| A | B |
|---|
| geologic time scale | record of Earth's history from its origin to the present used to correlate geologic events, environmental changes, and development of life-forms that are preserved in rocks |
| eon | longest time unit in the geological time scale, measured in billions of years |
| Precambrian | longest eon in geologic history of Earth |
| era | second-largest time unit in the geologic time scale, measured in hundreds of millions of years |
| period | third-longest time unit in the geological time scale, measured in tens of millions of years to hundreds of millions of years |
| epoch | time unit in the geological time scale, smaller than a period, measured in millions of years to tens of millions of years |
| mass extinction | many groups of organisms disappear from the rock record at the same time |
| decline | to approach the end |
| deposition | final stage in the erosional process in which the movement of transported materials slows and the materials are dropped in another location |
| uniformitarianism | states that processes occurring today have been occurring since Earth formed |
| relative-age dating | the order in which geologic events occurred |
| original horizontality | states that sedimentary rocks are deposited in horizontal or nearly horizontal layers |
| superposition | states that in an undisturbed rock sequence, the oldest rocks are at the bottom and each successive layer is younger than the layer beneath |
| cross-cutting relationships | states that an intrusion or a fault is younger than the rock it cuts across |
| priciple of inclusion | inclusions must be older than the rock layer that contains it |
| unconformity | a gap in the rock record caused by erosion or weathering |
| correlation | matching of outcrops of one geographic region to another |
| key bed | a rock or sediment layer used as a marker |
| drought | extended period of low rainfall, usually caused by shifts in global wind patterns |
| radioactive decay | emission of atomic particles at a constant rate from a radioactive substance and its resulting change into other elements over time |
| radiometric dating | process used to determine the absolute age of a rock or fossil by determining the ratio of parent nuclei to daughter nuclei within a given sample |
| half-life | period of time it takes for a radioactive isotope to decay to one-half of its original amount |
| dend'ochronology | science of comparing annual growth rings in trees to date events and environmental changes |
| varve | alternating light-colored and dark-colored sedimentary layers of sand, clay, and silt deposited in a lake that can be used to date cyclic events and changes in the environment |
| similar | nearly the same or alike |
| evolution | adaptation of life-forms to changing environmental conditions |
| original preservation | describes a fossil with soft and hard parts that has not undergone any change since the organism's death |
| altered hard part | a fossil whose organic material has been removed and whose hard parts have been changed by recrystallization or mineral replacement |
| mineral replacement | pores of the organism's buried hard parts are filled with minerals from ground water |
| permineralization | process in which pore spaces in a fossil are filled in with mineral substances |
| trace fossil | imprints left by the organism that give insight as to how the organism lived, moved, and obtained food |
| index fossil | the remains of a plant or animal that was abundant, widely distributed, and existed briefly that can be used by geologists to correlate or date rock layers |
| mold | a fossil that can form when a shelled organism decays in sedimentary rock and is weathered away, leaving a hollowed-out impression |
| cast | fossil formed when an earlier fossil of a plant or animal leaves a cavity that becomes filled with minerals or sediment |
