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PCS Earth Science
Chapter 21 - Fossils and the Rock Record
Term | Definition |
---|---|
altered hard parts | fossils whose organic material has been removed and whose hard parts have been changed by recrystalization or mineral replacement |
cast | fossil formed when an earlier fossil of a plant or animal leaves a cavity that becomes filled with minerals or sediment |
correlation | matching of rock outcrops of one geographic region to another |
cross-cutting relationships | principle stating that a fault or intrusion is younger than the rock it cuts across |
dendrochronology | the process of counting tree rings to determine the age of a tree |
eon | longest time unit in the geological time sale, measured in billions of years |
epoch | time unit in the geologic time scale, smaller than a period, measured in millions of years to tens of millions of years |
era | second-longest unit of time in the geological time scale, measured in hundreds of millions of years, and defined by differences in life forms that are preserved in rocks |
evolution | adaptation of life-forms to changing environmental conditions |
fossil | remains or evidence of a once-living plant or animal |
geologic time scale | record of Earth's history from its origin to the present used to correlate geologic events, environmental changes, and development of life forms that are preserved in rocks |
half-life | period of time it takes for a radioactive isotope, such as carbon-14, to decay to one-half of its original amount |
index fossil | remains of plants or animals that were abundant, widely distributed, and existed briefly that can be used by geologists to correlate or date rock layers |
key bed | a single, widespread rock layer that is unique and easily recognizable; used to correlate rock layers |
mold | fossil that can form when a shelled organism decays in a sedimentary rock and is weathered away, leaving a hollowed out impression |
original horizontality | principle stating that sedimentary rocks are deposited in horizontal or nearly horizontal layers |
original preservation | describes a fossil with soft and hard parts that have not undergone any change since the organism's death |
period | third-longest time unit in the geological time scale, measure in tens of millions of years to hundreds of millions of years, and defined by life-forms that were abundant or became extinct |
permineralization | process in which pore spaces in a fossil are filled in with mineral substances |
radioactive decay | emission of atomic particles at a constant rate from a radioactive substance and its resulting change into other elements over time |
radiometric dating | process used to determine the absolute age of a rock or fossil by determining the ratio of parent nuclei to daughter nuclei within a given sample |
superposition | principle stating that in an undisturbed rock sequence, the oldest rock layers are at the bottom and each successive layer is younger than the layer beneath |
uncomformity | gap in the rock record caused by erosion or weathering |
uniformitarianism | states that processes such as mountain building, erosion, and sea-level changes that are occurring today have been occurring since Earth formed |
varve | a banded layer of sand and silt that is deposited annually in a lake, especially near ice sheets or glaciers, and that can be used to determine absolute age |