How the radioactive decay of isotopes is used to determine the age of rocks?

Answer 1

The amount of the remaining radioactive isotopes it contains.

Answer 2

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Answer 3

We use carbon -14 isotope for dating. The half life of carbon-14 is 5570 years. BY estimating the ratio of Carbon 14 present at this moment against the carbon-12 we can compute the age of the relics. If 50% of carbon 14 remains then we can say the age will be 5570 years. If 25% remains then the age elapsed has to be 2*5570 ie 11140 years.

answer 2 Because of the relatively short half-life of C14, its use is mainly restricted to archaeological items, or recent organic remains.

Many other radioactive schemes are used for other geological studies.

Some of the potassium in your body is decaying into Argon, K40 - Ar40. [K40 has a half-life of 1.3 x109 years. ]

This transition is frequently used to determine the age of material (such as lava) that has solidified. Before it solidified, any Ar would have evaporated, so any Ar present today has most probably been made since the rock cooled. And the frozen structure of the rock would have held any 'new' Ar.

Similarly with various Uranium to Lead transitions. And even Uranium - Uranium transitions. There are many many of them.

Answer 4

This is Ar/Ar dating, (carbon dating is for living organisms). When Ar is expelled from the earth, it starts off as a specific radioactive isotope of argon. The argon will break down and form other Ar isotopes at a set rate known as a half-life. The concentration of the isotopes is used to determine the age of the rock

Answer 5

When a rock crystallises from a melt (igneous) certain radioactively unstable isotopes within the crystals develop at the time of formation. Starting when the melt solidifies these unstable parent isotopes begin to decay into their daughter isotopes. Radiometric/Isotopic dating uses the ratio of parent to daughter isotopes to determine when the rock formed. For example say you have a parent isotope A and it decays into its daughter isotope B and A takes 10000 years to reach its half life, if there is a 50:50 ratio of A:B then the rock is 10000 years old if there is a ratio of 75:25 then 5000 years have passed since its formation from melt and 25:75 then 15000 years have passed. The most common isotopes used are Rubidium:Strontium and Potassium:Argon but there are many others. In metamorphic rocks the parent daughter isotope method can only tell you the time of the metamorphosis not the formation of the original rock. Sedimentary rocks cannot be dated this way, the only dating of this kind that can be carried out on them is Carbon 14 dating and only gives dates as far as 50,000 years ago.

Answer 6

In rocks, radioisotopes such as Uranium decay with half-lives of millions or even perhaps billions of years, depending on the isotope. When the rock was first formed, there is a certain amount of Uranium, compared to a substance like Lead, which is a product when Uranium decays. As time passes, this Uranium decays into Lead and the amount of Uranium decreases while that of Lead increases. Using the ratios, the age of the rock can be determined easily. Carbon dating cannot be used on rocks because it is not accurate for objects older than 80 000 years old. Carbon dating is only suitable for finding the age of fossils.