| A | B |
|---|
| uniformitarianism | a principle that geologic processes that occurred in the past can be explained by current geologic processes |
| relative age | the age of an object in relation to the ages of other objects |
| law of superposition | the law that a sedimentary rock layer is older than the layer above it & younger than layers below if layers not disturbed |
| unconformity | a break in geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time |
| law of crosscutting relationships | the principle that a fault or body of rock is younger than any other body of rock that it cuts through |
| absolute age | numeric age of an object or event-stated in years before the present-such as radiometric dating |
| varve | banded layer of sand & silt that is deposited annually in a lake, especially near ice sheets or glaciers; can be used to determine absolute age |
| radiometric dating | method of determining the absolute age of an object by comparing the relative percentages of a radioactive isotope & a stable isotope |
| half-life | time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope |
| fossil | trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock |
| paleontology | the scientific study of fossils |
| trace fossils | a fossilized mark that formed in sedimentary rock by the movement of an animal on or within soft sediment |
| index fossil | fossil that is used to establish the age of rock layers-distinct, abundant & widespread; existed for short span of geologic time |
