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### Course: Modern Physics (Essentials) - Class 12th>Unit 4

Lesson 4: How do we determine the age of fossils?

# Potassium-argon (K-Ar) dating

How K-Ar dating can be used to date very old volcanic rock and the things that might be buried in between. Created by Sal Khan.

## Want to join the conversation?

• much like my question from the last video, how do we know what the rate of decay from k to ar is? what facts, experiments or processes led us to our current understanding of the rate?
• The rate of decay can be accurately found with particle detectors, by measuring decay events as a function of time. You could do this at home with a radioactive source (small one in smoke detectors), a Photo Multiplier Tube, and a simple circuit. The circuit/PMT would measure decay in events/hour. Once the decay events/hour has dropped by 50%, one half life has occurred. You don't have to wait millions of years though, you can simply use ga-jillions of atoms over a shorter duration.
• Is this the reason why there is a large amount of Argon in the atmosphere?
• Yes, exactly! Just like Helium, the Argon in our atmosphere (about 1%) is mostly produced by radioactive decay.
• If someone split a sample of the rock in half, would the Ar-40 be released, resetting the decay, and thus rendering the sample unusable?
• The Ar is bound in the minerals. Over geologic time Ar can diffuse out of minerals (especially at high temperatures), but at surface temperatures no Ar is released. In the lab samples are heated from 300°C to the melting point to release the trapped Ar.
• How do we know how much of a given isotope is inside something, whether it be finding carbon 14 or potassium/argon? How do we differentiate between isotopes of an element experimentally? What do we actually do? What experiment can we perform? Thanks for answering!
• We can measure isotope concentrations (or isotope ratios) in a mass spectrometer. K concentrations used to be measured with a flame spectrophotometer, but modern mass spectrometers have become so precise that this technique is not used anymore.
• My science teacher says that it has a half life of 1.3 billion years. Which is more accurate?
• 40K decays to 40Ar with a half-life of 11.93 Ga and to 40Ca with a half-life of 1.397 Ga. So for the two branches we have a combined half-life of 1.25 Ga.
• why do elements even decay ?
• If the nucleus is unstable, either from having too many protons compared to neutrons, neutrons compared to protons, or they are simply 'packed' together unstably, then it will decay to get into a more stable arrangement.
• Why is potassium-40 less stable and more rare than potassium-41? Normally the more neutrons you have, the more unstable you are. (Ex: Carbon-14 less stable than carbon-12.) Why is this the exception?
• Kevin Henke PhD from no answers in genesis states the 1/2 life of 40K is 1,250 million years. You state 1.25 billion years. Then someone else says 1300 million. I am doing a report and need to know which is correct please.
• 1250 million is the same as 1.25 billion, and they both round to 1300 million. So all those numbers are basically the same, Catherine.