uniformitarianism
a principle that states that geologic processes that occured in the past can be explained by current geological processes.
catastrophism
a principle that states the geologic change occurs suddenly
paleontology
the scientific study of fossils.
relative dating
any method of determining whether an event or object is older or younger than other events or objects.
superposition
a principle that states the younger rocks lie above older rocks in the layers that have not been disturbed.
geologic column
an arrangment of rock layers in which the oldest rocks are at the bottom.
unconformity
a break in the geologic record created when rock layers are eroded or when sediments is not deposited for along period of time.
absolute dating
any method of measuring the age of an event or object in years.
isotope
an atom that has the same number of protons as other atoms of the same element do but that has a different number of neutrons.
radioactive decay
the process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element.
half-life
the time needed for half of a sample of radioactive substance to undergo decay.
fossil
the remains or physical evidence of an organism preserved by geological processes.
trace fossil
a fossilized mark that is formed in soft sediment by the movement of an animal.
mold
a mark or cavity made in a sedimentary surface by a shell or other body.
cast
a type of fossil that forms when sediments fill in the cavity left by a decomposed organisim.
index fossil
the fossil that is found in the rock layers of only one geological age and that is used to establish the age of the rock layers.
geologic time scale
the standard method used to divide the Earth's long natural history into mangeable parts.
eon
the largest division of geological time
era
a unit of geological time that includes two or more periods
period
a unit of geologic time into which eras are divided.
epoch
a subdivision of a geologic period
extinction
the death of every member of a species.