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Earth History
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Animations and Other Links
absolute age - the measure of how many years ago an event occurred or an organism lived
cast - a type of fossil thatoccurs when minerals fill a mold and harden into the shape of the original organism
correlation - the matching of rock layers from one area to another
eon - the first division of geologic time and represents the longest segments of geologic history
epoch - an interval of geologic time smaller than a period
era - a subdivision of eon in geologic time
fossil - the remains or the evidence of a living thing
geologic column - an ideal sequence of rock layers created by combining data from all the known rock sequences at various locations
geologic cross-section - a diagram showing a side-view of the features below the surface
geologic time scale - a chronological arrangement of geologic events, commonly represented in a chart form with the oldest event and time unit at the bottom and the youngest at the top
half-life - the time it takes for half the radioactive atoms in a sample to decay to a stable product
index fossil - a fossil that is used to date the rock layers in which it is found; the original organism must have lived during a relatively short, well-defined time span and have a wide geographic distribution
mold - an empty space left in a rock that is in the shape of the organism; forms when the soft parts of the organism decay and/or the hard parts are dissolved by chemicals
original horizontality - the state of strata being flat lying or nearly so at the time they were deposited
period - a subdivision of a geologic era and is the fundamental unit of geologic time
principle of superposition - in undisturbed sedimentary rock layers (strata), the oldest rock is at the bottom and the youngest is at the top
radioactive dating - a technique used to measure the amounts of a parent and daughter isotope within a rock or mineral and use the ratio of the two to calculate the absolute age
radioactive decay - the process by which radioactive isotopes emit or capture tiny particles
relative age - the age of something compared to something else
strata - layers of sedimentary rock
subsidence - sinking or downward settling of the earth's surface
unconformity - a surface of erosion between rock layers that represents a missing gap in Earth history; a buried erosional surface
uniformitarianism - the fundamental principle that geological processes and natural laws now operating to modify earth's crust have acted in much the same manner throughout geologic time and that past events can be explained by forces observable today; "the present is the key to the past"
uplift - a raising of earth's surface produced by crustal movement
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Last Updated 4/06/05