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All terms in this list:

fossil: Any preserved evidence of ancient life, including shells, imprints, burrows, coprolites, and organically-produced chemicals.

trace fossil: A type of fossil reflecting the reworking of sediments and hard substrates by organisms, rather than the physical remains of the organism itself. This includes structures such as burrows, trails, impressions, coprolites and borings.

relative dating: A method of determining the age of a fossil by comparing its placement with that of fossils in other layers of rock.

law of superposition: the principle that states that younger rocks lie above older rocks if the layers have not been disturbed.

unconformity: A gap in time in rock strata, where erosion occurs while deposition slows or stops.

geologic column: an ordered arrangement of rock layers that is based on the relative ages of the rocks, with the oldest rocks at the bottom.

absolute dating: the determining of the age of an event or object in years.

radioactive decay: the breakdown of a radioactive isotope into a stable isotope of the same element or of another element.

half-life: The time required for half of the nuclei in a sample of a specific isotope to undergo radioactive decay.

radiometric dating: the finding of the absolute age of a sample by determining the relative percentages of a radioactive parent isotope and a stable daughter.

continental drift: the slow movement of continents explained by plate tectonics

climate: the average weather conditions in an area over a long period of time

ice core: a long cylinder of ice drilled down by scientists.

geologic time scale: the divided time units of Earth's natural history.

eon: the longest period of time

eras: subdivided eons

periods: divided eras

epochs: the smallest unit of geologic time

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