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Question		Answer
Fossil start learning		The preserved remains or traces of living things.
Sedimentary rock start learning		The type of rock that is made of hardened sediment.
Mold start learning		A fossil formed when an organism buried in sediment dissolves, leaving a hollow area.
Cast start learning		A fossil that is a copy of an organism’s shape, formed when minerals seep into a mold.
Petrified fossil start learning		A fossil in which minerals replace all or part of an organism.
Carbon film start learning		A type of fossil consisting of an extremely thin coating of carbon on rock.
Trace fossil start learning		A type of fossil that provides evidence of the activities of ancient organisms.
Paleontologist start learning		A scientist who studies fossils to learn about organisms that lived long ago.
Scientific theory start learning		A well-tested concept that explains a wide range of observations.
Evolution start learning		The process by which all the different kinds of living things have changed over time.
Extinct start learning		Describes a type of organism that no longer exists anywhere on Earth.
Relative age start learning		The age of a rock compared to the ages of rock layers.
Absolute age start learning		The age of a rock given as the number of years since the rock formed.
Law of superposition start learning		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 start learning		An igneous rock layer formed when lava flows onto Earth’s surface and hardens.
Intrusion start learning		An igneous rock layer formed when magma hardens beneath Earth’s surface.
Fault start learning		A break or crack in Earth’s lithosphere along which the rocks move.
Unconformity start learning		A place where an old, eroded rock surface is in contact with a newer rock layer.
Index fossil start learning		Fossils of widely distributed organisms that lived during only one short period.
Radioactive decay start learning		The breakdown of a radioactive element, releasing particles and energy.
Half-life start learning		The length of time needed for half of the atoms of a sample of a radioactive isotope to decay.
Geologic time scale start learning		A record of the geological events and life forms in Earth’s history.
Era start learning		One of the three long units of geologic time between the Precambrian and the present.
Period start learning		One of the units of geologic time into which geologist divide eras.
Continental drift start learning		The theory that the continents have moved over time.

Earth's History

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