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polarity subchron A very short time interval (generally less than 0.1 Ma) of alternate polarity occurring within a polarity chron, e.g. the Olduvai normal subchron within the Matuyama reversed polarity chron. The term ‘polarity subchron’ has been proposed by the ISSC to replace ‘polarity event’; however, at present both terms are in use. The corresponding polarity chronostratigraphic unit is polarity subchronozone. See also MAGNETOSTRATIGRAPHIC TIME-SCALE; and POLARITY SUPERCHRON.