Historical Geology Vocab

Uniformitarianism

processes have shaped earth in the past are essentially the same as those operating today

relative dating

rock layers are placed in their proper sequence or order chronologically

Unconformity

a surface that represents a break in the rock record, caused by erosion or lack of deposition

correlation

establishing the equivalence of rocks of similar age in different areas

law of superposition

in any undisturbed sequence of sedimentary rocks, each layer is older than the layers above and younger than the layers below

principle of original horizontality

in relative dating, layers of sediments are generally deposited in a horizontal or nearly horizontal position

principle of cross cutting relationships

in relative dating, a fault or intrusion is younger than any geologic feature through which it cuts

fossil

the remains or traces of an organism preserved from the geologic past

extinct

when an organism no longer exists on earth

index fossil

fossil that is useful for correlation because it is widespread, abundant, but limited to a particular span of geologic time

evolution

a change over time

ice age

a period of time when much of the earth's land is covered by glaciers

radiometric (absolute) dating

the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes

radioactivity

the spontaneous decay of certain unstable atomic nuclei

half-life

the time required for one half of the atoms of a radioactive substance to decay

radiocarbon dating

the method of determining age by comparing the amount of radioactive carbon-14 to the amount of stable carbon-12 in a sample

geologic time scale

timeline of earth's history divided into units of varying lengths

eon

the largest unit of time on the geologic timescale; can be broken into eras

era

the second largest unit of time on the geologic time scale; can be broken into periods

period

the second smallest unit of time on the geologic time scale; can be broken into epochs

epoch

the smallest unit of time on the geologic time scale