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Glossary of The History of the Earth - Terms to Know

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absolute age
the number of years since the rock formed
amber
hardened resin or sap of evergreen trees, sometimes preserved insects
cast
a copy of the shape of an organism
decay
the breakdown of dead organisms into the substances from which they are made
extinct
no longer exists and will never exist again on earth
extrusion
lava that hardens on the surface
fossil
preserved remains or traces of living things
igneous rock
formed when magma or lava hardens
imprint
impressions made by thin objects such as leaves or feathers
index fossil
fossil of an organism that only existed a short period of time; helps to tell the relative ages of the rock layers in which they occur
intrusion
an area below the surface where magma has cooled and hardened
law of superposition
in horizontal sedimentary rock layers the oldest layer is at the bottom and each higher layer is younger than the one below it
metamorphic rock
formed when sedimentary or igneous rocks are changed by heat, pressure or both – rarely contains fossils
mold
a hollow area in sediment in the shape of an organism or part of an organism
paleontologist
scientists who study fossils
petrified
turning into stone
petrified fossil
fossils in which minerals replace all or part of an organism
principle of uniformitarianism
– the processes that act on the erath’s surface today are the same as the processes that have acted on the earth’s surface in the past
relative age
the age of a rock as compared to other rocks
sediment
small pieces of rocks, shells, and other material that have been broken down over time; quick burial in this material slows down or stops decay
sedimentary rock
formed by hardened sediments; where most fossils are found
tar pit
large pools in Southern California that contain fossil remains of many different animals
trace fossil
– fossils which are the marks or evidence of the activities of the an organism
unconformity
a gap in the geologic record
amphibian
an animal that lives part of its life on land and part of its life in water
brachiopod
– marine organisms that lived more than 435 million years ago; indicate to scientists that the area was once covered with a shallow sea
Cenozoic Era
65 million years ago to the present; a time when mammals developed adaptations to live in many different environments
conifer
cone bearing plant
cycad
palm-like seed plants
decay element
the stable element into which a radioactive element breaks down
epoch
a subdivision of a geologic period
era
a long unit of time used for geologic history
geologic time
the method used to organize events in the history of the earth
half-life
the amount of time it takes for one-half of the atoms of a sample of the radioactive element to decay
inorganic
not living or never having lived
interglacial
the time between ice ages
invertebrate
animals without backbones
mammal
a warm blooded animal that feeds its young milk
mass extinction
when many types of organisms become extinct at the same time
Mesozoic Era
began about 225 million years ago; the age of reptiles
organic
living or once living
Paleozoic Era
– began about 570 million years ago; when a great number of different kinds of organisms evolved
period
a subdivision of a geologic era
Precambrian Era
the earliest geologic time period
radioactive decay
unstable elements that break down over time and release energy
radioactive element
an unstable element that gives off energy as it breaks down
reptile
cold blooded animals that were successful during the Mesozoic Era
trilobite
a group of hard shelled animals whose bodies had three distinct parts; often used as an index fossil
vertebrate
animals with backbones

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