1. Chapter 17-1: The Fossil Record Essential Questions: What is the fossil record? What is the fossil record? What is the difference between relative dating & radioactive dating? What is the difference between relative dating & radioactive dating? What are the main divisions of geologic time scale? What are the main divisions of geologic time scale?

2. Half of a Half of a Half... Some forms of chemical elements are unstable—that is, they break down into other substances. Like the decay of leftovers in your refrigerator, this breakdown takes place over time. Unlike those leftovers, however, the breakdown of unstable forms of an element progresses in a very orderly way—by decaying into halves. Some forms of chemical elements are unstable—that is, they break down into other substances. Like the decay of leftovers in your refrigerator, this breakdown takes place over time. Unlike those leftovers, however, the breakdown of unstable forms of an element progresses in a very orderly way—by decaying into halves.

3. 1.Using your ruler, draw a line 24 cm in length on a sheet of paper. Make a mark at the halfway point (12 cm). 1.Using your ruler, draw a line 24 cm in length on a sheet of paper. Make a mark at the halfway point (12 cm). 2.Then, divide this 12-cm segment in half, making a mark at 6 cm. Continue in this way, dividing each progressively smaller segment in half (ignoring all of the other segments) until it becomes too small to accurately measure. 2.Then, divide this 12-cm segment in half, making a mark at 6 cm. Continue in this way, dividing each progressively smaller segment in half (ignoring all of the other segments) until it becomes too small to accurately measure. 3.Now count each progressively smaller half- segment. How many segments did you count? 3.Now count each progressively smaller half- segment. How many segments did you count?

4. Fossils & ancient life Fossils & ancient life Paleontologist: study fossils Paleontologist: study fossils Fossil record Fossil record Provides evidence of history of life on Earth Provides evidence of history of life on Earth Shows how different organisms have changed over time Shows how different organisms have changed over time 99% of species that have lived on Earth now extinct 99% of species that have lived on Earth now extinct

5. How fossils form How fossils form

6. Interpreting fossil evidence Interpreting fossil evidence Relative dating Relative dating Position of sediment beds relative to each other gives relative ages Position of sediment beds relative to each other gives relative ages

7. Radioactive dating Radioactive dating Isotope: form of element with different numbers of neutrons Isotope: form of element with different numbers of neutrons Ex: C 12 & C 14 : 2 extra neutrons in C 14 Ex: C 12 & C 14 : 2 extra neutrons in C 14 While a sample is losing neutrons, it’s radioactive While a sample is losing neutrons, it’s radioactive Amount of time for ½ of isotopes in sample to revert to stable form is its half-life Amount of time for ½ of isotopes in sample to revert to stable form is its half-life For C 14 : λ = 5440 years For C 14 : λ = 5440 years If we look at % of C 14 & C 12 If we look at % of C 14 & C 12 Ex: 25% C 14 & 75% C 12 Ex: 25% C 14 & 75% C 12 2λ = 10,880 years old 2λ = 10,880 years old Other isotopes decay more slowly Other isotopes decay more slowly Potassium: λ = 1.3 billion years Potassium: λ = 1.3 billion years

8. Geologic time scale Geologic time scale Eras and periods defined by major changes in the fossil record Eras and periods defined by major changes in the fossil record Divisions are not Divisions are not even amounts Ex: Precambrian Ex: Precambrian era = about 90% of Earth’s history