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Lesson 1: Geologic Time Evidence That Earth Has Changed • Geologists use the principle of superposition, which states that in rock layers that have not been folded or deformed, the oldest rock layers are on the bottom. • The principle of superposition gives the relative ages of rock layers, which tells you whether the layers are younger or older than other rock layers Lesson 1: Geologic Time Evidence That Earth Has Changed • The preserved remains or evidence of past living organisms are called fossils. • Many fossils represent species that no longer live on Earth. Lesson 1: Geologic Time Evidence That Earth Has Changed • During radioactive decay, one element changes into another element. • The radioactive parent element transforms to the stable daughter element. • The half-life is the time it takes for half of the parent element to transform, or decay, to the daughter element. • By comparing the amount of parent element to the amount of daughter element in a sample, scientists can calculate the age of the sample. Lesson 1: Geologic Time Evidence That Earth Has Changed • The geologic time scale is a visual record of Earth’s history, with the individual units based on changes in the rocks and fossils. Lesson 1: Geologic Time The Geologic Time Scale • Geologists divide Earth’s history into eons, eras, periods, and epochs • According to the geologic time scale, Earth has changed over billions of years Lesson 1: Geologic Time The Geologic Time Scale • The beginning of the Cambrian period is marked by an abrupt appearance of complex life-forms. • Earth is 4.6 billion years old. Humans have experienced only a small part of Earth’s history. Lesson 1: Geologic Time Review 1) The preserved remains of organisms are called __________________. 2) Define geologic time scale in your own words. 3) Use the terms radioactive decay and half-life in a sentence. Lesson 1: Geologic Time Review 4) What is the longest unit of time on the geographic time scale? a) Eon b) Epoch c) Era d) Period 5) How were fossils used in the development of the geologic time scale? 6) Compare the ages of rocks determined by the principle of superposition and ages determined by radioactive decay. Lesson 1: Geologic Time Review 7) Suggest how the geologic time scale might be different if there were no radioactive elements. 8) In 1978, the start of the Paleozoic era was placed at 570 million years ago. Today, it is placed at 542 million years. Discuss how this change is possible. 9) If you begin with 140g of an element, how much remains after 3 half-lives? Lesson 1: Geologic Time Exit Ticket Quiz 1.Which items contain evidence that Earth is very old? A. rock layers B. living organisms C. daughter elements 2.The geologic time scale was developed using A. fossils and superposition. B. volcano and earthquake data. C. parent and daughter elements. 3. If Earth’s history lasted only one year, humans would have appeared A. in July. B. in February. C. in December. Lesson 1: Geologic Time Exit Ticket Quiz 4. process in which one element changes into another 5. states that older rock layers are generally under younger rock layers 6. the longest units of geologic time 7. time it takes for half of a radioactive element to break down into another element 8. preserved remains or evidence of ancient life-forms 9. stable substance formed from a radioactive one 10. a scale of Earth’s natural history A. Eons B. Daughter Elements C. Fossils D. Half-Life E. Geologic Time Scale F. Radioactiv e Decay G. Superposition