1. Dating Fossils & Rocks Fossils are the petrified remains of once- living organisms. There are a few kinds: Fossils are the petrified remains of once- living organisms. There are a few kinds: Mineralization: the bone or tissue is replaced by minerals (there are several types). Mineralization: the bone or tissue is replaced by minerals (there are several types). Molds and Casts Molds and Casts Trace fossils: tracks and burrows. Trace fossils: tracks and burrows.

2. How Old? There are two types of dating…relative and absolute. There are two types of dating…relative and absolute. Relative Dating: Sediment builds up over time…if the layers are undisturbed, the older stuff is on the bottom (this is called Superposition). Relative Dating: Sediment builds up over time…if the layers are undisturbed, the older stuff is on the bottom (this is called Superposition). You can’t get an actual time in years this way, but you can say fossil A is older or younger than fossil B. You can’t get an actual time in years this way, but you can say fossil A is older or younger than fossil B.

3. Absolute Dating: Radioactive isotopes are used to figure out an actual age in years. Absolute Dating: Radioactive isotopes are used to figure out an actual age in years. Isotopes are elements that are radioactive and unstable. They decay at a measurable rate. When this happens, they breakdown into another element. Isotopes are elements that are radioactive and unstable. They decay at a measurable rate. When this happens, they breakdown into another element. By measuring the ratio of isotope to breakdown product, we calculate the age of the rock. By measuring the ratio of isotope to breakdown product, we calculate the age of the rock. The time it takes for half of an isotope sample to break down is called…half-life. The time it takes for half of an isotope sample to break down is called…half-life.

4. Some Isotopes Used for Absolute Dating Parent-------Daughter Parent-------Daughter Carbon-14>Nitrogen: half-life= 5,730 years Carbon-14>Nitrogen: half-life= 5,730 years Potassium-40>Argon-40: HL= 1.3 bill. Yrs. Potassium-40>Argon-40: HL= 1.3 bill. Yrs. Uranium-235>Lead-207: HL= 700 mill. Yrs Uranium-235>Lead-207: HL= 700 mill. Yrs That means for example that if you had 100 grams of U- 235, after 700 million years had passed you’d have 50 grams left (along with 50 grams of Lead-207) That means for example that if you had 100 grams of U- 235, after 700 million years had passed you’d have 50 grams left (along with 50 grams of Lead-207) That also means…you can’t use C-14 to calculate the age of dinosaurs fossils. Why? Cuz…it breaks down too quickly! You can use other isotopes though That also means…you can’t use C-14 to calculate the age of dinosaurs fossils. Why? Cuz…it breaks down too quickly! You can use other isotopes though

5. As the amount of Parent Decreases the amount of Daughter Increases!

6. Try This… Carbon-14 has a half-life of 5,730 years. Carbon-14 has a half-life of 5,730 years. 1. If you had a 10 gram sample of C-14, how many years would take for there to be 2.5 grams left? 2. If you had a 20 gram sample of C-14, how much would remain after 22,920 years?