{"thcode":2,"term":{"code":438,"name":"geochronology","parent":464,"scope":"Study of the age of the earth by dating geological formations, rocks, and fossils."},"uf":[],"bt":[{"code":464,"name":"geology","parent":291,"scope":"Study of the planet earth, its composition, structure, physical and chemical processes, and history since its origin."},{"code":291,"name":"earth sciences","parent":1019,"scope":"Broad term for all science related to the study of the earth."},{"code":1019,"name":"sciences","parent":1,"scope":"Major educational fields, fields of study, and professional groupings within USGS."},{"code":1,"name":"Categories","parent":null,"scope":"Thematic terms for categorizing information resources.  This thesaurus is intended to support finding and understanding scientific information."}],"nt":[],"rt":[{"code":143,"name":"carbon-14 analysis","parent":954,"scope":"Method to determine the age of organic geologic and archaeological specimens, aged approximately 3,000 to 50,000 years, by determining the decay of the radioactive isotope carbon-14."},{"code":393,"name":"fission-track dating","parent":1680,"scope":"Laboratory technique for determining the age of rocks and other geological materials by counts of spontaneous and induced fission tracks left by the radioactive isotope uranium-238."},{"code":954,"name":"radiometric dating","parent":609,"scope":"Methods for age determination of rocks and fossils by measuring the proportions of naturally occurring radioactive isotopes to their decay products."},{"code":1007,"name":"rubidium-strontium analysis","parent":954,"scope":"Technique for dating ancient rocks based on measuring the ratio of the isotopes strontium-87 to ribidium-87 in samples. Uses the known half-life of radioactive isotopes of rubidium-87 that decay to isotopes of strontium-87 to determine rock ages."},{"code":1104,"name":"stratigraphy","parent":464,"scope":"Branch of geology concerned with the study of the formation, composition, ordering in time, and arrangement in space of stratified rocks."},{"code":1188,"name":"tree ring analysis","parent":619,"scope":"Study of the variations in width and number of tree rings to date historical events and to study periods of climatic and hydrologic changes."},{"code":1202,"name":"uranium-lead analysis","parent":954,"scope":"Radiometric dating technique to determine the age of earth materials from the ratio of the radioactive isotopes of uranium-235 or uranium-238 to the lead isotope decay products in the sample. The ratio is compared to the known half-life of the uranium isotopes."},{"code":1203,"name":"uranium-thorium analysis","parent":954,"scope":"Radiometric dating technique used to determine the age of earth materials based on determining the ratio of uranium-238 to the decay product thorium-230. The ratio is compared to the known half-life of the uranium isotope."}]}