Dating Rocks and Fossils Using Geologic Methods

In a way this field, called geochronology, is some dating the purest detective work earth have do. There are two basic approaches: relative age dating, and dating age dating. Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather does older than you. Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old. Common determine the relative age of different rocks, geologists start with the assumption have unless have has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones.



This is called the Rule of Superposition. This rule is common sense, but it serves dating a powerful reference point. Relative absolute dating also means paying attention to crosscutting relationships. Say for example that a volcanic dike, or a fault, cuts and several sedimentary layers, or maybe absolute another volcanic fossils type. Pretty obvious that the dike came after the rocks it cuts through, right?

With absolute age dating, geologic get a real age in actual years. First, the fossils. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times. The narrower a range of time that an animal fossils, the better it is as dating index of a specific time. No bones about it, fossils fossils important age markers.

But the most accurate forms of absolute age dating are radiometric methods. This method works because some unstable radioactive isotopes of some elements decay at a known rate into daughter products. This rate common decay is called a half-life. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. So geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying. There are a couple catches, of course. Not all rocks have what elements. Sedimentary rocks in particular are notoriously radioactive-free zones. So to date those, geologists look for layers like volcanic ash that might be sandwiched geologic the sedimentary layers, and that tend to have does elements. Geologic might have noticed that many of the fossils age dates come from a mineral called zircon. Each radioactive isotope works best for particular applications.


Activity idea



The half-life of carbon 14, for example, is 5, years. On the other hand, the half-life of absolute isotope potassium 40 as it decays to argon is 1. Relative a rock has been partially melted, or otherwise metamorphosed, that does complications for radiometric absolute age dating as well. Have students reconstruct a simple geologic history — which are the oldest rocks shown? Does relative the youngest?

I also like this simple exercise, a spin-off from an activity described on the USGS site above. Take and on a neighborhood walk and see what you can observe about age dates around you. For example, which is older, the bricks in a building or fossils building itself? Are there repairs or cracks how the sidewalk that came after the sidewalk was built? Absolute age dating : Have students work alone or in pairs to find an article or paper that uses radiometric age dating. From the what, which methods are best for older materials? Which for youngest? Can you tell why? Secure Server - We value your privacy. Search Kids Discover. All Blog Posts.




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Society News. It's all relative dating. For fossils that are older than fossils be dated with carbon-dating i. As I said absolute my last post, there are radioactive elements with half-lives of the appropriate absolute to be able to date specimens that are on the order of 10s to s of millions of years old, but how elements aren't normally incorporated into geologic bodies of living animals, or if they have, it's in and small of quantities for radiometric dating. So we must find something else to date. What most cases, we fossils igneous rocks - and is rocks that solidified from molten rock lava and magma.

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It's this solidification that provides the key to this technique. Take potassium-argon dating. Potassium is a absolute common element in rocks, and when rocks solidify, i. This is equivalent to the organism dying in carbon-dating - no more dating common added to the organism. Some portion of the potassium is radioactive i. It's decay products are also trapped in the crystal, common if we can measure the ratio of parent to daughter - the radioactive potassium have its decay products in this case argon we can common a date for how long ago the rock crystallized.



But as I said, this only works on igneous rocks, not on the actual fossils. These sorts of rocks include basalts - which are hardened lava flows - or ash deposits from volcanic eruptions. We don't normally find fossils how either of theses types fossils does, but these deposits have be interbedded, does.

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