Thanks for the question! Sedimentary rocks can be difficult to age-date if they are composed solely of pre-existing grains, but often there are materials in a sedimentary rock that can be age-dated directly. Fossils of calcium carbonate or calcium phosphate animal shells or planktonic tests can be dated with radiometric techniques, because the minerals will contain useful trace elements for dating (U/Pb, Rb/Sr, Sm/Nd etc.) and because the mineral content of the animal's shell formed essentially at the same time as the sediment was buried. Also, during sedimentary diagenesis new minerals can form as the sediment is compacted and heated. New calcium carbonate can precipitate between grains, mobilized from elsewhere in the rock, cementing the grains together. Trace elements in the cement can be used to date the cement, which provides a minimum age for the rock; diagenesis happens after deposition and burial, so the rock would need to be at least as old as its diagenetic formation time. Magnetic reversals can also be used, if the sedimentary rock contains diagenetic magnetite. Those are a few of the ways geologists age-date sedimentary rocks. There are some more specialized methods for particular rock types, too. I hope that helps!

I'm watching too many videos and they do not define exactly what does it mean that a rock is "created"

3:40 seemingly the zircon crystal does not pick up Pb when it's "forming"

@@SK-cb6wz Okay due to de electronegativity of the 'non-metallic' part of the salt of zircon, it can explain why it prefers uranium rather than lead

Thanks for watching, and thanks for your question. The half-lives of uranium & thorium isotopes were worked out early in the 20th century, and using tech even of that time it's possible to do accurately, although it does require a working laboratory with equipment set up to do the job. Horribly simplified, one sets up a radiation detector of appropriate type & sensitivity and measures the number of decays from a sample of known isotopic composition and prepared shape over time. Accounting for instrumental precision, sample geometry and many other annoying variables to control, one accumulates numbers and statistics over weeks & months, allowing determination of a reliable analysis of the decay rate. The reproducible results let us establish decay rates with great accuracy.

@@EarthParts would be great to see a video how we conclude a mineral that can have 4.5 years half life span, to be specific how did we found the baby zircon to take as reference so as to calculate its present age.

Helium (He) is a natural product of U decay. It tends to accumulate in rocks hosting kerogen, which accumulates U during burial and diagenesis because U complexes strongly with organic matter. Over time, helium tends to accumulate in kerogen-rich strata. A major commercial source of He is natural gas, from which the noble gas can be separated. Nothing to get around. We depend on U decay in the crust as a major source of helium for industrial use. Thanks for the question and thanks for watching!

Earth.Parts OK that explains it. You felt there was no need to mention helium because it's no longer in the rocks. It's in the nature gas pockets. And it's not relevant in rock dating?

Not really. Helium is a small atom and it doesn't react or bind with anything under most conditions, so it basically just wanders as a gas out of the rock, depending on the situation. It's not useful for rock dating as any kind of daughter product, or anything like that.

Err isn't helium is important in radiometric dating because its nucleus was alpha radiation? Hence the decay of uranium emits alpha particles which are ionising and form helium when they are able to grab 2 electrons?

Works for me. Thanks for watching!