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Magnetic minerals acquire natural remanent magnetization (NRM) during cooling through the Curie temperature (thermoremanent magnetization) or during deposition and diagenesis (depositional remanence). The strength of NRM depends on paleomagnetic field strength at the time of remanence acquisition and is independent of the current ambient field. Saturation magnetization indicates the maximum magnetization possible for a given mineral.
From your study of paleomagnetism, you know that rocks can carry a memory of ancient magnetic fields. But how exactly does a rock become magnetized, and what controls how strong that magnetization is? The answers lie in the behavior of magnetic minerals — primarily magnetite, hematite, and their solid-solution relatives — and the physical processes that lock magnetic signals into the rock record.
Every ferromagnetic or ferrimagnetic mineral has a characteristic Curie temperature above which thermal energy overwhelms the magnetic ordering of atoms, and the mineral becomes paramagnetic (essentially non-magnetic in the paleomagnetic sense). For magnetite, this temperature is about 580°C; for hematite, about 675°C. When a lava flow cools and its magnetic minerals drop below the Curie temperature, the minerals acquire a magnetization aligned with the ambient geomagnetic field. This is thermoremanent magnetization (TRM), and it is the strongest and most stable form of natural remanent magnetization. The key insight is that once acquired, TRM is locked in by the crystal structure of the mineral — it persists for billions of years unless the rock is reheated above the Curie temperature or chemically altered. The intensity of TRM is proportional to the strength of the ambient field at the time of cooling, which is why paleointensity studies can estimate how strong Earth's field was millions of years ago.
Sedimentary rocks acquire remanence through a different mechanism. As sediment settles through water, magnetic grains — tiny crystals of magnetite or hematite — physically rotate to align with the ambient field before being locked into place by compaction and cementation. This depositional remanent magnetization (DRM) is typically weaker than TRM because not all grains align perfectly and because post-depositional compaction can disturb the original orientation. A related process, chemical remanent magnetization (CRM), occurs when new magnetic minerals grow during diagenesis or low-grade metamorphism; these minerals acquire a magnetization reflecting the field at the time of their growth, not the time of original deposition. Together, TRM, DRM, and CRM constitute the natural remanent magnetization (NRM) that paleomagnetic studies seek to measure and interpret.
Saturation magnetization is a different but related concept. It describes the maximum magnetization a mineral can achieve when every magnetic domain is aligned in the same direction — the state reached when an external field strong enough to overcome all internal resistance is applied. Saturation magnetization is an intrinsic property of the mineral (about 480 kA/m for pure magnetite at room temperature) and does not depend on the rock's history. It matters in practice because it sets the upper limit on how strong a rock's remanence can be and because comparing measured NRM to saturation magnetization reveals what fraction of the mineral's magnetic capacity was utilized — a ratio that carries information about the paleomagnetic field strength, grain size, and domain state of the magnetic carriers.