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AMS Geologic Time
Geologic Time Vocabulary
Term | Definition |
---|---|
fossil | The remains, trace or imprint of a once-living plant or animal. |
geologic time scale | A record of Earth's history from its origin to the present. |
eon | The largest division of geologic time; it contains ALL the other divisions of time. |
era | A division of geologic time; usually tens to hundreds of millions of years in duration; may contain several periods. |
period | A division of geologic time; generally tens of millions of years in duration; may contain several epochs. |
epoch | pronounced EE-pahk; the smallest division of geologic time; generally hundreds of thousands to millions of years in duration. |
mass extinction | When many groups of organisms disappear from the rock record at about the same time. |
granite | A coarse-grained, intrusive igneous rock; component of continental crust/rocks |
relative-age dating | A method to study the order in which geologic events occurred. |
original horizontality | A principle of geology that states that sedimentary rocks are deposited in horizontal or nearly horizontal layers. |
superposition | A principle of geology that states that in an undisturbed rock sequence, the oldest rocks are at the bottom and each consecutive layer is younger than the layer beneath it. |
cross-cutting relationships | A principle of geology that states that an intrusion is younger than the rock it cuts across. |
principle of inclusions | A principle of geology that states that the eroded fragments, called inclusion, in a rock layer MUST be OLDER than the rock layer that contains them. |
unconformities | Buried surfaces of erosion |
correlation | The matching of unique rock outcrops or fossils exposed in one geographic region to similar outcrops exposed in other geographic regions. |
key bed | A rock or sediment layer used as a marker to correlate rock formations in different geographic areas. |
absolute-age dating | The science of determining the numerical age of rocks and other objects by radiometric dating or radiocarbon dating. |
radiometric dating | The process of obtaining the age of an object through the use of radioactive isotopes. |
radiocarbon dating | The science of determining the age of organic materials, which contain abundant carbon, by using C-14. |
dendrochronology | The science of using tree rings to determine absolute age; allows scientists to date recent geologic events such as volcanic eruptions, earthquakes and glaciation. |
varves | Alternating bands of light- and dark-colored sediments of sand, clay and silt. They represent seasonal deposition of sediments, usually in lakes. |
original preservation | These fossils are the remains of plants and animals that have been altered VERY LITTLE since the organism's death. Freezing, arid (dry) or oxygen-free environments aid in preservation. |
altered hard parts | Fossils created when the remaining hard parts of organisms, such as shells, bones or cell walls, are changed. This is the most common type of fossil. |
mold | A type of fossil that forms when sediments cover the original hard part of an organism, such as a shell, and the hard part if later removed by erosion or weathering. These fossils are hollow. |
cast | Forms when the mold is filled with material. This type of fossil can be 3-dimensional and look like the organism. |
trace fossils | Indirect fossils that include worm trails, footprints and tunneling burrows; can provide information about how an organism lived, moved and obtained food. |
index fossils | Fossils that are easily recognized, abundant and widely distributed geographically. Example: trilobites |
basalt | A fine-grained, extrusive igneous rock; component of oceanic crust (rocks). |
stromatolites | Large coral reef-like mounds of cyanobacteria that dominated the shallow oceans during the early Proterozoic Era; cyanobacteria are microscopic organisms that use photosynthesis to survive. |
Ediacaran biota | Fossils of the first multi-cellular organisms discovered in the Ediacara Hills of Australia. |