Paleomagnetic reversals create distinctive magnetic polarity zones in volcanic and sedimentary sequences. These zones can be correlated to the geomagnetic polarity time scale, providing precise dating without radiometric analysis. Magnetostratigraphy is particularly valuable for Neogene and younger sequences.
From your study of geomagnetic reversal chronology, you know that Earth's magnetic field periodically flips polarity — magnetic north becomes magnetic south and vice versa — at irregular intervals ranging from tens of thousands to millions of years. From stratigraphy, you know that rock layers record time sequences. Magnetostratigraphy exploits the intersection of these two facts: when rocks form, they lock in the magnetic field direction of the time they formed, creating a sequence of normal and reversed polarity zones that can be matched to the global geomagnetic polarity time scale (GPTS) like a barcode.
The recording mechanism differs between rock types. In volcanic rocks, iron-bearing minerals like magnetite crystallize from cooling lava and align with the ambient magnetic field. Once the rock cools below the Curie temperature (about 580°C for magnetite), the magnetic alignment is locked in permanently — this is thermoremanent magnetization. In sedimentary rocks, tiny magnetic grains (detrital magnetite or hematite) physically rotate to align with the field as they settle through water and become trapped during compaction. This detrital remanent magnetization is weaker and can be affected by post-depositional processes, but in fine-grained sediments deposited in quiet water, it faithfully records the field direction at the time of deposition.
The practical technique works by drilling oriented core samples at closely spaced intervals through a stratigraphic section and measuring each sample's magnetic polarity in the laboratory. The result is a column of normal (N) and reversed (R) polarity zones — a magnetic polarity stratigraphy for that section. This local pattern is then compared to the GPTS, which has been independently calibrated using radiometric dating of volcanic rocks with known polarities. The GPTS has a distinctive, irregular pattern of long and short polarity intervals — the Brunhes normal chron (0–0.78 Ma), the Matuyama reversed chron (0.78–2.58 Ma), and so on — that functions like a unique fingerprint. When the pattern of polarity zones in your section matches a segment of the GPTS, you have dated those rocks without needing any radiometric measurements from the section itself.
Magnetostratigraphy is especially powerful for Neogene and Quaternary sequences (the last ~23 million years), where the GPTS is most precisely calibrated and polarity intervals are short enough to provide high temporal resolution. It excels in sedimentary environments like deep-sea cores and continental basins where datable volcanic layers are absent, making it complementary to biostratigraphy and radiometric dating. The technique also provides global correlation capability: a polarity reversal happens everywhere on Earth simultaneously, so a reversal boundary identified in a marine core from the Pacific can be correlated directly with one in a terrestrial section in East Africa, bridging environments that share no fossil species in common.