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polarity superchron The longest polarity time interval in the magnetostratigraphic time-scale, comprising a number of polarity chrons, and indicating a period of time (varying between 30 and 100 Ma) during which the polarity of the Earth's geomagnetic field has a distinct bias. The bias may be towards a normal field, a reversed field, or an evenly alternating field (called a mixed field), e.g. the present Cretaceous—Tertiary—Quaternary mixed-polarity superchron. The corresponding polarity chronostratigraphic unit is the polarity superchronozone.