Rocks get better with age: HOW and WHY we date them

02.18.2021 - By PlanetGeo: The Geology Podcast

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In this episode we discuss the many ways in which we determine the ages of rocks.  This is a really common and important question that many people have.  Here, we cover some of the basics.  Basically, the methods used to date rocks range from simple intuition that anyone can do if you just spend some time looking at outcrops, to really advanced techniques involving laser and plasma instruments!  

We introduce this topic by describing the differences between relative dating, where we put events in order, and absolute dating where we assign numerical ages to specific events.  As you might tell from the discussion, the techniques geoscientists use vary widely!  We highlight a few of the key principles established by the OG geologists like Charles Lyell, such as the principle of original horizontality, or the law of cross cutting relations.  We also discuss the most memorable places that we have seen where these principles are clearly shown! 

We then move into discussing how radiometric dating works, and how radioactive decay is very important to our understanding of the age of events on Earth.  We highlight specifically the techniques that Jesse uses in the lab he is building at Penn State to date individual parts of tiny mineral grains that are the size of a human hair!  Amazingly, these tiny mineral grains can record events billions of years apart from one another!  The techniques we use to get actual ages out of these minerals include blasting them with a laser and injecting that material into a plasma so that we can measure the uranium and lead compositions of the mineral.  All fancy stuff that doesn't require a white lab coat!  

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