Historical Geology Week 2 Notes
Historical Geology Week 2 Notes GEOL1005
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This 0 page Class Notes was uploaded by Kate Notetaker on Thursday January 21, 2016. The Class Notes belongs to GEOL1005 at George Washington University taught by Catherine A. Forster in Winter 2016. Since its upload, it has received 102 views. For similar materials see Historical Geology in Geology at George Washington University.
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Date Created: 01/21/16
January 21 2016 0 Three different kinds of rock 0 Igneous o Sedimentary Clastic Precipitated l oversaturated ions 0 Metamorphic Navajo Sandstone Jurassic 0 Nearly pure rounded quartz sand in enormous desert dune deposits 0 Cross beds Gives an indication of the wind pattern 0 Remains of giant dune elds Kayenta Formation o Siltstones and sandstones with occasional thin limestone beds 0 Lake deposits 0 Evidence of owing and standing water Ripples in top view indicating l running water Shows a river channel with 7 ripples in sideview J H i 39L Also beds with mud cracks o A lot of fossils including small dinosaurs turtles frogs pterosaurs and mammals Navajo Sandstone lies ABOVE the Kayenta Formation o Kayenta is older Geologists view 0 Reconstruction of the sequence of rocks 0 From the evidence of all the sedimentary rocks 0 The environment has dried out o Became a big arid dune eld 0 Wind or air isn t as dense as water so it can t move things as well 0 Bedload 39 r a Material that is moving by Smpende load rolling orjumping at the quot quot quot t 39uquot bottom of a channel whim Coarser 7 a hi I 39M v 11771quot o Suspended load y The material that is being P if g F quot 4 Em rel carried by the water itself m3 f f r l E f Finer material I J I o What to know 0 Timing of events 0 Order of events what happened rst what happened last etc 0 Rates at which things happened Relative dating 0 Determining the order in which events happen 0 Using the law of superposition Oldest is at the bottom newest is at the top Things that can happen to disturb the rocks 0 Deformation Folding and tilting of the rocks Have to deposit the rocks 2 a before you fold them quot ft a 33 o Igneous intrusions Igneous rock is pushed into the a 39 f FOCk already there a Di kes they c ut a c ross 3 if Sills go with the strata k t 39u397mr M f 0 Ex Can be like lava owing across the top 4 o Faults break in the rock that comes straight through it The two resulting parts can move different ways This can make the rock become displaced Law of CrossCutting relationships 0 lgneous intrusion or a fault is always younger than the rock it cuts across 0 You can t cut across rock unless the rock is already there Vertical strata 0 Have to look very closely to nd out which way is up and which way is down Absolute Dating radiometric dating 0 How to nd out the actual numeric age of rocks 0 Radioactive decay l isotopes lsotopes are atoms of the same element but differ in the number of neutrons Atomic mass number of protons and neutrons Atomic number number of protons o Tells us which element it is Elements differ in their number of isotopes o Unstable l stable isotope Parent daughter 0 Loss of protons or neutrons l radioactive decay Change between parent to daughter happens over time Decay begins as soon as the crystal forms 0 You cannot date sedimentary rocks this way Can date minerals O O Radioactive decay happens at a predictable and measurable rate Rate decay constant 7 When half of the parent has converted to daughter that is the rst half life Half life 0693 I Minerals We can date it by the ratio of the amount of parent and the amount of daughter and its decay constant 7 Equation 0 T1Mln 1 Error is only 1 Carbon14 dating Half life of carbon is 5730 years Only used to date organic materials that are less than 80000 years old Used a lot in archaeology K40 I Ar40 Half life is 125 billion years U235 I Pb207 Half life is 704 million years 0 New techniques for dating precipitated rocks 0 The crystal in limestones often contains Uranium Can date them because they contain enough uranium o A lot of sedimentary rocks have igneous intrusions in them Helpful because you can date those If you know the order of things you can gure out around how old it is
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