Earth's History

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Fossil
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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.
Mold
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A fossil formed when an organism buried in sediment dissolves, leaving a hollow area.
Cast
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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.
Paleontologist
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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.
Evolution
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The process by which all the different kinds of living things have changed over time.
Extinct
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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.
Extrusion
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An igneous rock layer formed when lava flows onto Earth’s surface and hardens.
Intrusion
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An igneous rock layer formed when magma hardens beneath Earth’s surface.
Fault
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A break or crack in Earth’s lithosphere along which the rocks move.
Unconformity
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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.
Half-life
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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.
Era
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One of the three long units of geologic time between the Precambrian and the present.
Period
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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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