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rock layers

contain evidence that Earth is very old

the geologic time scale was developed using

fossils and superposition

If Earth's history lasted only one year, humans would have appeared

December

process in which one element changes into another

radioactive decay

states that older rock layers are generally under younger rocks

superposition

the longest units of geologic time

eons

time it takes for half of a radio active element to break down into another elelment

half-life

preserved remains or evidence of ancient life-forms

fossils

stable substance formed from a radioactive one

daughter element

a scale of Earth's natural history

geologic time scale

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