Earth History

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

a fossil that is associated with a particular span of geologic time

a law that states that in any undeformed sequence of sedimentary rocks, each bed is older than the layers above and younger than the layers below

process by which rocks are placed in their proper sequence or order

the determination of the age of an object with reference to a specific time scale, such as a fixed calendar or in years before present (BP)

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

the spontaneous decay of certain unstable atomic nuclei

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

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

the division of Earth's history into blocks of time—eons, eras, periods, & epochs

the largest unit of time on the geologic time scale (broken into smaller units called eras)

a major division on the geologic time scale (broken into smaller units called periods)

a basic unit of the geologic time scale that is a subdivision of an era (broken into smaller units called epochs)

smallest unit of geologic time

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