How old is old? Glossary

An igneous intrusion "bakes" (metamorphoses) surrounding rocks. The rock that has been baked must be older than the intrusion.

The temperature below which isotopes in a mineral are no longer free to move, so the radiometric clock starts.

The remarkable diversification of life, indicated by the fossil record, that occurred at the beginning of the Cambrian Period.

A radiometric dating process that can tell us the age of organic material containing carbon originally extracted from the atmosphere.

The boundary surface between two rock bodies (as between two stratigraphic formations, between an igneous intrusion and adjacent rock, between two igneous rock bodies, or between rocks juxtaposed by a fault).

The process of defining the age relations between the strata at one locality and the strata at another.

If one geologic feature cuts across another, the feature that has been cut is older. For example, if an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the dike.

The decay product of radioactive decay.

An unconformity in which the strata below were tilted or folded before the unconformity developed; strata below the unconformity therefore have a different tilt than strata above.

An unconformity parallel to the two sedimentary sequences it separates.

A type of unconformity at which sedimentary rocks overlie basement (older intrusive igneous rocks and/or metamorphic rocks).

The largest subdivision of geologic time.

An interval of geologic time representing the largest subdivision of a period.

An interval of geologic time representing the largest subdivision of the Phanerozoic Eon.

A line of damage formed in the crystal lattice of a mineral by the impact of an atomic particle ejected during the decay of a radioactive isotope.

A recognisable layer of a specific sedimentary rock type or set of rock types, deposited during a certain time interval, that can be traced over a broad region.

A group of fossil species found in a specific sequence of sedimentary rock.

In a stratigraphic sequence, different species of fossil organisms appear in a definite order; once a fossil species disappears in a sequence of strata, it never reappears higher in the sequence.

A composite stratigraphic chart that represents the entirety of the Earth's history.

The sequence of geologic events that has taken place in a region.

A map showing the distribution of rock units and structures across a region.

The span of time since the formation of the Earth.

A scale that describes the intervals of geologic time.

A rhythmic layering that develops in trees, travertine deposits, and shelly organisms as a consequence of seasonal changes.

The time it takes for half of a group of a radioactive element's isotopes to decay.

The interval of time between deposition of the youngest rock below an unconformity and deposition of the oldest rock above the unconformity.

If an igneous intrusion contains fragments of another rock (xenoliths), the fragments must be older than the intrusion.

The comparison of the pattern of magnetic reversals in a sequence of strata, with a reference column showing the succession of reversals through time.

The age specified in years.

Sediments generally accumulate in continuous sheets. If you find a sedimentary layer cut by a canyon, then you can assume that the layer once spanned the canyon but was later eroded by the river that formed the canyon.

Typically, the surfaces on which sediments accumulate (such as a floodplain or the bed of a lake or sea) are fairly horizontal. Therefore, layers of sediment when originally deposited are fairly horizontal.

A radioactive isotope that undergoes decay.

An interval of geologic time representing a subdivision of a geologic era.

The interval of geologic time between Earth's formation about 4.57Ga and the beginning of the Phanerozoic Eon 542Ma.

The process by which a radioactive atom undergoes fission or releases particles thereby transforming into a new element.

The science of dating geologic events in years by measuring the ratio of parent radioactive atoms to daughter product atoms.

The interval of a sequence of strata in which a specific fossil species appears.

The age of one geologic feature with respect to another.

A cross-section diagram of a sequence of strata summarising information about the sequence.

In a sequence of sedimentary rock layers, each layer must be younger than the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect.

A boundary between two different rock sequences representing an interval of time during which new strata were not deposited and/or were eroded.

Uniformitarianism assumes that the same natural laws and processes that operate in the universe now, have always operated in the universe in the past and apply everywhere within the universe: "the present is the key to the past".