by 
Drublic
ANSWERS: 7
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If it's a sedimentary rock they look at the layers and there thickness and associate the type of minerals in each layer with a geographical time period of that area
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RadioCarbon Dating
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the use of absolute dating... absolute datingi works by measuring the amount of a radioactive isotope in a mineral grain of part of that rock unit. The amount of daughter isotope vs parent isotope gives an absolute age as different radioactive elements decay at a STANDARD rate, meaning they all decay at that rate. geologists often measure age by relative dating however which is just age in comparison to the surrounding rocks
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Radio dating is known to be crap even by the universities that wrote the books on it. There is no way of knowing age as it's all based on a presuppositon that the earth is billions of years old. RC dating can only be reduced to 6000 or so years until it is no longer measurable -- so beyond that is just guess work. Yes, the first 6K happens at lineer rate, but there is no way of knowing if that rate slows, increases or stops beyond 6K years -- especially since it's been proven that temperature, atmospheric pressure, sunlight, radiation, etc. etc. etc. can and does change that rate -- and all those circumstances HAVE changed and continue to change since the beginning. Then, the same illogical dating is applied to geologic layers, then to fossils ... Notice that everytime a new fossil is found, they say its older than the last one. That the only way these people can get funding (and personal income BTW) for projects. Its all a lie to further their anti-theisic view of the universe; which is a whole other topic.
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Mostly radio-argon dating. Potassium-40, a metal isotope, decays at a very well known rate into argon-40, a gas. When the rock was molten, the gas would escape, so we know that when is solidified there was no argon. The can therefore determine time since the rock solidified by measuring the ratio of Potassium-40 to Argon-40 today.
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Radiometric Dating. not carbon dating tho, since rocks are WELL outside it's range. stronger isotopes are used for longer periods. . U/U dating, for example is relevant up to a few million years, while Uranium/Lead dating gives accurate results all the way to the formation of the earth.
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Only volcanic rocks can be dated absolutely. When the lava cools down, all 'clocks' start at zero, because there's almost no gas in lava. Then the radioactive elements decay at a constant rate and the gas they release gets trapped in small bubbles in the rock. For example (there are many more): Thorium-232 decays to Lead-208 at a half-life rate of 14 billion years. Potassium-40 decays to Argon-40 at a half-life rate of 1.26 billion years. Or Iodine-129 decays to Xenon-129 at a half-life rate of 17 million years. By comparing the ratio of these isotopes, one can calculate how much time has passed since the rocks were formed. If one wants to date sedimentary rocks and fossils in them, one uses relative dating. Rock layers are analyzed and places where they are 'sandwiched' between layers of volcanic rocks provide a timespan for them. By looking at various geological sites, the missing pieces of the puzzle can be found and the puzzle gets completed.
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