Measuring time in millions and billions of years.
Scientists can be certain of dates when results from different dating methods agree. Correlated evidence indicates that geologic time spans several billion years.
Click on photo for larger image. |
Atomic time scale. Time determined in years by measuring radioactive decay. Some atoms of the same element weigh more than others. Heavy atoms, known as isotopes, become unstable and lose nuclear particles. Isotopes, called parents, decay at a constant rate and form more stable daughter elements. The time it takes for half of an isotope's nuclei to decay is known as its half-life. By knowing an isotope's half-life or rate of decay, scientists can date an igneous (previously molten) rock by comparing amounts of parent and daughter elements within a sample. The exhibit also discusses dating by measuring radioactive damage and by counting annual lake deposits. Another section shows how scientists compare index fossils in fossil layers to measure relative time. |
Items on exhibit |
|
|---|---|
| Specimen | Comments |
| mica | |
| smoky quartz | Colorado |
| amethyst | Patterson, New Jersey |
| pink quartz | |
| uranophane | moderately radioactive, Mitchell County, North Carolina |
| granite | very mildly radioactive, Big Falls, Wisconsin |
| oil shale samples | with varves (layers added annually) |
| ammonite | Copper River, Alaska |
| graptolite fragments | genus and species unknown |
| free-floating foraminefera | nine genera shown |
