Earth's History

Fossil

The preserved remains or traces of living things.

Sedimentary rock

The type of rock that is made of hardened sediment.

Mold

A fossil formed when an organism buried in sediment dissolves, leaving a hollow area.

Cast

A fossil that is a copy of an organism’s shape, formed when minerals seep into a mold.

Petrified fossil

A fossil in which minerals replace all or part of an organism.

Carbon film

A type of fossil consisting of an extremely thin coating of carbon on rock.

Trace fossil

A type of fossil that provides evidence of the activities of ancient organisms.

Paleontologist

A scientist who studies fossils to learn about organisms that lived long ago.

Scientific theory

A well-tested concept that explains a wide range of observations.

Evolution

The process by which all the different kinds of living things have changed over time.

Extinct

Describes a type of organism that no longer exists anywhere on Earth.

Relative age

The age of a rock compared to the ages of rock layers.

Absolute age

The age of a rock given as the number of years since the rock formed.

Law of superposition

The geological principle that states that in horizontal layers of sedimentary rock each layer is older than the layer above it and younger than the layer below it.

Extrusion

An igneous rock layer formed when lava flows onto Earth’s surface and hardens.

Intrusion

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

Fault

A break or crack in Earth’s lithosphere along which the rocks move.

Unconformity

A place where an old, eroded rock surface is in contact with a newer rock layer.

Index fossil

Fossils of widely distributed organisms that lived during only one short period.

Radioactive decay

The breakdown of a radioactive element, releasing particles and energy.

Half-life

The length of time needed for half of the atoms of a sample of a radioactive isotope to decay.

Geologic time scale

A record of the geological events and life forms in Earth’s history.

Era

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

Period

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

Continental drift

The theory that the continents have moved over time.