Unit 6 - Geologic Time

fossils and the rock record, the precambrian earth, the paleozoic, masozoic, and cenozoic eras

geologic time scale

record of history from its origin 4.6 bya to the present.

eon

the largest time unit of the geologic time scale.

Precambrian

informal unit of geologic time consisting of the first three eons during which Earth formed and became habitable.

era

second-longest time unit in the geologic time scale, measure in tens to hundreds of millions of years, and defined by differences in life-forms that are preserved in rocks.

period

third-longest time unit in the geologic time scale, measured in tens of millions of years.

epoch

time unit in the geologic time scale, smaller than a period, measured in hundreds of thousand to millions of years.

mass extinction

occurs when an unusually large number of organisms disappear from the rock record at the same time.

uniformitarianism

the theory that geologic processes occuring today have been occurring since the Earth formed.

relative-age dating

establishing the order of past geologic events.

original horizontality

the principle that sedimentary rocks are deposited in horizontal or nearly horizontal layers.

superposition

the principle that, in an undisturbed rock sequence, the oldest rocks are on the bottom and each consecutive layer is younger than the layer beneath it.

cross-cutting relationship

the principle that intrusion or fault is younger than the rock it cuts across.

principle of inclusions

the principle that fragments, called inclusions, in a rock layer must be older than the rock layer that contains them.

unconformity

gap in the rock record caused by erosion or weathering.

correlation

matching of rock outcrops of one geographic region to another.

key bed

a rock. or sediment layer that serves as a time marker in the rock record and results from volcanic ash or meteorite-impact debris that spread out and covered lage areas of Earth.