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Chapter 8-Section 2

relative age

the age of a rock compared to the ages of other rocks

absolute age

the number of years since the rock was formed

law of superposition

in horizontal sedimentary rock layers the oldest layer is at the bottom. Each higher layer is younger than the layers below it


lava that hardens on the surface


when magma cools and hardens into a mass of igneous rock


a break in the Earth's crust


a gap in the geological record


a piece of rock that is contained in another rock

index fossil

a fossil that is used to match rock layers; it must be widely distributed and represent a type of organism that existed only briefly

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