Geological Time and Rock Formation

The age of a rock compared to the ages of other rocks.

• relative age

• half-life

• absolute age

• radioactive decay

The time it takes half of the atoms of a radioactive element to decay

• relative age

• half-life

• absolute age

• radioactive decay

One of three long units of geologic time between Precambrian and the present.

• evolution

• period

• geologic time scale

• era

Change over time for organisms.

• geologic time scale

• evolution

• era

• period

An igneous rock later formed when lava flows onto Earth's surface and hardens.

• extrusion

• index fossil

• intrusion

• uncomformity

A record of the geologic events and life forms in Earth's history.

• index fossil

• half-life

• radioactive dating

• geologic time scale

Fossils of widely distributed organisms that lived during a geologically short period.

• index fossil

• half-life

• radioactive dating

• geologic time scale

An igneous rock layer formed when magma hardens beneath Earth's surface.

• extrusion

• index fossil

• intrusion

• unconformity

In horizontal layers of sedimentary rock, each layer is older than the layer above it.

• mass extinct

• geologic time scale

• era

• law of superposition

One of the units of geologic time into which geologists divide eras.

• era

• geologic time scale

• period

• uncomformity