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Two studies, conducted 40 years apart, show how combining field observations and thermal modeling can reconstruct the history of massive lava flows and how they altered the surrounding landscape. 

Abstract In small‐volume igneous intrusions, the duration of magmatism can be difficult to determine because assembly of an intrusion from component magma pulses may occur within geochronologic uncertainties. We demonstrate that the paleomagnetic record of short‐term movement of the geomagnetic pole (secular variation) can place constraints on the duration of intrusion assembly over shorter time periods. An analysis of¹⁴C data paired with paleomagnetic data from lava flows illustrates this approach. The flows record paleosecular variation that, when combined with the maximum rate of secular variation from the Holocene, returns a minimum time elapsed between any two flows. Data from an Oligocene laccolith indicate that this system records a minimum of 49° of secular variation and therefore took at least 750 years to be emplaced. High‐precision radiometric geochronology would be unable to resolve this assembly, suggesting that the paleosecular variation record in shallow igneous rocks contains valuable temporal constraints on upper crustal magmatism.