Magnetism is one of the fundamental mechanisms that underpins physics, and becomes even more important while studying landscapes like a caldera. The Earth has a constant magnetic field, caused primarily by geodynamics in the outer core, however is also altered by different formations of rocks and minerals in the subsurface. This can be useful as an imaging tool, where measuring the magnetic field on land can tell us about the minerals we might find underground, or the geometry of what certain deposits of those materials may look like. Beyond that, magnetism is intricately tied to how parts of the caldera are even formed, dictating the direction of crystalline structures, or even flow patterns of magnetic materials over time.
During field surveys involving magnetism, the changes in this magnetic field are precisely what is being measured, in units of nanoteslas (nT). This can either be done passively, or by injecting an electric current into the Earth, and measuring the decay of the additional magnetic field that had been induced. Methods like the latter study the physical property of magnetic susceptibility, essentially a dimensionless ratio between how much magnetization the material picks up per unit of applied magnetic field. Remnant magnetization is slightly different, measured in amperes per meter (A/m), studying how much magnetization in a given rock is retained permanently, even after the external field is removed. Either way, magnetic methods are one of the best ways we have to image the subsurface, especially as the magnetic dipole decays at a cubic rate (1/r^3) with respect to the earth’s radius, meaning that changes near the local surface are easily readable.
The most prominent element that is measured in all of these survey methods is magnetite (Fe3O4). From the chemical formula we can immediately tell that iron rich rocks are highly susceptible to containing large amounts of magnetite; this kind of magnetism displayed by iron is called ferromagnetism. This directly plays into the geology of a caldera, for where igneous rocks like basalt will have a much higher ferromagnetic content then the sedimentary rocks around them. Within igneous rock, we have a distinction between Felsic and Mafic rocks, with higher and lower silica content percentages, of which Mafic rocks are more magnetic. We may even be able to infer the internal topography of a geothermal system using magnetism, since hot fluids consistently circulating through rock will chemically change the magnetic qualities of chambers it flows through.
-Leo