Geologic Time

Flashcards on Geologic Time

Relative Time

Determining which came first, second, etc. in a sequence of events.

Superposition

In a sequence of rocks, the rocks at the bottom are the oldest.

Cross-cutting relationships

Older rocks may be cut by younger rocks or features.

Original horizontality

Rocks are deposited in flat layers, typically at the bottom of the ocean or other body of water.

Unconformities

Eroded surfaces representing gaps in time in the geologic record.

Principle of Uniformitarianism

The same slow-acting geological processes that operate today have operated in the past; the present is the key to the past.

Index fossils

Species that existed for a relatively short period of time and are found over large geographic areas, used for precise correlations of rock layers.

Proterozoic

Earlier life.

Phanerozoic

Life revealed.

Cambrian explosion

Rapid diversification of life, with the explosion of organisms with hard skeletons at the beginning of the Cambrian period.

Mass extinctions

Events in which large numbers of species die off.

Numerical Time

Dating rocks and geologic events in specific units of time.

Isotopes

Atoms of the same element with different numbers of neutrons.

Radioactive decay

The process by which an unstable nucleus spontaneously changes to a more stable form, releasing energy.

Parent isotope

The unstable original isotope in radioactive decay.

Daughter isotope

The stable new isotope formed from radioactive decay.

Half-life

The time it takes for half of the parent isotopes to convert to daughter atoms.

Catastrophism

The idea that the Earth has been and can be affected by short duration, sometimes violent events that may be global in nature.