a principle that states that geologic processes that occured in the past can be explained by current geologic processes.
a principle that states that geologic change occurs suddenly.
the scientific study of fossils.
any method of determining whether an event or object is older or younger than other events or objects.
a principle that states that younger rocks lie above older rocks if the layers have not been disturbed.
an arrangement of rock layers in which the oldest rocks are at the bottom.
a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.
any method of measuring the age of an event or object in years
an atom that has the same number of protons [or the same atomic number] as other atoms of the same element do but that has a different number of neutrons [and thus a different atomic mass].
the process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element.
a method of determining the age of an object by estimating the relative percentages of a radioactive [parent] isotope and a stable [daughter] isotope.
the time needed for half of a sample of a radiactive substance to undergo radioative decay
the trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock.
a fossilized mark that is formed in soft sediment by the movement of an animal.
a mark or cavity made in a sedimentary surface by a shell or other body.
a type of fossil that forms when sediments fill in the cavity left by a decomposed organism.
Geologic Time Scale
the standard method used to divide the Earth's long natural history into manageable parts.
the largest division of geologic time.
a unit of geologic time that includes two or more periods.
a unit of geologic time into which eras are divided.
a subdivision of a geologic period.
the death of every member of a species.