1. Ch.3, Sec.3 – Absolute Dating: A Measure of Time
- with the discovery of the natural decay of uranium in 1896, French physicist Henri Becquerel provided a means of using radioactive elements as clocks to measure geologic time
- the process of establishing the age of an object by determining the number of years it has existed is called absolute dating
Henri Becquerel

2. Ch.3, Sec.3 – Absolute Dating: A Measure of Time
Radioactive Decay
- to determine the age of rocks & fossils, scientists analyze isotopes of radioactive elements
- atoms of the same element that have the same number of protons but have a different number of neutrons are called isotopes
- radioactive isotopes tend to break down into stable isotopes of the same or other elements in a process called radioactive decay

3. Ch.3, Sec.3 – Absolute Dating: A Measure of Time
- the radioactive decay process of parent isotopes often times creates daughter isotopes, which are stable elements
- the length of time it takes for elements to decay is always constant, so scientists can compare the amount of parent material to daughter material in rocks and fossils to date them
- the more daughter material there is, the older the rock or fossil will be!

4. Ch.3, Sec.3 – Absolute Dating: A Measure of Time
Radiometric Dating
- determining the age of an object by estimating the relative percentages of a radioactive (parent) isotope & a stable (daughter) isotope is called radiometric dating
- scientists measure this age by calculating the half-life, which is the time needed for half of a sample of a radioactive substance to undergo radioactive decay

5. Ch.3, Sec.3 – Absolute Dating: A Measure of Time
Types of Radiometric Dating
1. Potassium-Argon Method: potassium-40 has a half-life of 1.3 billion years & decays into calcium or argon (used to date rocks & fossils older than 100,000 years)
2. Uranium-Lead Method: uranium-238 has a half-life of 4.5 billion years & decays into lead (used to date rocks & fossils older than 10 million years)
3. Rubidium-Strontium Method: rubidium-87 has a half-life of 49 billion years & decays into strontium-87 (used to date rocks & fossils older than 10 million years)

6. Ch.3, Sec.3 – Absolute Dating: A Measure of Time
4. Carbon-14 Method: carbon-14 has a half-life of 5,730 years & decays into nitrogen-14 (used to date rocks & fossils younger than 50,000 years old)