Absolute Dating

Absolute Dating Absolute Dating determines the actual age of a rock or fossil in years. The main method of absolute dating is RADIOMETRIC DATING.

How Radiometric Dating Works Radioactive elements are unstable isotopes of elements, with atoms that give off particles (radiation) from their nuclei. This is called Radioactive Decay Atoms give off particles to become new lighter, more stable elements. Decay will continue until the new element is stable and not radioactive.

Radioactive decay

Half lives

Rate of Decay Radioactive elements decay at CONSTANT RATES. As soon as a rock forms with radioactive elements in it, decay begins. We use this to find the age of a rock. The rate at which an element decays into a new element is measured as a HALF LIFE.

Half Life A HALF LIFE is the amount of TIME it takes for HALF of atoms of a radioactive element to decay into a new, stable element. Terms used to describe material: Parent Material: the original, radioactive element Daughter Material: the new, stable element

Example 100 % Parent Material Time= 0 The rock formed 50 % Parent, 50% Daughter 1 Half Life 25% Parent, 75% Daughter 2 Half Lives 12.5 % Parent, 87.5 % Daughter 3 Half Lives 6.25% Parent, 93.75 % Daughter 4 Half Lives

Graph of data: Decay Curve 100 % Parent 50 25 12.5 6.25 1 2 4 5 3 Half Lives

Using Half Life to find Age If you know the number of years there are in an elements half-life, you can figure out the age of a rock. The number of years in an elements’ half-life would be the number of YEARS it takes for HALF of the radioactive elements in the rock to turn into stable elements.

Example The number of years in a half-life for Carbon-14 is 5,730. So….. If one half-life has passed, the rock is 5,730. If two half-lives have passed, the rock is 11,460 years old.

Samples of Half-Lives

Questions If a rock contains 50% parent material, how many half-lives have passed? If a rock contains 12.5 % parent material, how many half-lives have passed? If a rock’s half-life is 25 years, and two half-lives have passed, how old is the rock? If a rock’s half-life is 50 years, and three half-lives have passed, how old is the rock?

Half Life, 1/2