| A | B |
|---|
| Geologic Time Scale | A record of Earth's history from its origin 4.6 billion years from the present |
| Eons | The largest of these time units |
| Precambrian | Three earliest eons make up 90% of geologic time |
| Eras | All eons are made up of eras the next longest unit of time |
| Periods | All eras are divided into periods |
| Epochs | All periods are divided into epochs |
| Precambrian | No life on land, fossil bacteria and algae |
| Paleozoic | Also know as the "Age of Invertebrates" |
| Mass Extinction | Many groups of organisms disappear from the rock record at about the same time |
| Permian | The largest mass extinction in the history of the earth |
| Mesozoic Era | Also known as the age of reptiles |
| Cenozoic Era | Also known as the age of mammals |
| Uniformitarianism | States that geologic processes occurring today have been occurring since the earth formed |
| Original Horizontality | The principle that sedimentary rocks are desposited in horizontal or nearly horizontal layers |
| Superposition | In an undisturbed rock sequence, the oldest rocks are on the bottom and the youngest on the top |
| Cross-Cutting Relationships | An intrusion is younger than the rock it cuts across |
| Inclusions | Fragments called inclusions in a rock layer must be older than the rock layer than the rock layer that contains them |
| Unconformities | Buried surfaces of erosion that represent a gap in time |
| Disconformity | When a horizontal layer of sedimentary rock overlies another horizontal layer of sedimentary rock after a period of erosion |
| Nonconformity | Sedimentary rock over igneous or metamorphic rock |
| Angular Unconformity | A horizontal layer of sedimentary rock is later laid on top the tilted eroded layers |
| Correlation | Matching of unique rock outcrops or fossils exposed in one geographic region to similar outcrops exposed in other geographic regions |
| Key Beds | A rock of sediment layer that serves as a time marker in the rock record |
| Absolute Age Dating | Enables scientists to determine the numerical age of rocks and other objects |
| Radioactive Decay | The emission of radioactive particles and the resulting change into other isotopes over time |
| Radiometric Dating | When scientists date an object using radioactive isotopes |
| Half-Life | The length of time it takes for one half life of the original isotope to decay |
| Radiocarbon Dating | Using Carbon 14 to determine the age of organic material which contain abundant carbon |
| Dendrachronology | The science of using tree rings to determine absolute age |
| Ice Cores | Cyclinders of ice drilled from thick glaciers to determine the types and amounts of gases that existed in the atmosphere when the ice was formed |
| Varves | Bands of alternating light and dark colored sediments of sand, clay, and silt |
| Evolution | The change in species over time |
| Original Preservations | Remains of plants and animals that have been altered very little since the organisms death |
| Altered Hard Parts | Hard parts such as shells, bones or cell walls can become fossils |
| Mineral Replacements | The pore spaces of an organisms buried hard parts are filled in with mineral from groundwater |
| Mold | Creature is dissolved completely out of the rock leaving a depression |
| Trace Fossils | Indirect fossils include traces of worm trails, footprints, and tunneling burrows |
| Index Fossils | Fossils that are easily recognized, abundant, and widely distributed geographically |
