and older rocks. It is sometimes assumed that the maximum metamorphic temperature can be simply related to the temperature at which a magnetization is removed from a sample in the laboratory. By this line of reasoning, if a rock had been heated to 320 $^{o}$C during a metamorphic event, say for 1 million years, its magnetization isolated by heating in the laboratory to 320 $^{o}$C would record the magnetic field at the time of the metamorphic event. The magnetization isolated as temperatures greater than 320 $^{o}$C might record the field when the rock first cooled. In reality, many (if not most) bulk rock samples would be totally remagnetized during this metamorphic event, and/or their magnetic field strength records might be forever compromised. To understand why this is so, and how we might see through metamorphism, we must first consider the basis for measurement of past field strength (the subdiscipline of paleointensity).\

Paleointensity measurement
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The physical mechanism of magnetization that is best understood is that of thermoremanent magnetization (TRM), acquired when a sample’s constituent minerals cool through their Curie temperatures. To recover paleointensity from rocks carrying a TRM, the most robust method is the Thellier approach, named after seminal work 