geologic time study guide – Flashcards
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mass extinction
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a rapid event during which a significant part of all life forms on earth died
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relative age
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time expressed as the order in which rocks formed and geological events occurred
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absolute age
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time measured in yeats
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principle of origional horizontality
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most sediment is deposited as nearly horizontal beds, and therefore much sedimentary rocks started out with nearly horizontal layering
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principle of superposition
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in any undisturbed sequence of sediment or sedimentary rocks, the age becomes progressively younger from bottom to top
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principle of cross-cutting relationships
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a rock must exist before anything happens to it
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evolution
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the change in the physical and genetic characterisitics of a species over time
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fossil
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the preserved trace, imprint, or remains of a plant or animal
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principle of faunal succession
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fossil organisms succeed one another in a definite and reconizable sequence, so that sedimentary rocks of different ages contain different fossils, and rocks of the same age contain identical fossils. Therefore, the relative ages of rocks can be identified from their fossils
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conformable
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the condition in which sedimentary layers were deposited continuously without interruption
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uncanformity
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a gap in the geological record, such as an interruption of deposition of sediments, or a break between eroded igneous and overlying sedimentary strata, usually of long duration
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disconformity
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a type of unconformity in which the sedimentary layers above and below the unconformity are parallel
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angular unconformity
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an unconformity in which younger sediment or sedimentary rocks rest on the eroded surface of tilted or folded older rocks
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nonconformity
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a type of unconformity in which layered sedimentary rocks lie on an erosion surface cut into igneous or metamorphic rocks
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correlation
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demonstration of the equivalence of rocks or geologic features age from different locations
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index fossil
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indentifies and dates the layers in which it is found ; abundantly preserved in rocks, widespread geographically, and existed as a species or genus for only an relatively short time
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key bed
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a thin, widespread, easily recognized sedimentary layer that can be used for correlation
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isotopes
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atoms of the same element that have the same number of protons but different numbers of nutrons
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half-life
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the time it takes for half of the nuclei of a radioactive isotope in a sample to decompose
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radiometric dating
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the process of measuring the absolute age of geologic material by measuring the concentrations of radioactive isotopes and their decay products
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geologic column
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a composite columnar diagram that shows the sequence of rocks at a given place or region arranged to show their position in the geologic time scale
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geologic time scale
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a chronological arrangement of geologic time subdivided into units
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eon
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the longest unit of geologic time
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era
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a geologic time unit; eons are divided into these, and inturn are divide into periods
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period
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a geologic time unit longer than an epoch and shorter than an era
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epoch
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the smallest unit of geologic time; periods are divided into these
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Hadean Eon
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the earliest time in earth's history
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Archean Eon
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a division of geologic time 3.8-2.5 billion years ago ; the oldest known rocks formed at this time
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Proterozoic Eon
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the portion of geologic time from 2.5 billion to 570 million years ago
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Phanerozoic Eon
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the most recent 570 million years of geologic time represented by rocks that contain evident and abundant fossils
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Precambrian time
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all of geologic time before the Paleozoic era, encompassing approximately the first 4 billion years of earth's history ; all rocks formed during this time
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Paleozoic era
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the part of geologic time 570-245 million years ago ; invertebrates, fishes, amphibians, reptiles, ferns, and cone-bearing trees were dominant
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Mesozoic era
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the part of geologic time roughly 245-65 million years ago ; dinosaurs rose to prominence and became extinct
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Cenozoic era
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the latest of the four eras into which geologic time is subdivided ; 65 million years ago to the present
