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Lecture 7

EESA06 Lecture 7.docx

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Environmental Science
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Nick Eyles

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EESA06 Lecture 7 Page 1 EESA06 Lecture 7 Galapos Islands – in Equador, hotspots - Hotspot trails – old dead volcanoes, flat tops (from erosion), below seas level, dead volcano - Hotspots – majority occur on oceanic plates - Relative Ages of islands: the youngest island above the hotspot, eventually go under sea level, systematic aging of the rocks as you go down the trail - Gives us a clue of plate movement - Bathymetric – map that shows water depths - Islands after 3 million years become completely submerged - Phreatomagmatic – water + volcanic activity, violent releases - Geologists like this area because there is No vegetation = young landscape, still actively evolving - Barrel Cactus are the pioneer/first plants grow in cracks of lava, o Prickly pear cactus another pioneer plants, eaten by giant tortoises, evolution of long wooden stems to avoid being eaten o Blue footed booby – bird with nostrils underneath eyes, due to evolution - Aa lava (rocky), pohoehoe lava (ropey) - Pyroclastic layers of ash that falls on the land that blows out underwater, think of a ‘snow avalanche’ that happens under water - laminated sediments - Scoria – large pieces of burnt rock found around cinder cones - Trace fossil – track of organism, like footprints, turtle tracks to lay eggs Rocks and Minerals: Clues to Ancient Environments - Lithification – to turn a sediment into rock - Compaction – to squeeze water and air out of something Differences Between Rocks and Minerals - Rock – is a naturally formed consolidated material, composed of grains of one or more minerals - Mineral – is a naturally occurring, inorganic, crystalline solid that has a definite chemical composition. They are the building blocks of rocks. - Igneous rocks – to be born of fire, extrusive and intrusive - Crystallography – the study of different shapes of crystal and diagnostic diff mineral types Minerals 4,500 different types but - The most important group are the silicate minerals containing silicon and oxygen and accounts for 90% of all minerals. Few minerals dominate ex: o Quartz – (silica = SiO2= silicon plus oxygen) o Olivine (Mg, Fe SiO 4 – mantle o Feldspars (largest subgroup, shiny field crystals, common in shield rocks, composite soil, semi- precious stones) - crust o Pyroxenes (ex: augite) o Mica o Clay minerals such as kaolinite, montmorillonite – result of weather of other minerals, electrical properties, dictate whether soil is fertile or not EESA06 Lecture 7 Page 2 Silicate Structure and its effects on magma viscosity - According to how tetrahedron are assembled - Isolated silicate structure – found in basalts - Single Chains, Double Chains, Sheets and Framework - Controls the thickness of magma, rocks we find, shape of volcano etc. - Quartz is the key mineral that thickens magma (less in basalt) - Basic rocks lack silica, Acidic rocks have higher concentration of quartz (Ex: Granite, Andesite, more than 50% is quartz) How do we identify different minerals? - Color – most noticeable by least accurate - Streak – color when mineral group to a fine powder on streak plate o Ex: hematite – red/brown o Silicate minerals do not streak - Hardness (scratchability) o Moh’s hardness scale (1-10)  Talc (very soft) = 1  Diamond = 10 Sedimentary Environments - Locations where sediments accumulate - Highest mountains to deepest oceans - Different environments – produces diff type of sedimentary rock - Continental Shelf – shallow sea floor - Slope – going down into deep sea - Abyssal Plane – deep sea Grand Canyon – Metaphoric rock layer covered by sedimentary rock - Tells us about Fossils, plate movement EESA06 Lecture 7 Page 3 Rift Basin - Evaporites – rift fills up water then dries out repeatedly - Rock salt – common component of rift basins - Minerals – result from evaporation of salt water - Basalt –
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