Reversed magnetization in pyroclastics from the 1985 eruption of Nevado del Ruiz, Colombia

One of the fundamental principles of palaeomagnetism and rock magnetism states that the direction of natural remanent magnetization (NRM) is acquired parallel to the applied field. However, ferromagnetic mineral phases with complex microstructures may violate this rule1,2 and produce a magnetization vector which is antiparallel to the direction of the external field. Andesitic pumice, which was hurled several kilometres during the disastrous 1985 eruption of the Nevado del Ruiz volcano (Colombia), carries a stable but reversed NRM with southerly declination and negative inclination. Heating experiments show that this magnetization is due to a self-reversal mechanism which also induces a reversed thermoremanent magnetization (TRM) in the laboratory field. Thus we have used the Nevado del Ruiz 1985 pumice, which was magnetized in the present well-known geomagnetic field, to demonstrate for the first time that self reversal has actually controlled the process of NRM acquisition.