Index Fossils and Geologic Time Dating Fossils 1. Scientists use two methods to date fossils: absolute dating & relative dating 2. Scientists use radiometric dating to determine the absolute age of a rock or fossil. 3. Relative dating uses layers of rock to determine a relative age. Ex: Deeper rocks are older and therefore have older fossils Using Fossils to Date Rocks 4. Index fossils are fossils of organisms that scientists use to determine how old rock layers are. To be an index fossil the organism must: A. Be common in rocks from most of the world. B. Have lived for only a geologically short period of time(a few million years) C. Be easy to identify Ex: trilobites & ammonites Using Fossils to Date Rocks 5. Index fossils are useful to scientists because they can be used to help tell the relative ages of the rock layers in which they occur. Index fossil North America South America Africa Europe Geologic Time: Why Do We Use It? Geologic Time Scale 6. The geologic time scale is a record of the life forms and geologic events in Earth’s history. 7. Scientists use the geologic time scale instead of years because the span of Earth’s history is so great. Geologic Tim 8. The Earth is about 4.6 Billion Years old and is older than the oldest fossils on earth.