<descriptor thcode="2"><thesaurus thcode="2" name="USGS Thesaurus" scope="Topics and methods of scientific study carried out by USGS, with product types, scientific disciplines, geologic time, and types of institutional structure and activities.  Broad and shallow, used to help people find scientific information." creator="U.S. Geological Survey" rights="Public domain" edition="2.0" date="2020-01-09" codetype="number" prefix="topic" uri="https://www2.usgs.gov/science/USGSThesaurus" tblname="term" nonpref="nonpref" relterm="relterm" root_code="1" contact="pschweitzer"><category><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/></category><altlabel><altlabel>USGSThesaurus</altlabel><altlabel>USGSThesaurus</altlabel></altlabel></thesaurus><term thcode="2" code="437" name="geochemistry" parent="291" scope="Study of the distribution of chemical elements and natural compounds on the earth and in the atmosphere and the chemical processes that affect the earth."/><uf><usefor name="chemistry (Earth sciences)"/><usefor name="geochemical anomaly maps"><usewith><term thcode="2" code="700" name="maps and atlases" parent="808" scope="Representation, usually on a flat surface, of a part or whole of the Earth or other parts of the universe.  Collections of maps linked digitally or bound together in a book are called atlases."/></usewith></usefor></uf><bt><term thcode="2" code="291" name="earth sciences" parent="1019" scope="Broad term for all science related to the study of the earth."/><term thcode="2" code="1019" name="sciences" parent="1" scope="Major educational fields, fields of study, and professional groupings within USGS."/><term thcode="2" code="1" name="Categories" scope="Thematic terms for categorizing information resources.  This thesaurus is intended to support finding and understanding scientific information."/></bt><nt><term thcode="2" code="1078" name="soil chemistry" parent="437" scope="Branch of chemistry concerned with the elements and compounds that make up soils. Includes chemical processes involving soils."/><term thcode="2" code="1298" name="water chemistry" parent="437" scope="Branch of chemistry that deals with the elements and compounds in water."/></nt><rt><term thcode="2" code="69" name="atomic absorption analysis" parent="153" scope="Technique used to identify chemicals based on the measurement of the spectra produced by atoms and molecules with absorption of electromagnetic radiation."/><term thcode="2" code="93" name="biogeochemical cycling" parent="310" scope="The cycling of chemical constituents through a biological system."/><term thcode="2" code="141" name="carbon cycling" parent="93" scope="The circulation of carbon in the biosphere, atmosphere, and hydrosphere through a series of processes that include photosynthesis, consumption, and respiration."/><term thcode="2" code="142" name="carbon isotope analysis" parent="654" scope="Experimental determination of the proportion of a given stable carbon isotope (C12 or C13) in a sample."/><term thcode="2" code="153" name="chemical analysis" parent="619" scope="Chemical techniques used to identify the composition of substances."/><term thcode="2" code="158" name="chromatography" parent="153" scope="Analytical techniques used to separate or partition the components of a gaseous or liquid mixture by differences in absorption rates during flow around or over surface absorbents such as columns of silica, filter papers, or gels."/><term thcode="2" code="288" name="earth history" parent="455" scope="The structural and compositional changes that the earth has undergone from its formation. Use for discussions of the whole earth (or large sections of it) instead of regional studies."/><term thcode="2" code="426" name="gas chromatography" parent="158" scope="Chromatographic technique used to separate or partition the components of a mixture.  Solid and liquid samples are vaporized and passed through a column containing a liquid that differentially absorbs the  molecules."/><term thcode="2" code="435" name="geochemical processes" parent="101" scope="Processes affecting the amount, distribution, or structure of chemical elements in air, water, soil, rocks, and minerals."/><term thcode="2" code="609" name="isotopic analysis" parent="619" scope="Experimental determination of the proportion of a given isotope (or isotopes) in a sample."/><term thcode="2" code="654" name="light stable isotope analysis" parent="609" scope="Analytical technique using mass spectrometry to measure the different isotopic forms of low mass (light) elements such as oxygen, hydrogen, carbon, nitrogen and sulfur that occur in samples."/><term thcode="2" code="662" name="liquid chromatography" parent="158" scope="Chromatographic technique to separate or partition the components of a mixture in which samples are liquid and passed through a column."/><term thcode="2" code="711" name="mass spectroscopy" parent="153" scope="Instrumental technique to separate and identify molecules.  Gaseous ions are formed, with or without fragmentation.  Their mass/charge ratios and relative electrical abundance are then measured or the spectra are recorded."/><term thcode="2" code="727" name="metamorphism (geological)" parent="451" scope="Process by which rocks are altered in composition and texture by extreme heat, high pressure, or the action of chemicals."/><term thcode="2" code="783" name="neutron activation analysis" parent="153" scope="Sensitive laboratory technique for both qualitative and quantitative analysis of elements in samples.  Target nuclei are bombarded with neutron beams to start nuclear reactions which emit characteristic gamma ray radiations."/><term thcode="2" code="855" name="oxygen isotope analysis" parent="654" scope="Experimental determination of the proportion of a given stable oxygen isotope in a sample."/><term thcode="2" code="872" name="particle-beam spectroscopy" parent="153" scope="Analytical technique using a concentrated beam of charged subatomic particles."/><term thcode="2" code="1191" name="tritium analysis" parent="654" scope="Mass spectrometric measurement of the decay of the radioactive isotope of hydrogen in water samples as a means of dating and investigating changes in water."/></rt></descriptor>