Geomagnetic Reversal Chronology and Magnetostratigraphy

Core Idea

The geomagnetic field reverses at irregular intervals (averaging ~200 ka). A geomagnetic polarity time scale (GPTS) documents reversal patterns; magnetostratigraphy correlates magnetic reversals in sediments and lavas to the GPTS for dating.

Explainer

From your study of the geodynamo, you know that Earth's magnetic field periodically reverses polarity — north becomes south and south becomes north. From paleomagnetic dating, you know that rocks can record these polarity states and that the pattern of reversals can be matched across distant locations. Geomagnetic reversal chronology brings these ideas together into a single, precisely calibrated framework: the geomagnetic polarity time scale (GPTS), which is essentially a barcode of normal and reversed polarity intervals stretching back through geologic time.

The GPTS was originally constructed by measuring the magnetic polarity and radiometric ages of young volcanic rocks, particularly basalt flows in places like Iceland and the western United States. When many dated lava flows were arranged in time sequence, a pattern emerged: intervals of normal polarity (field oriented like today) alternating with intervals of reversed polarity at irregular spacings. The major polarity intervals, lasting roughly 0.5 to 5 million years, are called chrons and are named after pioneers of geomagnetism — Brunhes (current normal), Matuyama (reversed), Gauss (normal), and Gilbert (reversed) for the most recent four. Within chrons, shorter episodes of opposite polarity called subchrons add finer detail; the Olduvai and Jaramillo subchrons within the Matuyama, for example, are normal-polarity intervals lasting a few hundred thousand years each.

The power of the GPTS lies in the fact that reversals are globally synchronous — when the field reverses, it reverses everywhere on Earth simultaneously. This means the polarity sequence recorded in a sedimentary section in Italy must match the sequence in a deep-sea core from the Pacific, even if the sediments are completely different in composition and fossil content. Magnetostratigraphy exploits this principle: by measuring the polarity of closely spaced samples through a stratigraphic section, a geologist builds a local polarity column — a sequence of normal and reversed zones. That local column is then compared to the GPTS to find the best-fit correlation, assigning numerical ages to the section's boundaries.

The calibration of the GPTS itself relies on a combination of radiometric dating (especially ⁴⁰Ar/³⁹Ar dating of volcanic rocks that bracket reversal boundaries) and the pattern of marine magnetic anomalies — the symmetric stripes of alternating normal and reversed polarity recorded in oceanic crust as it forms at mid-ocean ridges and spreads away. By measuring the width of these anomaly stripes and knowing the spreading rate, geophysicists extended the GPTS back through the Cretaceous and into the Jurassic, well beyond the reach of dateable continental lava flows. The resulting timescale — refined over decades and now known in versions like CK95 or GTS2020 — provides one of geology's most powerful correlation tools, capable of resolving ages to within a few tens of thousands of years for Cenozoic strata and anchoring global stratigraphic frameworks independent of fossil biozonation.