Dating Methods / Essential Question How can scientists determine the ages of geologic material? What are strata? Layers of rock Strata Folded strata What is an angular unconformity? When rock deposited in horizontal layers is folded or tilted, then eroded, and then covered by more horizontal layering Angular unconformity What is the law of cross-cutting relationships? The fault or igneous intrusion is always younger than the rock layer it cuts through. Cross-cutting relationship What are nonconformities? Unstratified igneous or metamorphic rock is eroded and then covered by layers of sediment. Nonconformity What are disconformities? The boundary between younger sedimentary rock layers deposited on an eroded layer of older sedimentary rock disconformity What are faults? A break or crack in Earths crust along which rocks shift their positions Fault Earth history in 24 hours scale: 1 sec = 52,000 years 1 min = 3,125, 000 yrs 1 hour = 187,500,000 yrs What is correlation? Matching geologic materials from different regions to ages Correlating tree rings Rates of erosion and deposition Measure the recent rates Estimate age of geologic feature Becomes less reliable as age increases Most reliable within last 10,000-20,000 years Niagara Falls Varve count Similar to counting tree rings Count annual layers of sediment Common in glacial lake beds Distinguished by layers of dark fine particals and light heavy particles Radiometric dating A method of determining the absolute age of an object by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope. Atoms of the same element with different numbers of neutrons are called isotopes. Radioactivity and radiometric dating Radiometric dating Half-life the time for one-half of the radioactive nuclei to decay Requires a closed system Cross-checks are used for accuracy Complex procedure Yields numerical dates The radioactive decay curve Dating sedimentary strata using radiometric dating Radioactivity and radiometric dating Carbon-14 dating Half-life of only 5730 years Used to date very recent events Carbon-14 produced in upper atmosphere Incorporated into carbon dioxide Absorbed by living matter Useful tool for anthropologists, archeologists, historians, and geologists who study very recent Earth history