Geologic time

Geological time

uniformitarianism

the idea that the same geologic processes shaping the Earth today have been at work throughout Earth's history

catastrophism

the theory that changes in Earth's crust have resulted mainly from sudden violent and unusual events

paleontology

the science involved with the study of past life

relative dating

the process of determining whether an event or object is older or younger than other events

superposition

a principle that states that younger rocks lie above older rocks if the layers have not been disturbed

geologic column

an ideal sequence of rock layers that contains all the known fossils and rock formations on Earth arranged from oldest to youngest

fault

A break in Earth's crust along which rocks move.

intrusion

molten rock from the Earth's interior that squeezes into existing rock and cools

folding

When rock layers bend under stress

tilting

when internal forces in the Earth slant rock layers

unconformity

A break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.

disconformity

a type of unconformity in which part of a sequence of parallel rock layers is missing

nonconformity

A type of unconformity in which layered sedimentary rocks lie on an eroded surface of nonlayered igneous or metamorphic rock

angular unconformity

a type of unconformity between horizontal rock layers and rock layers that are tilted or folded

absolute dating

any method of measuring the age of an object or event in years

isotopes

Atoms with the same number of protons but different numbers 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

radiometric dating

determining the absolute age of a sample based on the ratio of parent material to daughter material

half-life

the time needed for half of a sample of a radioactive substance to undergo radioactive decay

fossil

A trace of an ancient organism that has been preserved in rock.

trace fossils

A type of fossil that provides evidence of the activities of ancient organisms

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 organism

index fossils

a fossil that is useful for dating the rock layer in which it is found

geologic time scale

the standard method used to divide the Earth's long natural history into manageable parts

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

extinction

The death of every member of a species