SECTION 10 3 Radioactive Decay Radioactive Dating RADIOACTIVE

SECTION 10: 3 Radioactive Decay

- Radioactive Dating RADIOACTIVE DECAY During radioactive decay, the atoms of one element break down to form atoms of another element.

- Radioactive Dating RADIOACTIVE DECAY The half-life of a radioactive element is the amount of time it takes for half of the radioactive atoms to decay.

- Radioactive Dating DETERMINING ABSOLUTE AGES Geologists use radioactive dating to determine the absolute ages of rocks.

- Radioactive Dating PERCENTAGES What percentage of a radioactive element will be left after three half-lives? First multiply 1/2 three times to determine what fraction of the element will remain. You can convert this fraction to a percentage by setting up a proportion: To find the value of d, begin by cross-multiplying, as for any proportion: 1 X 100 = 8 X d d= d = 12. 5%

- Radioactive Dating PERCENTAGES Practice Problem What percent of a radioactive element will remain after five half-lives? 3. 125%

- Radioactive Dating DETERMINING ABSOLUTE AGES The age of a sedimentary rock layer can be determined relative to the absolute age of an igneous intrusion or extrusion near the sedimentary rock.

- Radioactive Dating IDENTIFYING MAIN IDEAS As you read the section “Determining Absolute Ages, ” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea. Main Idea Using radioactive dating, scientists can determine… Detail the absolute ages of the most ancient rocks using potassium-40. the absolute ages of fossils up to about 50, 000 years ago using carbon 14. the ages of sedimentary rocks by dating the igneous intrusions and extrusions near the sedimentary rock.

SECTION 10: 4: DETERMINING ABSOLUTE AGES

- The Geologic Time Scale THE GEOLOGIC TIME SCALE Because the time span of Earth’s past is so great, geologists use the geologic time scale to show Earth’s history.

- The Geologic Time Scale SEQUENCING As you read, make a flowchart like the one below that shows the eras and periods of geologic time. Write the name of each era and period in the flowchart in the order in which it Geologic occurs. Time Scale Precambrian Time Paleozoic Era: Permian Paleozoic Era: Cambrian Period Mesozoic Era: Triassic Paleozoic Era: Ordovician Period Mesozoic Era: Jurassic Paleozoic Era: Silurian Mesozoic Era: Cretaceous Paleozoic Era: Devonian Cenozoic Era: Tertiary Paleozoic Era: Carboniferous Cenozoic Era: Quaternary