Variations in the Earth's magnetic field, as recorded by magnetic particles in rocks and sediments, may be used as a means of stratigraphic correlation. Major reversals of the Earth's magnetic field have been independently dated in many localities throughout the world. Consequently, the record of these reversals in sediments can be used as time markers or chronostratigraphic horizons. There have also been secular (century- to millennial-scale) variations in the nondipole field that were regional in scale. Two other general categories of dating methods are based on chemical changes within the samples being studied since they were emplaced. The first involves amino acid analysis of organic samples, generally used to assess the age of associated inorganic deposits or to estimate paleotemperatures from organic samples of known age. The second category involves methods that assess the amount of weathering an inorganic sample has experienced, primarily to assess the relative age of freshly exposed rock surfaces. Another dating method uses tephra (airborne pyroclastic material ejected during the course of a volcanic eruption). Extremely explosive eruptions may produce a blanket of tephra covering vast areas forming regional isochronous stratigraphic markers. Providing that the dated tephra layer can be uniquely identified in different areas, it can be used as a chronostratigraphic marker horizon to provide limiting dates on the sediments with which it is associated. Biological dating methods generally use the size of an individual species of plant or lichen as an index of the age of the substrate on which it is growing. They may be used to provide minimum age estimates only. Examples include lichen growth on rock surfaces, and tree growth on glacial moraines.
