Superchron

A superchron is an unusually long interval within Earth's geomagnetic polarity timescale during which the Earth's magnetic field does not reverse polarity. In paleomagnetism, the magnetic field normally reverses at irregular intervals, producing a sequence of normal and reversed polarity intervals known as chrons and subchrons. A superchron is defined by a multi‑million‑year duration in which a single polarity prevails.

The best-known example is the Cretaceous Normal Superchron (CNS), which lasted roughly from 120 to 83 million years ago, about 37 million years. During this interval the field remained in normal polarity. Other proposed superchrons include a late Paleozoic interval (the Kiaman Superchron) from the late Carboniferous to early Permian, lasting several tens of millions of years with a single polarity dominating.

Superchrons are identified from magnetostratigraphic records in marine sediments and volcanic rocks. By determining the polarity of rocks of known age and correlating them with the geomagnetic polarity timescale, scientists infer long intervals without reversals and anchor the geological timescale.

The existence of superchrons provides constraints for models of the geodynamo, as reversal frequency is linked to core dynamics and mantle convection. They serve as useful tempo anchors for dating rocks and correlating global stratigraphic records, though their causes and exact durations remain active topics of research.