Clocks in Rocks Flashcards
Absolute age
The actual number of years elapsed form a geologic event until now.
Eon
The largest division of geologic time, embracing several eras.
Epoch
A subdivision of a geologic period, often chosen to correspond to a stratigraphic sequence. Also used for a division of time corresponding to a paleomagnetic interval.
Era
A division of geologic time including several periods, but smaller than an eon. Commonly recognized eras are Paleozoic, Mesozoic, an d Cenozoic.
Geologic time scale
(1) A timeline based on a stratigraphic succession that provides a chronological record of the history of a region. (2) The entire span of time since the Earth formed.
Half-life
The timie required for one-half of the original number of radioactive atoms in an element to decay.
Isotopic dating
The use of naturally occurring radioactive elements to determine the ages of rocks.
Mass extinction
A short interval during which many species simply disappear from the geologic record.
Period
A division of geologic time representing one subdivision of an era.
Principle of faunal succession
The principle that the layers of sedimentary rocks in an outcrop contain fossils in a definite sequence.
Principle of original horizontality
Th eprinciple that sediments are deposited as essentially horizontal beds.
Principle of superposition
The principle that each layer of sedimentary rock. In a tectonically undisturbed sequence is younger than the one beneath it and older than the one under it.
Relative age
The age of one geologic event in relation to another.
Stratigraphic succession
A vertical set of rock strata that provides a chronological record of the geologic history of a region.
Unconformity
A surface between two rock layers that were not laid down in an unbroken sequence. An unconformity represents the passage of time.
