Question

How are radioactive isotopes used to date rocks?

Answer 1

it's complicated, but basically, when a rock forms, we can call this time zero. as time passes, some atoms decay (radioactive isotopes). by looking at the amount of radioactive isotopes in a mineral (which is part of a rock), we can determine how long it has been since time zero (when the rock formed). once an atom changes into another atom, it is permanently changed and can't change back. so we can essentially "count" how much of the new atom we have in the mineral/rock, and determine how long the rock must have existed.

Answer 2

Uranium isotopes decay extremely slowly (long half-life) to stable isotopes of lead.

Uranium-235 has a half-life of 700 million years.

Uranium-238 has a half-life of 4.4 billion years.

The relative amounts of Uranium and lead in an igneous rock can therefore be used to estimate the age of the rock.

Table showing the relative amounts of Uranium-238 and stable lead over time.



Time = Zero

After 1 Half-life

After 2 Half-lives

After 3 Half-lives

Amount of Uranium-238

100 %

50 %

25 %

12.5 %

Amount of Stable Lead

0 %

50 %

75 %

87.5 %

Ratio of Uranium : Lead

1 : 0

1 : 1

1 : 3

1 : 7

If a specimen of rock is found to have a ratio of Uranium to lead of 1 : 3 it means that a time of two half-lives of Uranium-238 have elapsed since the rock's formation.

The age of the rock would therefore be 2 x 4.4 billion years = 8.8 billion years old.

The decay of the radioisotope Potassium-40 can be used to date rocks. Potassium-40 decays into Argon-40, which is a stable gas. To get an accurate result, however, no argon-40 gas must escape from the rock in the years following its formation.