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AMS Geologic Time

Geologic Time Vocabulary

TermDefinition
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.
Created by: dianabowman
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