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polarity chron The basic time interval in the magnetostratigraphic time-scale, during which the Earth's geomagnetic field is constantly, or predominantly, of one polarity, e.g. the Gauss normal polarity chron and the Matuyama reversed polarity chron. The duration of polarity chrons is variable, but generally is longer than 0.1 Ma. Polarity chrons may be interrupted by polarity subchron(s), and grouped to form a polarity superchron. The term ‘polarity chron’ has been proposed by the ISSC to replace polarity epoch; however, at present both terms are in use. The corresponding chronostratigraphic unit is polarity chronozone.