</strong> most valuable minerals are the precious gemstones like diamond, which are hardand rare. Diamond is found in certain volcanic deposits, or has been eroded from themand deposited in rivers and beaches. Semi-precious stones which are fairly hard and rareinclude garnet and the coloured forms of quartz found in some mineral veins, like purpleamethyst quartz.Metallic minerals like hematite (Figure 1.13) and galena, can form valuable ore deposits,but one of the most valuable metal minerals is gold. Gold is an un-reactive element thatrarely combines with other elements and so is usually found on its own as native gold.Native gold can be found in mineral veins or, like diamond, eroded from veins anddeposited in sediments.Minerals can be identified by their properties; some important minerals are shown infigures 1.14 and 1.15.8
Page 1 and 2: Basic Books in ScienceBook 6<strong
Page 3 and 4: 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: Figure 1.8: A red garnet crystal in
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 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
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
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Figure 2.3: Rock fragments loosened
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Figure 2.6: The jo
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Figure 2.10: The m
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are forming them. Similarly the sha
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Figure 2.19: This photo was taken f
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Figure 2.25: Dinosaur tracks conser
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• On coastal sections, whenever p
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found evidence that the Earth was a
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Figure 3.3: James Hutton, the ‘Fo
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Figure 3.5: A page of Wegener’s 1
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Figure 3.7: The st
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Figure 3.10: The r
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Figure 3.13: The m
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Figure 3.15: The S
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Figure 3.19: An ocean versus contin
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Figure 3.23: Map of the major tecto
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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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