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How Old is that Rock?

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Uranium-Lead Dating using Zircon Crystals.
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Text Comments (31)
codenamerishi (23 days ago)
Great video! Thanks
naitham manku (1 month ago)
Sir thank you ,sir I whant some impermation
Andrew Black (3 months ago)
What I'm curious to know is how we know how fast uranium isotopes turn into lead isotopes.
Mark Anthony (6 months ago)
Thank you! This is the only video I've found that really shows the gritty "how it works in a lab". All the other videos were really dumbed down just explaining half life but not showing how the measurements were actually taken.
GNS Science (6 months ago)
Glad you like it!
Vikram Ramanathan (10 months ago)
Nice explanation!
Sandy Combs (10 months ago)
The curve indicates a decrease in the rate of decay, not constant rate. Why does the rate of decay slow down by 50% for the second have life and continue to slow down by the same amount for every half-life there after? What is affecting the rate of decay over time?
Sandy Combs (8 months ago)
Yes the decay rate does decrease with every "half-life" your explanations "because you have fewer uranium atoms in the crystal " is unsupported hypothesizing......maybe or maybe not, all that is known is that the measured rate of decay does decrease with each half-life and you said it's constant and it is not.
E.L. Wagner (8 months ago)
The absolute activity rate of decays per second will go down with time, because you have fewer uranium atoms in the crystal once some have decayed. Every time you draw a graph of something you halve, then halve again and again and again, you will get a curve. However, the decay constant does not change. Think of the decay of uranium as something akin to rolling dice or tossing a coin. You can't predict when an individual uranium atom will decay, any more than you can predict what an individual roll of dice will get you, but you can predict what the ratio of rolls will be over time.
Sandy Combs (9 months ago)
Indeed, the graph indicate a decrease in the rate of decay over time, a constant rate would be a straight line. Why do some isotope, of let's say C14, take 5730 yrs to decay and others take 23,000 years to decay? Why the variability within the same sample?
Simon Thomas (9 months ago)
Sandy Combs That's how half life's work. Plot a graph and see for yourself 😉
Paul Mackenzie (10 months ago)
How do they know how long a half life is.
Original Judah (10 months ago)
Paul Mackenzie Hello, my name is Maliek and I study chemistry at a University in Florida with hopes of becoming a nuclear chemist and I think I might be able to help you out. Half life by definition is the time it takes for a radioactive isotope of any element to be reduced in it's abundance from the original sample by half. Now because of the principles laid out in physics regarding the attraction of subatomic particles and possible stable and unstable configurations of these particles within an atom we know that the rate of decay for any particular atom/isotope (which are really just different configurations of the particles or in some cases a change in the number of the particles) is constant by virtue of the fact the particles that make them up have constant and measurable qualities. Because radioactive decay occurs at a constant rate we can observe any radioactive element at any given time and trace back the time of it's initial state of abundance provided that it's known. Pretend I am driving from a starting point let's say Miami to New York at a constant speed the entire time 60 miles an hour, Miami in this case would represent the radioactive sample in it's initial state with New York representing it in it's final state. Since we are driving at a constant speed or velocity V=d/t velocity is known and distance from the starting position is known and velocity remains constant we can mathematically determine the time that has gone by, same basic principal applies when we use half life.
M-ZAP (3 years ago)
How do we know whether the Lead present in the rocks are a result of the Uranium decay or if it's already a part of the rock due to Lead naturally occurring in nature?
Charmaine McGregor (1 year ago)
Matheas Zircon strongly rejects Pb during it's crystallisation. Therefore within it's crystal structure the Pb is produced as a result of radioactive decay.
krickrack (3 years ago)
+Matheas I think when zircon crystals are made (process called crystallization) some elements can be incorporated (elements that substitute for Zirconium). Uranium is one of these elements and Lead is not. So when zircon crystals are made, there is no Lead in them. So the Lead we find in zircon crystals are the result of the Uranium decay.
desertcolt07 (3 years ago)
how can we know if there is no existing daughter product in the zircon?
krickrack (3 years ago)
+desertcolt07 I think Lead is not an element that is incorporated in the process of crystallization of zircon crystals. So there is no Lead when zircon forms.
Osama84ca (4 years ago)
great video, nobody could answer this simple question before!
Jenny Tokumei (3 years ago)
This was answered by Clair Patterson many decades ago. It's just public schools tend to not teach about it...
Joseph Galan (4 years ago)
Just a quick question: How did scientists determine the half-life of Uranium @4:35 and other elements for that matter? Thanks!
Jenny Tokumei (1 year ago)
+Charles Carrigan Exactly, and as highly radioactive as uranium ore is, the isotope of uranium in the ore has a half life of 4.5 billion years, which nicely illustrates your point.
Jenny Tokumei (3 years ago)
Uranium decays as a very predictable rate. You only need to measure the number of uranium atoms in a sample, and look at the rate of decay over time to determine the decay rate, then extrapolate the decay rate to figure out how long it'll take for half of the uranium to decay.
Elese Eagle (4 years ago)
This is extremely helpful! Thanks so much!
Dilithium Crystals (4 years ago)
err so how old was that rock. 
theblue92 (5 years ago)
Ah, the relevant wikipedia article seemed to answer the question (Uranium-lead_dating). Feel free to elaborate and/or fact-check, though. :)
theblue92 (5 years ago)
Would one be correct in assuming that the formation of zircon crystals in a laboratory setting results in lead-free crystals? Presumably due to the composition of the molecular crystal latice rejecting the lead in some manner? As the formation of such heavy elements apparently only come from supernovae one might wonder if any dating method would ever effectively measure the time between atomic formation (last supernova) and the formation of the Earth/Solar System.
libertynindependence (5 years ago)
Is there a study that shows this?
libertynindependence (5 years ago)
What about dating some of the fresh magma after it has cooled, how old does that usually date?
Abdul Salim (5 years ago)
Awesome video
wordreet (5 years ago)
DOH! Will you make another video that tells us how old that particular rock on the beach was? :¬)

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