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Rock magnetization acquires a remanent magnetization (TRM in igneous rocks, DRM in sediments) parallel to Earth's magnetic field at the time of formation, preserving a record of ancient field directions. Paleomagnetic reversals—sudden switches of the dipole polarity (north ↔ south)—occur irregularly on timescales of 200,000 to millions of years; the reversal rate accelerated in the Cenozoic. The paleomagnetic record provides a dating tool and reveals true polar wander and apparent polar wander paths used in plate reconstruction.
From your understanding of Earth's magnetic dipole field, you know that our planet generates a roughly dipolar magnetic field through convection in the liquid outer core. This field has a north and south magnetic pole, and at any point on the surface it has a specific declination (the angle from geographic north) and inclination (the angle below horizontal, which varies with latitude). Paleomagnetism is the science of reading ancient field directions preserved in rocks — essentially using rocks as fossil compasses.
The recording mechanism depends on the rock type. When an igneous rock cools through its Curie temperature (about 580°C for magnetite), the magnetic minerals lock in a magnetization parallel to the ambient field. This thermoremanent magnetization (TRM) is strong and stable over billions of years. In sedimentary rocks, tiny magnetic grains physically rotate to align with the field as they settle through water, producing a detrital remanent magnetization (DRM) that is weaker but still preserves the field direction at the time of deposition. In both cases, the key insight is the same: the rock becomes a snapshot of the magnetic field at a specific moment in geological time.
The most dramatic feature of the paleomagnetic record is that Earth's field periodically reverses polarity — magnetic north and south swap places. During a reversal, the field weakens, becomes complex and multipolar for a few thousand years, then re-establishes with opposite polarity. These reversals are not periodic; they occur irregularly, with intervals between reversals ranging from tens of thousands to tens of millions of years. The record of normal and reversed polarity intervals has been compiled into the geomagnetic polarity timescale (GPTS), calibrated by radiometric dating of volcanic rocks. This timescale is one of the most powerful dating tools in geology: if you measure the polarity sequence in a sedimentary or volcanic section, you can correlate it to the GPTS like matching a barcode — a technique called magnetostratigraphy.
Beyond dating, paleomagnetism is the backbone of plate tectonic reconstructions. Because inclination depends on latitude (tan(I) = 2·tan(λ)), measuring the remanent inclination of an ancient rock tells you the latitude at which it formed. If that latitude differs from the rock's present position, the plate has moved. By compiling paleomagnetic directions from rocks of many ages on a single continent, you trace an apparent polar wander path (APWP) — a curve showing where the magnetic pole appeared to be over time. The pole did not actually wander that much; the continent moved. When APWPs from two continents diverge back in time and then converge, it reveals when the continents were joined and when they separated, providing quantitative constraints on paleogeography that no other method can match.