Why do fresh basaltic lavas at oceanic ridges become permanently magnetized?

Fresh basaltic lavas at oceanic ridges become permanently magnetized through a process known as thermoremanent magnetization (TRM).1

As molten magma erupts at the ridge crest and begins to cool, it contains iron-titanium oxide minerals. When these minerals cool below their Curie temperature—typically between 200–300 °C (392–572 °F)—the magnetic domains within the minerals align themselves with the orientation of Earth’s magnetic field at that time. This alignment is "locked" into the rock as it continues to cool, creating a permanent record of the magnetic field's polarity.123

Because seafloor spreading continuously moves this newly formed, magnetized crust away from the ridge axis, the oceanic floor acts as a conveyor belt that preserves a chronological record of Earth's periodic magnetic reversals. This process results in the symmetrical, linear magnetic anomalies observed parallel to oceanic ridges, which serve as critical evidence for the movement of tectonic plates.145