Earth's History

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The preserved remains or traces of living things.

Sedimentary rock
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The type of rock that is made of hardened sediment.

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

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

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

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

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

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

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

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

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

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

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

Law of superposition
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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.

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

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

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

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

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

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

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

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

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

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

Continental drift
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The theory that the continents have moved over time.

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