| A | B |
|---|
| uniformitarianism | a principle that states that the same geologic processes shaping the Earth today have been at work throughout Earth's history |
| catastrophism | a principle that states that all geologic change occurs suddenly |
| relative dating | determining whether an object or event is older or younger than other objects or events |
| superposition | a principle that states that younger rocks lie above older rocks in undisturbed sequences |
| geologic column | an ideal sequence of rock layers that contains all the known fossils and rock formations on Earth arranged from oldest to youngest |
| unconformity | a surface that represents a missing part of the geologic column |
| absolute dating | the process of establishing the age of an object, such as a fossil or rock layer, by determining the number of years it has existed |
| isotopes | atoms of the same element that have the same number of protons but have a different numbers of neutrons |
| radioactive decay | a process in which radioactive isotopes tend to break down into stable isotopes of other elements |
| radiometric dating | determining the absolute age of a sample based on the ratio of parent material to daughter material |
| half-life | for a particular radioactive sample, the time it takes for one-half of the sample to decay |
| fossil | any naturally preserved evidence of life |
| permineralization | a process in which minerals fill in pore spaces of an organism's tissues |
| petrification | a process in which an organism's tissues are completely replaced by minerals |
| trace fossils | any naturally preserved evidence of an aminal's activity |
| coprolites | preserved feces, or dung, from animals |
| mold | a cavity in the ground or rock where a plant or animal was buried |
| cast | an object created when sediment fills a mold and becomes rock |
| index fossil | a fossil of an organism that lived during a relatively short, well-defined time span; a fossil that is used to date the rock layers in which it is found |
| geologic time scale | a scale that divides Earth's 4.6-billion-year history into distinct intervals of time |
| eon | the largest division of geologic time |
| era | the second-largest division of geologic time |
| period | the third-largest division of geologic time |
| epoch | the fourth-largest division of geologic time |
