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The Sr geochronology method involves extracting these isotopes from fossil shell material (only several milligrams of sample are necessary for X-ray fluorescence spectroscopy).The ratio of these two isotopes derived from a sample is compared with a database of known samples to determine relative ages.Tephrochronology is the study of volcanic ash deposits.
Relatively young deposits can be sometimes dated using tree rings, varved-lake sediments, coral growth patterns, and other methods.
Paleontology is the study of life in past geologic periods (fossil plants and animals), incorporating knowledge of an organism's phylogeny, relationships to existing organisms, and correlation to an established chronology of Earth History.
Biostratigraphy is the science of correlation of sedimentary units base on the identifiable fossils they contain.
Paleontologists examine fossils of all kinds, but micropaleontology (the study of microscopic organisms) is perhaps the most useful method of dating because the remains of tiny organisms tend to be better preserved, more widely distributed, and may provide more precise age determinations than larger shells or bone material.
A geologic map or report typically is only a summary of investigations that frequently involve the collecting and processing of hundreds of rock samples, followed by the evaluation and interpretation of data from a variety of analytical techniques.