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ERS120H Discussion Board Q & A

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Department
Earth Science
Course
ERS120H5
Professor
Lindsay Schoenbohm
Semester
Winter

Description
Apr 10 2011Discussion Board QAHalf LifesWe start off with 100 parent atoms in a rock and no daughter atoms at all Over time the parent all gradually turns into daughter through radiogenic decay A halflife is a specific amount of time For every half life that passes 12 of the parent will decay into the daughter product Lets say we have a pair or atoms that decay from parent to daughter with a halflife of 1 million years After 1 million years pass we would have 50 parent and 50 daughter After another million years pass we would have 25 parent and 75 daughter and so onSo the way to figure out the age of a mineral or a rock is by measuring the ratio of parent to daughter AND by knowing the halflife So if you measure a ratio of 25 parent and 75 daughter you know that 2 halflives have passed and if the halflife is 1 million years then you know that 2 million years 2 halflives x 1 millions years have passedOne thing you need to think about is how long is the half life versus how old is the rock After 10 or so halflives have passed only 1210 percent of the parent remains which is a very small amount At some point it just becomes too small for even the best instruments to measure C14 dating for example has a halflife of around 6000 years so it cant be used to date rocks in which more than about 60000 years or the half life x 10 So if you thought you had the oldest rock in the world on your hands you would not date it with C14 but if you had an archeological find you might To date the oldest rock in the world you might use U238 to Pb206 which has a half life of 45 billion yearsWhy is radiometric dating good for igneous rocks Is it because it allows scientists to figure out when exactly the rock solidified Yes thats more or less right In igneous rocks new crystals form as the rock cools so if you date a mineral within an igneous rock you know it formed at the same time as the rock itselfSedimentary rocks are often composed of eroded bits of other rocks So you can image a 1 billion year old granite igneous rock being eroded transported deposited and lithified into a sandstone a million years ago If you dated minerals in the sandstone you would get an age of 1 billion years when the sandstone only formed 1 millions years ago Such information can actually be quite usefulbut not for figuring out the age of the sedimentary rockThe same thing happens with metamorphic rocks You could metamorphose a much older igneous rock Sometimes the temperatures of metamorphism are hot enough to reset or partially reset the age of the rock but sometimes notFor dating a rock using half lives which are normally the parent ion and the daughter ion uranium or leadOr is it different depending on the rock type and would it be specified which were the parent ion and which were the daughter ionOn that note are we allowed to bring a calculator to do the calculations of halflivesUranium is radioactive so its unstable and its always the parent ion Most isotopesof lead are stable and in our case it is always the daughter There are a bunch ofdifferent possible parentdaughter pairs out there U238Pb206 U235Pb207 KPage 1 of 15Apr 10 2011Discussion Board QA40Ar40 and of course C14 which technically is C14N14 In each of these examples Ive listed the unstable parent first and the stable daughter second In each case as the rock gets older the amount of parent decreases and the daughter increasesCould anyone answer What is the protolith ofphyllite and schist Is the protolith of Gneiss and slate sanstoneThe protolith of slate phyllite schist and gneiss is mudstoneshaleThe protolith of quartzite is sandstoneAnd the protolith of marble is limestoneWhen were using fossils to determine relative age what are we doing Do we use fossils to determine the age of rock layers or do we use the rock layers to find out the age of the fossils I dont know what the question is asking Plus what does it mean by relationships between life history and the timescale Is it just when life began and so on Finally how do we use igneous rocks to find out about sedimetenary rocks I thought it wasnt possible to use dating on sedimentary rocks ThanksIn relation to the fossils I think if you have bedding at a certain location that formed at a certain time plus ANOTHER location of bedding with the same forming time if you find fossils that are the same age at both locations it signifies the two bedding locations correlate As for the other questions beats me Oscars answer is spot on for fossils we use them to figure out which rocks are of the same age in different locationsThe point of my question about life history and the geologic timescale is that virtually every boundary on the timescale for example between the Precambrian and the Cambrian corresponds to a major event in life history such as the Cambrian explosion Its also common for boundaries to occur at the time of major extinctions This is because geologists devised the geologic timescale based mostly on the fossil record long before radiometric dating had been invented They used these lifeevents as markers to correlate rocks around the worldIt isnt possible to date sedimentary rocks directly using radiometric dating But you can use crosscutting or other relationships to BRACKET the age of sedimentary rocks For example if you find a lava flow in the rocks below a sedimentary unit and a dike that cuts across it you can figure out the upper and lower limits on the possible age of the sandstone by dating theseSo I know that ground water is water that resides under the surface of the Earth mostly in pores or cracks of rocks or sedimentHowever I am having some trouble understanding how groundwater is different from cave systems I was unable to find it in the text and lecturesFrom my understanding the difference between cave water and the ground water is 1 one is above ground and 2 the cave water system seems more like a river or Page 2 of 15
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