Earth's History/Geologic Time Reduced Unit

Unconformity

A gap or missing layer in a rock column

Superposition

A principle that states that younger rocks lie above older rocks if the layers have not been disturbed

Relative Age

An object or event being older or younger than another

Era

Shorter than an eon, longer than a period

Period

Shorter than an era, longer than an epoch

Epoch

Smallest unit of geologic time

4.6 billion years

The estimated age of Earth and the solar system

Correlation

The matching of rock layers from different areas in terms of age

Geologic Time Scale

Broken into eons, eras, periods, and epochs

Fossil

Evidence of past life; most commonly preserved in sedimentary rock

Eon

Largest unit of geologic time

Index fossil

Must come from a species that existed for a short period of time

Older

rocks on the bottom of a rock column

Younger

rocks on the top of a rock column

Precambrian

the name of the era that accounts for about 90% of Earth's history