| A | B |
|---|
| Hutton's theory changed Earth's age how | developed uniformitarianism principle, rates of process vary over time |
| uniformitarianism | geologic processes from past explain current events like volcanism and erosion |
| relative age | age of an object in relation to age of other objects |
| law of superposition | a sedimentary rock layer is older than layers above it if undisturbed |
| original horizontality | orientation of sedimentary rocks will be horizonal if undisturbed |
| graded bedding | arrangement of layers; coarse, heavy particles in bottom layers |
| cross-beds | sand deposited; forms curbed beds at angle to bedding plane; slanting layers |
| ripple marks | small waves formed on surface of sand by wind or water |
| unconformity | break in geologic record due to erosion of rock layers, or no deposition for a while |
| erosion produces unconformity by | wearing away sections/layers, break in geologic history |
| nonconformity | stratified rock resting on unstratified rock |
| angular unconformity | rock deposited in horizontal layers; folded/tilted then eroded |
| disconformity | boundary between horizontal layers of old and young sedimentary rock layers; deposited on eroded surface |
| law of crosscutting | a fault/body of rock is younger than rock it cuts through |
| absolute age | numerical age of an object/event determined by mechanical/chemical methods |
| better method to calculate absolute age | chemical method |
| rates of erosion | good to determine features formed w/i past 10,000-20,000 years |
| rates of deposition | uses data to estimate average rate of deposition in sedimentary rocks |
| varve count | banded layers of sand deposited annually in a lake, esp. near ice sheets or glaciers |
| radiometric dating | determines absolute age by comparing relative percentae of radioactive/stable isotopes |
| isotopes | atoms of same element that have different numbers of neutrons |
| half-life | time required for half a radioactive isotope to break down by radioactive decay |
| 7 half-lives | limit for radioactive decay dating due to not enough to measure accurately |
| carbon-14 | decays over a period of 5,700 years (half-life); dates recent organic remains |
| uranium-238 | decays to form let-208; half-life 4.5 billion years; dates igneous rocks older than 10 million years |
| rubidium-87 | decays to form strontium-87; half-life 49 billion years; dates extremely old rocks |
| potassium-40 | decays to form calcium-40/argon-40; dates igneous/metamorphic/sedimentary; |
| fossils | remains or traces of living organisms in sedimentary rocks |
| paleontology | scientific study of fossils |
| why are fossils in sedimentary rocks | dead organisms get buried quickly |
| mumification | fossils formed in dry places |
| tar seeps | fossils formed when organism caught in tar |
| amber | fossils formed in hardened tree sap; dna preserved |
| freezing | fossils frozen in ice |
| petrification | fossils formed when minerals replace original organic materials in ground water |
| molds and casts | fossils where sedimentation fills open spaces or shells leave open space |
| imprints | fossils of carbonized imprints of leaves, stems, flowers and fish in soft mud or clay |
| coprolites | fossilized feces |
| gastroliths | fossilized stones in disgestive systems of organism |
| trace fossils | fossilized marks formed in sedimentary rock by movement of animals |
| index fossils | fossils used to establish age of rock layers; distinct, abundant, widespread and limited time |
| geologic column | order arrangement of rock layers based on relative ages of rocks; |
| eons | largest unit of geologic time; 88% Earth's history |
| eras | two or more periods in geologic time |
| periods | geologic time named for location where first fossils found |
| epochs | geologic time divided into smaller units; age |
| ages | geologic time defined by occurance of distinct fossils |
