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PHS ES Chp12

PHS ES Geologic Time Chapter 12

TermDefinition
uniformitatianism the concept that the processes that have shaped Earth in the geologic past are essentially the same as those operating today.
relative dating process by which rocks are placed in their proper sequence or order, only the chronological order of events is determined, not the absolute age in years.
law of superposition a law that states that in any undeformed sequence of sedimentary rocks, each bed is older than the layers above and younger than the layers below.
principle of original horizontality a principle of relative dating; layers of sediments are generally deposited in a horizontal or nearly horizontal position.
principle of cross-cutting relationships a principle of relative dating; a rock or fault is younger than any rock or fault through which it cuts.
unconformity a surface that represents a break in the rock record, caused by erosion or lack of deposition
correlation establishing the equivalence of rocks of similar age in different areas
extinct term used to describe a type of organism that no longer exists anywhere on Earth
fossil the remains or traces of an organism preserved from the geologic past.
principle of fossil succession geologic principle stating that fossil organisms succeed one another in a definite order in the rock record.
theory of evolution theory developed by Charles Darwin stating that all the different kinds of living things have changed over time.
natural selection process by which characteristics that make an individual better suited to its environmnet become more common in a species.
adaptation a trait that helps an organism survive and reproduce
index fossil a fossil that is associated with a particular span of geologic time
radioactivity the spontaneous decay of certain unstable atomic nuclei
half-life the time required for one half of the atoms of a radioactive substance to decay
radiometric dating the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.
radiocarbon dating method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample.
geologic time scale the division of Earth history into blocks of time - eons, eras, periods and epochs; the time scale was created using relative dating principles.
eon the largest time unit on the geologic time scale, next in order of magnitude and above era.
precambrian time the long time span from Earth's formation to the beginning of the Cambrian period; made up of the Hadean, Archaen, and Proterzoic eons.
era a major division on the geologic time scale, Eras are divided into shorter units called periods.
period a basic unit of the geologic time scale that is a subdivision of an era; periods may be divided into smaller units called epochs.
epoch a unit of the geologic time scale that is a subdivision of a period.
Created by: melaniebird
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