| A | B |
|---|
| fossil | remains or traces of a once-living organism preserved in rock |
| petrified remains | fossils that form when original materials that made up the organisms are replaced with minerals |
| carbonaceous film | fossil formed when the remains of a once-living organism are subjected to heat and pressure, leaving only a thin film of carbon behind |
| mold | fossil formed when an organism is buried, decays, and leaves behind a hollow place in the rock |
| cast | fossil formed when sediments fill a mold and harden into rock |
| index fossil | fossils of species that existed on Earth for only a short time, were abundant, and were widespread geographically |
| principle of superposition | states that for undisturbed layers of rocks, older rocks lie underneath younger and younger rocks |
| relative dating | method to determine the order of events and relative age of the rocks by examining the position of rocks in a sequence |
| unconformity | gaps in the rock layers due to erosion, nondeposition, or both |
| absolute dating | process that uses the properties of atoms in rocks and other ob jects to determine their exact ages, in years |
| radioactive decay | release of nuclear particles and energy from unstable atomic nuclei |
| half-life | time needed for one-half the mass of a sample of a radioactive isotope to decay |
| radiometric dating | process to determine the absolute ages of rocks by measuring the amounts of parent and daughter materials in a rock and by knowing the half life of the parent |
| uniformitarianism | states that Earth processes happening today are similar to those that happened in the past |
