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the concept that processes that have shaped Earth in the 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 undefined sequence of sedimentary rocks, each bed is older than the layers above and younger than the layers below.

principle of orginial horizonality

a principle of relative dating; layers of sediments are generally depostited in a horizontal or nearly horzontial position.

principle of cross-cuting relationships

a principle of relative dating; a rock or fault is younger than any rock or fault through which it cuts.


surface that represents a break in the rock record, caused by erosion or lack of deposition.


establishing the equivalent of rocks of similar age in different areas.


the remains or traces of an organism preserved from the geologic past.

index fossil

a fossil that is associated with a particular span of geologic time.


the spontaneous decay of certain unstable atomic nuclei.


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 bblocks of timeeons, periods, and epochs; the time scale was created using relative dating principles.


the largest time unit on the geologic time scale, next, in order of magnitude above era.


a major division on the geologic time scale; Eras are divided into shorter, units called periods.


a basic unit of the geologic time scale that is a subdivision of an era; periods may be divided into smaller units called epochs.


a unit of the geologictime scale that is a subdivision of a period.

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