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Magnetostratigraphy uses the pattern of paleomagnetic reversals (magnetic polarity zones) preserved in sedimentary and volcanic sequences to establish age correlations across regions, independent of fossils. The geomagnetic polarity time scale (GPTS) calibrates reversal boundaries using radiometric dates; reversal patterns are globally synchronous, enabling correlation and dating of undated sequences. Combined with biostratigraphy and radiometric dating, magnetostratigraphy yields high-resolution chronologies for Phanerozoic and Cenozoic strata.
From your study of paleomagnetism and reversals, you know that rocks can preserve a record of Earth's magnetic field at the time they formed — volcanic rocks lock in the field direction when magnetic minerals cool through their Curie temperature, and sedimentary rocks record the field as magnetic grains align during deposition. You also know that Earth's field periodically reverses polarity. Magnetostratigraphy exploits these facts to build a dating and correlation tool that works independently of fossils, lithology, or geographic location.
The basic procedure begins with collecting oriented samples at closely spaced intervals through a stratigraphic section — a cliff face, a road cut, a drill core. Each sample is brought to the laboratory and subjected to progressive demagnetization (either by heating in steps or by exposing it to alternating magnetic fields of increasing strength) to strip away secondary magnetization components acquired after the rock formed. What remains is the characteristic remanent magnetization (ChRM), which reflects the ambient field at the time of formation. By measuring the declination and inclination of the ChRM for each sample, the geologist determines whether the field was normal (like today) or reversed at the time that layer was deposited. Plotting polarity against stratigraphic position produces a local magnetic polarity column: a vertical barcode of normal and reversed zones.
This local polarity column is then compared to the geomagnetic polarity time scale (GPTS) — the master reference sequence of dated reversals compiled from radiometrically dated volcanic rocks and marine magnetic anomalies. Because reversals are globally synchronous (the field reverses everywhere at once), the pattern of normal and reversed intervals in any section on Earth should match some segment of the GPTS. The task is pattern matching: finding the unique stretch of the GPTS that best fits the observed local column. This is rarely unambiguous from magnetics alone — a sequence of three or four polarity zones could match multiple parts of the timescale. Independent age constraints from biostratigraphy (fossil assemblages that restrict the possible age range) or radiometric dates (from interbedded ash layers or lava flows) narrow the possibilities and lock the local column into the correct position on the GPTS.
Once the correlation is established, every polarity boundary in the section receives a numerical age from the GPTS, providing a chronological framework with resolution on the order of tens to hundreds of thousands of years — significantly finer than most biostratigraphic zonations. This makes magnetostratigraphy particularly valuable for correlating marine and continental sections (where fossil assemblages differ), for dating sediments that lack suitable fossils, and for calibrating the timing of evolutionary, climatic, and tectonic events across the Cenozoic and Mesozoic. The method's independence from lithology and biological content gives it a uniquely global reach among stratigraphic tools.