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Volcanic ash layer dating

The most important tools for paleontologists are collections of fossils and paleontological reports (with fossil plates for identification) from other locations in the region or around the world.

Micropaleontologists and palynologists work with microscopes or scanning electron microscopes (SEM).

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.

Oxygen isotope concentrations in mollusk shell and calcareous algal material normalize with seawater while the organisms are alive.

Paleontologists frequently work in conjunction with other scientists utilizing any number of other geochronology methods.

Like fossils, the chemical and physical characteristics of rocks, minerals, and organic materials can be used for correlation.

Great volcanic eruptions in the Western United States in the geologic past produced airfall deposits that have been recognized as far away as the East Coast.

The USGS maintains a tephrochronology laboratory in Menlo Park, CA.


  1. A tephra layer which bears its own unique chemistry and character may be used as a temporal marker horizon in archaeological and geological sites. Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples.

  2. In addition to the chemical and physical characteristics of volcanic ash, select igneous minerals in the ash can be used for absolute dating discussed below. For more information, contact Andrei Sarna-Wojcicki.

  3. Correlation may be difficult or erroneous if several different ash eruptions occurred, and a layer deposited in one is correlated with that from another. Even then, the correlation may be justified if the two ash deposits represent the same volcanic episode. Much work has been undertaken to characterize ash layers both physically and chemically.

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