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But the most accurate forms of absolute age dating are radiometric methods. Sedimentary rocks in particular are notoriously radioactive-free zones.
This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: 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. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.
Similar ripples occur in tidal environments and correct interpretation requires that the local facies content be taken into account.
Before you begin this activity, read the book chapter listed below, which is available online through Library Reserves.
Unlike people, you can’t really guess the age of a rock from looking at it.
Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.
In this lab, you will apply many of these laws and principles to determine the relative sequence of geologic events that created a particular set of rock layers and intrusions.
After all, a dinosaur wouldn’t be caught dead next to a trilobite.
The narrower a range of time that an animal lived, the better it is as an index of a specific time.
No bones about it, fossils are important age markers.
Fossils are generally found in sedimentary rock not igneous rock.
Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years.