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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 3 Half Lives 4 5

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 halflives have passed, how old is the rock?

Half Life, 1/2

- Slides: 15