and just googling about how things are dated.... so this has been interesting.
For organic matter (or used-to-be organic matter f.e. ancient papyrus) there is used Carbon C-14 radio-dating method.
C-14 isotope is created in upper level of atmosphere where atoms are bombarded by cosmic rays from the Sun, and for example following reaction occurs:
Neutron is captured and proton is ejected to conserve energy, momentum, charge and Baryon number.
Carbon C-14 isotope is unstable and decays via following path exclusively:
Its half-life time is 5730 years.
That means if you would have 1 kg of C-14 at the beginning,
after 5730 years, you would have 500 grams,
after yet another 5730 years (11500 years total), you would have 250 grams,
and so on, so on.
This allows dating organic matter to something like 50k years.
Calcium Carbonate rocks have organic source, but were made a long long time before 50k years ago.
Carbon C-14 produced in atmosphere is absorbed by living plants in Carbon Dioxide (CO2).
Plants are eaten by animals, and either of them are eaten by human.
Dead body, dead plant, does not absorb Carbon C-14 anymore, so its amount is decreased when it's decaying.
So scientist can take sample of ancient dead body, and measure amount of Carbon C-14 isotope remaining in it.
To measure date of non-organic matter, rocks, there could be used radioactive isotope of Rubidium Rb-87 which is decaying exclusively to Strontium Sr-87
It has astonishing high half-life 49.23 billions years. Therefor only a tiny fraction of it decayed after rocks were made (melted the last time).
Edited by Sensei, 15 May 2017 - 08:51 AM.