A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Paleomagnetic reversals create alternating zones of normal and reversed polarity in sedimentary and volcanic sequences. These polarity zones (magnetozones and magnetochrons) can be matched to the geomagnetic polarity time scale (GPTS), providing age constraints independent of radiometric dating. Magnetostratigraphy is valuable for dating sediments too young for radiometric methods or lacking datable minerals.
You already know from paleomagnetism that Earth's magnetic field has repeatedly reversed its polarity throughout geologic history — the north magnetic pole becomes the south pole and vice versa. These reversals are global and essentially instantaneous in geologic terms (typically completing within a few thousand years). Every rock that forms during a normal-polarity interval records a northward-pointing magnetic direction, and every rock that forms during a reversed-polarity interval records a southward-pointing direction. Magnetostratigraphy uses this binary signal — normal or reversed — as a dating tool by matching the pattern of polarity zones in a rock sequence to the known timeline of reversals.
The reference framework is the Geomagnetic Polarity Time Scale (GPTS), a detailed record of when every reversal occurred over the past ~170 million years. The GPTS was originally constructed from magnetic anomaly patterns on the seafloor (symmetrical stripes of normal and reversed polarity flanking mid-ocean ridges) and calibrated with radiometric dates from volcanic rocks. Each named interval of constant polarity is called a chron (or magnetochron), and shorter events within chrons are called subchrons. For example, the current normal-polarity interval — the Brunhes chron — began about 780,000 years ago, preceded by the reversed-polarity Matuyama chron. The GPTS provides a barcode-like pattern of long and short polarity intervals that is unique enough to be matched against patterns found in rock sections.
In practice, a magnetostratigraphic study begins by collecting oriented samples at closely spaced intervals through a sedimentary or volcanic section. Each sample is demagnetized in the laboratory to isolate its primary magnetic direction, and the polarity (normal or reversed) is determined. The result is a local polarity column — a sequence of normal and reversed zones stacked in stratigraphic order. The geophysicist then correlates this local column against the GPTS, looking for a match between the pattern of thick and thin polarity zones. A single reversal is ambiguous — many reversals look alike — but a sequence of five or more polarity zones with distinctive relative thicknesses usually produces a unique match, pinning the section to specific ages.
Magnetostratigraphy is especially powerful when combined with other dating methods. A single radiometric date or biostratigraphic datum within the section anchors the polarity pattern to the GPTS, resolving any ambiguity. Once correlated, every reversal boundary in the section becomes a dated horizon, providing age control at intervals of roughly 200,000 to 500,000 years throughout the section — far denser than most radiometric dating can achieve. This makes magnetostratigraphy invaluable for dating continuous sedimentary sequences like deep-sea cores, loess deposits, and lacustrine sections where datable volcanic ash layers are rare or absent.